015 Sillitoe

Economic Geology Vol. 80, 1985, pp. 1467-1514

Ore-Related Breccias in Volcanoplutonic Arcs RICHARD H. SILLITOE

8 West Hill Park, Highgate Village, London N6 6ND, England

Abstract

An overview of breccias related to a v•riety of base metal, precious metal, and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation.

1. Release ofmagmatic-hydrothermal fluids from high-level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and/or ascent. Single or multiple intrusion-related breccia pipes and pre- and intermineral breccias in porphyry copper deposits provide widespread examples.

2. Magmatic heating and expansion of meteoric pore fluids may lead to brecciation, commonly of late or postmineral age and including pebble dikes, in porphyry-type and related deposits. Magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of Kuroko-type massive sulfide deposits; many of the resultant breccias underwent limited sedimentary transport. Overpressuring of heated fluids beneath semipermeable, partly self-sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings; magmatic heating or tectonic disturbance may have triggered brecciation.

3. Interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions. Postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious (4- base) metal deposits were generated in this manner; these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters.

4. Magmatic-hydrothermal brecciation may lead to disruption of rocks through to the palcosurface, decompression, and fragmentation and eruption of the top part of an underlying magma chamber. Pre- and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry-type and other base and precious metal systems; they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits.

5. Breccias may result from mechanical disruption of wall rocks during subsurface movement of magma. Any intrusion-related deposit may include such intrusion breccias.

6. Tectonic breccias resulting from fault displacement may accompany any type of ore deposit.

A continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction.

Introduction

BRECCIAS with an enormous variety of characteristics are common, perhaps ubiquitous, accompaniments to a wide spectrum of hydrothermal ore deposits. They have fascinated and perplexed miners and geologists for at least 200 years. Ore-related breccias were identified correctly during the late 19th century (e.g., in Cornwall, England; Hunt, 1887, p. 421-422), and in 1896, Emmons provided an explicit description of the Bassick and Bull-Domingo breccia pipes in Col- orado. The common occurrence of breccias as hosts for, or associates of, hydrothermal ore deposits was generally appreciated by the early 20th century, as evidenced by perceptive reviews of their characteristics and proposals for their origin by Locke (1926), Walker (1928), and Emroohs (1938). Notwithstanding their early recognition, however, it has only been during the last decade or so that some of the more subtle varieties and expressions of brecciation have

been appreciated. Even today, large matrix-rich bodies of breccia are often confused with volcanosedi- mentary formations and elongate matrix-poor breccias are incorrectly assigned a tectonic origin. Worse still, ore-related breccias not uncommonly pass unnoticed.

Ore-related breccias were last reviewed by Bryner (1961). Mayo (1976) presented an historical overview of subsurface breccias of igneous affiliation, but only a few of his examples are associated with ore deposits. This paper begins with a brief discussion of classification problems and proceeds to a description of the characteristics, alteration and mineralization features, and possible origins of six categories of ore-related breccias. The treatment is based on the writer's field studies combined with a perusal of the voluminous literature on ore-related breccias.

Attention is restricted to volcanoplutonic arcs because they contain a greater number and variety of ore-related breccias than any other metallogenic set-

0361-0128/85/439/1467-4852.50 1467

1468 RICHARD H. SILLITOE

ting and have provided most of the examples described in the literature. Discussion is focused on breccias that were generated in subsurface environments by hypogene processes in association with economically significant base metal, precious metal, and lithophile element deposits. Subaerial volcanic breccias are not dealt with, except for those that accumulated in close proximity to their subsurface feeders.

Although this paper is restricted to ore-related breccias, it should be emphasized that numerous examples of apparently similar breccias devoid of even subeconomic amounts of mineralization are known from arc terranes in many parts of the world (e.g., Gates, 1959; Morris and Kopf, 1967; Bussell and McCourt, 1977).

Classification

A comprehensive genetic classification of ore-related breccias remains elusive. The proliferation of genetic terms used to describe breccias tends to ob- scure rather than illuminate the subject: intrusion, intrusive, explosion, eruption, collapse, phreatic, phreatomagmatic, hydrothermal, fiuidization, gas fluxion, steam blast, hydraulic fracture (hydrofrac), and tuffisitic are just some of the qualifters used, commonly loosely or even erroneously, in the literature. The difficult question of origin has been further com- pounded by attempts to explain the formation of breccias in general by a single mechanism. In common with Bryner (1961) and Richard (1969), the writer prefers the notion of multiple origins for ore-related breccias and is in sympathy with Joralemon (1952, p. 256) when he stated: "It is inconceivable that all breccia chimneys were formed by the same process," and "Nature evidently loves a breccia, and if no violent phenomenon is available, the breccia is formed just the same"!

In principle, ore-related breccias are amenable to classification on the basis of either genetic or descriptive criteria, in the same way as Recent volcanic rocks (e.g., Wright et al., 1980). Ideally, the descriptive criteria would be diagnostic of a breccia's genesis. In

the case of ore-related breccias, however, it has proved impossible to infer the process reliably from observed geometric, lithologic, and textural characteristics. Existing classification schemes, such as those by Wright and Bowes (1963), Kents (1964), and Bry- ner( 1968), are inadequate because of the subjectivity of many of the descriptive parameters employed, as well as because of the lack of support for many of the resulting genetic assumptions.

In this paper, ore-related breccias are discussed in the context of a broad genetic framework, which takes into account the overlap now widely recognized between intrusive, volcanic, and hydrothermal processes. With the exception of tectonic breccias, the primary division is based on the inferred role of magma and/or aqueous fluids in breccia formation, and further subdivision is on the basis of ore deposit type. The resulting scheme, which dictates the or- ganization of this paper, is summarized in Table 1. Assignment of a breccia to the appropriate category does not rely solely on breccia characteristics but also takes cognizance of the overall environment of brecciation, in particular the relationship to, and conditions of, accompanying ore deposition. The recognition of modern analogs for several types of ore-related breccias also proves useful.

Magmatic-hydrothermal breccias are products of the release of hydrothermal fluids from magma chambers, irrespective of the original source of the fluids concerned (magmatic, meteoric, connate, or ocean waters). Hydromagmatic (including hydrovolcanic) breccias, as defined by Macdonald (1972) and Sher- idan and Wohletz (1981), are generated by the interaction of magma and an external source of water, such as ground or surface (ocean, lake) waters. The hydromagmatic category is subdivided into phreatomagmatic breccias, where both water and magma directly contributed to formation of the observed products, and phreatic breccias, in which only magmatic heat had access to the external water source. Mag- matic (including volcanic) breccias result from fragmentation and eruption of magma from subsurface

TABLE 1. Subdivision of Ore-Related Breccias Employed in this Paper

Magmatic-hydrothermal breccias

Hydromagmatic (hydrovolcanic) breccias

Magmatic (volcanic) breccias Intrusion breccias

Tectonic breccias

Phreatic breccias

Phreatomagmatic breccias

Pipes related to intrusions Porphyry-type deposits

Epithermal precious (4- base) metal deposits Porphyry-type and other intrusion-related deposits Kuroko-type massive sulfide deposits

Porphyry-type and epithermal precious (4- base) metal deposits

Porphyry-type and other base and precious metal deposits Any intrusion-related deposits

Any type of ore deposit

Jose Javier Rojas

Jose Javier Rojas

Jose Javier Rojas

Jose Javier Rojas

Jose Javier Rojas

ORE-RELATED BRECCIAS IN VOLCANOPLUTONIC ARCS 1469

chambers. The remaining categories of subsurface breccia--intrusion and tectonic--are only briefly considered for the sake of completeness. Intrusion breccias are a direct product of the passive subsurface movement of magma. Tectonic breccias are primarily the products of tectonic processes, in which water may or may not have participated. The widely employed term hydrothermal breccia describes the products of magmatic-hydrothermal and hydromagmatic processes and therefore provides a valuable designation for many ore-related breccias.

An additional category, amagmatic-hydrothermal, may be introduced to include breccias generated by hydrothermal fluids of, say, meteoric or cormate origin, uninfiuenced by magmatism. The breccias recognized from Mississippi Valley-type lead-zinc deposits, sediment-hosted massive sulfide lead-zinc deposits, unconformity-type uranium deposits, and sediment-hosted pipes and bodies are all assignable to this category. However, since these ore deposit types are generally absent from arc terranes, amagmatic-hydrothermal breccias are not considered further.

Magmatic-Hydrothermal Breccias Pipes related to intrusions

General remarks: This section describes breccias, confined to single or multiple pipes, that possess a close genetic connection with unaltered and unmin- eralized intrusive rocks, either batholiths or stocks. There seems to be a gradation from districts characterized by one or more breccia pipes associated with fresh intrusive rocks to districts in which the pipes constitute only parts of larger volumes of pervasive alteration-mineralization of porphyry type (see below). Although most of the breccias summarized in Table '2 are demonstrably not parts of porphyry systems, and therefore are not underlain by porphyry- type mineralization, Copper Creek (Grimour, 1977) and Kidston (R. H. Sillitoe, unpub. rept., 1980) could be the high-level manifestations of largely concealed bodies of porphyry copper-molybdenum and Climax- type porphyry molybdenum mineralization, respectively.

It is clear from Table 2 that there is no age restriction for mineralized breccia pipes. Known examples range from Archcan through Proterozoic and Paleo- zoic to Meso-Cenozoic. Most of the western American breccia pipes are Mesozoic or Cenozoic in age, although the absence of examples in Table 2 younger than Eocene is noteworthy. This observation is interpreted to reflect eraplacement of the breccias at hyp- abyssal depths (1-3.6 kin; So and Shelton, 1983) and the time required for their subsequent unroofing.

Characteristics: The intrusion-related breccias under consideration here are restricted to pipes that may occur individually or in closely spaced clusters of up

to 200 or more (Table 2). Pipes (also termed chimneys or columns) are generally roughly circular to ovoid in cross section and possess vertical dimensions which are observed or inferred to be several times greater than their maximum horizontal dimensions. Horizon- tal dimensions are commonly in the range of 50 to 300 m but are as great as 1,300 X 900 m at Kidston (Placer Exploration Ltd., 1981) or as little as 3 m in the Cabeza de Vaca district (Sillitoe and Sawkins, 1971). The full vertical extent of a pipe is nowhere observable, although minimum vertical dimensions of 725 to 860 m are known for four districts (Table 2). Unless tilted subsequent to emplacement, pipes are only uncommonly inclined at more than 15 ø from the vertical.

Several examples of partly bifid pipes have been recorded. The San Antonio de La Huerta pipe in Sonora, Mexico, divides downward into two prongs (R. H. Sillitoe, unpub. rept., 1975), whereas the Childs-Aldwinkle pipe in the Copper Creek district (Kuhn, 1941), the Ilkwang pipe (Fletcher, 1977), and the A-B pipe at Inguar/tn (Sawkins, 1979) all bifurcate upward.

The contacts between breccia pipes and their wall rocks are commonly abrupt, and in many cases, marked by a zone of closely spaced vertical fractures (or sheeting) from 1 to 5 m wide (Fig. 1). Fractures may be mineralized or lined with fault gouge. Sheet- ing is not present as a single uninterrupted annulus but is made up of several straight to gently curved bands of fractures, which commonly tend to be more markedly curved at one of their ends. Overlap of these several lengths of sheeting tends to give a polygonal outline to pipes. Alternatively, breccia and unfractured wall rocks may grade into each other over distances of several meters.

The upward and downward terminations of pipes are not commonly observed. Locally, as in the San Pedro de Cachiyuyo district (Sillitoe and Sawkins, 1971), pipes are seen to be capped by dome-shaped roofs overlain by columns of altered but unbrecciated rock, and it seems unlikely that many of these breccia pipes approached the palcosurface. Where the bottoms of pipes have been observed, as in the A-B pipe at Inguar•m (Sawkins, 1979) and the San Antonio de La Huerta pipe (R. H. Sillitoe, unpub. rept., 1975), they are irregular but grossly fiat, and breccia ter- minates abruptly against less altered intrusive or country rocks. The Copper Prince pipe in the Copper Creek district is underlain by a mineralized open fissure (Kuhn, 1941; Joralemon, 1952), whereas the lensoid Extensi6n San Luis pipe at Inguar•m is transitional downward to a shear zone (V. F. J. Escand0n, unpub. talk 1974).

The breccias are normally characterized by angular to subrounded fragments ranging in size from a few centimeters to several meters and, locally, several tens


TABLE 2. Selected Examples of Mineralized

Locality Host rocks Age (m.y.)

No. of pipes Surface Vertical (total/ dimensions dimension

mineralized) (m) (m) Fragment

form Rock flour

Tribag, On- Granite, mafic vol- tario, Canada canics, felsite

1,055 4/3 up to 700 X 300 >860 Angular Absent (except East breccia)

Chadbourne, Andesitic + rhyo- Ontario, Can- litic volcanics ada

Golden Sun- Calcareous sedi- light, Mon- ments, latite por- tana phyry

Victoria, Limestone, sand- Nevada stone

Copper Creek, Granodiorite, an- Arizona desitic volcanics

Ortiz, New Quartzite, pyroclas- Mexico tics

Los Pilares, Latitic q- andesitic Sonora, volcanics Mexico

Washington Andesitic, latitic dist., Sonora, q- trachytic Mexico volcanics

La Colorada, Trachytic q- rhyoli- Zacatecas, tic pyroclastics Mexico

Inguarfm, Mi- Granite, granodio- choac•tn, rite, granodiorite Mexico porphyry

Tu'rmalina, Peru

Granodiorite

Archean

Early Ter- tiary

135(?)

68

Oligocene

-55(?)

45.7 •

53.6 t

35.6 •

Tertiary

1/1 300 x 120 >750

1/1 200 x 200 , >550

>4/1 >200 x 75 >800

>200/8 up to 180 >270

3/1 970 X up to 600 >150

1/1 600 X 300 >725

13/2 up to 100 >400

9/6 up to 100 X 40 >300 (600 inferred)

10/3 up to 600 X 300 225

1/1 150 X 150 >600

Angular Absent

Angular to sub- Absent rounded

Angular, locally Present in rounded parts

Angular to Absent rounded

Angular to Locally pres- rounded ent

Angular Absent

Angular or Present in rounded some pipes

Mainly Abundant rounded

Angular to 10 to >50% rounded


San Pedro de Cachiyuyo, Chile

Cabeza de Vaca, Chile

El Bolsico, Chile

San Francisco

de Los Andes, Argentina

Granodiorite

Granodiorite, andesitic volcanics

Quartz diorite, quartz diorite porphyry

Sandstone, shale, siltstone

Paleocene 24/10 up to 250 X 130 216

62 >100/5 up to 70 > 100

Paleocene

Late Carbon- iferous-

Early Permian

4/1 180 x 95 >170

3/1 70 x 15-30 >35


Angular to locally rounded

Angular to rounded

Angular

Absent

Abundant

Absent


Breccia Pipes Related to Intrusive Rocks

Hydro- thermal

alteration Principal hypogene (t = tour- metallic minerals maline) (in order of abundance)

Principal gangue minerals

Structural control

Related intrusive rock

Ore reserve

and/or mined (M = million, t = metric tons) Reœerence

Sericitic, Pyrite, chalcopyrite, Quartz, calcite, chloritic, pyrrhotite, magnetite, ankerite, lau- argillic molybdenite montite

Sericite-cal- Pyrite cite

Silicifica- tion, sericitic

Calc-silicate

Sericitic (t), K silicate

Sericitic

Sericitic, chloritic

Sericitic, K silicate, chloritic

Sericitic

Propylitic (t)

Sericitic, chloritic

(t)

Sericitic (t)

Sericitic (t)

Sericitic (t)

Silicifica-

tion (t)

Pyrite, chalcopyrite, bornitc, galena, sphalerite

Pyrite, chalcopyrite

Pyrite, chalcopyrite, molybdenite, bornitc

Pyrite, magnetite, hematite, scheelite

Specularitc, pyrite, chalo copyrite, scheelite

Pyrite, chalcopyrite, molybdenite, scheelite

Pyrite, sphalerite, galena, tetrahedrite, chalcopyrite

Chalcopyrite, pyrite, scheelite

Pyrite, chalcopyrite, molybdenite, arsenopyrite, wolframite, scheelite


Pyrite, chalcopyrite, specularitc, scheelite

Chalcopyrite, molybdenite, pyrite, specularitc

Pyrite, arsenopyrite, bismuthinite, chalcopyrite

Quartz, albite, calcite, anker- itc, dolomite

Quartz, barite, sericite, fiuora- patire

Calcite, diopside, garnet, quartz

Quartz, sericite, chlorite, tourmaline

Calcite

Quartz, calcite, chlorite

Quartz, tourmaline

Quartz

Quartz, epidote, tourmaline, chlorite, calcite



Quartz, tourmaline, K-feldspar, calcite

Quartz, tourmaline, sericite, calcite

Tourmaline, quartz

Faults, joints, contacts

Fault related

Not recognized

Absent

Probably absent

Not recognized

Not recognized

At least partly fault related

Not recognized

N 20 ø W + N

70 ø E faults(?)

Not recognized

Absent

Absent

Not recognized

Jointing

Felsite stock(?)

Syenite(?) body

Latite porphyry stock(?)

Quartz fatire porphyry stock(?)

Latite

Quartz latite porphyry(?)

Unknown

Granodiorite pluton(?)

Quartz monzonite(?)

Granodiorite q- granodiorite porphyry stock

Granodiorite pluton

Granodiorite pluton

Granodiorite pluton

Granodiorite pluton

Granodiorite pluton

i Mr, 1.6% Cu; 40 Mt, 0.2% Cu (Breton pipe)

1.8 Mr, 4.5 ppm Au

31 Mr, 1.9 ppm Au

2.2 Mr, 2.4% Cu, 0.05% Bi

3,714 t Cu, 3,151 t Mo

7 Mt, 1.7 ppm Au, 0.05% WOa

19 Mr, 2.6% Cu; 44 Mt, 0.8% Cu

1.2 Mr, 1.7% Cu, 0.14% W, O.O6% Mo

2 Mr, 4% Pb q- Zn, 120 ppm Ag

6 Mt, 1.2 to 1.5% Cu, 0.04% WOa

13,600 t Cu, 1,360 t Mo

>0.6 Mt, 3.7% Cu

High-grade Cu, minor W

2.7 Mr, 1.27% Cu, 0.12% Mo

38 t Bi

Armbrust (1969), Blecha (1974), Norman and Sawkins (1985)

Walker and Cregh- cur (1982)

Porter and RipIcy (1985)

Atkinson et al. (1982)

Kuhn (1941), Jora- lemon (1952), Simons (1964)

Lindquist (1980), Wright (1983)

Wade and Wandtke (1920), Locke (1926), Thorns (1978)

Sillitoe (1976), Simmons and Sawkins (1983)

Albinson (1973)

Escand6n (unpub. talk, 1974), Silli- toe (1976), Sawkins (1979)

Carlson and Sawk- ins (1980)

Sillitoe and Sawkins (1971)

Parker et al.

(1963), Sillitoe and Sawkins (1971)

Pimentel (1979), C. Llaumett (unpub. rept., 1981)

Llambias and Mal- vicini (1969)

14 7 2 RICHARD H. SILLITOE

TABLE 2--(Continued)

Locality Host rocks Age (m.y.)

No. of pipes Surface Vertical (total/ dimensions dimension Fragment

mineralized) (m) (m) form Rock flour

Y16j'firvi, Fin- Intermediate volca- 1,800 to land nics 1,900

Ilkwang, Quartz monzonite 69 S. Korea

2/1 700 X 5-80 380 Angular Absent

1/1 80 X 50 >100 Angular to Absent rounded

Khao Soon, Argiilaceous sedi- Thailand ments

Redbank, Trachytic volcanics, Northern dolomite, sand- Territory, stone, shale Australia

Triassic(?)

1,575(?)

Kidston, Gneiss, granodio- Middle Car- Queensland, rite boniferous Australia

1/1 800 X 400 >300 Angular to sub- Absent rounded

50/9 up to 135 >330 Angular Generally absent

1/1 1,300 X 900 >250 Angular to sub- <5% rounded

After Damon et al. (1983)

of meters. Megafragments > 100 m across have been defined at Kidston (Placer Exploration Ltd., 1981). Several pipes exhibit a marked decrease in the degree of breeeiation both inward and downward, as at E1 Bolsleo (Pimentel, 1979), Turmalina (Carlson and Sawkins, 1980), Ilkwang (Fletcher, 1977), and Los Pilares (Wade and Wandtke, 1920). The last is characterized by an unbreeciated cylindrical core. Large fragments in pipe interiors may also display zones of marginal sheeting similar to those around pipes.

Breeeia fragments were separated by 5 to 30 vol percent open space prior to complete or partial ee- menration by gangue and sulfide minerals. In a few pipes, silt- to sand-size clastic material, commonly termed rock flour, is present as a matrix and is generally accompanied by fragments with a greater degree of rounding. Fragment rounding and rock flour are both attributed to interfragment attrition. Rock flour-bearing breccia may be present in pipes as local patches, as distinctly separate, commonly late bodies (e.g., Victoria, Atkinson et al., 1982; Redbank, Knut- son et al., 1979), or as the only material present (e.g., La Colorada, Albinson, 1973; Inguarftn, V. F. J. Es- eand6n, unpub. talk, 1974). The informal descriptive terms open space breccia and rock flour breceia may be used to distinguish between these two end-member types. These terms are preferred to the roughly equivalent collapse and intrusion breecias of Bryner (1968) and many subsequent workers because they do not connote formational mechanisms. Both types of breccias may be clast supported, but in many ex-

ampies a matrix of either rock flour or hydrothermal cement completely separates fragments.

Some breccias, or more generally, their upper parts, are characterized by tabular fragments to which the descriptive terms shingle breccia or domino breccia have been applied (Fig. 2). In extreme eases, aspect ratios of tabular fragments attain 1:30. In many examples, it is clear that the tabular form of fragments is not attributable to closely spaced jointing or bedding of prebreeeiation lithologies. Characteristically, tabular fragments are aligned parallel to one another, like shingles on a roof, with attitudes changing progressively from steep in close proximity to the pipe walls through gently inward dipping to subhorizontal in the central parts of pipes. Shingle breccia is probably produced by the regular breakage and detachment of zones of sheeting like those around pipe walls and large fragments. The progressive decrease in dip of tabular fragments inward from pipe walls suggests a process of slabbing from the walls and/or roof of a pipe followed by downward settling.

Some breccias contain highly rounded, spheroidal clasts, locally up to 1 m in diameter (Fig. 3). Such fragments may be isolated in angular breecias or may constitute the majority of the fragments throughout, or in part of, a pipe, as at Bull-Domingo, Colorado (Emmons, 1896). The outer portions of some spheroidal clasts are characterized by closely spaced concentric fractures, which give rise to an onionlike ap- pearance termed hypogene exfoliation by Farmin (1937). Locally, the outermost concentric layer(s) is


Hydro- thermal

alteration

(t = tourmaline)

Principal hypogene metallic minerals

(in order of abundance) Principal gangue Structural Related

minerals control intrusive rock

Ore reserve

and/or mined (M = million, t = metric tons) Reference

Silicifica- tion, chloritic

Sericitic

Sericitic, si- lieifiea- tion

Arsenopyrite, chalcopy- Tourmaline Not recognized Granodiorite 4 Mt, 1.4% Cu, rite, pyrrhotite, pluton •0.04% scheelite WOa

Pyrrhotite, chalcopy- Quartz, tourma- Absent Quartz monzo- 3,500 t Cu, 40 rite, arsenopyrite, line nite stock t W wolframite

Ferberite, pyrite Quartz Nearby fault Unknown W

K-feldspar- Chalcopyrite chlorite

Sericitic, Pyrite, sphalerite, ga- carbonate lena

Dolomite, quartz, E to NE linea- Trachyte chlorite ments plugs(?)

Quartz, calcite, Not recognized Rhyolite dikes sericite + stock(?)

Himmi et al. (1979)

Fletcher (1977)

Ishihara et al.

(1980)

3.5 Mt, 1.8% Orridge and Mason Cu (1975), Knutson

et al. (1979)

39 Mt, 1.76 Bain et al. (1978), ppm Au Placer Explora-

tion Ltd. (1981)

partially detached and, in places, disaggregated to produce tabular fragments.

Intrusion-related breccias rarely reveal evidence to suggest appreciable vertical displacement of fragments during pipe emplacement. In fact, in parts of some pipes, fragments appear merely to have been pulled apart and can be fitted back into their original positions as in a jigsaw (Fig. 4). Normally the lithologies of fragments closely match those of their wall rocks, thereby commonly producing monolithologic breccias. Where several rock types adjoin a pipe, little mixing of fragments of different lithologies has taken place and contacts beyond the pipe may be extended through the breccia (Fig. 5). There is, however, normally a relatively small downward displacement of all fragments at most levels within a pipe. This has been quantified by comparison with distinctive wall-rock lithologies at several localities and amounts to 25 m at Washington (Simmons and Sawkins, 1983), 100 m at Redbank (Orridge and Mason, 1975) and Tribag (Norman and Sawkins, 1985), >125 m at Panuco, Mexico (Buchanan, 1983), and a maximum of 160 m at Los Pilares (Wade and Wandtke, 1920; Fig. 5). Locally, however, there is evidence for some mixing and upward transport of fragments, as at La Colorada and Kidston.

Breccias are commonly located in the upper parts of, or immediately above, plutons or stocks, or are distributed around their sloping margins. In some districts, pipes may be interpreted to have extended from the upper parts of a pluton into its roof rocks.

In several districts, including some confined to sizable plutons, small volumes of fine-grained porphyritic intrusive rock are temporally, spatially, and probably genetically associated with the brecciation process. The intrusive rock may occur as dikes and small bodies, angular breccia fragments, and irregular, partly disaggregated masses within the pipe. The last type of occurrence provides evidence that the magma was plastic during brecciation. These minor intrusions have been emphasized from the Chilean districts (Parker et al., 1963; Sillitoe and Sawkins, 1971), Copper Creek (Simons, 1964), Tribag (Blecha, 1974), Victoria (Atkinson et al., 1982), and Kidston (Placer Exploration Ltd., 1981), and suggest the presence in depth of larger bodies of the same intrusive rock with which pipe formation was linked. Such a body was encountered by drilling some 800 m beneath the out- crop of the Breton pipe at Tribag (Blecha, 1974).

Table 2 suggests that there is no general agreement on the role of structure in localization ofbreccia pipes. The impression is gained from the literature that the importance assigned to structural control says more about the proclivity of the observer than it does about the localization of breccia pipes! This statement is borne out by comparing the interpretations of Kuhn (1941) and Simons (1964) for the Copper Creek district. On the basis of available evidence, it is tentatively concluded that major regional structures play little part in breccia pipe formation and, if structural control is significant, it is likely to be by minor faults, fractures and joints. One of the most detailed struc-


FIG. 1. A typical sheeted zone bordering a breccia pipe. Ilk- wang, southern Korea.

tural studies of a breccia pipe and its environs was undertaken at Chacritas, Chile, by Reyes and Charrier (1976), who concluded that neither the position nor the shape of the pipe was structurally determined.

-?

FIG. 3. Spheroidal fragment and its mould.

Alteration and mineralization: Most intrusion-related breccias carry copper mineralization, although molybdenum, tungsten and/or gold are commonly also economically important commodities (Table 2), and a minor tonnage of bismuth ore was exploited at San Francisco de Los Andes (Llamblas and Malvicini, 1969). Breccias at Chadbourne, Golden Sunlight, Or- tiz, and Kidston (Table 2) are exploitable solely for their gold (and subordinate silver) contents. A few breccias are different and contain silver-lead-zinc or tungsten mineralization (Table 2).

All breccias of this type underwent to some degree the hydrothermal replacement and open-space-filling stages referred to below, a fact which strongly suggests that alteration and mineralization were necessary consequences of the brecciation process. How- ever, (50 percent of breccias in any duster of pipes are ore bearing (Table 2), a characteristic that has often frustrated the explorationist (Joralemon, 1952).

Sericitization is the most common alteration type

FIG. 2. Shingle breccia cemented by massive tourmaline from a breccia pipe. Yabricoya district, Chile. Geology pick handle as

FIG. 4. Typical jigsaw breccia cemented by tourmaline and sericitized along fragment margins and fractures. Approximately one-third natural size.

ORE-RELATED BRECCIAS 1N VOLCANOPLUTONIC ARCS 1475

E W

SHEE•EDI v .•• ' "'"'•K•v • •,• [••1 VOLCANIC

1001, , 2.5-3.0• Cu ORE "=• 0 melers ]00

FIO. 5. Cross section through the Los Pilares breccia pipe, Sonora, Mexico. It shows the distribution of copper orebodies as an annulus in the m•ginal p•t of the breccia • well as smaller bodies within it, and the depression of the latite-andesite contact within the breccia pipe. Taken from Wade and Wandtke (19•0), with lithologic nomenclature from Thorns (1978).

in the breccia pipes discussed in this section and is commonly accompanied by tourmaline (Table 2; Figs. 2 and 4). Chloritization and silicification were also commonly developed, propylitic and K silicate assemblages are recorded in a few pipes or parts thereof, and calc-silicate alteration is present at Victoria (At- kinson et al., 1982). Alteration generally ends abruptly around the margins of pipes, especially at sheeted zones, but in some examples (e.g., Ilkwang; Fletcher, 1977) may extend a few meters or even tens of meters into the wall rocks. Marked changes in alteration type are observed in some pipes: sericitization changes downward to propylitization at Los Pi- lares (Wade and Wandtke, 1920) and transitions from sericitic to K silicate assemblages have been noted in the lowermost portions of pipes at Washington (Sim- mons and Sawkins, 1983), Childs-Aldwinkle, Copper Creek district (Kuhn, 1941), and Los Verdes, Buena Esperanza district, Mexico (R. H. Sillitoe, unpub. rept., 1975).

The alteration (replacement) stage in breccia pipes took place immediately after, and perhaps also during, fragmentation. It was followed by an episode of open- space filling, during which both gangue and metallic minerals were precipitated (Table 2). Both are commonly coarse grained and well crystallized, and pegmatitic textures are common. In copper-bearing pipes, the open-space-filling stage commenced with the outward growth from fragments of tourmaline and/ or quartz, followed by any scheelite, wolframite, or arsenopyrite and finally by pyrite (and/or pyrrhotite), chalcopyrite, and molybdenite. Sphalerite and galena followed by carbonates and/or late quartz may constitute a final filling. Ore minerals at Inguar•tn, E1 Bol- sico, and La Colorada are dispersed in interfragment

rock flour instead of present as open-space fillings. In contrast to many breccia varieties (see below), most of the intrusion-related breccias considered here underwent only single mineralization events and generally lack evidence for rebreeciation of early mineralization; Golden Sunlight and Kidston are, however, exceptions.

Instead of being homogeneously mineralized, many breccias contain only restricted volumes of ore-grade material. This is commonly present along part of a pipe margin, immediately adjoining the sheeted zone, as at Victoria, Los Pilares (Fig. 5), Turmalina, E1 Bol- sico (Fig. 6), Ilkwang, and San Francisco de Los Andes. At Los Pilares, the marginal annulus of ore thickens substantially at both ends of the ovoid pipe. At Y18- jSrvi, the four steep ore shoots are located close to the northeastern end of the extremely elongate pipe (Himmi et al., 1979). Enhanced permeability resulting from more original open space between fragments, and proximity to the sheeted zone, is believed to account for the higher-grade mineralization in the marginal parts of pipes. The highest grade of gold ore at Kidston occurs at the southwestern end of the pipe in an exceptionally wide (up to 300 m), inward-dipping, quartz-filled sheeted zone, which cuts Precam- brian granite wall rocks, the breccia, and postbreccia rhyolite dikes (Bain et al., 1978; Fig. 7).

Ore may be restricted to portions of pipe interiors. The gold orebody at Ortiz coincides with the part of the star-shaped breccia that carries the least rock flour (Lindquist, 1980). Orebodies in the Breton breceia at Tribag are confined to domal fractures, which are oval to circular in plan, extend into the wall rocks of the breccia (Blecha, 1974), and probably resulted from late subsidence (Norman and Sawkins, 1985).

Total Cu O. 30

7 .... Mo '0.25 ,,

,,,•. .,., '"' ' ø"ø 2 ','i ' ' ^ I:j ø'øs

o "• •' • '•g---• ..... • '• • "---• o IN SlTU BRECCIATION ' ,

C•STS: CL^STS '?ST• C•STS J SHEE•D o 2• 5oo m SHE.D

I I • i • i Z•E Z•E

•. 6. Relationship betwee. copper a.d molybde.um co.- re.rs a.d breccia ch•actedstics across the •1 Bolsico breccia pipe, Chile. Mappi• a.d sampli.• carried o.t alo• the SV] adit o• the 3,030-m level. Compiled [rom Pime•teJ (]gTg) a•d C. •Jau- mett (u.pub. rept., ]gS]).


Sheeted quartz veins Contact

Late { Rhyolite & microgranite '•'•.-'.':• Paleozoic Breccia pipe •

Precambrian { Granite • Metamorphic rocks •

FIG. 7. Surface map of the breccia pipe at Kidston, Queensland, Australia, to show distribution of gold-bearing annular fractures and postbreccia dikes. Taken from Bain et al. (1978).

At Chadbourne, gold is concentrated in cylindrical shoots ofbreccia, up to 40 m wide, that have the same plunge as the pipe (Walker and Cregheur, 1982).

Metals are commonly zoned at the scale of a pipe. For example, at Turmalina the molybdenum content exceeds that of copper in the upper parts of the pipe but decreases steadily downward (Carlson and Saw- kins, 1980), whereas in the Childs-Aldwinkle pipe at Copper Creek the molybdenum content remains un- changed (0.6-1.2%), but the copper content increases from i percent at the top to 6 to 8 percent on the 800-ft level (Kuhn, 1941). In contrast, molybdenum increases in grade downward in the Washington pipe (Simmons and Sawkins, 1983). Horizontal metal zoning may also be present, as at E1 Bolsico, where Pi- mentel (1979) reported a zonation from copper-molybdenum through molybdenum to a low-grade core inward from the sheeted contact (Fig. 6).

Studies of fluid inclusions in open-space-filling minerals from intrusion-related breccias reveal that the mineralizing fluids ranged in temperature from 310 ø to 470øC and in salinity from 1 to 50 equiv. wt percent NaC1 (see So and Shelton, 1983). The higher temperature and higher salinity fluids are similar to those involved in early (K silicate) stages of porphyry deposit formation (Sheppard et al., 1971) and like them may be reasonably inferred as at least partly of magmatic-hydrothermal origin.

Origin: All the principal mechanisms for breccia

pipe formation were proposed, at least in basic form, many years ago and recent studies of breccia pipe formation have all utilized one of these mechanisms with at most minor modification or embellishment (Table 3). Bearing in mind the downward movement of fragments and the existence of up to 20 percent open space in many pipes, any brecciation mechanism must be capable of generating an appreciable void. Five hypotheses have been entertained for the production of a void (Table 3): (1) localized dissolution and upward removal of rock material by fluids released from cooling magma (Locke, 1926), (2) release, perhaps explosively, of volatiles from magma with material carried physically upward (Walker, 1928; Emmons, 1938), (3) downward movement of magma by either shrinkage or withdrawal (Hulin, 1948; Perry, 1961), (4) development of a bubble on the roof of a stock or pluton by accumulation of exsolved fluids (Norton and Cathies, 1973), and (5) production of dilatent zones on major faults during displacement (Mitcham, 1974).

The first four hypotheses all account for the ubiquitous association observed between breccia pipes, intrusive rocks, and alteration-mineralization, whereas the fifth does not and therefore is discounted as a general brecciation mechanism.

The four proposed mechanisms for breccia pipe formation may not necessarily be considered as mutually exclusive and might all contribute in varying degrees to brecciation if considered in the context of Burnham's (1979, 1985) model for energy release during eraplacement and solidification of hydrous magmas at high crustal levels. As quantified by Burn- ham (1985), energy is dissipated from hydrous magma during exsolution of an aqueous fluid phase by the second boiling reaction (water-saturated melt--• crystals + aqueous fluid), and then by decompression of both the exsolved low-density aqueous fluid and the water-saturated residual melt. Decompression causes expansion of previously exsolved fluid, exsolution of additional fluid, and the expenditure of a greater amount of energy than during second boiling. As discussed by Allman-Ward et al. (1982) and Burn- ham (1985), processes triggered by and accompanying decompression appear to account satisfactorily for the formation of breccia pipes, especially where fluid is released from the top of a restricted cupola (giving a single pipe) or is preferentially channeled by inhomogeneous structurally prepared wall rocks above a more extensive pluton (giving a swarm of pipes).

Violent and rapid expulsion of fluid from magma would be capable of generating steep tensile fractures, or reopening existing faults or fractures, and further widening them by hydraulic fracture of their walls. Decompression caused by propagation of fractures into higher level, lower pressure (perhaps hydrostatic)


TABLE 3. Some Suggested Mechanisms for Formation of Breccia Pipes

Principal mechanism Modification

Violent release of fluid from magma (Emmons, 1938; Llambias and Malvicini, 1969•; Knutson et al., 1979•; Allman-Ward et al., 1982•; Burnham, 1985; Porter and Ripley, 1985 •)

Subsurface shock metamorphism (Godwin, 1973)

Collapse due to excavation of exsolved vapor bubble (Norton and Cathies, 1973)

Collapse into void formed by rock dissolution (mineralization stoping; Locke, 1926; McKinstry, 1955; Sillitoe and Sawkins, 1971; Mills, 1972)

Rock dissolution along minor faults with only subsidiary collapse (Kuhn, 1941 •; Johnston and Lowell, 1961 •)

Readjustment upon cooling of underlying magma with only subsidiary collapse (Butler, 1913 •)

Collapse into void formed by magma withdrawal (Perry, 1961; Blecha, 1974•; Atkinson et al., 1982 •)

Collapse into void formed by shrinkage due to cooling of magma (Hulin, 1948)

Collapse into dilatent zone formed on major fault (Mitcham, 1974)

Chemical brecciation in situ following pipe formation by another mechanism (Sawkins, 1969)

Combined with decrease in magma pressure (Armbrust, 1969 •) Due to magma advance (hydraulic ramming; Kents, 1964) Due to magma advance and followed by solution-induced col-

lapse (Fletcher, 1977 •) With venting of rock flour to give void for collapse (Scherken-

bach, 1982•; Simmons and Sawkins, 1983 •)

Following fracturing due to magmatic pulsations (Reyes and Charrier, 1976 •)

Due to release of fluid (Walker and Cregheur, 1982 •)

Mechanism proposed for single pipe or group of pipes

regimes would result in increased fluid release from the magma, and an increased rate of fluid "streaming" (Burnham, 1985), both of which could result in mixing and milling of fragments, production of rock flour matrix, and varying degrees of upward transport of material. Such conditions would also facilitate intrusion of small volumes of magma into and around developing breccia pipes.

If fluid pressures dropped to values below those necessary to maintain the channel open at depth, caving and spalling of the walls of the partly evacuated conduit might be induced. Open-space and shingle breccias, sheeted zones, arching roof fractures, and exfoliated fragments might all be produced in this way. The close association of rock flour and open space breccias in the same pipe swarm and, locally, even in a single pipe accords well with such fluctua- tions in fluid pressure during decompression.

It is uncertain if the fracturing and fragmentation involved in the generation of sheeting and shingle breccia can be attributed solely to the effects of decompression or whether the preexistence of an array of concentric and radial fractures produced by upward-directed (fluid) pressures (Reyes and Chattier, 1976) is also required. As a cause for hypogene ex-

foliation in these and other breccias (see below), an instantaneous drop in confining pressure during decompression (Godwin, 1973; Sillitoe, 1976; Allman- Ward et al., 1982) is preferred to other proposed mechanisms, such as interclast attrition (e.g., Gavasci and Kerr, 1968), mechanical detachment of altered clast rims (e.g., Simons, 1964; Sillitoe and Sawkins, 1971), and thermal spalling of fluid-heated clasts (e.g., McBirney, 1959; Warnaars, 1983).

Features such as fragment rounding and mixing, rock flour generation, and differential vertical displacement of fragments have been considered by many workers (e.g., Mayo, 1976; Woolsey et al., 1975; McCallurn, 1985) to be compatible with the operation of fiuidization as a transport mechanism during the formation of subsurface breccias, including some of those under consideration in this section. However, in view of the great disparity in particle sizes in rock flour breccias, it seems unlikely that more than a small fraction of a breccia was ever truly fluidized (cf. Wolfe, 1980). If particles of a given size were fluidized, then finer grained material would undergo elutriation to accumulate at the top of the pipe above fines-depleted breccia (cf. Wilson, 1980); this vertical zoning is never observed. It is more likely


that breccias which underwent significant upward movement did so as slurries, in much the same way as the chaotic fragment assemblages in debris flows (P. T. Delaney, writ. commun., 1984).

A discrete void filled by fluid could also be produced on a pluton's roof as a result of either localized lifting of the roof rocks during fluid release (Burnham, 1985; Fig. 8a) or, perhaps less probably, by withdrawal of magma (Perry, 1961; Fig. 8c). Burnham (1985) calculated that energy released instanta- neously during decompression by a unit mass of magma would be sufficient to lift an equivalent mass of rock for a height of 990 m, given no frictional re- sistance, and therefore confirmed the feasibility of generating a void in this way. The reality of fluid- filled voids at the tops of magma chambers is confirmed by the existence at Panasqueira, Portugal, of a lensoid mass of quartz that was precipitated in a cavity at the apex of a granite cupola (Kelly and Rye, !979). However, breccia pipe formation was inhibited at Panasqueira either because fluid pressures were insufficient to instigate horizontal extension failure or because the 14-m height of the cavity was too little to induce appreciable caving.

Fluid corrosion of quartz-rich rocks might also be effective in producing or enlarging voids near the tops of plutons or in their immediate roof rocks (Locke, 1926; Fig. 8b). The mechanism is viable during cooling of a fluid from 520 ø to 340øC at a constant pressure not exceeding 900 bars (the region of retrograde solubility for quartz; Fournier, 1983). Sericitization of feldspars also results in the production of significant void space (15-20% of the feldspar volume; W. C. Burnham, writ. commun., 1984). Evidence for partial dissolution of igneous rocks is provided both by the corroded and porous fragments found in some breccias and by the existence of unbrecciated replacement pipes. These are particularly common near the roofs

a b c

_ ___

..... ........................................................... .....

•G. 8. Schematic representation ofbreccia pipes above a pluton roof that were formed with three different types of transitory void development: (a) doming of roof rocks by accumulation of exsolved fluid, (b) dissolution of roof rocks by exsolved fluid, and (c) magma withdrawal.

WHIPSTICK MINE Extrapolated former position of contact

:::++::':{.• REPLACEMENT PiPE • 0 meters 100

FIG. 9. The bismuth- and molybdenum-bearing Whipstick replacement pipes, New South Wales, Australia. Taken from Weber et al. (1978).

of felsic plutons in eastern Australia and comprise steep, narrow (1-10 m), branching bodies, of roughly circular to elliptical cross section, filled with remnants of sericitized intrusive rock and pegmatitic aggregates of quartz, molybdenite, bismuthinite, wolframite, and other minerals (Blanchard, 1947; Fig. 9). The evidence favors production of premineralization openings by rock solution, with the pipes perhaps not being wide enough to have permitted caving and breccia formation (McKinstry, 1955).

Geometric relationships near the bottoms of pipes, as schematized in Figure 8, may prove useful for dis- tinguishing between voids formed by fluid overpres- sures, rock dissolution, and magma withdrawal.

In most intrusion-related breccias, only one brecciation event occurred and was probably accom- plished by low-density aqueous fluids (W. C. Burn- ham, writ. commun., 1984). It was followed by the open-space-filling stage of mineralization, in which high-salinity fluids played an important role (see So and Shelton, 1983). Fluid flow through many breccias seems to have been sluggish if the coarse, locally pegmatitic texture of ore and gangue minerals is attributed to slow crystallization rather than to a low degree of fluid supersaturation.

Porphyry-type deposits

General remarks: Most porphyry systems, be they dominated by copper, molybdenum, gold, tin, or tungsten, contain one or more varieties ofbreccia (cf. Richard, 1969). Breccias are reported from 50 to 60 percent of porphyry systems, as in western Canada (Seraphim and Hollister, 1976) or the Philippines (Sillitoe and Gappe, 1984). More are certainly present but either are not exposed or have not been recognized. The breccias range from minor adjuncts to deposits to the economically dominant parts of some porphyry systems, as at Boss Mountain, Copper Flat, Cumobabi, Los Bronces (Disputada), and Ardlethan (Table 4). Even porphyry-type mineralization as old


as early Archcan is well endowed with breccias (Bar- ley, 1982).

Characteristics: The most abundant and widespread breccias in porphyry systems are grouped under this category. They exhibit a broad spectrum of characteristics (Table 4), many of them shared with the intrusion-related breccia pipes dealt with above.

The breccias commonly occur as lensoid, ovoid, or circular pipelike bodies with steep to vertical dips (Table 4). Pipes may.occur singly or in groups of as many as 25 at Copper Basin (Johnston and Lowell, 1961) and 35 at Cumobabi (Scherkenbach et al., 1985). Additional geometries include dikes, irregular bodies, carapaces to dikes or plugs (e.g., Island Cop- per, Cargill et al., 1976; and E1 Abra, Ambrus, 1977), and annular configurations (e.g., around an unbrecciated core at Duluth, Cananea, Perry, 1935).

The breccia bodies range in horizontal dimensions from a few meters to a maximum of 2 X 0.7 km for the composite pipe at Los Bronces (Warnaars, 1983). Known vertical dimensions are likewise considerable and commonly range from 500 to 1,000 m at Red Mountain (Quinlan, 1981; Fig. 10), Cananea (Perry, 1935, 1961), and Ardlethan (Paterson, 1976) to at least 1;100 m at Los Bronces (Warnaars et al., 1985). An upward increase in the rock volume occupied by breccia is recorded from some localities, e.g., Sierrita- Esperanza (West and Aiken, 1982) and Toquepala, Peru (Zweng and Clark, 1984).

The form of pipelike breccias in porphyry systems is, in general, less regular than that of breccia pipes divorced from porphyry systems. Irregular embay- ments and offshoots from the main breccia bodies are commonplace and contacts with the enclosing parts of the porphyry system are commonly gradational, although they can be sheeted and abrupt (e.g, Whim Hill breccia at Santa Rita; Norton and Cathies, 1973). A number of examples of both the tops and bottoms of porphyry-related breccias have been described. Examples of bottoming, characterized by a rapid transition from breccia to stockworked or fractured rock, include the Transvaal breccia at Cumobabi (at 350 m; Scherkenbach, 1982) and the Whim Hill breccia at Santa Rita (at about 100 m as two separate lobes; Norton and Cathies, 1973). Upward terminations ofbreccias have been described from the Capote pipe at Cananea, which fades out into a mineralized limestone horizon 100 m beneath the surface (Perry, 1935; Meinert, 1982), and the 148-155 pipe at Red Mountain, which tops out about 1,200 m below the surface (Quinlan, 1981; Fig. 10). Given this evidence from Cananea and Red Mountain, and observations elsewhere (e.g., Copper Flat, Dunn, 1982; and Santo Nifio, Philippines, Sillitoe and Gappe, 1984) suggesting marked upward decrease in the size ofbreccia bodies, it is inferred that most porphyry-related breccias were originally "blind."

It is clear from Table 4 that breccia fragments range from angular to rounded and that comminuted rock flour may or may not contribute to their matrices. It would appear that heterolithologic breccias with subrounded or rounded fragments and a rock flour matrix (rock flour breccias; Fig. 11) are more widespread than intrusion-related breccia pipes (Table 4). The rock flour matrix locally exhibits irregular but generally steep alignment of its constituent particles, a fabric attributed to upward fluid streaming (e.g., Central breccia at Los Bronces, Warnaars et al., 1985; Llallagua, Fig. 12; and Ok Tedi, Arnold and Fitzger- ald, 1977). Tabular fragments are uncommon. Rem- nant open space between fragments is frequently observed but in many cases amounts to only a few volume percent of the breccia and comprises isolated, roughly triangular openings in tightly fitting fragment arrays. Clast-supported breccias are the norm (Fig. 11) although every gradation to bodies composed entirely of rock flour is known. Only a small percentage of breccias possesses an igneous matrix (in the sense that it is composed of an intrusive rock). Examples include a small part of the breccias at Boss Mountain (Soregaroli, 1975), Bethlehem (Briskey and Bellamy, 1976), Granisle (Kirkham, 1971), and Ok Tedi (Ar- nold and Fitzgerald, 1977).

Individual porphyry-related breccias also seem to exhibit a greater variety of textures than isolated breccia pipes. This feature attains its extreme development at Los Bronces, where a sequence of seven principal breccias each distinguished on the basis of the size and form of clasts, the nature and amount of matrix, and the degree and type of alteration-mineralization constitutes a single composite pipe (War- naars, 1983; Warnaars et al., 1985).

The degree of fragment displacement in porphyry- related breccias is varied but, in general, is greater than in intrusion-related breccia pipes, an observation supported by the frequency of heterolithologic breccia. Particularly noteworthy is the increased evidence for the ascent of clasts--intrusive clasts were dis- placed upward by 200 m in the Infiernillo breccia at Los Bronces (Warnaars, 1983) and K silicate-altered clasts were carried upward at least 100 m at Mocoa (Sillitoe et al., 1984a). Descent of fragments is also documented, however, and amounts to 250 to 300 m at Los Bronces (Warnaars, 1983; Warnaars et al., 1985) and >330 m in the Capote pipe at Cananea (Perry, 1961). Elsewhere, however, as at Copper Flat (Dunn, 1982), fragment displacement is considered to be minimal.

The breccias described in this section generally are closely related to one or more porphyry stocks. Most breccias are rooted in porphyry intrusions, although in some cases, as at Cananea (Perry, 1935), Questa (Leonardson et al., 1984), Red Mountain (Quinlan, 1981), and Ardlethan (Paterson, 1976), much of the


TABLE 4. Selected Examples of Magmatic-Hydrothermal

Hydrothermal Principal host Form of breccia alteration (t --

Locality rocks Age (m.y.) body Fragment form Rock flour tourmaline)

Bethlehem, Granodiorite 200 Steep elongate Angular to B.C., Can- anastomosing rounded ada bodies

Boss Mountain, Granodiorite 105 Irregular lenslike Angular to B.C., Can- vertical body rounded ada

Galore Creek, Alkalic volcanics, 174 to 198 Steep pipelike Angular to B.C., Can- syenite por- bodies rounded ada phyry

Island Copper, Quartz-feldspar 154 Carapace to Rounded B.C., Can- porphyry, an- steep dike ada desitic volca-

nics

Mt. Pleasant, Granite por- 330 to 340 Pipelike body Angular and N. B., Can- phyry rounded ada

Sacaton, Ari- Quartz monzo- 64.5 Large irregular Mainly subangular zona nite porphyry, body to subrounded

monzonite

porphyry, granite

Sierrita-Esper- Quartz monzo- 57 Irregular up- Angular to anza, Ari- nite porphyry, ward-flared rounded zona quartz monzo- bodies

nite, quartz diorite, andesitic volcanics

Copper Basin, Quartz diorite, 64 25 vertical pipes Angular to Arizona quartz monzo- rounded

nite, quartz monzonite

porphyry

Red Mountain, Latitic and an- •60 Steep pipe Angular Arizona desitic volca-

nics

Copper Flat, Quartz monzo- 73.4 Steep elongate Angular, little dis- New Mexico nite pipe placed

Santa Rita, Granodiorite 63 Elongate pipe Angular, sub- New Mexico porphyry (Whim Hill rounded

breccia)

Questa, New Andesitic volca- 23 Mexico nics

Cananea, Son- Granite, lime- 59.9 • ora, Mexico stone, quartz-

ite, rhyolitic to andesitic volcanics

Cumobabi, Quartz monzo- 40.0 • Sonora, Mex- nite porphyry ico or andesitic

volcanics

Body above cu- Subangular(?) pola of aplite porphyry

Eight principal Angular to sub- pipes rounded

•35 irregular Angular but pipes and rounded at La bodies Verde pipe

Abundant Biotitic

0 to 70% Biotitic

Present lo- K silicate cally (+ garnet)

Abundant Pyrophyllite- sericite

Abundant Quartz-topaz

5 to 20% K silicate

Abundant in K silicate upper parts

Absent Quartz-K-feldspar

Absent K silicate + •sericitic

Absent K silicate

Present K silicate

Absent

Absent

Absent, present at La Verde pipe

K silicate

Sericitic, K silicate, skarn destruction

K silicate or sericitic (t)


Breccias Associated with Porphyry-type Deposits

Principal metallic Principal gangue Age relative to Economic minerals minerals porphyry deposit significance Reference

Chalcopyrite, bornite, pyrite, molybdenite

Molybdenite, pyrite


Pyrite, chalcopyrite, molybdenite

Biotite, chlorite, Largely premineral High-grade parts Briskey and Bellamy tourmaline, of orebodies (1976) quartz

Quartz

Biotite, garnet, anhydrite

Early intermineral Ore largely re- Soregaroli (1975), stricted to Soregaroli and breccias Nelson (1976)

Premineral Part of orebody Allen et al. (1976)

Quartz, pyrophyllite Premineral Part of orebody Cargill et al. (1976)

Wolframite, molybdenite, arsenopyrite, native bismuth, bismuthinite

Pyrite, chalcopyrite, molybdenite, specularite

Quartz, fluorite Premineral

Quartz Premineral

Main part of W- Kooiman et al. Mo orebody (1984)

Hosts much of Cummings (1982) West orebody


Quartz, biotite Early mineral High-grade ore West and Aiken (19S2)


Quartz Largely premineral Three pipes carry Johnston and Lowell high-grade Cu- (1961) Mo ore

Chalcopyrite, pyrite, mo- Quartz, K-feldspar, Premineral lybdenite anhydrite, calcite

Pyrite, chalcopyrite, Quartz, biotite, K- Early mineral magnetite, molybde- feldspar, fluorite, nite calcite, apatite

Pyrite, chalcopyrite, Quartz, K-feldspar, Early mineral magnetite, molybde- biotite nite

Molybdenite Quartz, K-feldspar, Premineral biotite

High-grade ore, especially on contacts

High-grade central part of orebody

Part of supergene orebody

Main orebody

Quinlan (1981)

Dunn (1982)

Kerr et al. (1950), Rose and Baltosser (1966), Norton and Cathies (1973)

Leonardson et al. (1984)

Chalcopyrite, bornite, pyrite, sphalerite, molybdenite, galena

Pyrite, molybdenite, chalcopyrite, tetrahedrite

Quartz, carbonate, phlogopite (La Colorada), chlorite

Quartz, biotite, K- feldspar, anhydrite, apatite, siderite or quartz, tourmaline

Intermineral

Premineral

High-grade ore

Four bodies carry Mo ore

Perry (1935, 1961), Meinert (1982)

Sillitoe (1976), Scherkenbach et al. (1985)


TABLE 4. (Continued)

Hydrothermal Principal host Form of breccia alteration (t --

Locality rocks Age (m.y.) body Fragment form Rock flour tourmaline)

La Caridad, Quartz monzo- 54.5 • Sonora, Mex- nite porphyry, ico diorite, grano-

diorite

Mocoa, Colom- Dacite porphyry, 166 bia andesitic-daci-

tic volcanics

Quebrada Quartz monzo- 38 Bianca, nite, quartz Chile and feldspar

porphyries E1 Abra, Chile Diorite 33 to 35

Los Bronees, Chile

Llallagua, Bolivia

Panguna, Papua New Guinea

Quartz monzo- 7.4 to 4.9 nite, andesitic volcanics

Quartz latite 20 porphyry, ar- gillite

Andesitc, diorite, 3 to 5 granodiorite

Ok Tedi, Quartz monzo- Papua New nite porphyry Guinea

Ardlethan, Adamellite, N. S.W., quartz-feldspar Australia porphyry

Irregular to Rounded to sub- <10% Sericitic + pipelike angular silicate bodies

Irregular bodies Angular to sub- 0 to 20% K silicate on roof and rounded q- sericitic flank of stock

Lens-shaped Rounded to Minor Sericitic (t), K composite angular silicate body + pipe(s) (2 X 1 km)

Hoodlike bodies Angular to Minor K silicate to dacite por- rounded phyry plugs

Seven steep bod- Angular to suban- 0 to 40% Sericitic (t), ies comprising gular propylitic 2 X 0.7-km complex

Pipes, dikes, and Subangular to 20 to 100% Sericitic (t) irregular rounded bodies

Irregular pipe- Angular to 0 to 80% Biotitic like bodies rounded

1.1 Dikes and irreg- Angular to 0 to 100% ular bodies rounded (10-20% of stock)

411 to 412 Four irregular Angular to Abundant pipelike rounded bodies

K silicate

Sericitic (t), chloritic (t)

After Damon et al. (1983)

brecciation is in overlying rocks. Locally, specific intermineral intrusions may be singled out as closely related both spatially and genetically to brecciation. Examples include dacite porphyry plugs at E1 Abra (Ambrus, 1977), a biotite granodiorite at Panguna (Baldwin et al., 1978), and quartz diorite porphyries at Biga (Atlas) and Santo Tomas II in the Philippines (Sillitoe and Gappe, 1984).

Breccias occupy a wide variety of positions within porphyry systems. Many of them are centrally placed, as at Copper Flat (Dunn, 1982), Questa (Leonardson et al., 1984), and Red Mountain (Quinlan, 1981; Fig. 10), whereas others are eccentrically located (e.g., Mocoa, Sillitoe et al., 1984a; Los Bronces, Warnaars, 1983). Elsewhere breccia pipes constitute partial halos to porphyry copper deposits, as at Bingham, Utah, where a variety of poorly documented breccia pipes, perhaps not all of the same origin, is present

in the peripheral lead-zinc zone (Rubright and Hart, 1968), and at Bagdad, Arizona, where the most important pipe (the Black Mesa) carries copper and molybdenum mineralization (Anderson et al., 1955).

Fault control ofbreccias in porphyry systems is not widely recognized, although it is likely at some localities, such as Bethlehem, where the marked elon- gation of some of the breccia bodies is suggestive of structural localization (Briskey and Bellamy, 1976). Contacts between different lithologies, especially between intrusions and their wall rocks, seem to have provided a more widespread locus for brecciation.

Alteration and mineralization: K silicate alteration dominated by biotite, K-feldspar, or both minerals is the most widespread alteration type in porphyry-related breccias, although sericitization is also relatively common (Table 4). In addition, advanced argillic alteration occurs at Island Copper (Cargill et al., 1976),


Principal metallic Principal gangue Age relative to Economic minerals minerals porphyry deposit significance Reference

Pyrite, chalcopyrite Quartz, tourmaline Intermineral Part of chalcocite ore zone

Chalcopyrite, pyrite, mo- Quartz, K-feldspar, Intermineral Partly high-grade |ybdenite sericite, chlorite ore

Pyrite, chalcopyrite, bor- Quartz, biotite, K- Intermineral Lens-shaped nite, molybdenite feldspar, sericite, body contains

tourmaline Cu-Mo ore

Chalcopyrite, bornite Biotite Intermineral Part of ore zone

Pyrite, chalcopyrite, Tourmaline, quartz, Intermineral Parts of four specularite, mo- ohiorite, sericite, breccias consti- lybdenite anhydrite tute hypogene

ore

Cassiterite, pyrite

Chalcopyrite, bornite


Tourmaline, quartz Pre- and inter- Partly ore mineral

Quartz, biotite, K- Intermineral High-grade ore feldspar

Quartz, biotite Intermineral Part of orebody

Saegart et al. (1974), R. H. Sillitoe (unpub. rept., 1975)

Sillitoe et al. (1984a)

Hunt et al. (1983)

R. H. Sillitoe and H. Neumann (unpub. rept., 1970), Am- brus (1977)

Warnaars (1983), Warnaars et al. (1085)

$illitoe et al. (1975), Grant et al. (1980)

Baldwin et al. (1978)

Arnold and Fitzger- ald (1977)

Pyrite, arsenopyrite, sphalerite, galena, chalcopyrite, cassiterq itc

Quartz, tourmaline, Early and inter- Comprises most sericite, chlorite, mineral of the nine ore- siderite, fluorite bodies

Paterson (1976), P. J. Eadington and R. G. Paterson (unpub. rept., 1984)

propylitization at Los Bronces (Warnaars et al., 1985), quartz-topaz alteration at Mt. Pleasant (Kooiman et al., 1984), and skarn-destructive quartz-chlorite-carbonate-hematite alteration at Cananea (Meinert, 1982). K silicate alteration is notably more abundant than in intrusion-related breccia pipes. At some localities, both K silicate-altered and sericitized breccias are present in close proximity (e.g., Mocoa; Sillitoe et al., 1984a); elsewhere sericitic alteration over- printed early K silicate assemblages (e.g., Cumobabi; Scherkenbach, 1982) or characterizes the apex and flanks of a largely K silicate-altered pipe (e.g., Red Mountain; Quinlan, 1981). At Cumobabi, breccias located near the center of the hydrothermal system are K silicate altered and constitute molybdenum ore whereas more peripheral breccias are propylitized and/or sericitized and are devoid of ore to explored depths (Sillitoe, 1976; Scherkenbach, 1982).

Quartz is the most widespread cementing mineral, although it is absent or minor at E1 Abra and Galore Creek. In K silicate-altered breccias it is accompanied by K-feldspar and/or biotite, to which one or more of chlorite, fluorite, apatite, siderite, tourmaline, magnetite, and specularitc may be added. The K silicate assemblage present as a matrix to breccias at Questa (Leonardson et al., 1984), Copper Flat (Dunn, 1982), and the Colorada pipe at Cananea (Perry, 1935, 1961) is pegmatitic in texture. Tourmaline tends to be a more common constituent of sericitized breccias (Ta- ble 4). Garnet occurs as both an alteration and matrix mineral at Galore Creek (Allen et al., 1976). One or more of chalcopyrite, pyrite, and molybdenite is also present as a matrix component, even in rock flour breccias. Cassiterite is the economically most important cementing mineral at Llallagua and Ardlethan, as is wolframite at Mt. Pleasant.


148-155 B•ECCIA riPE

1 /I I

o m•.. '•

FIG. 10. Diagrammatic cross section through the Red Mountain porphyry copper system, Arizona, to show the central position of the 148-155 breccia pipe. Taken from Quinlan (1981).

FIG. 12. Swirly flow texture in rock flour matrix to heterolithologic breccia. Oruro tin deposit, Bolivia. Approximately half natural size.

Bre•cias in porphyry systems are commonly characterized by higher contents of exploitable metals than the surrounding stockworks. The situation reaches an extreme at Boss Mountain (Soregaroli, 1975), Copper Flat (Dunn, 1982), Cumobabi (Scher- kenbach, 1982), and Los Bronces (Warnaars et al., 1985), where the porphyry copper stockworks beyond the breccias do not attain ore grades. Elsewhere, however, including Island Copper, Cananea, Mocoa, Quebrada Bianca, Questa, Llallagua, Mt. Pleasant, and Ardlethan, breccias constitute the highest grade parts of the orebodies. Locally, as at Los Bronces (Warnaars, 1983) and Mocoa (Sillitoe et al., 1984a), metal grades are appreciably enhanced by the presence of previously mineralized clasts in the breccias. In some breccias, the metal budget is distinctly different from that characteristic of the porphyry deposit as a whole. As examples, at Quebrada Bianca, a dikelike breccia car-

,, •-- _•. ,,' -• , • • *•. • ; •.• • • '•

. -.. • ß ... q ß .• • . -• , • '- *•

.- ..• **":•-•:'..../.,.;•:•.:.•.,d -• .%•" ..'.- •;•-..• ,[ ,,•

•O. 11. T•ie•! rock flour breccia. EI Abra porphyry copper depo,it, •hile.

ries more than 15 times the average molybdenum grade of the rest of the deposit (Hunt et al., 1983) and at Santo Tomas II, Philippines, small pipelike breccias have markedly higher Mo to Cu and Mo to Au ratios than the rest of the deposit (Sillit.oe and Gappe, 1984).

In common with intrusion-related breccia pipes, some breccias in porphyry systems are characterized by a preferred distribution of ore minerals. Examples may be cited from the 148-155 pipe at Red Mountain, where copper, molybdenum, and silver grades around the margins are several times greater than those in its interior (Quinlan, 1981), and from the Donoso pipe at Los Bronces, where copper is concentrated in a series of downward-closing shells (Warnaars, 1983).

Based on the examples selected for Table 4, breccia emplacement in porphyry systems ranges in age from premineral to intermineral. In premineral examples there is no evidence of any earlier stages of mineralization, and at some localities, such as Bethlehem (Briskey and Bellamy, 1976), the main mineralized stockwork crosscuts the breccia bodies. Where breccias are designated as early mineral (Table 4), there is only minor evidence from constituent fragments for prebrecciation alteration and mineralization. This is exemplified by low-grade pyrite-chalcopyrite mineralization related to pervasive sericitization that predated the brecciation-K silicate alteration event at Copper Flat (Dunn, 1982), a barren prebreccia stage of quartz-K-feldspar veining at Sierrita-Esper- anza (West and Aiken, 1982), and prebreccia quartz- topaz alteration at Ardlethan (P. J. Eadington and R. G. Paterson, unpub. rept., 1984). In contrast, intermineral breccias were emplaced later than one or more main stages of alteration and mineralization. Evidence for this conclusion is commonly provided


by the restriction of ore-bearing veinlets to individual fragments in a breccia (Fig. 13), as emphasized for Granisle and elsewhere by Kirkham (1971), or by the truncation of alteration and stockwork veinlets by an entire breccia body. Intermineral breccias may also contain clasts of mineralized breccia derived from preexisting bodies, a relatively common feature at Los Bronces (Warnaars, 1983) and elsewhere.

In some cases, copper and molybdenum introduction to intermineral breeeias accompanied renewed (or continued) K silicate alteration, whereas elsewhere it was associated with localized serieitie, or in most Philippine examples, ohiotitle alteration (Si!litoe and Gappe, 1984).

Stable isotope studies to determine the source of fluids responsible for alteration-mineralization of breeeias in porphyry systems have not been carried out. However, the coincidence of brecciation and K silicate alteration during the early development stages of many porphyry systems (Table 4) suggests that meteoric-hydrothermal fluids generally were subordinate to fluids of direct magmatic-hydrothermal par- entage (Sheppard et al., 1971).

Origin: Most workers in the last two decades have attributed the principal breeeias in porphyry systems to the violent release of magmatic-hydrothermal fluids from cooling stocks (e.g., Phillips, 1973; Seraphim and Hollister, 1976). It is clear that the model of Burnham (1979, 1985) and others for brecciation by fluid liberation during second boiling, followed by decompression of the released fluids, is as effective in explaining the wide variety of breeeias in porphyry systems as it is the isolated intrusion-related breeeia pipes described above. Furthermore, the widespread stockwork fractures in porphyry systems may also be

FIG. 13. Intermineral breeeia with quartz veinlet confined to clast near the middle of photograph. Chlorite-bearing rock flour matrix.

attributed to the same late magmatic processes (Burnham, 1979).

The spectrum of textures and relationships summarized above for breccias in porphyry systems may be attributed to the same mechanisms used to explain comparable features in isolated breccia pipes. It is therefore no longer necessary to invoke separate origins for texturally and geometrically different breccias that occur in close proximity in many porphyry systems; they may all be related to the same Overall mechanism.

Rock flour breccias showing evidence of mixing and upward transport of fragments are apparently more widespread in porphyry systems than in isolated breccia pipes and may be due to the efficient release of larger volumes of fluids from subvolcanic porphyry stocks than from the roofs of deeper seated plutons (see Burnham, 1985). A more protracted release of fluids, or several stages of release as a result of multiple intrusion, effectively explains the intermineral position of many breccias in porphyry systems. In- termineral brecciation may also be favored by the re- duction of rock permeability resulting from early stages of K silicate alteration (particularly quartz precipitation) and mineralization (see Fournier, 1983).

Phreatic (Hydromagmatic) Breccias Epithetreal precious (_ base) metal deposits

General remarks: Epithermal precious metal deposits may be subdivided conveniently into three principal categories (Bonham and Giles, 1983): volcanic-hosted deposits, hot spring-related deposits, and carbonate-hosted (Carlin-type) deposits. A shallow (< 1,000 m) level of emplacement is inferred for most epithermal deposits. An association of epithermal deposits with volcanic structures or landforms, including flow-dome complexes, maar-diatreme systems, and caldera ring fractures (Table 5), emphasizes the shallow depths of emplacement. In fact, several of the hot spring-related deposits attained the contemporary surface as shown by their association with sinters (Ta- ble 5). As a consequence of their shallow settings, most deposits range from Miocene to Pleistocene in age and lack large volumes of associated intrusive rocks (Table 5).

It is widely accepted that breccias are a common accompaniment to volcanic-hosted and hot spring-related epithermal deposits and are considered by Ber- ger and Eimon (1983) and Bonham and Giles (1983) as an integral part of the latter category. Their importance in many Carlin-type deposits has also been emphasized recently (Sillitoe, 1983a).

Characteristics: A broad range of breccia types is found in epithermal systems (Table 5). Their geometries range from small veins and veinlets (Fig. 14) to large pipes, tabular masses, and irregular anastomos-

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Jose Javier Rojas

Jose Javier Rojas

TABLE 5. Selected Examples of Phreatic Breccias

Locality Host rocks Age (m.y.) Volcanic setting Form Fragment characteristics

Equity Silver, B.C., Canada

Dacitic tuffs 59 None known Irregular tabular Angular to rounded, at body least two generations

Cinola, B.C., Canada

Delamar, Idaho

Cripple Creek (Globe Hill), Colorado

Summitville, Colorado

Red Mountain, Colorado

Round Mountain, Nevada

Buckhorn, Nevada

Buckskin, Nevada

Hasbrouck Mountain, Nevada

Conglomerates, siltstones

Rhyolite domes, plugs, flows

Latite-phonolite intrusions

Quartz latite porphyry dome

Rhyolitic to quartz latitic volcanics, quartz latite porphyry plugs

Metasediments, ignimbrite

Basaltic andesitic volcanics, argil- lite

Rhyolitic pyroclastics

Volcaniclastic sediments, ignimbrite

Late Ceno- None known zoic

15 Rhyolite flow- dome com-

plex

27 to 28 Interior of diatreme

22 to 23

22.5

25

Late Tertiary

15.5

16.3

Dome on older

caldera ring fracture

Ring fracture of older caldera

On caldera ring fracture

Graben

Rhyolite flow- dome com-

plex Rhyolite flow-

dome com-

plex

Extensive, poorly Angular to rounded(?) defined bodies

Irregular vein Angular, monolithologic and pipelike to subrounded, het- bodies erolithologic

Irregular bodies Angular, monolithologic and pipes to to rounded, hetero- >330 m lithologic; three

generations

Pods, pipes, and Angular to subrounded, tabular bodies mono- or heterolitho-

logic, three generations

Pipes to >370 m Angular to rounded, heterolithologic

Upward-flared pipelike body to >350 m

Pipelike body + subaerial(?) patches

Pipelike body

Extensive irregular bodies

Angular to subangular, heterolithologic, moved upward

Angular

Angular to rounded, sorted parallel to contact

Angular to rounded, heterolithologic, moved upward

Northumberland, Nevada

Alligator Ridge, Nevada

Limestone, dolomite, shale, siltstone

Limestones, shales

84.6(?)

Tertiary(?)

None known

None known

Structurally and stratigraphi- cally controlled bodies

Irregular(?) bodies

Angular

Angular

La Coipa, Chile

Rosia Montana, Romania

Chinkuashih, Taiwan

Wau, Papua New Guinea

Siltstone, dacitic ignimbrite 4- tuff

Dacite porphyry

Sandstone, shale

Phyllites, explosion breccia

Miocene(?)

Late Mio- cene

Pleistocene

<4 to >2.4

Dacite domes

Probable flow- dome complex

Dacite porphyry flow-dome

complex Tuff ring around

maar

Irregular pipes and bodies

Breccia pipes to 500 m

Small pipes and dikes to >200 m

Anastomosing veins and pods, subaerial apron

Angular to subrounded

Angular to rounded(?)

Angular or rounded, moved upward, heterolithologic

Angular to rounded, heterolithologic

1486

Associated with Precious Metal Deposits

Hydrothermal Principal hypogene alteration minerals

Ore deposit type and ore reserve

(M = million, t: metric tons)

Relation to

palcosurface Relation to

orebody Reference

Advanced Quartz, pyrite, arseno- Bulk Ag-Cu-Au-Sb; Subsurface Constitutes ore argillic pyrite, tetrahedrite, 28 Mt, 106 ppm (1,000 m?)

chalcopyrite, sphaler- Ag, 0.38% Cu, itc, galena 0.96 ppm Au

Silicification Quartz, pyrite, marcasite Bulk Au; 41 Mt, Subsurface, proba- Constitutes ore 1.85 ppm Au bly shallow

Silicification, Quartz, pyrite, nauman- Bulk Ag-Au; 9 Mt, Shallow subsurface Partly ore argillic nite, argentitc 86 ppm Ag, i and at paleosur-

ppm Au face (sinter)

Quartz, sericite, Quartz, fluorite, carbon- Bulk Au, 2 orebod- Subsurface chlorite, ate, celestite, anhy- ies; •4 Mt, 1.3 montmoril- drite, pyrite, galena, to 1.8 ppm Au 1onite sphalerite, chalcopy-

rite, pyrrhotite Silicification, Quartz, alunite, pyrite, Au-Ag-Cu lodes Shallow subsurface

advanced enargite, covellite, na- and pipes argillic rive sulfur

Silicification, Quartz, clays, pyrite, en- advanced argite, chalcocite, co- argillic veilitc, bornitc, sphal-

erite, galena Silicification Pyrite

Silicification, Quartz, pyrite, marcasite kaolinitc, adularia, sericite

Silicification, Quartz, pyrite, stibnite, alunite sulfosalts, cinnabar

Silicification, Quartz, pyrite, acan- adularia, illite thite, stibnite, pyrar-

gyrite, chalcopyrite

Silicification Quartz, barite, pyrite (jasperold)

Silicification Quartz, calcite, barite, (jasperoid) pyrite, stibnite

Silicification, Quartz, pyrite, sphaler- advanced itc, galena, chalcopy- argillic rite, sulfosalts

Silicification, Quartz, rhodochrosite, adularia, pyrite, sphalerite, ga- argillic lena, chalcopyrite

Silicification Pyrite, enargite, quartz, alunite

Minor Quartz, calcite, manga- nocalcite, pyrite, galena,

sphalerite

Cu-Au-Ag pods and pipes

Bulk Au; 204 Mt, 1.2 ppm Au

Bulk Au; 4.6 Mt, 1.54 ppm Au

Vein and stock-

work Au-Ag

Bulk Au-Ag

Carlin-type Au; •40 Mt, 2.4 ppm Au

Carlin-type Au; 4.5 Mr, 4.1 ppm Au

Bulk Ag-Au prospect

Au

Cu-Au veins + breccias

Bulk Au-Ag

Subsurface

Shallow subsurface

Shallow subsurface and paleosurface(?) (sinter fragments)

Shallow subsurface

and palcosurface (sinter)

Shallow subsurface (<150 m) to paleosurface (sinter)

Subsurface

Subsurface

Shallow subsurface

and palcosurface (sinter)

Subsurface

Subsurface

Shallow subsurface and paleosurface

Constitutes ore

Ore bearing and postore

Contains orebodies

Barren, central to orebody

Constitutes part of orebody

Cut by veins

Constitutes ore-

body

Constitutes ore

Partly ore

Partly mineralized

Partly ore

Partly ore

Constitutes ore

Cyr et al. (1984), Wodjak and Sinclair (1984)

Cruson et al. (1983)

Pansze (1975), R. H. Sillitoe and H. F.

Bonham (unpub. observations, 1981)

Thompson et al. (1985)

Steven and Ratt•

(1960), Perkins and Nieman (1983)

Burbank (1941), Fisher and Leedy (1973), Lipman et al. (1976)

Mills (1982), Berger and Eimon (1983)

Monroe and Plahuta

(1984)

Vikre (1983)

Bonham and Carside

(1979), R. H. Silli- toe and H. F. Bon-

ham (unpub. observations, 1981), Gra- ney (1984)

Motter and Chapman (1984), R. H. Silli- toe and H. F. Bon-

ham (unpub. observations, 1981)

Klessig (1984), R. H. Sillitoe and H. F.

Bonham (unpub. observations, 1981)

R. H. Sillitoe (unpub. rept., i980)

R5dulescu et al.

(1981)

Chu (1975)

Sillitoe et al. (1984b)

1487

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FIG. 14. Typical breccia veinlet resulting from hydraulic fracture. Matrix comprises silicified rock flour. Epithermal precious metal prospect, Chile.

ing bodies. Several epithermal breccias possess known vertical extents of 200 to 500 m (Table 5). The reg- ularly shaped pipes at Red Mountain (Burbank, 1941) and Chinkuashih (Chu, 1975) are reminiscent of the intrusion-related pipes described above. It is therefore significant that marginal sheeted zones and a close relation of breccias to quartz latite porphyry plugs are characteristic of several pipes at Red Mountain (Burbank, 1941; Fisher and Leedy, 1973) and large isolated spheroidal fragments were reported from Chinkuashih (Chu, 1975).

At several localities, such as Round Mountain (Mills, 1972), there is a marked upward flare to pipe-shaped bodies, which is interpreted to be due to their approach to the contemporary land surface. In fact, at Buckhorn (Monroe and Plahuta, 1984), Buckskin (Vikre, 1983), Hasbrouck Mountain (Graney, 1984), Delamar (R. H. Sillitoe and H. F. Bonham, Jr., unpub. observations, 1981), La Coipa (R. H. Sillitoe, unpub. rept., 1980), and Wau (Sillitoe et al., 1984b), brecciation breached the palcosurface. Remnants of subaerial breccia aprons are still preserved at Wau. Es- sentially subaerial breccias at the Milestone prospect (Delamar), Buckhorn, and Hasbrouck Mountain contain fragments of sinter as well as a variety of underlying rocks, whereas at La Coipa (and in places at McLaughlin, California) surface hot spring sinters underwent brecciation more or less in situ. Silicified logs accompany sinter fragments at Milestone.

The textures of epithermal breccias are extremely varied. Rock flour and open-space breccias are both widespread (Berger and Eimon, 1983) and both may occur in individual breccia bodies. Rock flour is commonly masked by silicification (Fig. 14). There is commonly evidence for relatively restricted upward displacement of fragments, but this is claimed to attain 200 m in rock flour breccias at Chinkuashih (Chu, 1975). Appreciable open space is widespread in some

breccias, especially those that underwent hypogene leaching during advanced argillic alteration, as at Red Mountain (Burbank, 1941). Some epithermal breccias display a clear gradation to stockwork fracturing (e.g., Delamar, Hasbrouck Mountain, Globe Hill at Cripple Creek, Alligator Ridge, and Equity Silver).

Many epithermal breccias provide evidence of multiple stages of silicification, mineralization, and brecciation, and at some localities a temporal sequence, with each breccia exhibiting its own distinctive characteristics, may be determined. For example, Thompson et al. (1985) proposed four stages ofbrecciation, each accompanied by mineralization, in the Globe Hill area at Cripple Creek. The intermineral (and, locally, even postmineral) timing of brecciation is emphasized at many localities by the restriction of distinctive types of silicification or sulfide veining to isolated fragments. For example, as many as four varieties of silicified limestone occur .in breccia in the Taylor district, Nevada (Lovering and Heyl, 1974).

A structural control of epithermal breccias is emphasized more frequently than for deeper seated breccias associated more closely with plutons and stocks. Minor faults are considered to have localized the Red Mountain pipes (Burbank, 1941; Fisher and Leedy, 1973) and the Chinkuashih breccia dikes and pipes (Chu, 1975), whereas a major oblique-slip fault abuts and probably localized the Cinola breccias (Cruson et al., 1983). In the Globe Hill area at Cripple Creek, faulting took place during brecciation and acted as an important spatial control (Thompson et al., 1985). High-angle faults and stratigraphic horizons, especially limestone-shale contacts, localized much of the silicification and brecciation in carbonate- hosted epithermal deposits, as at Northumberland (Motter and Chapman, 1984) and Alligator Ridge (Klessig, 1984). Structures of volcanic origin also controlled brecciation and mineralization in several epithermal districts, as at Wau where short low-angle extensional structures between a diatreme ring fault (see below) and a regional fault localized brecciation (Sillitoe et al., 1984b).

Alteration and mineralization: The dominant feature that distinguishes epithermal breccias from most magmatic-hydrothermal breccias is the widespread occurrence of quartz as both a pervasive replacement of, and a cement to, fragments. It is generally fine grained and commonly chalcedonic, and characterizes all but three of the examples cited in Table 5. Silicified carbonate rocks .are generally referred to as jasperoid.

In epithermal precious metal deposits where silicification is widespread, there is a close relationship between the development of pervasive chalcedonic silica and brecciation, as seen at Summitville (Steven and Ratt6, 1960), in the carbonate-hosted epithermal deposits (Table 5), and elsewhere. The breccia pipes at Red Mountain (Burbank, 1941) are capped by mas-

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sive silicification. Silicification is accompanied by, or grades into, advanced argillic alteration rich in alunite at Summitville, Red Mountain, La Coipa, and Chink- uashih but is surrounded by less acid alteration types at the other localities listed in Table 5. Patches of silicification and associated brecciation are also typical of the similar zones of advanced argillic alteration that characterize the upper (volcanic) parts of porphyry copper systems (Sillitoe, 1983b).

The presence or absence of advanced argillic alteration is the dominant control on the sulfide and gangue mineralogy of the breccias. Sulfur-rich sUl- fides, especially pyrite, enargite, luzonite, and cov- ellitc, generally cement silicified breccias within zones of advanced argillic alteration, whereas much smaller amounts of pyrite, either alone or accompanied by sphalerite, galena, chalcopyrite, tennantite- tetrahedrite and/or argentitc occur where advanced argillic alteration is absent. Breccias associated with Carlin-type deposits tend to be cemented by a restricted number of minerals, of which quartz, calcite, pyrite, barite, and stibnite are the most widespread.

Epithermal breccias commonly constitute gold and/ or silver ore. Breccias may provide the main loci for ore, as at Red Mountain (Burbank, 1941), or may simply host some of the highest grade portions of an ore body, as at Hasbrouck Mountain (Graney, 1984) or Northumberland (Motter and Chapman, 1984). In these and most of the other examples in Table 5 the precious metal mineralization is present mainly in the breccia cement. Locally, however, as at Buckskin (Vikre, 1983), precious metals are present only in veins and stockworks that cut breccia. At Wan, much of the gold in the breccias is present in clasts of vein material ($illitoe et al., 1984b). In contrast, the breccia pipe at Round Mountain is barren, although it is surrounded by ore (Mills, 1982). Many of the brecciated jasperoids associated with carbonate-hosted epithermal deposits contain only trace amounts of precious metals, although at Northumberland and Al- ligator Ridge (Table 5) they are integral parts of the orebodies.

Only sparse information is available concerning the fluids involved in the formation of the epithermal deposits listed in Table 5 (e.g., Cripple Creek, Thomp- son et al, 1985; Equity Silver, Wodjak and Sinclair, 1984). In common with most epithermal precious metal deposits, however, the ore fluids are assumed to have been dominated by meteoric water (e.g., O'Neil and Silberman, 1974; Radtke et al., 1980). The most likely exceptions to this generalization are the volcanic-hosted deposits that contain ena•gite and gold as components of advanced argillic alteration (Summitville, Red Mountain, and Chinkuashih in Ta- ble 5), in which magmatic-hydrothermal fluids could conceivably have been important at least during early stages of mineralization. This appears to have been

the case in the zone of advanced argillic alteration at Julcani, Peru, where a radial swarm of tourmaline-- bearing breccia dikes emplaced prior to the main base and precious metal mineralization is interpreted by Shelnutt and Noble (1985) to be a product of deep- seated magmatic-hydrothermal fluids.

Modern analogs: Some epithermal breccias may be compared directly to the products of brecciation associated with active meteoric water-dominated geothermal systems in the Taupo volcanic zone of North Island, New Zealand, the western United States, and elsewhere. Brecciation is a common phenomenon at shallow levels in geothermal systems, and locally it breached the land surface to produce craters surrounded by aprons of breccia (Fig. 15). In keeping with the inferred mechanism of formation, the subaerial ejecta have been called hydrothermal explosion breccias (Muffler et al., 1971) or hydrothermal eruption breccias (Lloyd, 1959). Hydrothermal eruptions (hydroexplosions; Sheridan and Wohletz, 1983) were observed at Waimangu, New Zealand, in 1900-1904 and again in 1917 (Lloyd and Keam, 1965; Fig. 15), and at La Soufri•re, Guadeloupe, French West Indies in 1976 (Heiken et al., 1980).

In North Island, New Zealand, an area southwest of the Tarawera flow-dome complex, on the edge of the Okataina caldera (Fig. 16), is characterized by many hydrothermal explosion breccias. During the emplacement of the youngest Tarawera domes about 900 years ago (Cole, 1979), a series of hydrothermal eruptions took place in the nearby Waiotapu geothermal system, many of them localized by the Nga- pouri and associated faults (Lloyd, 1959; Cross, 1963; Hedenquist, 1983; Hedenquist and Henley, 1985). The craters along the trace of the Ngapouri fault (Fig. 16) are occupied by lakes measuring 100 to 750 m in diameter, which are surrounded by circular to oval

FIG. 15. Hydrothermal eruption of the Waimangu "geyser," probably in early 1904. Note the apron of breccia alongside the eruption crater. Taken from Lloyd and Keam (1976) after an original by Iles Photo, Rotorua.

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I 176•15 ß •.

Lake raraweca

l

/ /

38015 '- $.

L. Ngdkoro • 0 5 10km. • L __ • -- -- I 1

I

Waimongu Lake Rerewhakaaitu

ß (•) Lote Pleistocene dome rhyolites • Lore (;luotemory. foults: ... o bserved/conceeled ß ß Hydrothermel eruption craters ::.• Lake Rotomehane pre-1886

•.,.t Limit of Okatoino Volcanic Center

FIG. 16. Map of the Tarawera volcanic complex and associated hydrothermal eruption craters, North Island, New Zealand. Compiled from Lloyd (1959), Cross (1963), and Healy et al. (1964).

ejecta aprons ranging from i to 6 km 2 in area (Cross, 1963). In the southern Waiotapu area, an eruption crater 60 to 65 m across is occupied by Champagne Pool (Fig. 16), around which metal-bearing sinter terraces are currently accumulating. On the basis of the lithologies of clasts in the breccia apron around Champagne Pool, Hedenquist and Henley (1985) determined that the eruption extended downward to a depth of about 170 m. Fragments in nearby breccia aprons originated from depths as great as 300 m.

The Tarawera volcanic complex became active again in 1886, when a fissure (the Tarawera rift; Fig. 16) bisecting the domes erupted basaltic tephra. At the southwestern end of the fissure, at Waimangu (Fig. 16), minor basaltic eruption was followed four years later in 1900 by the inception of hydrothermal eruptions (Lloyd and Keam, 1965, 1976). The so-called Waimangu geyser (Fig. 15) erupted in Echo Crater and threw jets of debris, mud, and water to heights as great as 460 m and was characterized for four years by four- to nine-hour eruptions recurring at intervals of 30 to 36 hours (Lloyd and Keam, 1965). In August 1903, the crater measured 120 X 75 m and was 14.6 m deep (Lloyd and Keam, 1965). Some 12 years later, another eruption crater was created nearby. The initial steam-charged blast carrying rocks and mud partly

destroyed a building, 0.8 km away, and claimed two lives. Ejecta attained heights of 300 m, but activity had nearly ceased only two weeks later (Lloyd and Keam, 1965).

Several hydrothermal eruption events took place in the Kawerau and Orakeikorako geothermal fields of North Island since 16,000 years ago (Nairn and Solia, 1980; Lloyd, 1972). At Kawerau, three co- alesced eruption craters, each estimated to be 300 to 500 m wide, resulted from eruption of material from depths of at least 200 m (Nairn and Solia, 1980).

In 1976, phreatic eruptions were observed from fissures on the flanks of a volcanic dome at La Souf- ri•re (Heiken et al., 1980). Clouds of steam and fine- grained tephra rose buoyantly and were then pushed downward as density currents along surrounding val- leys.

The breccias produced by hydrothermal eruptions constitute aprons that decrease in both thickness and constituent clast size outward from crater rims. A maximum thickness of 13 m was reported for breccia on the rim of the Okaro crater at Waiotapu (Cross, 1963; Fig. 16). The breccias are heterolithologic and matrix supported, with some partial rounding of fragments observable locally. Clasts are up to 2 m across and commonly include hydrothermally altered ma-


terial derived from preexisting alteration zones; py- ritized, silicified, veined, and/or hydraulically brecciated fragments are widespread. At Kawerau, Nairn and Solia (1980) distinguished three episodes of hydraulic brecciation from features displayed by breccia clasts. Carbonized wood also occurs locally, accretionary lapilli were reported at La Soufri•re (Heiken et al., 1980), and fragments ofsinter were recognized in breccia around Lake Ngapouri (Fig. 16; Hedenquist and Henley, 1985). No juvenile clasts are present. The breccia matrices comprise clay-rich rock flour, which is typically muddy when wet. Breccias are mostly chaotic but may be weakly bedded.

Ore deposits are not known to be associated with any of the recent hydrothermal eruption craters and associated breccias but may well be in the process of formation in the breccia-filled vents inferred to underlie the craters. Evidence for this notion is provided by contents of up to 80 ppm Au and 175 ppm Ag in sinter around the rim of the Champagne Pool crater (Weissberg, 1969) and by the model presented by Hedenquist and Henley (1985).

Origin: Work by Henley and Thornley (1979), Nairn and Solia (1980), Hedenquist (1983), Berger and Eimon (1983), Fournier (1983), Hedenquist and Henley (1985), and Nelson and Giles (1985) has led to a good understanding of the likely mechanisms for phreatic brecciation associated with epithermal precious metal deposits and analogous geothermal systems. The brecciation seems generally to be dependent upon a buildup of hydrostatic pressure beneath a local barrier of low permeability. Permeability is commonly reduced by localized self-sealing (Facca and Tonani, 1967) in response to dumping of silica as fluids cooled on approach to the surface. Hence the widespread occurrence of silicification in and around epithermal breccias and the presence of silicified (or silica-carbonate) clasts in most hydrothermal breccias observed at the tops of hydrothermal systems. Alternatively, zones of low permeability may be provided by gouge-filled fault zones, shale beds, or densely welded volcanic rocks. Hydrostatic pressure increase beneath a localized barrier may be due directly to ascending fluids or, as proposed by Hed- enquist and Henley (1985), to the transmission of deeper fluid pressures to the barrier via a compress- ible cap of gas (particularly CO2) that separated and accumulated during boiling. Magmatic heating also provides an effective means of increasing fluid pressures (Nelson and Giles, 1985).

The trigger for phreatic brecciation is commonly attributed to intrusion of magma and/or seismically induced faulting, although more transient effects may also prove adequate (e.g., earth tides; Heiken et al., 1980). At Wau, brecciation was attributed to rapid reductions of confining pressure induced by sliding of rock masses into a maar crater (Sillitoe et al.,

1984b), but it could also have been a response to high-level dike intrusion. Activation of the Ngapouri and subsidiary faults along with magmatic intrusion were thought to have triggered hydrothermal eruptions at Waiotapu (Lloyd, 1959; Cross, 1963), whereas faulting alone was favored as a cause for these eruptions by Hedenquist and Henley (1985) and for those at Kawerau (Nairn and Solia, 1980). At La Souf- ri•re, there is no evidence that faulting played any part in the phreatic activity, which is more reasonably related to magmatic heating (Heiken et al., 1980).

As the prelude to hydrothermal eruption, a semipermeable barrier undergoes rupture by hydraulic fracturing, which is dependent upon the fluid pressure exceeding the sum of the lithostatic pressure and the tensile strength of the rock. Hydraulic fracturing, with or without the assistance of faulting, causes decompression of the fluid-filled fissures, which in turn causes disruption of their enclosing rocks and, commonly, the violent conversion (flashing) of water to steam. Continued violent discharge of water, steam, and entrained debris progressively widens initial fissures to form larger breccia bodies and pipes. The first eruptions at Waimangu in 1900 and at Kawerau are suspected to have been base surges (Lloyd and Keam, 1976; Nairn and Solia, 1980).

Available energy can be dissipated in the subsurface to give rise to "blind" breccias or it may be sufficient, to maximum depths of about 1 km (Nelson and Giles, 1985), to cause fissure propagation to the paleosurface and hydrothermal eruption to take place. Evi- dence is summarized above for hydrothermal eruption in several hot spring-related precious metal deposits, but brecciation is likely to have been an entirely subsurface phenomenon in many carbonate-hosted epithermal deposits and probably also in some volcanic- hosted epithermal deposits. The self-sealing-rupture sequence is likely to be episodic in most epithermal environments, as shown by evidence for multiple brecciation and silicification cited above. Boiling and chemical changes accompanying or immediately following brecciation may be instrumental in precious metal precipitation (Berger and Eimon, 1983; Hed- enquist and Henley, 1985).

Porphyry-type and other intrusion-related deposits General remarks: This section treats a variety of

generally poorly altered and mineralized breccias associated with porphyry-type and other deposits. The breccias differ from those assigned above to a magmatic-hydrothermal origin.

Characteristics: Breccias included in this category (Table 6) may be broadly subdivided into two geometries: irregular to pipelike bodies, and dikes and their offshoots. Many examples of the latter type are called pebble dikes, a term of some antiquity (e.g.,

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TABLE 6. Selected Examples of Phreatic Breccias

Locality Control Age (m.y.) Form Fragment

characteristics Matrix

Butte, Montana Quartz porphyry dike contacts

Butte, Montana Above quartz latite porphyry dike

62.8 to 57.7

62.8 to 57.7

Urad, Colorado Rhyolite porphyry "-30 contacts

Mt. Emmons, None known "-16 Colorado

Central City, Partly east-north- Colorado east fractures (The Patch)

59

Irregular bodies (Modoc breccias)

Dikes, pipes (Mtn. View breccias)

Shallow irregular bodies

Dikes (up to 750 m)

Steep pipe (230 X 140, 480 m deep)

Angular to rounded, Rock flour monolithologic

Angular to rounded, Rock flour heterolithologic

Angular to rounded, Rock flour or mono- to heterolith- none ologic

Subrounded Rock flour

Angular to rounded Rock flour

Leadville, Col- Postmineral quartz Early Ter- orado monzonite por- tiary

phyry

Tintic, Utah Fractures, monzo- Oligocene nite porphyry dikes

Bisbee, Ari- Faults, bedding 163 zona planes

Cuajone, Peru Northwest faults, 51 latite porphyry

Toquepala, Latite porphyry 59 Peru

E1 Salvador, Northwest + ra- 41 Chile dial fractures,

latite porphyry

Dikes Angular to rounded, Rock flour heterolithologic + porphyry

Dikes, lenses, Angular to rounded, Rock flour, mi- sills heterolithologic nor porphyry

Dikes, sills, pipes

Steep tabular + irregular bodies

Irregular pipe, dikes

Dikes

Subangular to rounded, Rock flour heterolithologic

Rounded, heterolitho- Latite porphyry, logic minor rock

flour

Rounded Rock flour

Angular to rounded, Rock flour heterolithologic

Mt. Morgan, Partly northeast Middle De- Queensland, fault vonian Australia

Dikes Angular to rounded, Rock flour heterolithologic

Parsons, 1925) used to describe dikelike bodies of breccia in which the fragments are well rounded.

In Table 6, the first type is represented by the Red Mountain breccias described by Wallace et al. (1978) at Red Mountain (Urad), the Modoc breccias studied by Minervini (1975) at Butte, and The Patch referred to by Bastin and Hill (1917) and Sims et al. (1963) in the Central City district. Fragments in these breccias range from angular to rounded and are set in variable amounts of rock flour. At Urad, both the Junk Rock and Rubble Rock breccias vary from open space- to

rock flour-dominated over short distances. There is a tendency for the breccias to be monolithologic, with little evidence for appreciable fragment transport. At both Urad and Butte, this type of breccia is closely related to particular phases of porphyry intrusion and tends to be concentrated as sleeves or envelopes around the resulting intrusive bodies.

The second type comprises mainly dikes (Fig. 17), which at some localities are accompanied by sill-like bodies, pipes, and irregular bodies. Abrupt transitions from one geometry to another are commonplace, as

ORE-RELATED BRECCIAS 1N VOLCANOPLUTONIC ARCS

Associated with Porphyry and Related Deposits

1493

Upward fragment

Hydrothermal Cementing displace- Relationship to alteration minerals ment (m) Ore deposit type orebody Reference

Propylitic Chlorite, epidote, Present Porphyry Cu-Mo Pre-Main Stage pyrite, sphaler- + veins veins, barren ire, chalcopyrite

Minor None > 120 Porphyry Cu-Mo Post-Main Stage + veins veins

Partly sericitic None Minor Mo lode in porphyry Postore Mo system

None None 500 Porphyry Mo Postore

Sericitic Quartz, pyrite, Unknown Au-Ag-Cu-Pb-Zn Preore sphalerite, veins + breccia chalcopyrite, galena, tetrahedrite

None None Present Pb-Zn-AgoAu re- Postore placement + vein

Partly silicified Quartz, pyrite, up to 1,800 Pb-ZnoAg-Au veins Preore, largely local Pb-Zn ore + replacements barren

None None >1,000 Porphyry Cu + re- Postore placement Cu + Pb-Zn

Partly silicified Quartz, pyrite Present Porphyry Cu-Mo Postmineral

Minervini (1975)

Sales (1914), Meyer et al. (1968)

Wallace et al. (1978)

Thomas and Galey (1982)

Bastin and Hill (1917), Sims et al. (1963)

Thompson et al. (1983)

Farmin (1934), Lovering et al. (1949), Morris and Lovering (1979)

Bryant and Metz (1966), Bryant (1968, 1983)

Satchwell (1983)

Minor

Chlorite, calcite (deep); sericitic, advanced argillic (shallow)

None

Minor Present Porphyry Cu-Mo Late to postmineral

None Present Porphyry Cu-Mo Late to postmineral

None Unknown Cu-Au pyritic re- Postmineral placement

Richard and Courtright (1958)

Gustafson and Hunt (1975)

Cornelius (1967)

at Bisbee (Bryant, 1968). Dikes range in thickness from 1 cm to 10 m and commonly tend to pinch and swell both vertically and horizontally. They are con- tinuous in a vertical sense for at least 600 m (e.g., Tintic, Morris and Lovering, 1979; E1 Salvador, Gus- tarson and Hunt, 1975) and possess strike extents as great as 1 km at E1 Salvador (Gustarson and Hunt, 1975) and 0.75 km at Mt. Emmons (Thomas and Galey, 1982). Pebble dikes may occur singly (e.g., Mt. Emmons) or in swarms. Sill-like bodies have similar thicknesses but are generally less extensive. Pipe-

like bodies are up to 150 m in diameter at Bisbee (Bryant, 1968). All bodies possess abrupt contacts with wall rocks (Fig. 17). Faults and fractures of well- defined strike appear to have localized breccia dikes at the majority of deposits (Tintic, Bisbee, Cuajone, E1 Salvador, and Mt. Morgan). At E1 Salvador, pebble dikes become less numerous downward and die out completely some 600 m beneath the surface (Gustaf- son and Hunt, 1975).

The breccia dikes and associated bodies all contain angular to rounded clasts of a variety of rock types in

1494 I•ICH.4•,D H. SILLITOE

FIG. 17. Pebble dike. Rio Blanco porphyry copper deposit, Chile.

a sand- to silt-size rock flour matrix (Fig. 17). The matrix comprises from about 30 to nearly 100 percent of a breccia. Fragments become progressively more rounded upward at Tintic (Farmin, 1937; Morris and Lovering, 1979) and El Salvador (Gustafson and Hunt, 1975).' Exceptionally well rounded fragments (Fig. 17), some exhibiting hypogene exfoliation, are common, as at Tintic (Farmin, 1934, 1937) and Bisbee (Bryant, 1968). Flow-banding is present in the rock flour matrix of some dikes. Comparison of clast lithologies with the local geology has enabled determi- nation of an appreciable upward transport of some fragments in most pebble dikes: 500 m at Mr. Emmons (Thomas and Galey, 1982), >1,000 m at Bisbee (Bryant, 1968), and at least 1,800 m at Tintic (Farmin, 1934).

A close spatial (and probably temporal) relationship is commonly apparent between breccia dikes and specific phases of intrusion, which are commonly late to postmineral in age and dikelike in form. This close relationship is manifested by the occupancy of fattits or 'fractures by both igneous and pebble dikes and by the local occurrence of an igneous rock as an unbrecciated matrix to breccia dikes. Pebble dikes give

way downward to monzonite porphyry dikes at Tintic (Farmin, 1934; Morris and Lovering, 1979) and quartz latite porphyry at Butte (Meyer et al., 1968). Availability of magma at the time of pebble dike emplacement is shown by mutually crosscutting relationships between latite porphyry and •ebble dikes at El Salvador (Gustarson and Hunt, 1975).

Alteration and mineralization: Both types of breccias discussed in this section, except for those at Tintic and Central City, were emplaced !ate in their re- spective mineralization sequences. Breccia dikes were characteristically the last additions and postdated all mineralization (and other brecciation) in many .districts. At Red Mountain (Urad), the breccias postdated the Urad molybdenum orebody and only constitute ore locally because of high concentrations of molyb- denitc-bearing clasts (Wallace et al., 1978). The Mo- doe breccias at Butte carry clasts of Pre-Main Stage (i.e., porphyry type) mineralization but were cut by Main Stage rhodochrosite veinlets and locally cemented by minor amounts of base metal sulfides (Mi- nervini, 1975).

Breccia dikes are generally unaltered, or only slightly altered (Table 6), and carry sulfides mainly as components of fragments. At Bisbee, for example, pebble breccias locally make ore on account of high concentrations of fragmental sulfides, especially where they abut replacement copper orebodies (Bryant, 1983). Ore-bearing fragments are also present, albeit in lesser concentrations, at Leadville, El Salvador, Mt. Morgan, and Mt. Emmons, with molybdenum-bearing fragments having ascended at least 500 m at the. last locality (Thomas and Galey, 1982). In contrast to the majority of pebble dikes, those'at El Salvador were emplaced while sericitic and advanced argillic alteration was active at shallow levels of the porphyry system,. although ore-related alteration events had ceased at depth (Gustarson and Hunt, 1975). The pebble dikes at Tintic only locally make ore but were interpreted by Lovering 6t al. (1949) and Morris and Lovering (1979) to have been emplaced prior to mineralization.

Although the nature of the fluids responsible for this weak alteration and mineralization has not been determined, a meteoric-hydrothermal origin is not inconsistent with field relationships.

Origin: Many of these breccias were empiaced in close association with poorly mineralized and fractured intrusive rocks after much of the associated alteration and mineralization had ceased. These relationships suggest that brecciation may have been induced by meteoric fluids under the influence of heat from an igneous intrusion (cf. Gustarson and Hunt, 1975; Morris and Lovering, 1979).

It is possible that thermal expansion of meteoric fluids in the wall rocks of stocks or dikes, as proposed by McBirney (1963) and modeled by Knapp and


Knight (1977) and Delaney (1982), could provide a suitable mechanism for the generation of both little- moved monolithologic breccias around or above porphyry stocks or dikes, as at Urad, Butte, and Central City, and heterolithologic breccia dikes.

Application of the mechanism assumes that wall rocks were saturated with relatively cool meteoric water or had become recharged with meteoric water following completion of the main stages of magmatic- hydrothermal mineralization--as they certainly had at E1 Salvador, where a shallow hot spring system was shown to have operated during pebble dike formation (Gustarson and Hunt, 1975). Delaney's (1982) cal- culations showed that pressurization and expansion of ground waters are most effective during rapid emplacement of magma into relatively impermeable rocks at shallow depths (•1 km). Late-stage dike emplacement into hydrothermally healed rocks in the upper parts of porphyry systems would therefore provide a suitable environment. Brittle failure and brecciation would result from rapid increases of pore fluid pressure sufficient to exceed the lithostatic load plus tensile strength of the rock. If water approached the boiling point curve for a given depth, then it is likely to have flashed to steam during dike intrusion, as envisaged for the E1 Salvador pebble dikes by Gus- tarson and Hunt (1975). Violently expanding steam could then have opened fractures above the dikes and perhaps caused their propagation to the paleosurface. Repeated steam production during intrusion would have resulted in shattering, entrainment, and upward transport of material along the walls of fractures above the dikes. A high proportion of steam in the upfiowing fluid-rock mixture would have given rise to the extreme mobility suggested by the transport of material over large vertical distances and its injection along tortuous pathways. Decompressive events consequent upon repeated opening of fractures could have caused the widespread hypogene exfoliation of fragments and would have favored localized ascent and penetration of magma into still-mobile breccia. Kuroko-type massive sulfide deposits

General remarks: Kuroko-type deposits were generated at and immediately beneath the sea floor in a physically unstable environment characterized by volcanic, hydrothermal, and mechanical activity. It is not surprising therefore that a variety of fragmental accumulations and textures characterize both Kuroko- type deposits and their immediate host rocks (Clark, 1971, 1983; Eldridge et al., 1983). Selected examples ofbreccias associated with Kuroko-type deposits from Archean to mid-Miocene in age are considered in this section.

Characteristics: Most of the sulfide-bearing breccias considered here (Table 7) are located on the flanks of, or as aprons around, felsic lava domes and overlie

felsic lava or fragmental volcanic rocks (Fig. 18). The breccias occur as lateral extensions of bodies of massive ore or, less commonly, overlie them. Most of the breccias generally are not underlain by stockwork mineralization. It is clear that many of these breccia bodies are allochthonous distal accumulations of sulfide-bearing fragmental material. The geometry of the breccia bodies ranges from lenses through elongate tabular bodies to sheets, with thicknesses up to a maximum of about 25 m (Fig. 18). The linear breccia body that constitutes the Maclean orebody at Buchans is 870 m long (Thurlow and Swanson, 1981). Com- monly the breccias occupy paleotopographic depres- sions which in places, as at Buchans, have been defined as elongate troughs (Thurlow et al., 1975).

Many of these sulfide-bearing breccias are heterolithologic and comprise variable amounts of felsic to basic volcanic rocks, argillaceous rocks, massive sulfide, and barite. Clasts of gypsum or siliceous stockwork are also present, as are pieces of earlier breccias. The clasts, up to > 10 m in size, are generally angular to subrounded in outline, but well-rounded fragments have also been described (e.g., Kurosawa; Motegi, 1974). At Buchans, for example, sulfide-poor "breccia conglomerate" and "granite conglomerate" were described by Thurlow et al. (1975) and Thurlow and Swanson (1981), names that reflect the spheroidal form of many of the fragments. The granite breccia contains spheroidal granite fragments up to 6 m in size of an unknown, but presumably a deeper level, source (Thurlow and Swanson, 1981), which appear to have gained their form through hypogene exfoliation. At Buchans (Thurlow and Swanson, 1981), Ainai (Ishikawa and Yanagisawa, 1974), and elsewhere, some of the massive sulfide clasts exhibit bent, wispy outlines interpreted to result from fragmentation and incorporation while still in a semilithified state. The breccias range from clast to matrix supported. Matrix is generally fine-grained clastic material, which may be dominated by comminuted lithics, sulfides, and/or barite. The breccias range from chaotic, unbedded aggregates, as at Buchans and Vauze, to well-bedded accumulations characterized by graded bedding and other sedimentary structures, as at several Japanese localities (Table 7).

There is some evidence for the existence of a spectrum ofbreccia types in the Kuroko environment that range from those composed entirely of massive sulfide clasts (e.g., Ainai) through breccias with both massive sulfide and lithic clasts (Table 7) to several end-member types dominated by lithic clasts. The breccias composed mainly of sulfide clasts generally constitute ore (Table 7). Lithic breccias pre- and postdate ore formation; the preore variety includes the Motoyama- type breccias of Horikoshi (1969), which commonly underlie massive sulfide ore.

The crosscutting breccia summarized in Table 7



.0..*• BEDDED FRAGMENTAL ORE • MUDSTONE • MASSIVE ORE •'•'] RHYOLITIC BRECCIA :• STOCKWORK ORE +J'•+-J FELSIC LAVA DOME

FIG. 18. Schematic section through a Kuroko-type massive sulfide deposit to show relationships of fragmental ore. Based on Lee et al.'s (1974) interpretation of the Kamikita deposit, Japan.

from Mt. Chalmers is distinctly different from those breccias described above. It occurs as a siliceous, chimneylike body and is flanked by polymetallic massive sulfide deposits (Large and Both, 1980). The chimney comprises angular to subrounded fragments of chalcedony and jasper recemented by chalcedony and minor barite and sericite. The breccia carries 2 to 10 wt percent disseminated pyrite and was exploited for its gold content (Large and Both, 1980). The Mt. Chalmers breccia is similar in many ways to the silicified breccias associated with many epithermal precious metal deposits generated in subaerial settings (see above).

Origin: Horikoshi (1969) proposed an origin by phreatic steam explosions for the Motoyama-type breccias associated with many Japanese Kuroko deposits. He envisaged explosive activity to have been triggered by ingress of cool seawater to consolidated but still hot felsic lava domes on the sea floor. A similar mechanism has been applied by several workers, including Clark (1971, 1983), Ishikawa and Yanagisawa (1974), Spence (1975), Thurlow et al. (1975), Walker et al. (1975), Henley and Thornley (1979, 1981), and Thurlow and Swanson (1981), to the massive sulfide- bearing breccias that constitute parts of some Kuroko- type deposits. By analogy with near-surface brecciation in subaerial geothermal systems (see above), Henley and Thornley (1979, 1981) attributed generation of the massive sulfide-bearing breccias to hydrothermal eruptions triggered by separation of volatiles (including CO2) from ascending fluids, with accumulation of the volatiles beneath semipermeable cap rocks. A decrease in permeability could be caused by silicification of the subsea-floor conduit (as at Mt. Chalmers) or capping of the conduit by massive sulfide. Rapid rupture of the cap rock would have been accompanied by fragmentation of massive sulfides with or without underlying mineralized and/or un- mineralized rocks and by their ejection on to the sea floor. As noted by Henley and Thornley (1979), the process is potentially repetitive, thereby explaining the formation of the Motoyama-type and other preore

breccias, synore breccias carrying plastically de- formed sulfides, and postore breccias.

In most cases, the ejected fragmental material appears to have been transported variable distances downslope away from the eruption site, presumably because fragments came to rest on a steep slope or, alternatively, because fragment accumulation caused oversteepening of an existing slope. Earthquake activity or continued or renewed dome emplacement could also have caused landsliding of near-vent breccia accumulations. In the case of poorly sorted breccias lacking internal structure, either transport was limited, as at Vauze where the breccia resembles a talus-covered slope (Spence, 1975), or over greater distances, as at Buchans, transport is inferred to have taken place as density flows (Thurlow and Swanson, 1981; Walker and Barbour, 1981). Mass flowage appears to have been transitional to turbidity currents, which were probably the main transporting agents for well-bedded breccias that exhibit a variety of sedimentary structures.

The presence of a variety of fragment lithologies, including argillized and chloritized volcanic rocks, stockwork ore, and previously formed breccias, is strong evidence for the origin of heterolithologic breccias by hydrothermal eruption. It must be ad- mitted, however, that some breccias, especially those composed essentially of massive sulfide fragments, could have formed simply by fragmentation during slumping and landsliding.

Phreatomagmatic (Hydromagmatic) Breccias Porphyry-type and epithermal precious (_ base) metal deposits

General remarks: The breccias in this section are associated mainly with epithermal deposits and porphyry copper deposits and appear to be appreciably less widespread than other varieties of breccia described above from these two ore deposit types. These breccias were first recognized as associates of ore deposits by Sillitoe and Bonham (1984), although they would appear to include some of the "prehydro- thermal" breccias of Bryner (1961) and to constitute the breccia category addressed by Wolfe (1980).

The term diatreme is preferred to that of breccia pipe for breccia-filled conduits of this type because of their intrinsic differences and because they are believed to have been generated in a manner that com- plies with Daubrf•e's (1891) original definition of a diatreme as a vent produced by volcanic explosion.

Characteristics: This category possesses a number of unifying characteristics that help to distinguish it from other types of breccia. Many examples of this breccia type, especially those at Cripple Creek, Mon- tana Tunnels (Fig. 19), Cerro de Pasco, E1 Teniente, Guinaoang, Dizon, and Acupan (Table 8), are fine


FIG. 19. Highly sericitized, matrix-rich phreatomagmatic breccia. Note polished subrounded clast near end of pocket knife. Montana Tunnels, Montana.

grained and largely matrix supported; from 50 to 90 percent matrix material is usual. At Montana Tunnels, matrix material is <2 mm in grain size but is deficient in silt- and clay-size fractions (Sillitoe et al., 1985). Many of the breccias have a juvenile tuffaeeous component, besides rock flour, in their matrices. The tuffaceous material commonly approximates daeite in composition and comprises both broken and unbroken crystals of quartz, biotite, and feldspar. The tuffaeeous component is often difficult to recognize where it is intensely altered, as at Montana Tunnels (Fig. 19). Breccias .with a wholly or partly tuffaceous matrix were denominated "tuffisite" by Cloos (1941).

The abundant matrix material generally precludes the.presence of significant open space in most breccias, although large cavelike openings have been encountered at El Teniente and Dizon. Those at El Ten-

TABLE 8. Selected Phreatomagmatic Diatremes and

Probable Locality Horizontal Vertical juvenile

(diatreme name) Host rocks Age (m.y.) dimensions dimensions component Other features .

Montana Tunnels, Late Cretaceous vol- 45 to 50 2.1 X up to 0.6 >310 Quartz latitie Cut by quartz Montana eanies, Eocene ig- tuff latite por-

nimbrites phyry dikes Cripple Creek, Precambrian granite, 27.9to 29.3 5.9 X 2.7 >1,000 Latite-phonolite Cut by bodies

Colorado gneiss, schist tuff of alkaline rock, basaltic breccia

Bassick, Colorado Precambrian gneiss, Oligocene 1.3 x 0.85 >430 Andesitc tuff(?) granite

Cerro de Pasco, Silurian-Devonian 14 to 15 2.7 X 2.3 >800 Felsic tuff Quartz latite Peru (Rumial- phyllite, Permian porphyry !ana Agglomer- red beds, Trias-]u- flow-dome ate) rassic limestones complex,

dikes

Mi Vida, Argen- Late Precambrian 6.8 2.3 x 1.1 >500 Rhyolitic pyro- tina (Carudo schists, migmatites, clastics breceia) granite; Miocene

syenodiorite

El Teniente, Miocene andesitic 4 to 5 1.3 x 1.3 >1,600 Minor felsic Cut by daeite Chile (Braden voleanies tuff(?) ' porphyry "pipe") bodies

Guinaoang, Phil- Mesozoic(?) schist, 2.9 8.5 X 3.5 >400 Dacitic tuff Cut by daeite ippines Mio-Plioeene an- porphyry

desitic volcanics dome

Acupan, Philip- Cretaceous-Paleogene Pleisto- 1.0 X 0.6 > 1,000 None pines (Balatuc andesitic voleanics, cene(?) "plug") Mioeene diorite

Dizon, Philip- Late Tertiary andes- Late Tertiary > 1.0 >300 Dacitic tuff Cut by dacite pines (Pua dia- itic voltanits, porphyry treme) microdiorite dikes,

dome(?) Cut by domes,

dikes Wau, Papua New Late Cretaceous-Pa- <4 to >2.4 1.4 X 1.4 >200 Dacitic pyro-

Guinea leogene phyllite, clastics Pliocene ignimbrite + clastic sediments


iente were up to 8 m in diameter, filled with water, and lined with crystals of gypsum, up to 3 m long, and other hypogene minerals (A. Enrione, pers. com- mum, 1983).

Clasts in these breccias are heterolithologic and comprise all known wall rocks to the diatremes. In addition to these accidental lithic clasts, some diatremes also contain rhyolitic to dacitic (latitic-pho- nolitic at Cripple Creek) clasts of apparently juvenile origin (e.g., Fig. 20). Most of the clasts are poorly vesiculated (cognate lithics), but dacitic pumice has also been recognized at Dizon (Sillitoe and Gappe, 1984) and Guinaoang (Sillitoe and Angeles, 1985), where the pumice is flattened parallel to the inclined contact of the diatreme. Clasts range up to 10 or more meters in diameter and are subangular to rounded in form. Clasts, especially the more common smaller

ones, are polished. Large spheroidal clasts, some of them displaying hypogene exfoliation features, are present at several localities, including Mi Vida (Kouk- harsky and Mirr•, 1976) and Dizon (Malihan, 1982; Fig. 21).

Several facies ofbreccia are recognized in a number of diatremes. For example, at Guinaoang, an early tuffaceous facies is cut by, and incorporated as fragments in, coarse- and fine-grained lithic breccias (Sil- litoe and Angeles, 1985). At Acupan, 95 percent of the Balatoc diatreme is occupied by a late breccia that becomes progressively finer grained inward. An early, even finer grained breccia occurs as remnants around the walls of the diatreme (Damasco and de Guzman, 1977). At Cripple Creek, a distinctive late phase of breccia with a basaltic tuff component in its matrix constitutes the pipelike Cresson "Blowout"

Maars Associated with Precious and Base Metal Deposits

Alteration and mineralization of Evidence for surface Ore deposit Location of Timing of

breccia connection type mineralization brecciation Reference

Sericite, siderite, Logs, base surge as Disseminated Within diatreme Pre- and inter- manganocal- blocks Au-Ag-Zn- mineral cite, pyrite Pb

Sericite, dolo- Logs, lacustrine sedi- Au veins, Mainly within Premineral mite, pyrite ments, accretionary breccias diatreme

lapilli

Clays(?) Logs, base surge de- Au-Ag-Pb-Zn Within southern Premineral posits(?) pipe part of dia-

treme

Propylitic, pyrite Base surge deposits Pb-Zn-Ag-Cu Mainly south- Premineral veins, re- east edge of placements diatreme

Advanced argil- None Porphyry Pipe in dia- Late mineral lic, pyrite, co- Cu-Mo, treme vellite, char- Cu-Pb-Zn- gite Ag pipe

Weak sericite, Bedding of base(?) Porphyry Surrounds dia- Postmineral tourmaline, py- surge origin Cu-Mo treme rite

Weak chlorite, Base surge deposits, calcite, clays, accretionary lapilli pyrite

Chlorite, calcite, Logs sericite, pyrite

Weak chlorite, Logs, base surge de- calcite, clay, posits, accretionary specularitc lapilli

Quartz, calcite, Lacustrine sediments, clays, pyrite; base surge, accre- kaolinte, alu- tionary lapilli nite, pyrite

Porphyry Southeast of Postmineral Cu-Au diatreme

Au in pipe- Annulus to dia- Intermineral(?) like brec- treme cias

Porphyry North of dia- Postmineral Cu-Au treme

Au veins, In tuff ring near Premineral stockwork maar ring

fault

Sillitoe et al. (1985)

Lindgren and Ransome (1906), Loughlin and Koschmann (1935), Thompson et al. (1985)

Cross (1896), Emmons (•896)

Geologic staff of Cerro de Pasco Corporation (1950), Silberman and Noble (1977)

Koukharsky and Mirr• (1976)

Lindgren and Bastin (1922), Howell and Molloy (1960)

Sillitoe and Angeles (1985)

Worley (1967), Da- masco and de Guz- man (1977)

Mallhah (1982), Sillitoe and Gappe (1984)

Sillitoe et al. (1984b)


FIG. 20. Heterolithologic (explosion) breccia from degraded tuff ring around maar crater. Phyllite (black) and dacite porphyry (white) clasts are prominent. Namie breccia from Wau, Papua New Guinea.

(Loughlin and Koschmann, 1935). A coarser breccia typically characterizes the marginal parts of some diatremes, as exemplified by the pebble breccia at Dizon and the coarse lithie breccia at Guinaoang.

Diatremes are typically larger than most breccia pipes. With only one exception, the examples in Table 8 all occupy > 1 km 2 at surface. The two largest, Crip- ple Creek and Guinaoang, have maximum surface dimensions of 5.9 and 8.5 km, respectively, and appear to have formed by coalescence of several smaller diatremes. The vertical extents of diatremes are also large, with Cripple Creek, El Teniente, and Acupan all exceeding 1,000 m. The Braden pipe at E1Teniente possesses an irregular dogtooth contact with a postmineral dacite porphyry at a depth of about 1,600 m and may not extend any deeper (A. Enrione, pers.

commun., 1983). Many of the diatremes possess inward-dipping walls' and several of them are funnel shaped, a form that is believed to typify their upper parts. The contacts are generally abrupt and defined by ring faults in which gouge and fault breccia are common. The wall rocks abuting the ring faults are shattered and, in some cases, brecciated. Discontin- uous annuli of open-space breccia adjoin diatremes at E1Teniente (Howell and Molloy, 1960) and Acupan (Damasco and de Guzman, 1977; Fig. 22); they appear to predate diatreme eraplacement. Large blocks of wall rock, up to several hundred meters long, became detached from the walls of some diatremes and are particularly widespread in the marginal parts of the breccias; some of those at Montana Tunnels are nearly vertical (Sillitoe et al., 1985).

At Wan, Sillitoe et al. (1984b) presented evidence for partial preservation of a maar crater and encircling tuff ring, which are inferred to be undedain by a breccia-filled diatreme comparable to those described above. Two varieties of breccia are present both

FIG. 21. Hypogene exfoliation exhibited by fragment from the marginal part of the Pua diatreme, Dizon porphyry copper- gold deposit, Philippines.

• Diotreme Ixeccia (ac•,toc 'thug') Diorite

vl• • Andesilk: volconics • v•.l• stri-tun

FIG. 22. Plan of the 1,500-m level of the Acupan gold mine, Philippines, to show open-space breccia and the contained G.W. orebodies as an annulus to the Balatoc diatreme ("plug"). Taken from Damasco and de Guzman (1977).


within the maar crater and as the sole components of the tuff ring. About 90 percent of the breccia is massive, unbedded, angular, heterolithologic and matrix supported (Fig. 20). It is similar to the "explosion breccia" defined by Wohletz and Sheridan (1983) from basaltic tuff rings and tuff cones and is therefore probably of ballistic fall origin. The remaining 10 percent is sand to pebble size and displays low-angle crossbedding and dune forms. These finer grained horizons, which are up to several meters thick, also contain abundant accretionary lapilli. These characteristics are typical of pyroclastic base surge deposits (e.g., Moore, 1967; Fisher and Waters, 1970), which are common constituents of maar volcanoes (e.g, Lor- enz, 1973; Sheridan and Wohletz, 1983). Lacustrine sediments, rich in plant remains, dominate the upper parts of the intramaar sequence at Wau and are overlain by blocks of basement phyllite and its tuff ring cover that slid into the maar following the cessation of explosive activity (Sillitoe et al., 1984b).

Fine-grained, cross-stratified breccia similar in ap- pearance to the base surge deposits at Wau, and apparently of the same origin, has been observed in restricted parts of the diatremes at Montana Tunnels (Sillitoe et al., 1985), Cripple Creek (Lindgren and Ransome, 1906), Cerro de Pasco (Silberman and No- ble, 1977), E1Teniente (Lindgren and Bastin, 1922), Guinaoang (Sillitoe and Angeles, 1985), Dizon (Sil- litoe and Gappe, 1984), and possibly, Bassick (Cross, 1896). Accretionary lapilli are present in these base surge deposits at Cripple Creek (Thompson et al., 1985), Guinaoang, and Dizon. The base surge deposits clearly constitute blocks at Montana Tunnels, Cripple Creek, Guinaoang, and Dizon, but this is less certain at Cerro de Pasco, Bassick, and El Teniente. These base surge deposits are believed to have subsided into the diatremes from subaerial tuff rings. The presence of fiuviolacustrine sediments, characterized by ripple marks and dessication cracks, to depths of >300 m below the present surface at Cripple Creek is ex- plained in the same manner (Thompson et al., 1985). Further evidence that diatremes intersected the palcosurface is provided by the presence of pieces of carbonized wood at Montana Tunnels, Bassick, Crip- ple Creek, Acupan, and Dizon (Table 8).

Several diatremes were cut by dikes or irregular bodies of intrusive rock, as at Montana Tunnels, Crip- ple Creek, Cerro de Pasco, E1 Teniente, Dizon, and Wau (Table 8). At Montana Tunnels and E1Teniente, brecciation was still active during intrusion, as evidenced by the irregular, swirly, and mutually crosscutting contacts between breccia and intrusive rock and, at the former locality, by chilled margins to pieces of dike rock incorporated in the breccia (Sil- litoe et al., 1985). At several localities where erosion is minimal or not far advanced, endogenous domes are recognizable, e.g., Cerro de Pasco (Silberman and

Noble, 1977), Guinaoang (Sillitoe and Angeles, 1985), Wau (Sillitoe et al., 1984b), and possibly, Dizon (Sil- litoe and Gappe, 1984). The dikes and bodies of porphyry encountered in a number of diatremes are likely to have fed domes at higher, now-eroded levels.

A structural control of diatreme emplacement is apparent at Montana Tunnels, Cerro de Pasco, and Wau, which all lie on major regional faults.

Alteration and mineralization: It may be appreciated from Table 8 that diatremes associated with epithermal precious metal deposits were emplaced either before mineralization commenced or, at Mon- tana Tunnels and possibly also at Acupan, while it was taking place. In contrast, the diatremes that accompany porphyry copper deposits are commonly very late or postmineral in age.

There is a tendency for precious (_ base) metal mineralization to be concentrated around the edges of diatremes, although their interiors may also be ore bearing. Examples of marginal ore include: a huge silica-pyrite body and associated Ag-Pb-Zn-Cu mineralization at Cerro de Pasco (Cerro de Pasco Cor- poration, 1950), an annulus of gold-bearing open- space breccia at Acupan (Fig. 22), and shallow gold lodes and associated stockworks at Wau (Sillitoe et al., 1984b). Gold telluride ore is also concentrated around the Cresson Blowout, a late facies of the Crip- ple Creek diatreme (Loughlin and Koschmann, 1935). Enhanced permeability provided by ring faults and associated shattering or brecciation of wall rocks is the prime reason for ore deposition around the margins of diatremes. At Acupan, 11 principal lenslike bodies of breccia parallel the diatreme contact over vertical intervals of up to 600 m and are thought to have been supplied with mineralizing fluids where they are intersected by auriferous veins (Worley, 1967; Damasco and de Guzman, 1977; Fig. 22).

Precious metal mineralization is also present within diatremes: gold-bearing veins and phreatic breccias (see above) at Cripple Creek (Thompson et al., 1985), a precious metal-bearing pipe at Bassick (Emmons, 1896), and a zone of largely disseminated Au-Ag-Zn- Pb mineralization at Montana Tunnels (Sillitoe et al., 1985). At Montana Tunnels, the presence of an appreciable amount of clastic sulfides (including veinlet fragments) in the breccia shows that mineralization continued during the period of active brecciation, probably during pauses in explosive activity.

In the case of the four porphyry copper deposits in Table 8, ore is located beyond the limits of the diatremes, although mineralized clasts are widespread in the diatreme breccias themselves. At Mi Vida, however, significant late-stage advanced argillic alteration affected much of the breccia and gave rise to a zoned, pipelike body of copper and lead-zinc mineralization in the diatreme's interior (Koukharsky and Mirr•, 1976).


Echo Crete, • • 1886 explosion crater '-'-':• Pre-I• •kes. WAIMANGU

• •r•t• of • su•, •.• C•tou• in • : • 3 km inferr• f• cross•i•. •1• •nt --- ' •ke levi.

FIG. 23. Volcanic explosion craters formed in 1886 along the Tarawera rift at Lake Rotomahana and Waimangu, North Island, New Zealand. The main phreatomagmatic base surge deposit originated from Great Crater basin. Pre-1886 and present lake levels shown. Taken from Nairn (1979).

The diatreme breccias are weakly but pervasively altered irrespective of whether or not their emplacement was pre- or postmineral. The alteration (except for that at Montana Tunnels and Mi Vida) appears to bear no direct relation to mineralization and is generally characterized by one or more of sericite, clays, chlorite, carbonate, zeolites, specularite, and pyrite (Table 8). However, part of the pyrite is invariably clastic. This alteration is attributed to the high fluid contents of the breccias at the times of their formation (see below).

Modern analogs: Maar volcanoes are widespread volcanic phenomena, although those involving magmas of rhyolitic to dacitic composition appear to be less common than their basic counterparts. This may be in part due to their destruction during later dome emplacement, as at Julcani, Peru (Shelnutt and Noble, 1985). From the standpoint of epithermal mineralization, the 1886 eruption of Rotomahana in the Taupo volcanic zone of North Island, New Zealand (Fig. 16), is of some interest although it did not give rise to a typical monogenetic maar volcano.

At the time of basaltic eruption from the Tarawera rift (see above), basaltic magma ascending along the southwestward continuation of the fissure is believed to have interacted with meteoric fluids of the Roto- mahana geothermal system to trigger a catastrophic phreatomagmatic eruption (Nairn, 1979). Water-saturated base surges traveled westward for at least 6 km from vents now 95 percent concealed beneath Lake Rotomahana (Fig. œ3) to produce the Rotoma- hana mud. The explosions disintegrated two large hot spring sinter aprons that capped part of the Roto- mahana geothermal system, as well as a large volume of altered rock from the system itself. Hydrothermal

activity took place after Rotomahana became quies- cent, as shown by the hydrothermal eruptions at nearby Waimangu some four years later (Figs. 16 and 23; see above).

Origin: Sheridan and Wohletz (1981, 1983) concluded that the phreatomagmatic explosive activity responsible for maar-diatreme generation may result from a fuel-coolant type of interaction between magma and an external water source. In the case of the diatremes under discussion here, the source is believed to have been an aquifer charged with ground water at depths of 1 to œ km, or even more (e.g., E1 Teniente), beneath the paleosurface. Fault zones also may have facilitated ground-water access in some places. Lindgren and Bastin (1922) were the first to propose the explosive interaction of magma and meteoric water as a mechanism for formation of one of the diatremes discussed here--the Braden pipe at E1 Teniente.

It is clear that the diatremes (Table 8) were generated by multiple explosions, each involving expansion and vaporization of ground water, and fragmentation and entrainment of magma particles (Sheridan and Wohletz, 1983). The essential (magmatic) and accidental (wall-rock) components of the resulting pyroclastic products are characterized by a high degree of comminution (Self and Sparks, 1978; Sheridan and Wohletz, 1983), as observed in many of the breccias. Ascent of fragmented magma, rock, steam, and water in diatremes gave rise to distinctive eruption products (Wohletz and Sheridan, 1983), among which pyroclastic base surge deposits and accretionary lapilli are particularly diagnostic. However, eruption was characterized by pyroclastic fall activity as well as by laterally directed, water-saturated base surges, with


the former becoming dominant as the availability of ground water was reduced (Sheridan and Wohletz, 1981). Erupted products constructed ejecta aprons, which commonly have the form of tuff rings or tuff cones (Wohletz and Sheridan, 1983).

Subsidence, as well as explosive activity, played a major role in the construction of diatremes and the maar craters that overlie them (Lorenz, 1973, 1975). Subsidence of rocks enclosed within ring faults was interspersed with and followed explosive activity. Rock masses became detached from the ring faults at depth by spalling (e.g., Montana Tunnels) and near the surface by slumping and landsliding (e.g., Wau). As a result of this gravity-controlled detachment of rock masses, the walls of maar craters retreated until they attained up to twice the diameter of the subjacent subvertical diatremes. Landsliding and fiuviolacustrine sedimentation dominated maar craters after explosive activity waned (e.g., Wau).

Intrusion of magma as irregular or dikelike bodies and its emplacement as endogenous domes at the paleosurface took place during (probably interspersed with) explosive activity but became dominant during the final stages of diatreme emplacement, probably due to a diminution of the meteoric water supply (Lorenz, 1975).

Where mineralization was produced largely by magmatic fluids, as with the case of porphyry copper deposits, diatreme emplacement tended to be a late- stage event. In contrast, where mineralization processes were dominated by meteoric fluids, as with the case of epithermal deposits, ore formation tended to accompany or follow diatreme emplacement. It is concluded therefore that significant quantities of ground water could not gain ready access to deep (K silicate-altered) levels of porphyry copper systems during magmatic-hydrothermal activity. It is not until late-stage collapse of convectively circulating meteoric fluids took place that external fluids had access to residual bodies of magma and could instigate phreatomagmatic activity. In epithermal settings, ore deposition was either controlled by existing permeability, such as ring faults, shattering and brecciation around maars or diatremes (e.g., Acupan, Cerro de Pasco, Wau), and by the poorly lithified nature of the diatreme breccia itself (e.g., Montana Tunnels), or accompanied generation of the required permeability by phreatic brecciation (e.g., Cripple Creek, Wau).

The mechanism responsible for the brecciated annuli around some diatremes, as at Acupan and E1 Ten- iente, remains problematic. These breccias were partly generated before diatreme emplacement (e.g., Howell and Molloy, 1960), to which they seem to have been precursors. Could the annuli have been localized by high magmatic and/or fluid pressures, in the manner envisaged by Koide and Bhattacharji (1975)?

Magmatie Breeeias

Porphyry-type and other base and precious metal deposits

General remarks: This class ofbreccias is not widely recognized in association with ore deposits and is poorly documented, but it is believed to include the examples summarized in Table 9. Although these breccias are distinct from the phreatomagmatic breccias described above, the term diatreme is also employed to describe the vents that contain them (cf. Daubr•e, 1891).

Characteristics: These diatremes contain breccias made up of angular to rounded clasts of juvenile and, in subordinate amounts, accidental origins. From available descriptions (Table 9), it appears that most of the juvenile material comprises poorly vesiculated cognate lithic clasts of dacitic to rhyolitic composition. However, vesiculated material may be more abundant than suggested in the literature: ignimbrite was erupted at Rio Blanco-Los Bronces and is still partly preserved (Stambuk et al., 1982; Vergara and Latorre, 1984; Warnaars et al., 1985) and rhyolitic tuffs at Ashio are described as highly welded in the central parts of the diatreme (Nakamura, 1970), suggesting that they may be ignimbritic in character. Matrix-rich breccias are not common, except at Casino (Godwin, 1976) and Ashio (Nakamura, 1970), where rhyolitic tuffs are described. Matrix is generally tuffaceous, although aphanitic rhyolite occurs at Redwell Basin and Cave Peak. Residual open space is scarce. Patches of breccia charged with large accidental lithic clasts are found around the borders of the Casino (Godwin, 1976), Rio Blanco-Los Bronces (Stambuk et al., 1982), and Ashio (Nakamura, 1970; Imai et al., 1975; Fig. 24) diatremes, with those at the first locality carrying large spheroidal clasts.

The diatremes vary greatly in horizontal dimensions. Those at Casino, Redwell Basin, and Moonmera are comparable in size with large intrusion-related breccia pipes, whereas the others are much larger (Table 9). Observed vertical dimensions are mainly in the 500- to 1,000-m range. The Redwell Basin diatreme bottoms at a depth of about 515 m at a thin (30-120 m) hornfels horizon, which partially separates it from an underlying rhyolite cupola (Sharp, 1978; Thomas and Galey, 1982). The diatremes range from nearly vertical to upward flared in form, with the most extreme case of the latter geometry being provided by the open funnel shape at Ashio (Naka- mura, 1970; Fig. 24). The breccia at Cave Peak is hourglass shaped and is preserved as an annulus around a quartz monzonite plug (Sharp, 1979).

Intrusive rocks are present in the diatremes. Felsitic rhyolite occurs as a matrix to breccias at Redwell Basin and Cave Peak and is steeply flow banded (Sharp, 1978, 1979). Larger masses of porphyry are also de-


TABLE 9. Selected Examples ofMagmatic Breccias

Vertical Horizontal dimension Probable juvenile

Locality Host rocks Age (m.y.) dimensions (km) (m) component

Casino, Yukon, Cretaceous quartz 70.3 0.7 X 0.4 >360 Rhyolitic tuff Canada monzonite + quartz + tuff breccia

monzonite porphyry

Redwell Basin, Mesozoic sedimentary 15.8 0.45 x 0.27 515 Rhyolitic breccia Colorado rocks + flow-banded

rhyolite

Cave Peak, Cambrian-Permian 37.4 to 36.1 0.76 X 0.76 >750 Rhyolitic breccia Texas sandstone + lime-

stone

Ortiz, New Cretaceous quartzite, Oligocene 2.5 X 0.9 >500 Latitic breccia Mexico Oligocene monzo- + tuff

nite

Toquepala, Peru Paleogene felsic volca- 59 1.3 X 1.0 >500 Dacitic pyroclas- nics tics

R•o Blanco-Los Miocene andesitic vol- 4 1.8 x 1.8 >600 Rhyolitic (ignim- Bronces, canics + granodio- britic) + dacitic Chile rite pyroclastics

Ashio, Japan Permo-Carboniferous Neogene 4.4 X 3.3 > 1,000 Rhyolitic tuff sedimentary rocks + rhyolite

Moonmera, Permian quartz diorite 245 0.42 x 0.18 Queensland, + granodiorite por- + 0.12 x 0.06 Australia phyry

>500 Tuff

scribed by Sharp (1978) from the Redwell Basin diatreme and are also present at Toquepala (Richard and Courtright, 1958), Rio Blanco-Los Bronces (Stambuk et al., 1982), and Ashio (Nakamura, 1970). At Rio- Blanco-Los Bronces, the early dacite porphyry may constitute the roots of domes (Vergara and Latorre, 1984).

Alteration and mineralization: The diatremes in Table 9 are divided between pre- (or early) mineral and post- (or late) mineral examples. Premineral diatremes are present in porphyry copper systems at Casino (Godwin, 1976) and Moonmera (Dummett, 1978), where they constitute the foei of K silicate alteration and associated eopper-molybdenum mineralization. The porphyry molybdenum mineralization at Cave Peak is centered on a quartz monzonite plug and only overlaps into the surrounding annulus ofbreeeia (Sharp, 1979). In contrast, diatremes were emplaeed late in the evolution of the Redwell Basin, Toquepala, and Rio Blanco-Los Bronees porphyry systems. The Redwell Basin breeeia overlies porphyry molybdenum mineralization and carries only minor lead-zinc mineralization in its upper parts (Sharp, 1978; Thomas and Galey, 1982). The Toquepala and

Rio Blanco-Los Bronces diatremes postdate all significant mineralization.

The diatremes at Ortiz and Ashio are both premineral. Gold-bearing magmatic-hydrothermal breccias were localized around the periphery of the diatreme at Ortiz (see above) and massive sulfide replacements of chert are concentrated around the diatreme at Ashio, which is also cut by a swarm of Cu-, Sn-, Bi-, and Zn-bearing veins (Nakamura, 1970; Imai et al., 1975; Fig. 24).

Modern analogs: The probable surface expressions of the ore-related, pyroclastic-filled vents discussed in this section are widespread, but two examples suf- fice: Novarupta basin in the Valley of Ten Thousand Smokes, Katmai, Alaska, and La Soufri•re, Guade- loupe, French West Indies.

The 2-km-wide Novarupta basin formed in 1912 over a flared, funnel-shaped conduit by initial explosive ejection of lithic debris, inward slumping of the walls, and continued eruptive reaming of the widening orifice (Hildreth, 1983). Pyroclastic flows were then erupted and followed by emplacement of the Nova- rupta rhyolite dome and accompanying ejecta apron (Hildreth, 1983). Final activity at Novarupta was hy-


Associated with Porphyry and Other Deposits

Evidence for Alteration of surface Ore deposit Location of Timing of

Other features breccia connection type mineralization brecciation Reference

Lithic-rich border K silicate, seri- None Porphyry Centered on dia- Premineral Godwin (1976) phase citic Cu-Mo treme

Cut by phreatic Sericitic None Porphyry Mo Mo beneath brec- Late mineral Sharp (1978), Thomas breccia dikes cia, Pb-Zn in and Galey (1982)

breccia

Breccia as annulus Biotitic around quartz monzonite plug

Cut by latite por- Weak sericitic, phyry dikes argillic

Cut by latite porphyry + pebble dikes

Cut by andesite-dacite + phreatic breccia dikes

Lithic breccia on contacts, breccia dikes parallel contact

Minor

Weak sericitic

None Porphyry Mo Centered on plug, Premineral Sharp (1979) overlaps brec-

None Au-bearing On diatreme con- Premineral breccia tact pipe

None Porphyry Southwest of dia- Late mineral Cu-Mo treme

Ignimbrite flow Porphyry Cu-Mo

Propylitic, pipe None of advanced argillic

Biotitic None

South ofdiatreme Postmineral

Lindquist (1980), Wright (1983)

Richard and Courtright (19S8)

Stambuk et al. (1982), Vergara and Latorre (1984), Warnaars et al. (1985)

Cu, Sn, Zn in Within and sur- Premineral Nakamura (1970), Imai veins + re- rounding dia- et al. (1975) placements treme

Porphyry Centered on dia- Cu-Mo treme

Early mineral Dummett (1978)

drothermal and gave rise to fumarolic activity (Fen- ner, 1938).

During the late Pleistocene, formation of an explosion crater, eruption of pyroclastic flows, and dome emplacement took place as a similar series of events at La Soufri•re and, as noted above, phreatic eruptions then occurred on the flanks of the dome (Heiken et al., 1980).

Origin: These diatremes are believed to result from energy release during crystallization and/or decompression of shallowly emplaced bodies of hydrous magma, as discussed by Burnham (1985) and summarized above under the section dealing with magmatic-hydrothermal breccias. Magmatic-hydrothermal brecciation of the outer carapaces and wall rocks of stocks was ascribed to the exsolution of fluids by the second boiling reaction followed by decompression. However, if the release of energy by these processes is of sufficient magnitude, brittle failure can attain the surface, cause disruption of the entire column of suprajacent rock, and trigger volcanic eruption (Burnham, 1972, 1985). Weakening of the overlying rocks by previous structural or brecciation events may predispose them to catastrophic failure.

The decompression caused by disruption of a 1- to 3-km-high column of rock through to the palcosurface could have triggered explosive fragmentation of ve- siculating magma in the underlying chamber and the consequent surface eruption of pyroclastic fall or flow deposits. The magmatic diatremes discussed here are believed to be the conduits through which fragmented magma was erupted. As in the case of phreatomagmatic diatremes, cessation of explosive activity was commonly followed by passive ascent of magma to give dikes, irregular bodies, and at surface, domes.

Intrusion Breccias

The term intrusion breccia was first proposed by Harker (1908) and subsequently adopted by Wright and Bowes (1963) for the products of the mechanical fragmentation and incorporation of wall rocks by intrusive magma. Angular to subrounded fragments are cemented by an igneous matrix and the resulting breccialike material grades into both intrusive rock (with or without wall-rock xenoliths) and unfractured wall rocks. Intrusion breccias are common as irregular patches near the walls and roofs of subvolcanic stocks, as in porphyry-type deposits.


A

O RHYOLITIC PYROCLASTICS.

(•) RHYOLI?E. I REPLACEMENT ORE •(• LI?HIC BRECCIA. VEINS :::'i!O SANDS?ONE, CHER?, SLATE.

FIG. 24. Plan and section of a magmatic diatreme at Ashio, Japan. Taken from Nakamura (1970).

Tectonic Breccias

Brittle fracture at high strain rates during movement on faults of various types gives rise to the generation of breccias. Given the localization of many ore deposits by faults (e.g., Newhouse, 1942), fault breccias are common in close association with mineralization. In many places, tabular or lenslike bodies of fault breccia are ore bearing.

Discussion

In this overview, a rigid categorization of ore-related breccias has been adopted in order to emphasize a number of different possible mechanisms for brecciation. In reality, however, ore-related breccias are thought likely to constitute a continuum rather than a series of discrete types. This conclusion is emphasized both by the occurrences of breccias with intermediate characteristics irreconcilable with any single category and by the combination of breccia types in single pipes or diatremes. Some of the most likely transitions (and confusions) are between the following breccia types:

1. Magmatic-hydrothermal-phreatic. For example, in porphyry copper systems a distinction between well-mineralized magmatic-hydrothermal breccias emplaced early in the deep, central parts of systems, and barren phreatic breccias emplaced late and pe- ripherally is relatively simple. However, in the case of weakly mineralized intermineral breccias located on the edges of porphyry copper ore zones, the role of magmatic-hydrothermal vis-h-vis meteoric-hydrothermal fluids is less clear. Similarly, with present understanding, distinction between some pipelike breccias listed as of phreatic (e.g., Red Mountain, Colo- rado) and magmatic-hydrothermal (e.g., Golden Sunlight) origins is, at best, difficult.

2. Phreatic-phreatomagmatic. The difficulty in this case is to assess, often with limited exposures of a breccia, whether an underlying magma body contributed only heat or heat plus a small volume of fragmented melt. For example, diatremes like that at Bal- atoc (Acupan) are assigned a phreatomagmatic origin even though no juvenile component has been identified with certainty. It is salutory to recall, however, that the distinction is even difficult to make for historic deposits: a phreatic (steam explosion) origin was widely accepted for the Rotomahana mud, New Zea- land, prior to Nairn's (1979) demonstration that basaltic tephra were directly involved in the 1886 eruption. All phreatomagmatic breccias summarized here are confined to large diatremes, whereas all the epithermal phreatic breccias occupy much smaller conduits. The possibility exists, however, that a phreatomagmatic mechanism could account for some small near-surface bodies of breccia given the evidence for at least some phreatic (hydrothermal) eruptions being caused by ascent of magma (e.g., Waimangu). An example may be provided by a near-surface breccia at McLaughlin, California, in which rhyolitic pumice was tentatively identified by C. Nelson and the writer.

3. Phreatomagmatic-magmatic. Sheridan and Wohletz (1981, 1983) have quantified the transition from phreatomagmatic to magmatic explosive activity, with the latter becoming dominant when the quantity of ground water gaining access to a magma chamber diminishes. It is therefore probable that some diatremes acted as conduits for both types of products during their active lives; those at Guinaoang and Rio Blanco-Los Bronces might be examples.

4. Magmatic-hydrothermal-magmatic. Since magmatic breccias are inferred to have been generated by a natural progression of the magmatic-hydrothermal brecciation mechanism, transitional examples are inevitable. Although breccia pipes that were in- truded by small volumes of magma may safely be considered as magmatic-hydrothermal, examples such as Kidston that contain an abundance of felsite and rhyolite porphyry fragments and are cut by rhyolite porphyry dikes are probably transitional to magmatic diatremes like those at Redwell Basin and Cave Peak.


5. Subsurface magmatic-subaerial volcanic. These two categories are arbitrarily defined, with subsurface breccias clearly being transitional to subaerial breccias. The same transition is of course also the case with phreatomagmatic breccias. Volcanic breccias, assignable to auto (flow) breccia, pyroclastic fall, flow and surge, and epiclastic types (Wright et al., 1980) are not specifically dealt with here but may cause considerable confusion in field situations, especially if they have undergone pervasive hypogene and/or supergene alteration and if exposure is poor. Confu- sion is prevalent if the volcanic breccia is coarse, poorly bedded, of appreciable thickness, and areally restricted. Examples that come to mind include: crumble (talus) breccias or pyroclastic block and ash flows as aprons around domes, coignimbrite lag-fall deposits marking the sites of collapse of eruption columns at the proximal ends of pyroclastic (ignimbrite) flows (Wright and Walker, 1977), the finer grained portions of landslide breccias (mesobreccias) as wedges around caldera walls (Lipman, 1976), and laharic (volcanic mud-flow) breccias.

6. Tectonic-other types. Because ikults are believed to have localized many types of nontectonic brecciation, fault breccia may commonly be associated with or transitional to other breccia varieties. This is particularly the case with phreatic breccias because faulting and hydraulic fracturing are commonly inti- mately related events and tectonic displacements may act as a trigger for hydraulic fracture. Fine-grained cataclasites (gouge) and slickensides are indicators of tectonic rather than hydrothermal origin.

7. Intrusion-other types. Small outcrops of intrusion breccia may be difficult to distinguish from magmatic-hydrothermal or phreatic breccias that underwent limited invasion by magma during decompressive events. However, a gradation to xenolith-rich intrusive rock is indicative only of intrusion breccia.

A disappointment of this overview is the failure to isolate diagnostic geometric, textural, or lithologic criteria for most types of breccias. However, it has proved possible to generalize a number of their characteristics, as summarized in Table 10. A number of features also are instructive from the standpoint of genesis:

1. Exfoliated spheroidal fragments are characteristic features of some magmatic-hydrothermal and phreatic (especially pebble dike) breccias as well as some phreatomagmatic and magmatic diatremes (Ta- ble 10) and are taken as indicators of decompressive events.

2. A juvenile component in breccia provides evidence for a magmatic or phreatomagmatic origin.

3. Base surge deposits with or without accretionary lapilli, either in subaerial aprons or as subsided blocks in diatremes, confirm phreatomagmatic (or perhaps

also phreatic; e.g., Kawerau) explosive activity of the base surge type.

4. Blocks of base surge deposits, fragments of carbonized wood, or pieces of hot spring sinter in breccia pipes or diatremes confirm that brecciation breached the palcosurface.

5. Restriction of alteration and/or mineralization to individual clasts attests to an inter- or postmineral timing of brecciation. Uncritical application of this criteri. on can lead to pitfalls, however, because mineralized clasts are also known from subaerial volcanic breccias. For example, Triassic laharic breccias at Cariboo-Bell, Canada (Bailey and Hodgson, 1979), and the 1982 pyroclastic fall breccias at E1 Chich6n, Mexico (Luhr, 1983), contain porphyry copper fragments.

This overview leads to a tentative statement on the genetic connection between brecciation and ore deposition. If the mechanisms proposed above for brecciation are correct, then the fluids responsible for rock fragmentation and subsequent mineralization are likely, in many cases, to have been parts of the same extended pulse, as exemplified by magmatic-hydrothermal breccias in intrusion-related pipes and porphyry copper systems and by phreatic breccias in epithermal precious metal deposits. The connection is more tenuous for some phreatomagmatic and magmatic diatremes, although in most cases it is reason- able to conclude that the mineralization event(s) was closely tied in time as well as space to brecciation. Intrusion breccias and many tectonic breccias were generated without fluid involvement, and if mineralized, were subjected to the passage of fluids at a later time. All breccias, especially their contacts with wall rocks and irrespective of their origins or geometries, provide low-pressure locales conducive to the focused flow of hydrothermal fluids. Ultimately it is for this reason that breccias carry ore, commonly of abnormally high grade.

Although the data base for ore-related breccias is extensive, there is still a chronic shortage of careful, detailed observations on the geometries, textures, and lithologies of breccias. In particular, more documen- tation is required of the upward and downward terminations of all types of breccias. Theoretical and modeling studies are also required in an effort to sim- ulate both the various brecciation mechanisms discussed above and the specific features of breccias, such as sheeted zones, shingle breccia, and exfoliated fragments. By analogy with the methodology employed in the study of pyroclastic rocks, particle size analysis (e.g., Walker, 1971) and SEM grain surface analysis (e.g., Sheridan and Wohletz, 1983) ofbreccia components may evolve criteria of use for determin- ing formational mechanisms. The writer is confident that the combination of observational data on breccias with further fluid inclusion and, in particular, stable


TABLE 10. Generalized Characteristics of

Ore

deposit Breccia category type Geometry Diameter (m) Fragment form Rock flour matrix

Magmatic-hydro- Isolated Single or multiple 50-300, locally Angular-subrounded, Locally present thermal pipes pipes > 1,000 locally rounded (<30%)

Porphyry Single or multiple up to 2,000 Angular to rounded Commonly present pipes, irregular (<50%) bodies

Phreatic

Phreatomagmatic

Epithermal Pipelike but com- up to •500 Angular to rounded Commonly present monly irregular (<50%)

Porphyry Pipes, pebble up to •500 Angular to rounded Commonly present dikes (up to 100%)

Kuroko Sheets, lenses up to 1,000 Angular to rounded Present (<30%) long

Porphyry Diatreme 1,000-3,000 Subrounded to Present (<90%) rounded

Epithermal Diatreme 1,000-3,000 Subrounded to Present (<90%) rounded

Magmatic Porphyry Diatreme 500-5,000 Subrounded to Present rounded

Epithermal Diatreme 500-5,000 Subrounded to Present rounded

Intrusion Intrusion- related deposits

Tectonic Any deposit

Irregular patches up to • 100 Angular Absent

Steep tabular up to •50 Angular to rounded Present (up to bodies wide 100%)

isotope studies of their contained alteration and mineralization will lead eventually to a fuller understanding of brecciation mechanisms, and even a viable breccia classification.

Acknowledgments Many geologists have stood (and sat) with the writer

on breccia outcrops during discussions ofbrecciation mechanisms; their ideas and patience are acknowl- edged with gratitude. The manuscript was improved by the comments ofW. C. Burnham and two Economic Geology reviewers, and by discussions with F. J. Sawkins; none of them necessarily agrees with my interpretations.

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Surface Timing of Economic Juvenile component Other features connection Alteration brecciation potential

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Tuff matrix, cognate Wall-rock blocks, base Maar, tuff ring, Generally Late or post- Generally bar- lithics surge deposits, Io- domes minor mineral ren

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Tuff matrix, pumice, Wall-rock blocks, lo- Pyroclastic fall Generally Pre-, inter-, and May be ore cognate lithics cally exfoliated q- flow depos- minor postmineral

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Tuff matrix, pumice, Wall-rock blocks, lo- Pyroclastic fall Generally cognate lithics cally exfoliated + flow depos- minor

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None Slickensides, gouge None

Variable, generally minor

Premineral May be ore, especially on contacts.

Commonly Generally bar- premineral ren

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