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TYPES OF SULFIDE-RICH EPITHERMAL DEPOSITS AND THEIR

AFFILIATION TO PORPHYRY SYSTEMS: LEPANTO–VICTORIA–FAR

SOUTHEAST DEPOSITS, PHILIPPINES, AS EXAMPLES

Jeffrey W. Hedenquist1, Rene Juna R. Claveria and Gener P. Villafuerte2

1Consulting Economic Geologist – Canada2BA - Lepanto Consolidated Mining Company

SUMMARY

The Lepanto and Victoria epithermal deposits contain in excess of 8 Moz Au,

associated with massive enargite-cemented breccia and quartz-carbonate-base

metal veins, respectively. The two deposits are separated by <500 m, but the

latter was not discovered until the former was nearly mined out. The sulfide

assemblages of these adjacent deposits are characterized by high- and

intermediate-sulfidation states, respectively. However, gold was introduced after

enargite at Lepanto, associated with an assemblage that is similar to that of the

Victoria ore. An even larger gold resource is present in similar to that of the

Victoria ore. An even larger gold resource is present in the Far Southeast

porphyry Cu-Au deposit, the top of which lies 200-400 m beneath and adjacent

to these two epithermal deposits. The timing of ore deposition at FarSoutheast

and Lepanto was about 1.4 to 1.3 Ma, whereas ore formed slightly later at

Victoria, at 1.15 Ma. Although a genetic relationship has yet to be proven for

these three deposits, their spatial and temporal affiliation highlights the potential

for similar associations in other porphyry and/or epithermal districts.

INTRODUCTION

A variety of epithermal deposit types have long been recognized (Lindgren,

1933), and their possible affiliation with porphyry systems has been suggested

(Sillitoe, 1975) and debated (Sillitoe, 1989). There is broad recognition and

acceptance of two end-member types of epithermal system (Heald et al., 1987;

White and Hedenquist, 1990; Sillitoe, 1993), although variations in style are

noted (Bonham, 1986; Sillitoe, 1993). There has been much debate on whether

or not these types are related genetically to each other, and to the underlying

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magmatic engine, which in some cases has formed a porphyry deposit. We

review the types of epithermal deposit from the basis of genetic affiliation, and

note those with evidence for a spatial, and in some cases genetic relationship

with porphyry deposits. In order to highlight these relationships, we discuss a

case study of one district in Luzon, Philippines, that hosts two types of

epithermal deposit located supradjacent to a large porphyry Cu-Au deposit.

TYPES OF EPITHERMAL DEPOSIT

The formation of one end-member type involves an early acid fluid that leaches

the rock prior to deposition of high-sulfidation state Cu sulfides and gold by a

less acid fluid. The other type of deposit forms from a near-neutral pH solution

that deposits precious ± base metal ore in quartz veins, accompanied by

alteration halos. These epithermal deposits have distinctly different

characteristics (Hedenquist et al., 2000), and are called high sulfidation and low

sulfidation, respectively, to refer to the sulfidation state of their characteristic

sulfide assemblages, an intrinsic feature of the deposit.

Characteristics

Within the low-sulfidation (LS) type of deposit, widely different Ag:Au ratios have

been noted, with some of the Ag-rich deposits having a high base-metal

content. Although base-metal contents tend to increase with increasing depth in

some deposits (Buchanan, 1981), the precious- and base-metal signature

appears to be a fundamental characteristic of the deposit style (White et al.,

1995; Simmons, 1995).

The Au-rich LS deposits are typically hosted by rhyolitic-dacitic rocks that have

a bimodal character and formed in an extensional setting (Sillitoe, 1993; John et

al., 1999). The deposits show crustiform vein textures in which chalcedony

dominates, suggesting a relatively low-temperature and shallow depth of

formation (Saunders, 1994). Adularia is a common gangue mineral, and illite

typically forms an alteration halo. There is a broadly similar style that has a

relation to alkalic volcanic host rocks (Bonham, 1986; Sillitoe, 1993). Sulfide

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minerals are minor, and selenides (or tellurides, in the alkalic case) are

common. The sulfides that occur record a low sulfidation state, including pyrite,

pyrrhotite, arsenopyrite, and high-Fe sphalerite (Figure1; John et al., 1999).

Type examples are listed in Table 1, and are best known from Nevada and

California, USA, and Kyushu, Japan.

By contrast, Ag-rich deposits, where Ag:Au typically has a minimum value of 10

or 20:1, up to 100s:1, are hosted by andesite-rhyodacite volcanic rocks in arc

settings, akin to the setting of most porphyry Cu deposits. Quartz veins are

crystalline, massive and comb textured, and sericite is a common alteration

mineral and gangue, whereas adularia is relatively uncommon. Sulfides are

abundant, particularly in the base metal-rich variety. In addition to pyrite, Fe-

poor sphalerite, galena, chalcopyrite, and tennantite-tetrahedrite occur, along

with Mn carbonates. This sulfide assemblage indicates an intermediate

sulfidation state (Figure 1). Examples with type characteristics occur in Nevada

and Colorado, USA, throughout Mexico and Perú, and in many parts of the

Philippines and Japan as well as in New Zealand and Romania (Table 1).

The high-sulfidation (HS) epithermal deposits, like the Ag ± base metal-rich

style of the low-sulfidation type, also are hosted by andesite-rhyodacite volcanic

rocks in an arc setting, and in many cases show a close spatial association with

porphyry deposits (Sillitoe, 1999). The deposits form after leaching of host rocks

by an acid fluid, the latter condensed from a magmatic vapor. The silicic

leached core, commonly vuggy quartz, has a halo of quartz-alunite, and

typically is underlain by roots of sericite or pyrophyllite. The early sulfide

minerals are typically euhedral pyrite with enargite-luzonite, a high-sulfidation

state assemblage. Where paragenetic studies have been conducted, gold is

introduced after enargite deposition, along with fine pyrite, Fe-poor sphalerite,

chalcopyrite, galena, and tennantite-tetrahedrite (Jannas et al., 1990;

Hedenquist et al., 1998), essentially the same assemblage of the Ag-Au ± base

metal-rich intermediate sulfidation style deposit.

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Styles and Terminology

Continued study on epithermal deposits is helping to identify fundamental

distinctions in characteristics. This now allows classification that reflects basic

variables that are related to formation, and which may be relevant to consider

during exploration.

John et al. (1999) and John (2001) have examined the various types of

epithermal deposit in Nevada. They note that in addition to the HS type of

deposit, there are distinctly different characteristics of two styles of LS deposit.

These distinctions follow those outlined above. In one style, typified by bonanza

Au deposits in the Northern Nevada Rift, including Sleeper, Midas and Ivanhoe,

the Au-rich, sulfide-poor veins have a very low sulfidation state assemblage

(Figure 1). By contrast, within the Western Andesite Arc, Ag-rich deposits such

as Comstock Lode and Tonopah have an intermediate sulfidation state

assemblage. We refer to these two styles as end-member LS and intermediate

sulfidation (IS), respectively, stressing their distinct tectonic settings, magmatic

affiliations, metal complements, sulfide and gangue mineral assemblages, and

in some cases, deposit form (Sillitoe, 1993).

We retain the HS terminology for deposits hosted by silicic and quartz-alunite

altered bodies, with enargite-pyrite being the dominant sulfide assemblage, but

we note that the precious metals in HS deposits are associated with a

paragenetically late assemblage that has an intermediate sulfidation state,

similar to that of IS deposits.

CAUSES OF VARIATION AMONG EPITHERMAL DEPOSITS

Fluid Composition

As noted above (Table 1), HS deposits form after the host rock has been

leached by a very acid fluid. The salinity of the mineralizing fluid is known from

only a few studies of fluid inclusions hosted by enargite, and ranges from 4 to

15 wt% NaCl equiv. However, gangue quartz contains inclusions that have

salinities ranging from 1 to 40 wt% NaCl, and HS deposits are commonly

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underlain by veins that contain hypersaline inclusion fluids (Arribas et al.,

1995a; Hedenquist et al., 1995). This hypersaline fluid is related to an

underlying porphyry system, formed by vapor separation (White, 1991; Arribas,

1995). By contrast, the intermediate salinity solution responsible for depositing

the enargite may be an unseparated fluid of bulk magmatic composition, similar

in composition and origin to the fluid that forms the sericitic overprint of porphyry

deposits (Hedenquist et al., 1998).

The end-member LS deposits that are Au-rich typically have salinities <1 wt%

NaCl, although the equivalent salinity is commonly reported to be higher, up to

several wt%. The low freezing-point depressions that lead to the high equivalent

salinity estimates are due to dissolved gases, largely CO2 but also including

H2S (Hedenquist and Henley, 1985). Fluids of such composition account for the

Au-rich but Ag and base metal-poor content of LS deposits, as gold is

transported as a bisulfide complex whereas most of the silver and base metals

are transported as chloride complexes (Henley, 1990).

The higher the salinity, the higher the concentration of dissolved Ag and base

metals in solution (Henley, 1990). This accounts for the change from Au-Ag to

Ag-Au to Ag-(Au)-base metal character of epithermal deposits, and the close

match with increasing salinity (Hedenquist and Henley, 1985). This relationship

is clearly seen in Mexican epithermal deposits, where the salinities are <1 wt%

NaCl equiv for Au-Ag deposits, to 3-5 wt% NaCl for Ag-Au deposits, to 10-20+

wt% NaCl for Ag and base-metal rich deposits (Albinson et al., 2001).

If fluid composition, both salinity and gas content, is so critical in determining

the metal complement of a deposit, just as fluid pH is critical in determining the

alteration assemblages and zoning (Sillitoe, 1993), what controls this

fundamental variable?

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Tectonic and Magmatic Setting

Low salinity (<1 wt% NaCl) but gassy fluids are typical of back-arc settings with

bimodal volcanism (Hedenquist and Henley, 1985), consistent with the nature of

the LS epithermal deposits formed in this setting. In these settings, large

magma chambers are thought to lie relatively deep, possibly explaining the

rarity of porphyry Cu deposits in such settings.

By contrast, volcanic arcs commonly host active geothermal systems with

salinities that are an order of magnitude greater, up to about 2-3 wt% NaCl

(Hedenquist and Henley, 1985). Geothermal systems of higher salinity are not

well known, except for the demonstrably amagmatic systems that owe their high

salinities, >20 wt% NaCl, to dissolution of evaporites (McKibben and Hardie,

1997). The high salinities that are recorded by fluid inclusions of Ag-rich IS

deposits may reflect episodic incursions into low-salinity systems, based on

evidence found at Fresnillo (Simmons, 1991). Thus, Ag ± base metal-rich brines

many underlie many geothermal systems in arc settings, with tectonic or

magmatic triggers causing periodic ascent and mineralization. This would

explain also why such brines are not observed in active systems, as they are

present for only a small portion of the life of the system due to density

constraints. Porphyry deposits typically form within the roots of volcanic arcs,

and thus there should be a supply of moderately saline fluid at sub-volcanic

depths in these arcs.

Further evidence on the tectonic and magmatic relationship between fluid

composition and epithermal style comes from igneous and volcanic rocks

associated with epithermal deposits. John (2001) has shown that the oxidation

state of the bimodal magma suites, determined from Fe-Ti oxide compositions,

are 3-4 orders of magnitude more reduced in extensional settings associated

with LS deposits, compared with the andesite arc magmas that host IS and HS

deposits. This difference is similar to the oxidation state distinction based on

sulfide mineral assemblages in the associated LS versus IS and HS deposits

(Figure1). This may be the result of hydrothermal fluid simply equilibrating with

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the volcanic and intrusive host rocks, or it may reflect a more intrinsic magmatic

affiliation. If the latter is shown to be true, then reduced magmas may contribute

reduced fluid to the hydrothermal systems that form LS deposits. By contrast,

the more oxidized magmas that form during subduction processes –close to the

SO2-H2S gas buffer that is characteristic of porphyry Cu deposits (Burnham and

Ohmoto, 1980)– may control the higher oxidation states of fluids that form IS

deposits. Generation of the very oxidized fluid that forms high sulfidation state

sulfides in HS deposits may be a function mainly of the host rock –a leached,

silicic material– having no buffer capacity to prevent a cooling fluid from

evolving to enargite stability on cooling (Barton and Skinner, 1979).

RELATIONSHIPS AMONG EPITHERMAL DEPOSIT TYPES:

THE PORPHYRY CONNECTION

Following on from the previous discussion, we have compiled a list of several

HS and IS deposits (Table 2), noting the presence of related epithermal veins in

a district, and the evidence for porphyry deposits. A similar but more thorough

compilation (Sillitoe, 1999) was used to examine the HS-porphyry relationship.

This followed an earlier argument (Sillitoe, 1983) that a spatial association

reflected a genetic affiliation.

Based on the arguments of John et al. (1999) and John (2001), we conclude

that end-member LS deposits form in a distinctly different tectonic environment

and magmatic affiliation from IS and HS deposits. In addition, LS epithermal

deposits are not known to occur in porphyry districts, perhaps because they are

related to a more deeply seated magma chamber. For these reasons, we now

focus only on the relationship between IS and HS epithermal deposits, and the

evidence for a transition from these deposits to the porphyry environment, using

a case study from the Philippines.

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LEPANTO – VICTORIA – FAR SOUTHEAST, PHILIPPINES

Introduction

There are many examples of spatially associated epithermal and porphyry ore

deposits (Table 2). Nowhere is this spatial association better seen than in the

Mankayan mineral district in northern Luzon, where the Lepanto HS Cu-Au and

Victoria IS Au-Ag-base metal deposit overlie the Far Southeast (FSE) porphyry

Cu-Au ore body to the northwest and south, respectively (Figure 2). This is one

of the richest mining districts in the Philippine archipelago in terms of economic

value and abundance and diversity of hydrothermal ore deposits. Within an area

of <25 km2, the district contains several porphyry Cu-Au and epithermal

precious- and base-metal deposits, both HS and IS types (Sillitoe and Angeles,

1985; Hedenquist et al., 1998; Cuizon et al., 1998).

Lepanto was discovered in pre-Spanish time, as the massive enargite ore

outcropped at the northwest end of the deposit (Figure 2). Large-scale mining

commenced in 1936 and ceased in 1996. The FSE porphyry deposit was

discovered in the 1980s during a drilling program that was investigating, among

other indications, fragments of altered and mineralized porphyry within young

volcanic outcrops. The Victoria deposit was discovered during the course of

exploration to extend the Lepanto reserves. Drilling near the Nayak workings

intersected a vein, and in 1995 horizontal drilling from the 900 m L of Lepanto

cut eight veins with grades of 1.3 to 193 g/t Au. roduction began in 1997. The

reserve of the IS veins may eventually compete in size with the total production

and reserves of HS ore from Lepanto (Table 2).

Geology and Age of Deposits

There are four main lithologic units in the district, three of which are intersected

by a longitudinal section through the Lepanto-FSE-Victoria deposits (Figure 3).

1) The volcanic to epiclastic basement consists of several Late Cretaceous to

middle Miocene sequences, and includes the Lepanto metavolcanic rocks and

Balili volcaniclastic rocks. 2) A large Miocene tonalitic intrusion forms the

western margin of the mineral district (Figure 1). 3) A Pliocene dacitic to

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andesitic breccia and porphyry unit, the Imbanguila hornblende dacite, predates

Cu-Au mineralization. (4) Unaltered Pleistocene dacite to andesite porphyritic

lava domes and pyroclastic flow units, called the Bato hornblende-biotite dacite,

are post-mineralization in age, although intrusion and volcanism was nearly

continuous (Figure 4). A large part of the Lepanto ore body and most of the FSE

ore body are hosted by basement metavolcanic or volcaniclastic rocks, whereas

most of the presently known Victoria veins are hosted by the Imbanguila units

(figures 5 and 6).

The ore deposits in the Mankayan district are spatially and temporally related to

the Pliocene to Pleistocene event of intermediate-composition volcanism

(Figure 2). The geologic and temporal relations at Lepanto-FSE indicate that

mineralization occurred in the middle of this event, associated with quartz diorite

porphyry dikes and bodies which are intersected in drill holes at depths of 800

to >1200 m below the present-day surface (Figure 3).

K-silicate alteration related to the FSE porphyry is centered on dikes

(Concepción and Cinco, 1989; Hedenquist et al., 1998), and formed at 1.41 ±

0.05 Ma (n=6 K-Ar biotite dates; Figure 4). Leaching and quartz-alunite

alteration occurred at 1.42 ± 0.08 Ma (alunite, n=5). This alteration style formed

over the top of the porphyry and extended NW >4 km along the basement-

pyroclastic rock contact, synchronous with potassic alteration. Quartz-illite-

sulfide veins with illite-chlorite halos that cut the porphyry system followed at

1.30 ± 0.07 Ma (n=10). New K-Ar dating results (Sajona et al., 2000; LCMC,

unpublished ages) indicates that illite-altered wall rock, dated at 1.50 ± 0.14 Ma

and indistinguishable from the FSE illite ages, was cut by the Victoria veins at

1.15 ± 0.03 Ma (n=2).

The dating results suggest that the Victoria veins are younger than the principal

sericite (illite) overprint at FSE, perhaps by as much as 50,000-100,000 years.

Although the Lepanto enargite was deposited after much of the alunite, based

on cross-cutting relationships, some alunite is intergrown with sulfides.

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Hedenquist et al. (1998) argued that the enargite stage was related to the

sericite overprint, based on the similarity of fluid inclusion compositions and

trends. Where Lepanto-style enargite and Victoria-style carbonate-base metal

veins are seen in the same location (e.g., at the base of the Lepanto deposit,

and in Victoria at Zone 8, 1000 mL), the latter clearly cut the former, indicating

that the Victoria ore was indeed the youngest event.

Lepanto Ore

Lepanto ore is closely associated with brecciated, massive or vuggy residual

quartz and a halo of quartz-alunite. This silicic and quartz-alunite alteration zone

mushrooms along the unconformity, and is exposed where this contact crops

out in the vicinity of the mine (Figure 2). Gonzalez (1959) mapped this silicic

and quartz-alunite alteration along a strike length of >4 km to the west and

northwest of FSE, partially outlining the Lepanto ore body. This alteration style

also crops out to the southeast and south, near the Guinaoang (Sillitoe and

Angeles, 1985) and Palidan porphyry occurrences (Figure 2).

The leached silicic zones are best developed at the unconformity and in the

dacite breccia. The quartz-alunite halo to the silicic zones that host ore include

local occurrences of kaolinite, dickite, diaspore, pyrophyllite, and native sulfur.

Within the metavolcanic sedimentary basement, below the unconformity,

quartz-alunite is present immediately beneath or adjacent to the silicic zone.

Beneath the unconformity, the advanced argillic zone grades downward or

outward to pervasive chlorite alteration. By contrast, quartz-alunite is less well

developed in the dacite, except at higher elevations distal from the FSE ore

body (Garcia, 1991). In the dacite, there is a zone of kaolinite adjacent to the

quartz-alunite zone that overlies the silicic core, and the kaolinite zone grades

upward to illite-smectite. This last alteration type passes vertically and laterally

to less altered and eventually fresh rock (Gonzalez, 1959; Garcia, 1991).

The ore interval rises in elevation from <700 to >1200 m as the unconformity

rises to the northwest (Figure 3). Ore is dominated by veins of massive pyrite

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and enargite occurring as open-space fillings, matrix or fragments in breccias

and subsidiary faults, and as replacements (Gonzalez, 1959; Tejada, 1989).

Where this fault intersects the unconformity, ore extends outward along the

unconformity. Many subparallel veins, called the Branch veins, splay off to the

west from the Lepanto fault. In the vicinity of the FSE porphyry, enargite ore is

hosted entirely by the Imbanguila dacite brecca, in veins called the Easterlies

(Garcia, 1991). Ore also occurs as stratabound lenses above and below the

unconformity (Garcia, 1991).

The Lepanto ore is divided into early and late stages, postdating silicic and

much of the quartz-alunite events (Figure 7). The high-sulfidation-state

sulfosalts, enargite and luzonite, are the principal Cu minerals and occur with

abundant euhedral pyrite (Stage I). Later fine-grained, anhedral pyrite is

accompanied by tennantite-tetrahedrite, chalcopyrite, sphalerite, and galena,

electrum (typically 900 fine), tellurides (including petzite, calaverite, hessite, and

krennerite), selenides, and Bi- and Sn-bearing minerals. Gold ore is associated

with tennantite-tetrahedrite and chalcopyrite, most of which appears

paragenetically later than enargite-luzonite (Gonzalez, 1956; Tejada, 1989;

Claveria, 1998). Anhydrite, barite and, less commonly, alunite occur as gangue,

followed by late quartz crystals and nacrite with minor kaolinite. Covellite occurs

as a late alteration product of Cu-sulfide minerals. Enargite fluid inclusion

studies indicate that the temperature and salinity decreased with increasing

distance from the porphyry (4-2 wt% equiv., Th = 285-190°C).

Victoria Ore

The Victoria deposit consists of quartz-carbonate-sulfide veins with crustiform

and banded textures and pinch and swell features common. Cymoid loops and

ladder veins typify the extensional structures, which have a variable strike, from

east-west and northeast to north northeast (Figure 5) and steep southeasterly

dip, changing to a northwesterly dip to the southeast (Figure 6). Veins are

relatively continuous and have been traced along strike for up to 600 m, with 3-

9 g/t Au ore continuous over a 400-m vertical interval. The high-grade ore, >30

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g/t, is more restricted, but still extends up to 250-m vertical intervals (Claveria et

al., 1999, 2000). Fourteen zones of vein sets have been defined to date (Figure

3), with underground workings on the 900, 950 and 1000 mL. Silver grades

range from an average of 76 g/t to 49 g/t from Zone 0 to Zone 8. In some zones,

the Ag:Au ratio increases with decreasing elevation, although this pattern is not

everywhere consistent.

The quartz veins contain abundant pyrite, sphalerite, galena and chalcopyrite.

Sphalerite and galena increases in zones of high gold grades, whereas

tetrahedrite is associated with massive chalcopyrite, locally accompanied by

traces of bornite and chalcocite, the latter as a replacement (Claveria et al.,

1999). Gold occurs in quartz and sulfides, particularly sphalerite.

Enargite replaced by tennantite-tetrahedrite occurs in east-west veins on the

northern margin, and is probably related to Lepanto veins. The paragenesis of

the Victoria veins indicates three stages of mineral deposition (Figure 7;

Claveria et al., 1999; Sajona et al., 2000). The early quartz stage is

accompanied by sulfides, including dark, Fe-rich sphalerite, and some gold. The

middle stage constitutes Fe-poor sphalerite ± galena ± quartz, followed by

rhodochrosite (with rhodonite) and late quartz. The decrease in Fe content of

the sphalerite suggests progressively more reducing conditions with

paragenetic sequence, similar to that noted in the enargite to tennantite-

tetrahedrite sequence at the neighboring Lepanto deposit (Figure 1). This trend

may reflect an increase in the degree of wall rock interaction.

The second stage was the dominant period of gold deposition, with gold content

following both sulfide and carbonate abundance. Silver minerals occur as

veinlets in sulfides, including acanthite, proustite, pyargyrite, tetrahedrite, and

tennantite. Massive crustiform to botryoidal pyrite-chalcopyrite veins are late.

The Mn carbonate occurrence is restricted to the north and western portions of

the deposit, with decreasing amounts of carbonate at greater depths and to the

southwest. This may indicate the northern area is proximal to the fluid source,

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consistent with rhodochrosite and rhodonite at higher temperature in similar

deposits elsewhere. The final stage of barren quartz and anhydrite cement

brecciated vein material. Fluid inclusions in sphalerite and rhodochrosite have

Th values of 200 to 250 C, with Tm data indicating salinities of 2 to 4 wt% NaCl,

similar overall to the enargite data from Lepanto.

The alteration halos are narrower around Victoria veins compared with Lepanto

veins. Silicification occurs adjacent to veins, followed by a sericite-clay (locally

kaolinite) assemblage outward to a pyropylitic zone. Mapping at the surface

prior to drilling the discovery drill hole identified narrow, sulfide-poor quartz

(+gypsum) veins. The Imbanguila dacite pyroclastic rocks at the surface, at an

elevation of about 1150 m, are argillically altered, with little evidence of

silicification. Access to this area is presently limited.

Far Southeast Ore

FSE porphyry Cu and Au grades are concentric around the dikes and irregular

intrusive bodies of melanocratic quartz diorite porphyry (Figure 3; Concepción

and Cinco, 1989). Grades are lower in the later leucocratic porphyry compared

with the melanocratic unit, and decrease together with the alteration intensity

from intrusive contacts inward toward the center of these later dikes. The lower

grade of the later dikes is due to their intramineral timing of intrusion.

K-silicate alteration consists of a biotite-magnetite±K-feldspar assemblage and

is associated with veins of vitreous, anhedral quartz. This alteration is partially

to pervasively overprinted by alteration assemblages of chlorite plus hematite

and/or sericite. Sillitoe and Gappe (1984) used the term sericite-clay-chlorite

(SCC) to describe this assemblage, typical of porphyry deposits in the

Philippines.

Definitive paragenetic evidence linking Cu sulfide minerals to the early veins of

vitreous, anhedral quartz veins was not found (Hedenquist et al., 1998).

However, there is petrographic evidence for Cu sulfides to be associated mainly

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with a later event characterized by formation of euhedral quartz crystals with

anhydrite (Hedenquist et al., 1998; Imai, 2000). Cathodoluminescent images

show that the early anhedral quartz is overgrown by euhedral quartz (P.

Redmond and J. Reynolds, pers. comm., 2000), the latter associated with

sulfide deposition. Bleached halos of sericite, cm to m wide and including illite,

accompany these euhedral quartz veins that also contain anhydrite-white mica-

hematite-pyrite-chalcopyrite-bornite; these veins cut SCC alteration. Gold in the

FSE deposit is present as free grains of electrum associated with chalcopyrite

and bornite (Concepción and Cinco, 1989), and locally is accompanied by Bi-

Te-bearing tennantite (Imai, 2000).

Upward and outward from the core of economic porphyry mineralization the

pervasive SCC assemblage grades from sericite-dominated with minor

pyrophyllite locally to an assemblage in which pyrophyllite is abundant, variably

accompanied by quartz, anhydrite, and kandite minerals (dickite, nacrite and

kaolinite). This pervasively altered rock is overlain and, locally, cut by a zone of

quartz-alunite that hosts the Lepanto ore, with variable anhydrite-diaspore-

dickite-pyrophyllite.

Genetic Relationships

Based on studies at FSE-Lepanto, K-silicate and quartz-alunite assemblages

had a coupled formation, and were associated with hypersaline liquid and low-

salinity vapor phases, respectively (Hedenquist et al., 1998). Phase separation

occurred across the ductile-brittle transition, allowing vapor to ascend and form

the silicic and quartz-alunite lithocap. The late sericite overprint was caused by

a low salinity magmatic water, and this fluid deposited much of the Cu sulfide

ore in both the FSE porphyry and Lepanto HS deposits, possibly remobilized

from deeper protore. Exploration for either style of ore deposit should be based

on the conclusion that they are genetically related to one another.

The Victoria deposit is closely related to the FSE porphyry and Lepanto HS

deposits, possibly representing a late stage of ore deposition in fractures

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opened subsequent to Lepanto formation. The alteration and ore mineral

assemblages of Victoria are similar to those of the gold stage at Lepanto,

suggesting that Victoria may be the late, lower sulfidation (Figure 1) equivalent

of Lepanto, but formed in structures that did not intersect silicic altered rock.

Alternatively, Victoria may have formed as the result of a later pulse of activity

related to intrusion southeast of the FSE dikes.

EXPLORATION IMPLICATIONS

There is a clear genetic relationship between some HS epithermal prospects

and underlying porphyry systems. In some cases, both environments host

deposits that constitute ore bodies in their own right. As discussed elsewhere

(Sillitoe, 1995, 1999), the quartz-alunite lithocaps that remain over or adjacent

to porphyry Cu deposits constitute exploration targets for veins and

disseminations of HS gold ore, although unless oxidized or particularly high

grade, the ubiquitous and refractory Cu-As sulfides are a drawback.

Conversely, where an advanced argillic lithocap outcrops, with or without HS

ore, there is indication of a magmatic-hydrothermal system that may have been

related to porphyry intrusion. In these cases, however, the depth to potential

porphyry ore, unless relief or tilting are favorable, may limit the attractiveness of

the discovery.

The common association of Au-Ag and/or Ag-base metal quartz veins on the

periphery of HS occurrences and porphyry deposits has long been noted.

Although a genetic relationship is not yet demonstrated between HS and IS

ores, their similarity suggest that geological factors such as the nature of the

host and the evolution of permeability may be factors that control which style of

ore forms. Although few IS veins have these been economic to mine due to

their typically small size, some (e.g., at Bingham) contain appreciable base

metal ore. Other IS deposits (e.g., Victoria) can contain as much precious metal

as the spatially associated HS ore body, the latter typically more prominent. For

these reasons, old HS and porphyry districts, as well as ongoing prospects,

should be examined carefully for their IS epithermal vein potential. The shallow

16

depth of their formation relative to a porphyry, and their typically free-milling Au

ore relative to HS enargite, mean that these veins can have the position and

metallurgy to be attractive if tonnage and grade can be found.

ACKNOWLEDGEMENTS

We thank Mr. A.F. Disini, President of Lepanto Consolidated Mining Company,

for permission to publish this paper, and the many Lepanto staff who

contributed to the information presented here. Discussions with Antonio Arribas

R., Marco Einaudi and Dick Sillitoe have contributed to the ideas presented

here.

REFERENCES

Albinson, T., Norman, D. I., Cole, D., and Chomiak, B., 2001, Fluid inclusion, gasanalysis, and stable isotope characteristics of epithermal deposits in Mexico,in Albinson, T. and Nelson, C., Society of Economic Geologist SpecialPublication, in press.

Arribas, A., Jr., 1995, Characteristics of high-sulfidation epithermal deposits, andtheir relation to magmatic fluid, in Thompson, J.F.H., ed., Magmas, fluidsand ore deposits: Mineralogical Association of Canada Shortcourse Series,v. 23, p. 419-454.

Arribas, A., Jr., Cunningham, C.G., Rytuba, J.J., Rye, R.O., Kelly, W.C.,Podwysocki, M.H., McKee, E.H., and Tosdal, R.M., 1995a, Geology,geochronology, fluid inclusions, and isotope geochemistry of the Rodalquilargold-alunite deposit, Spain: Economic Geology, v. 90, p. 795-822.

Arribas, A., Jr., Hedenquist, J.W., Itaya, T., Okada, T., Concepción, R.A., andGarcia, J.S., Jr., 1995b, Contemporaneous formation of adjacent porphyryand epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines:Geology, v. 23, p. 337-340.

Barton, P.B., Jr., Bethke, 1977, Economic Geology, v. 72, p.Barton, P.B., Jr., and Skinner, B.J., 1979, Sulfide mineral stabilities, in Barnes,

H.L., ed., 2nd edition, Geochemistry of hydrothermal ore deposits: NewYork, J. Wiley and Sons, p. 278-403.

Bonham, H.F., Jr. 1986, Models for volcanic-hosted epithermal precious metaldeposits: A review, in Proceedings Int. Volcanological Congress,Symposium 5, Hamilton, New Zealand, 1986: University of Auckland, CenterContinuing Education, Auckland, New Zealand, p.13-17.

Burnham, C.W., and Ohmoto, H., 1980, Late-stage processes in felsic magmatism.Mining Geology Special Issue, No. 8, p. 1–11.

Buchanan, L.J., 1981, Precious metal deposits associated with volcanicenvironments in the Southwest. in Dickson, W.R. and Payne, W.D., eds.,Relations of tectonics to ore deposits in the Southern Cordillera: ArizonaGeological Society Digest 14, 237-262.

17

Claveria, R.J.R., 1998, A paragenetic study of the different sulfides and tellurides inthe Lepanto enargite deposit, Mankayan, Benguet, Philippines: GEOCON98, Manila, Philippines, p. 199-210.

Claveria, R.J.R., Cuison, A.G., and Andam, B.V., 1999, The Victoria gold deposit inthe Mankayan mineral district, Luzon, Philippines, in Australian Institute ofMining and Metallurgy, PacRim ’99, Bali, Indonesia, 10-13 October,Proceedings, p. 73-80.

Claveria, R.J.R., Villafuerte, G.P. and Francisco, D.G., 2000, Ore shootdevelopment in the Lepanto Victoria gold ore shoot: GEOCON 2000,Manila, Philippines, in press.

Concepción, R.A., and Cinco, J.C., Jr., 1989, Geology of the Lepanto-FarSoutheast gold-rich copper deposit [abs]: International Geological Congress,Washington, D.C., Proceedings, v. 1, p. 319-320, and preprint, 46 p

Cooke, D. et al., 1996, Economic Geology, v. 91, p.Cuizon, A.L.G., Claveria, R.J.R., and Andam, B.V., 1998, The discovery of the

Lepanto Victoria gold deposit, Mankayan, Benguet, Philippines: GEOCON98, Manila, Philippines, p. 211-219.

Garcia, J.S., Jr., 1991, Geology and mineralization characteristics of the Mankayanmineral district, Philippines, in Matsuhisa, Y., Aoki, M., and Hedenquist,J.W., eds., High-temperature acid fluids and associated alteration andmineralization: Geological Survey of Japan Report 277, p. 21-30.

Gatter, 1999, Society of Economic Geologists, Guidebook, v. 31.Gonzalez, A.G., 1959, Geology and genesis of the Lepanto copper deposit,

Mankayan, Mountain Province, Philippines: Unpub. Ph.D. dissert., StanfordUniv., 102 p.

Harvey, B.A., Myers, S.A., and Klein, T., 1999, Yanacocha gold district, northernPeru, in Australian Institute of Mining and Metallurgy, PacRim ’99, Bali,Indonesia, 10-13 October, Proceedings, p. 445-459.

Heald, P., Foley, N.K., and Hayba, D.O., 1987, Comparative anatomy of volcanic-hosted epithermal deposits: Acid-sulfate and adularia-sericite types:Economic Geology, v. 82, p. 1-26.

Hedenquist, J.W., and Henley, R.W., 1985, The importance of CO2 on freezingpoint measurements of fluid inclusions; evidence from active geothermalsystems and implications for epithermal ore deposition: Economic Geology,v. 80, p. 1379-1406.

Hedenquist, J.W., Matsuhisa, Y., Izawa, E., White, N.C., Giggenbach, W.F., andAoki, M., 1994, Geology and geochemistry of high-sulfidation Cu-Aumineralization in the Nansatsu district, Japan: Economic Geology, v. 89, p.1-30.

Hedenquist, J.W., Arribas Jr., A., and Reynolds, T.J., 1998, Evolution of anintrusion-centered hydrothermal system: Far Southeast-Lepanto porphyry-epithermal Cu-Au deposits, Philippines: Economic Geology: v. 93, p. 373-404.

Hedenquist, J.W., Arribas R., A., and Urien-Gonzalez, E., 2000, Exploration forepithermal gold deposits, in Gold in 2000, Society of Economic Geologists,Reviews in Economic Geology, v. 13, p. 245-277.

Henley, R.W., 1990, Ore transport and deposition in epithermal ore environments,in Herbert, H.K. and Ho, S.E., eds., Stable isotopes and fluid processes inmineralization: University of Western Australia, Geology DepartmentPublication 23, p. 51-69.

18

Hudson, D.M., 1993, The Comstock district, Nevada, in Lahre, M.M., Trexler, J.H.,Jr., and Spinosa, C., eds, Crustal evolution of the Great Basin and SierraNevada: Cordilleran – Rocky Mountain section, Geological Society ofAmerica Guidebook, p. 481-496.

Imai, A., 2000, Mineral paragenesis, fluid inclusions and sulfur isotope systematicsof the Lepanto Far Southeast porphyry Cu-Au deposit, Mankayan,Philippines: Resource Geology, v. 50, p. 151-168.

Izawa, E., Urashima, Y., Ibaraki, K., Suzuki, R. Yokoyama, T., Kawasaki, K., Koga,A., and Taguchi, S., 1990, The Hishikari gold deposit: High-grade epithermalveins in Quaternary volcanic of southern Kyushu, Japan, in Hedenquist,J.W., White, N.C. and Siddeley, G., eds., Epithermal gold deposits of theCircum-Pacific: Journal of Geochemical Exploration, v. 36, p. 1-56.

Jannas, R.R., Beane, R.E., Ahler, B.A., and Brosnahan, D.R., 1990, Gold andcopper mineralization at the El indio deposit, Chile, in Hedenquist, J.W.,White, N.C. and Siddeley, G., eds., Epithermal gold deposits of the Circum-Pacific: Journal of Geochemical Exploration, v. 36, p. 233-266.

John, D.A., 2001, Miocene and early Pliocene epithermal gold-silver deposits in thenorthern Great Basin: Characteristics, distribution, and relationship tomagmatism: Economic Geology, v. 96, in press.

John, D.A., Garside, L.J., and Wallace, A.R., 1999, Magmatic and tectonic settingof late Ceonozic epithermal gold-silver deposits in northern Nevada, with anemphasis on the Pah Rah and Virginia ranges and the northern Nevada rift:Geological Society of Nevada, Special Publication no. 29, p. 65-158.

Lindgren, W., 1933, Mineral deposits, 4th edition: New York, McGraw-Hill, 930 p.Losada-Calderón, A.J., and McPhail, D.C., 1996, Porphyry and high-sulfidation

epithermal mineralization in the Nevados del Famitina mining district,Argentina, in Camus, F., Sillitoe, R.H., and Petersen, R., eds., Society ofEconomic Geology Special Publication 5, p. 91-117.

McKibben, M.A., and Hardie, L.A., 1997, Ore-forming brines in active continentalrifts, in Barnes, H.L., 3rd edition, Geochemistry of hydrothermal ore deposits:New York, John Wiley, p. 877-935.

Saunders, J.A., 1994, Silica and gold textures in bonanza ores of the Sleeperdeposit, Humboldt County, Nevada: Evidence for colloids and implicationsfor epithermal ore-forming processes. Economic Geology, v. 89, p. 628–638.

Sillitoe, R.H., 1975, Lead-silver, manganese, and native sulfur mineralization withina stratovolcano, El Queva, northwest Argentina: Economic Geology, v. 70,p. 1190-1201.

Sillitoe, R.H., l983, Enargite-bearing massive sulfide deposits high in porphyrycopper systems: Economic Geology, v. 78, p. 348-352.

Sillitoe, R.H., 1989, Gold 88, Economic Geology Monograph.Sillitoe, R.H., 1993, Epithermal models: Genetic types, geometrical controls and

shallow features, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke,J.M., eds., Geological Association of Canada Special Paper 40, p. 403-417.

Sillitoe, R.H., 1995, Exploration of porphyry copper lithocaps, in Mauk, J.L., and St.George, J.D., eds., PACRIM Congress 1995: Australasian Institute of Miningand Metallurgy, Publication Series No. 9/95, p. 527-532.

Sillitoe, R.H., 1999, Styles of high-sulphidation gold, silver and coppermineralization in the porphyry and epithermal environments, in AustralianInstitute of Mining and Metallurgy, PacRim ’99, Bali, Indonesia, 10-13

19

October, Proceedings, p. 29-44.Sillitoe, R.H., 2000, Enigmatic origins of giant gold deposits, in Geology and Ore

Deposits: The Great Basin and Beyond, Geological Society of NevadaSymposium Proceedings, Reno, May 15-18, p. 1-18.

Sillitoe, R.H., and Angeles, C.A., Jr., 1985, Geological characteristics and evolutionof a gold-rich porphyry copper deposit at Guinaoang, Luzon, Philippines, inAsian Mining ‘85: London, Institution of Mining and Metallurgy, p. 15-26.

Sillitoe, R.H., and Gappe, I.M. Jr., 1984, Philippine porphyry copper deposits:Geologic setting and characteristics: Bangkok, United Nations ESCAP,CCOP Technical Publication 14, 89 p.

Simmons, S.F., 1991, Hydrologic implications of alteration and fluid inclusionstudies in the Fresnillo district, Mexico: Evidence for a brine reservoir and adescending water table during the formation of hydrothermal Ag-Pb-Zn orebodies: Economic Geology, v. 86, p. 1579-1601.

Simmons, S.F., 1995, Magmatic contributions to low-sulfidation epithermaldeposits, in Thompson, J.F.H., ed., Magmas, Fluids, and Ore Deposits:Mineralogical Association of Canada, Shortcourse 23, p. 455-477.

Tan, 1991, Society of Economic Geologists Newsletter.Tejada, 1989Vikre, P.G., 1998, Quartz-alunite alteration in the western part of the Virginia

Range, Washoe and Storey Counties, Nevada: Economic Geology, v. 93, p.338-346.

White, N.C., 1991, High sulfidation epithermal gold deposits: Characteristics, and amodel for their origin, in Matsuhisa, Y., Aoki, M. and Hedenquist, J.W., eds.,Acid hydrothermal systems: Geological Survey of Japan Report 277, p. 9-20.

White, N.C., and Hedenquist, J.W., 1990, Epithermal environments and styles ofmineralization: variations and their causes, and guidelines for theirexploration, in Hedenquist, J.W., White, N.C. and Siddeley, G., eds.,Epithermal gold deposits of the Circum-Pacific: Journal of GeochemicalExploration, v. 36, p. 445-474.

White, N.C., and Hedenquist, J.W., 1995, Epithermal gold deposits: Styles,characteristics and exploration: Society of Economic Geologists Newsletter,October, p. 1, 9-13.

White, N.C., Leake, M.J., McCaughey, S.N., and Parris, B.W., 1995, Epithermaldeposits of the southwest Pacific: Journal of Geochemical Exploration, v.54, p. 87-136.

20

Table 1. Low-, intermediate- and high-sulfidation deposit characteristics

Low sulfidation Intermediate sulfidation High sulfidation

Setting, relatedvolcanic rocks

Bimodal rhyolite-basalt; extension

Andesite-rhyodacite; arcs Andesite-rhyodacite, dominated by calc-alkalicmagmas; arcs

Depth offormation

0-400 m 300-800 m(rarely >1000 m)

<100-1000 m >1000 m

Setting, typicalhost rock

Domes; pyroclasticand sedimentaryrocks

Domes; diatremes; .pyroclastic andsedimentary rocks

Domes, central vent;pyroclastic andsedimentary rocks

Dome-diatreme.Porphyry, volcanic,sedimentary rocks

Deposit form Vein, vein swarm,stockwork,disseminated

Vein, breccia body,disseminated

Disseminated, breccia,veinlet to* massiveveins

Dissemination,veinlets, breccia

Ore textures Fine bands, combs,crustiform, breccia

Coarse bands Vuggy quartz hostsreplacement, tomassive sulfide

Replacement

Alteration Alunite-kaoliniteblanket, clay halo

Clays, sericite,carbonates; roscoellite

Silicic (vuggy), quartz-alunite to pyrophyllite-dickite-sericite

Pyrophyllite-sericite, quartz-sericite

Gangue Chalcedony-adularia-illite-calcite

Quartz-carbonate-rhodonite-sericite±adularia-barite- anhydrite-hematite- chlorite

Alunite, barite, kaoliniteto anhydrite, dickite

Sericite,pyrophyllite

Sulfides Cinnabar, stibnite;pyrite/marcasite-arsenopyrite, Fe-richsphalerite, pyrrhotite,Au-Ag selenides, Sesulfosalts,

Pyrite-Au-Agsulfides/sulfosalts,variable sphalerite,galena, chalcopyrite,tetrahedrite/tennantite

Enargite/luzonite,covellite, pyrite to later(deeper) tetrahedrite-tennantite, chalcopyrite,Fe-poor sphalerite

Bornite, digenite,chalcocite, covellite

Metals Au-Ag-As-Sb-Se-(Te)-Hg-TlLow Ag:Au (~1:1);<0.1-1% base metals

Ag-Au-Pb-Zn, Ba, Mn, SeHigh Ag:Au (10:1-100s:1);2-10 (20+)% base metals

Au-Ag, Cu leached (Hgoverprint) to Cu-Au-Ag-Bi-Te-Sn

Cu-Au

Notablefeatures

Sinter, chalcedonyblanket

Some IS veins adjacent toHS ore

Steam-heated blanketto vuggy quartz host

Overprinted onporphyry features

Fluids <1% NaCl, gas-rich,<220°C

3-5 and 10-20% NaCl,220-280+°C

4-15+ wt% NaCl Variable, typicallyhypersaline

Examples McLaughlin, Sleeper,Midas, Ivanhoe,Hishikari (RoundMountain)

Comstock, Tonopah,Creede, Fresnillo,Pachuca, Guanajuato,Casapalca, Arcata,Orcopampa, Victoria,Baguio, Toyoha, Thames,Baia Mare

Yanacocha, PuebloViejo, Pierina, La Coipa,Tambo, Pascua,Summitville, Kasuga,Rodalquilar, to El Indio,Lepanto, Chinkuashih,Goldfield, Lahóca

Bisbee, MM,Chuquicamata, toFar Southeast,Resck

Based on Lindgren, 1933; Buchanan, 1981; Heald et al., 1987; Sillitoe, 1993a, 1999; White etal., 1995; John et al., 1999; Albinson et al., 2000, Hedenquist et al., 2000.

* Use of the term “to” refers to transition with increasing depth.

21

Table 2: Alternative nomenclature used for the two end-member epithermalenvironments and correspondence to active hydrothermal systems

Geothermal(dominated by neutral pH

andreduced hypogene fluid)

Volcanic-hydrothermal(dominated by early acidic and

oxidized hypogene fluid)

Reference

Au-qtz veins in andesite & rhyoliteAg-Au, Ag, Au-Te, & Au-Se veinsBase-metal veins with Au, Ag

Goldfield type, Au-alunite Lindgren, 1933; Ransome,1909

Hot spring Secondary quartzite; enargite-Au Giles and Nelson, 1982;Nakovnik, 1933, Ashley, 1982

Low sulfur High sulfur Bonham, 1986

Adularia-sericite Acid sulfate Heald et al., 1987

Adularia-sericite Alunite-kaolinite Berger and Henley, 1989

Low sulfidation High sulfidation Hedenquist, 1987

Intermediate sulfidation Hedenquist et al., 2000

Barren quartz-alunite lithocap Sillitoe, 1995

22

Table 3. Association of HS and IS epithermal deposits, and affiliation with porphyry deposits

Ore deposit Associated depositor prospect

Known porphyry Relationships References

Lepanto, Philippines, HS;36.3 Mt @ 3.4 g/t Au, 10.8g/t Ag, 2.9% Cu (P); 4.4 Mt@ 2.4 g/t Au, 1.7% Cu (R)

Victoria IS veins; >78 tAu (R), + Ag-Cu-Zn

Far Southeast; 650Mt @ 0.65% Cu,1.33 g/t Au (0.7% Cueq cutoff) (r)

HS, IS above and adjacent toporphyry, within 0.25 myrperiod

Hedenquist etal., 1998;Claveria et al.,1999

Acupan-Antamok, Baguio,Philippines, IS; >700 t Au

Qtz-alunite lithocaparound district, local Auvalues

Subjacent porphyryveins

Porphyry veins overprinted by ISveins, area capped by early qtz-alunite zone

Cooke et al.,1996

Chinkuashih, Taiwan, HS; 20Mt ore w/ >63 t Au, 183 t Ag,119 kt Cu (P)

Chiufen-Wutanshan ISveins; 29 t Au (P)

Proposed at depthfrom alterationpatterns

IS veins 1 km W of HS veins,within continuous altered zoneand same age as HS system

Tan, 1991

Kasuga, Japan, HS; 4 Mt @2.8 g/t Au (P)

Kago qtz veins, 0.6 tAu (P)

Qtz veins 5 km N of HS bodies Hedenquist etal., 1994

Ladolam, Papua NewGuinea, IS; 1190 t Au (r)

Porphyry Cuoverprinted by ISepithermal breccia

Sector collapse and overprint ofepithermal on low-gradeporphyry Cu

Sillitoe, 2000

Emperor, Fiji, alkalicepithermal veins; 136 t Au(1993 P)

Au-bearing veins oncaldera margin

Nasivi porphyry Cuprospect with qtz-alunite-Au HS veins

Porphyry prospect 3 km E, 0.4myr older than epithermal veins

Eaton andSetterfield, 1993

Lahóca, Hungary, HS; 3.1 Mt@ 0.63% Cu, 2 g/t Au (P),5.5 Mt @ 1.4 g/t Au (0.5 g/tcutoff) (r)

Replacement-veins36.6 Mt @ 3.1-3.5%Zn, 1.2-2.1%Pb.Paráfürdö IS veins

Resck; 109.4 Mt @0.96% Cu (O.8% Cucutoff); skarn 36 Mt@ 2.2% Cu, 11.5 Mt@ 5% Zn

Porphyry-skarn under HS body,replacement adjacent toporphyry. IS veins overprint HSsulfides 3 km SW Lahóca, nearadv arg zone

Gatter et al.,1999

La Mejicana, Argentina, HS;1 Mt @ 11 g/t Au, 80 g/t Ag,3% Cu (P), 0.25 Mt @ 6.1 g/tAu, 64 g/t Ag, 1.08% Cu (R)

Nevados delFamatina; 300 Mt @0.37% Cu, 0.3 g/t Au,0.06% Mo (r)

Spatial and/or temporalevolution from porphyry toperipheral epithermal ores

Losada-Calderónand McPhail,1996

El Indio, Chile, HS: 23.2 Mt@ 6.6 g/t Au, 50 g/t Ag, 0.2Mt @ 209 g/t Au (P)

Tambo HS Au-barite;37.2 Mt @ 4.2 g/t Au,42 Mt @ 1 g/t Au (P).Río del Medio Au-Agveins

Porphyry prospectsin district

HS shallow breccia Au deposit5 km to SE and qtz veins 5 kmN of HS deposit

Jannas et al.,1990

Yanacocha, Perú, HS; 128 tAu (1998 P), 1395 t Au (R, r)

Porphyry prospectdrilled in late 1960s

Older porphyrydeposits 15+ km E

Pyrophyllite on periphery ofsilicic lithocaps

Harvey et al.,1999

Chelopech, Bulgaria, HS;52.1 Mt @ 3.3 g/t Au, 1.4%Cu (P)

Au – base metal veinsat basement contact

Porphyry Cu-Moadjacent; sericiticroots to HS body

Deep drilling encounteredandalusite-diaspore

Sillitoe, 1999;personalobservations

Rodalquilar, Spain, HS; 10 tAu (P)

IS base-metal veins Sericitic roots to HSbody

Qtz-base metal IS veins within2 km of HS body

Arribas et al.,1995a

Comstock, US, IS; 260 t Au,5980 t Ag P

In district with Hg-rich lithocaps

In district withporphyry prospects

Pyrophyllite-alunite halos toveins, older than IS event?

Hudson, 1993;Vikre, 1998

HS, high sulfidation; IS, intermediate sulfidation; P, production; R, reserve; r, resource

23

FIGURE CAPTIONS

Figure 1: Oxidation state vs pH at 250°C for alteration and sulfide minerals ofepithermal interest (modified from Barton et al., 1977; John et al., 1999). Earlyleaching (vuggy quartz) and quartz-alunite form from a low pH, oxidized fluid (red),creating a barren advanced argillic lithocap that may host subsequent high-sulfidation(HS) state sulfide ore (orange), including enargite (en) with pyrite (py), e.g., theLepanto Cu-Au deposit. Gold typically follows enargite, associated with intermediate-sulfidation (IS) state minerals such as tennantite-tetrahedrite (tn), chalcopyrite (ccp)and sphalerite (spl) with low Fe content, and is sericite stable (blue). IS assemblagesare also typical of Ag-Au base-metal vein deposits such as Comstock Lode, Nevada(John et al., 1999) and the Victoria Au-Ag-Cu-Pb-Zn deposit. By contrast, high-gradeepithermal gold deposits such as Midas and Sleeper, Nevada, and Hishikari, Japan,are Ag- and base-metal poor. The sulfide assemblage of pyrite, pyrrhotite (po),arsenopyrite and high-Fe sphalerite indicates a very low-sulfidation (LS) state (green).cv-covellite, bn-bornite, di-digenite, ang-anglesite, gn-galena, hm-hematite, chl-chlorite(superimposed on magnetite stability).

24

Figure 2: Map of the Mankayan district, northern Luzon (inset), Philippines, showing thesimplified geology, and location and type of known hydrothermal deposits. Outlines ofthe economically most important deposits, Far Southeast (FSE), Lepanto, Victoria andGuinaoang are shown projected to the surface. Modified from Garcia (1991). A-B-Cshows line of section, Fig. 3.

Figure 3: Schematic northwest-southeast longitudinal section through the Lepanto HSand underlying Far Southeast (FSE) porphyry Cu-Au deposits, turning south (at B; Fig.2) through the Victoria IS vein deposit, showing the geological units and extent ofporphyry ore (from Concepción and Cinco, 1989; Garcia, 1991, Cuison et al., 1998).

25

Figure 4: K-Ar ages for mineral separates from igneous and hydothermal mineralsassociated with the FSE, Lepanto, and Victoria deposits (Arribas et al., 1995a; Sajonaet al., 2000; unpublished Lepanto Consolidated Mining Co. data). Samples arrangedby lithologic or mineral groups; analytical uncertainty at 2σ level contained within sizeof symbol, except where indicated by error bar. See Arribas et al. (1995a) andHedenquist et al. (1998) for details on all but Victoria samples. Ages (±2 sigma errors)and K concentrations for illite concentrates from Victoria samples: 5M P22 (quartz vein)1.14 ± 0.02 Ma, 5.53 wt%; 8M P1W (quartz vein) 1.16 ± 0.02 Ma, 5.50 wt% (LCMCunpublished data; analyzed at BGR, Germany). 8K stope (altered wall rock) 1.5 ± 0.14Ma, 6.7 wt% (Sajona et al., 2000; analyzed at IGNS, New Zealand).

26

Figure 5: a) 1000 m L plan map of the Victoria veins, showing their arcuate trend, east-west to the NE, to south southwest to the SW. b) 900 m L plan map, showing continuedarcuate vein trends.

27

Figure 6: Northwest-southeast section (Fig. 5) through the Victoria veins; note changein direction of dip across deposit, creating an upward flaring pattern.

28

Figure 7: Paragenetic sequences of ore and related minerals for Lepanto and Victoriadeposits (Claveria, 1998; Claveria et al., 1999).