Magmatic Anhydrite

www.elsevier.com/locate/lithos

Lithos 72 (2004) 147–161

Magmatic anhydrite and calcite in the ore-forming

quartz-monzodiorite magma at Santa Rita, New Mexico (USA):

genetic constraints on porphyry-Cu mineralization

A. Audetata,*, T. Pettkeb, D. Dolejsc

a Institute of Mineralogy, Universitat Tubingen, Wilhelmstrasse 56, Tubingen D-72074, Germanyb Isotope Geochemistry and Mineral Resources, ETH Zentrum, Zurich 8092, SwitzerlandcEarth and Planetary Sciences, McGill University, Montreal, Quebec, Canada H3A 2A7

Received 13 November 2002; accepted 7 October 2003

Abstract

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to

constrain physico-chemical factors (P, T, fO2, and volatile content) responsible for mineralization. The dike contains a low-

variance mineral assemblage of amphibole, plagioclase (An30–50), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite

and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that

anhydrite originally was abundant in the form of phenocrysts (1–2 vol.%), but later was replaced by either quartz or calcite.

Hornblende–plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-

monzodiorite, with one magma having ascended directly from z 14 km depth. Rapid magma ascent is supported by the

presence of intact calcite inclusions within quartz phenocrysts.

The assemblage quartz + sphene +magnetite +Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen

fugacity at logfO2>NNO+ 1, in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides. The same

reasoning generally applies for rocks hosting porphyry-Cu deposits, seemingly speaking against a major role of magmatic

sulfides in the formation of such mineralizations. There is increasing evidence, however, that magmatic sulfides play an

important role in earlier stages of porphyry-Cu evolution, the record of which is often obliterated by later processes.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Magmatic; Anhydrite; Calcite; Sulfur; Porphyry copper

1. Introduction

There is relatively little known about the role of

sulfur in the formation of porphyry-Cu (FMo, Au)

0024-4937/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2003.10.003

* Corresponding author. Tel.: +49-7071-29-7890; fax: +49-

7071-29-3060.

E-mail address: [email protected]

(A. Audetat).

deposits, despite the fact that primary ore minerals are

predominantly sulfides. Of particular interest is the

question of whether the simultaneous enrichment of

sulfur and metals is just ‘‘accidental,’’ or whether

sulfur was actively involved in the metal enrichment

process. Sulfur can be linked to ore-forming metals by

various processes, including (i) preferential melting of

a S- and Cu (FMo, Au)-bearing source; (ii) coupled

assimilation of S and metals during magma ascent;

A. Audetat et al. / Lithos 72 (2004) 147–161148

(iii) sequestering of Cu, Mo, and Au in magmatic

sulfides or immiscible sulfide melts; and (iv) simul-

taneous partitioning of sulfur and metals into exsolv-

ing aqueous fluids.

Of these processes, the sequestering of Cu, Mo,

and Au in magmatic sulfides or an immiscible sulfide

melt has drawn particular attention in the past few

years (Keith et al., 1991, 1997; Lynton et al., 1993;

Cygan and Candela, 1995; Jugo et al., 1999; Laroc-

que et al., 2000; Halter et al., 2002b) because it not

only explains the close relationship between sulfur

and metals, but also provides a viable mechanism for

metal enrichment. It has been proposed that sulfide

minerals or immiscible sulfide melts (in the following

collectively called ‘‘sulfides’’) could act as temporal

storage medium for chalcophile ore metals like Cu

and Au, thus preventing them to be incorporated at

trace element levels within crystallizing minerals.

Towards the end of crystallization, sulfides would

get oxidized and release the metals together with

sulfur to the exsolving aqueous fluids (Candela and

Holland, 1986; Candela, 1989, 1992). Such a model

is supported by experimental studies demonstrating

that Cu and Au strongly partition into magmatic

sulfide phases (Lynton et al., 1993; Cygan and

Candela, 1995; Jugo et al., 1999), as well as by

recent discoveries of primary sulfides in porphyry-

Cu-related magmas (Keith et al., 1991, 1997; Halter

et al., 2002b).

Some observations, however, argue against a major

role of sulfides in the formation of porphyry-Cu

deposits. Most importantly, magmas spatially associ-

ated with porphyry-Cu mineralization are generally

known to be oxidizing (e.g., Burnham and Ohmoto,

1980; Blevin and Chappell, 1992; Keith and Swan,

1995) and therefore should contain sulfates rather than

sulfides. The studies of Dilles (1987) and Streck and

Dilles (1998), for example, demonstrate that at Yer-

ington, the magmas leading to porphyry-Cu mineral-

ization were saturated in anhydrite during their whole

evolution from quartz-monzodiorite to granite. Hence,

it seems that the formation of magmatic sulfides (and

their subsequent oxidation) is not a prerequisite for

economic porphyry-Cu mineralization.

The aim of this paper is to present a new example

of an oxidized, porphyry-Cu-related magma (Santa

Rita, NM, USA) and, in a second part, to discuss the

controversy outlined above on a more general basis.

2. The porphyry-Cu deposit at Santa Rita

2.1. Geological setting

The porphyry-Cu deposit at Santa Rita (also called

Chino Mine) in southwestern New Mexico belongs to

a suite of approximately 50 other porphyry-type Cu

deposits that formed between ca. 52 and 72 Ma during

the so-called Laramide age in the American South-

west (Titley, 1993). These deposits represent one of

the most significant metallogenic provinces on Earth.

Their formation is linked to subduction of oceanic

crust, as suggested by the following observations: (i)

all deposits occur in a belt about 350–400 km

landward of the reconstructed continental margin;

(ii) the occurrences show a close spatial and temporal

association with andesitic to dacitic, calc-alkaline

volcanism; and (iii) there is evidence for compressive

stress and uplift. Most deposits are associated with

large (z 2 km diameter), multiple intrusions ranging

in composition from monzonite to granite, with usu-

ally only the most felsic members being spatially

related to mineralization (Titley, 1993).

Santa Rita is a classical quartz-monzodiorite-

hosted ore deposit in which remnants of essentially

the entire igneous sequence are exposed as either

volcanic or intrusive rocks. An excellent summary

of the geology of Santa Rita can be found in Rose and

Baltosser (1966), whereas detailed geologic and pet-

rographic descriptions are given by Jones et al.

(1967). Igneous activity in the region began in the

Late Cretaceous with the intrusion of dioritic to

quartz-dioritic sills into a Precambrian basement over-

lain by a ca. 1-km-thick sequence of Paleozoic and

Mesozoic sediments. Shortly afterwards, andesites

and andesitic breccias erupted to the surface, whereas

concomitant dioritic to gabbroic rocks formed at

depth. This was followed by multiple intrusion of

quartz-monzodioritic magma-forming dikes and the

major stocks of Santa Rita and Hanover–Fierro. The

Santa Rita stock has been dated at 63 Ma (Paleocene)

by the K–Ar method on hydrothermally altered rock

(Rose and Baltosser, 1966). Many quartz-monzodior-

ite dikes are apophyses of the larger quartz-monzo-

diorite intrusions, whereas others are younger and cut

through them. Later generations of dikes are more

siliceous and have compositions ranging from quartz-

monzonite to quartz latite. Most of the mineralization

A. Audetat et al. / Lithos 72 (2004) 147–161 149

occurred after the formation of quartz-monzodiorite

dikes, and was contemporary with the emplacement of

quartz-monzonite dikes (Rose and Baltosser, 1966).

Between 1911 and 1966, about 250 million tons of

copper ore were extracted from Santa Rita, with an

average Cu content of 0.8–0.9 wt.% Cu (Rose and

Baltosser, 1966). The mine is presently operating at an

ore grade of 0.2 wt.% Cu and 0.02 wt.% Mo (Robert

North, personal communication, 2000).

2.2. Methods

To constrain the magmatic processes leading to

copper mineralization at Santa Rita, we collected

samples from quartz-monzodiorite stocks and dikes

that were emplaced immediately before mineraliza-

tion. Dikes appear more useful than stocks because

they were quenched at a specific stage of crystalliza-

tion, are less altered, and are less deformed. Samples

described in this article were collected from one

particular dike that crops out between the villages of

Vanadium and Hanover, at coordinates 32j46.97VN/108j06.47VW along state road 356, ca. 4 km west of

the southern pit of Santa Rita. This dike (in the

following named ‘‘SR8’’) belongs to a group of about

50 quartz-monzodiorite dikes extending north and

west of the Santa Rita stock (Hernon et al., 1964).

Based on their structural, mineralogical, and geo-

chemical characteristics (average of three analyses:

63.4 wt.% SiO2; 16.1 wt.% Al2O3; 5.1 wt.% CaO; 4.4

wt.% FeOtot; 2.5 wt.% Na2O; 2.5 wt.% K2O; 2.0 wt.%

MgO; 0.5 wt.% TiO2; 0.5 wt.% MnO), there is no

doubt that these dikes are comagmatic with the

mineralized intrusions at Santa Rita and Hanover–

Fierro (Jones et al., 1967).

The collected rock samples were cut into 5- to 10-

mm-thick slices and examined with a binocular mi-

croscope. Well-preserved phenocrysts of quartz,

sphene, and apatite were marked and cut out for

thick-section preparation. From ca. 200 phenocrysts,

so-called ‘‘quick plates’’ (i.e., nonpolished sections)

were made, which were covered with immersion oil

and studied with a standard petrographic microscope

to recognize primary inclusion assemblages. The best

50 phenocrysts were polished on both sides and

prepared for microanalysis.

Unknown mineral inclusions were identified by

Raman spectroscopy, using a Dilor XY Raman mi-

croprobe with a resolution of 1800 lines/mm, a focal

length of 500 mm, and a Peltier-cooled CCD detector

with 1024 elements. Electron microprobe (EMP)

analyses of apatite, anhydrite, and amphibole phenoc-

rysts were performed on a Cameca SX50, using 15 kV

acceleration voltage, 20 nA sample current, a fixed

beam of 5 Am diameter, and peak counting times

between 20 and 100 s.

Quantitative analysis of selected minerals and

inclusions was performed by EMP and laser ablation

inductively coupled plasma mass spectrometry (LA-

ICP-MS). The latter method has the advantage that it

allows analysis of samples that became heterogeneous

after entrapment (e.g., feldspars, Fe–Ti oxides, sul-

fides, melts, and fluid inclusions) without the prereq-

uisite of prior homogenization. The main disadvantage

of LA-ICP-MS is that it produces relative element

abundances only, which need to be transformed into

absolute values by means of an internal standard

(Gunther et al., 1998; Audetat et al., 2000; Halter et

al., 2002a). The LA-ICP-MS system used in this study

is composed of a 193-nm excimer laser (Lambda

Physik, Germany), special energy homogenization

optics (Microlas, Germany), and an Elan 6100 quad-

rupole mass spectrometer (Perkin Elmer, Canada).

Technical details can be found in Gunther et al.

(1997, 1998), Gunther and Heinrich (1999a,b), and

Halter et al. (2002a).

2.3. Results

2.3.1. General petrology

Hand specimens collected from quartz-monzodior-

ite dike ‘‘SR8’’ appear dark-green and contain phe-

nocrysts of white plagioclase (f 20 vol.%),

amphibole (f 5 vol.%), biotite (f 5 vol.%), and

quartz (f 2 vol.%) disseminated in a fine-grained

matrix of the same minerals plus alkali feldspar. The

rock also contains 1–2 vol.% lath-shaped cavities

filled with an orange powder (Fig. 1a and b), which

seem to represent former anhydrite phenocrysts (see

below). Other minor phenocryst phases include apa-

tite, sphene, and magnetite. Phenocrystic orthoclase

was probably present as well (see Jones et al., 1967),

but could not be recognized in ‘‘SR8’’ due to the

strong alteration of the feldspar crystals. Jones et al.

(1967) determined the anorthite content of unaltered

plagioclase phenocrysts in several quartz-monzodior-

Fig. 1. (a) Photograph of a quartz-monzodiorite sample taken from dike ‘‘SR8’’ at Santa Rita. The circle marks a lath-shaped cavity filled with

orange-brown powder, which we interpret to represent a decomposed anhydrite phenocryst. (b) Digitally enhanced picture of the same view

area, with the orange-brown shades replaced by dark color. Based on such pictures the total volume of former anhydrite phenocrysts in this rock

was estimated at 1–2 vol.%. (c) Photomicrograph of an apatite phenocryst containing primary inclusions of anhydrite, magnetite, and

amphibole (doubly polished thick section viewed in transmitted light). (d) Schematic view of petrographic relationships observed between

apatite, anhydrite, and orange-brown powder. The upper part shows an apatite phenocryst with inclusions of anhydrite arranged along an early

growth zone. One inclusion is intersected by a crack and filled with the same orange-brown powder as that present in the cavities. The latter

often contain apatite crystals within and attached to them, suggesting a preferential intergrowth between apatite and anhydrite.



ite dikes at An30–An50, whereas our LA-ICP-MS

analyses of five plagioclase inclusions in quartz phe-

nocrysts reveal a more narrow composition of An32–

An42. The characteristics of each phenocryst phase

and contained primary inclusions are summarized in

Table 1, together with the method of inclusion anal-

ysis. A primary inclusion origin was inferred if the

following criteria were fulfilled: (i) occurrence along a

defined growth zone, (ii) complete enclosure within

the host crystal, and (iii) no intersection by any visible

crack or fluid inclusion trail.

All minerals except calcite and ilmenite occur both

as a phenocryst phase as well as inclusions within

other minerals, suggesting that the assemblage plagio-

Table 1

Phenocryst phases and their inclusions in granodiorite sample ‘‘SR8’’

Phenocryst Appearance Abundance

(vol.%)

Quartz Mostly euhedral; some grains embayed 2

Apatite Euhedral prisms < 1

Sphene Euhedral crystals containing < 1

numerous inclusions

Hornblende Dark-green prismatic grains 5

Plagioclase Strongly altered; some grains rounded 20

Anhydrite Decomposed to an orange-brown powder 1–2

Biotite Totally altered 5

Magnetite Some grains embayed < 1

K-feldspar (Presence inferred only) ?

a LA-ICP-MS analysis.b Electron microprobe analysis.c Identification by Raman spectroscopy.

clase + amphibole + quartz + biotite + magnetite +

sphene + apatite + anhydriteFK-feldspar was in sta-

ble equilibrium at the time of dike emplacement. An

example of coeval inclusions of magnetite, amphibole,

and anhydrite trapped within a single apatite pheno-

cryst is shown in Fig. 1c.

2.3.2. Anhydrite

The following petrographic observations suggest

that the 1–2 vol.% cavities in ‘‘SR8’’ represent

decomposed anhydrite phenocrysts: (1) anhydrite

occurs as primary inclusions within apatite (demon-

strating that the melt was actually anhydrite-saturated;

Fig. 1c); (2) cracked anhydrite inclusions within apa-

Maximum Contains Analyzed by

size (mm) inclusions ofLAa EMPb Ramanc

5 Plagioclase x x

Apatite x x

Hornblende x x

Sphene

Opaque

Calcite x

Melt inclusions x

Zircon x

6 Anhydrite x x

Magnetite x

Sphene x

Hornblende x x

Biotite x

Melt inclusions x

8 Apatite x x

Zircon x x

Ilmenite x x

Magnetite x x

Melt inclusions x

Anhydrite

5 Sphene x

Magnetite

Melt inclusions

Apatite x

Plagioclase x

10 Apatite

Hornblende

5 Apatite

4

1

?

Table 2

Electron microprobe analyses of apatite and anhydrite in ‘‘SR8’’

P2O5 SO3 CaO Na2O F Cl Total

Apatite

Phenocryst M;

spot 1 (core)

40.99 0.20 54.04 0.12 2.04 1.31 98.69

Phenocryst M;

spot 2

41.07 0.18 53.97 0.11 2.11 1.27 98.72

Phenocryst M;

spot 3

41.47 0.14 54.39 0.10 2.20 1.27 99.56

Phenocryst M;

spot 4

40.92 0.17 54.28 0.10 2.10 1.28 98.85

Phenocryst M;

spot 5 (rim)

41.11 0.24 54.17 0.09 2.11 1.30 99.01

Phenocryst N 41.44 0.18 54.01 0.08 2.39 1.38 99.48

Phenocryst F 41.85 0.16 54.33 0.08 2.12 1.30 99.84

Phenocryst C 42.25 0.15 53.58 0.06 2.59 1.25 99.89

Phenocryst K 41.87 0.13 54.54 0.11 2.37 1.14 100.16

Phenocryst I;

core

42.34 0.20 54.27 0.12 2.45 1.30 100.68

Inclusion in

quartz

41.93 0.35 53.91 0.13 2.06 1.45 99.83

Inclusion in

sphene 1

40.81 0.52 54.08 0.21 2.16 1.39 99.16

Inclusion in

sphene 2

41.50 0.31 54.30 0.17 2.11 1.40 99.79

Anhydrite

Anhydrite

inclusion

in crystal M

0.18 56.39 41.40 0.01 0.00 0.01 97.99

Anhydrite

inclusion in

crystal N

0.11 56.69 41.84 0.03 0.00 0.00 98.68

All values are in weight percent.


tite are replaced by the same orange-brown powder as

that present in the cavities (i.e., Fig. 1d); (3) anhydrite

is unstable in most groundwaters (explaining its de-

composition); (4) the cavities show the typical lath-

shaped outline of anhydrite crystals; and (5) there is a

close spatial relationship between apatite crystals and

cavities (Fig. 1d), suggesting anhydrite–apatite inter-

growths similar to those described from Mt. Pinatubo

(Baker and Rutherford, 1996; Pallister et al., 1997). X-

ray diffraction (XRD) analysis of the orange-brown

powder revealed that it consists essentially of micro-

crystalline quartz. In two samples from other quartz-

monzodiorite dikes in the area, however, both the

cavities in the matrix and opened anhydrite inclusions

within apatite phenocrysts are filled with calcite. The

latter contain ca. 20% void space, which corresponds

to the reduction in mineral volume if every SO42�

anion of the former anhydrite is replaced by CO32�. In

view of this evidence, we regard it as certain that the

1–2 vol.% cavities in ‘‘SR8’’ represent former anhy-

drite phenocrysts. Assuming a density of 2.9 g/cm3 for

the anhydrite and 2.7 g/cm3 for the quartz-monzodior-

ite, this amount of anhydrite phenocrysts translates

into a magma sulfur content of 2500–5000 ppm S.

Despite careful examination, we could not find any

traces of magmatic sulfides in ‘‘SR8,’’ neither as

inclusions within phenocryst phases, nor as oxidized

remains in the matrix (i.e., Larocque et al., 2000).

Most of the sulfur present in this magma was therefore

dissolved as sulfate, suggesting high fO2. High mag-

matic oxygen fugacities are suggested also by the

assemblage quartz + magnetite + sphene +Mg-rich

hornblende (Wones, 1989), which requires a mini-

mum of log fO2zNNO+ 1 at the reconstructed crys-

tallization temperature of 770F 50 jC (see below).

2.3.3. Apatite

Electron microprobe analyses of six apatite phe-

nocrysts and three apatite inclusions in sphene and

quartz are listed in Table 2. Due to the potential use of

apatite as a tool for monitoring magmatic sulfur

contents (e.g., Streck and Dilles, 1998; Parat et al.,

2002), we focus this discussion on their SO3 content.

Sulfur incorporation in apatite is possible through

several substitution mechanisms, which are coupled

such that Sc [Si-REE3 + +Na] (Streck and Dilles,

1998). Dsulfur, apatite/melt, therefore, not only depends

on melt temperature and oxygen fugacity (Baker and

Rutherford, 1996; Peng et al., 1997), but also on melt

chemistry.

Crystallization conditions of apatite in dike ‘‘SR8’’

are approximated by a laboratory experiment of an-

hydrite-saturated Pinatubo dacite at 760 jC, 2200 bar,

and MNO-buffered oxygen fugacity (run H12 of

Baker and Rutherford, 1996; MNO buffer equals to

NNO+ 3). In this experiment, Dsulfur, apatite/melt was

determined at about 9. Using this distribution coeffi-

cient and a sulfur content of 520–2080 ppm S in

apatite of ‘‘SR8,’’ the sulfur concentration in the

coexisting melt is calculated at 60–230 ppm. This

value is in agreement with experimentally determined

sulfur solubilities in anhydrite-saturated magmas

(Carroll and Rutherford, 1987; Luhr, 1990; Baker

and Rutherford, 1996). A major implication of the


discussion above is that the amount of sulfur stored in

the melt of ‘‘SR8’’ was negligibly small compared to

the 2500–5000 ppm S present in the form of anhy-

drite phenocrysts.

2.3.4. Amphibole

Electron microprobe and LA-ICP-MS analyses of

amphiboles are listed in Table 3 and summarized in

Fig. 2. A plot of Al2O3 vs. FeO reveals three

compositionally distinct groups (Fig. 2a). One com-

prises amphibole inclusions within apatite, whereas

the other two comprise amphibole phenocrysts and a

single quartz-hosted amphibole inclusion. The com-

position of amphibole preserved as inclusions and

phenocrysts can be used to constrain P–T conditions

of amphibole crystallization by means of Al-in-horn-

blende barometry and hornblende–plagioclase ther-

mometry. Fig. 2b summarizes the results of

calculations performed with a spreadsheet kindly

provided by J.L. Anderson.

The thermometry part in this spreadsheet is based

on the reaction edenite + albite = richterite + anorthite,

as described in Holland and Blundy (1994). A prob-

lem arises from the fact that the plagioclase phenocrysts

in ‘‘SR8’’ are zoned, which renders it impossible to

assign a particular plagioclase composition to a

specific amphibole composition. Hence, for the cal-

culation of data points in Fig. 2b, we used an average

plagioclase composition of An40 and added error bars

displaying the change in temperature (and cor-

responding effect on pressure) when the plagioclase

Table 3

Electron microprobe and LA-ICP-MS analyses of hornblende in ‘‘SR8’’ (

Hornblende sample SiO2 TiO2 Al2O3 FeOtot MgO MnO CaO N

Phenocryst P; core 47.5 1.2 7.4 14.4 14.0 0.5 11.5 1

Phenocryst P; middle 48.4 1.2 6.8 14.2 14.3 0.5 11.7 1

Phenocryst P; rim 48.1 1.3 7.1 14.3 14.1 0.5 11.7 1

Phenocryst F; core 45.1 1.8 12.0 6.9 18.2 0.1 11.3 2

Phenocryst G; core 44.4 1.2 13.2 7.9 17.0 0.1 11.5 2

Phenocryst G; rim 44.7 1.7 12.4 7.2 17.8 0.1 11.5 2

Phenocryst F; core 46.0 1.4 13.4 6.3 17.0 0.1 11.1 2

Inclusion in quartz 48.5 1.1 7.0 14.4 14.2 0.6 11.5 1

Inclusion in apatite Q 44.6 1.4 9.5 16.7 11.8 0.5 11.5 1

Inclusion in apatite J 44.3 1.5 9.6 16.5 11.8 0.5 11.6 1

Inclusion in apatite K 44.4 1.6 10.0 17.1 11.7 0.5 11.5 1

Thermobarometric results based on three different plagioclase compositio

composition is varied from An30 to An50 (cf. Jones et

al., 1967). Calculated temperatures are the same

(within error) for all three amphibole populations

analyzed, suggesting crystallization temperatures of

770F 50 jC.The Al-in-hornblende barometer applies to low-

variance granitic rocks containing an equilibrium as-

semblage of seven solid phases (quartz, K-feldspar,

An25–35 plagioclase, biotite, hornblende, sphene, and

Fe–Ti oxides) in the presence of melt and a free vapor

phase. Originally formulated by Hammarstrom and

Zen (1986) and Hollister et al. (1987), the barometer

has been extended by Anderson and Smith (1995) to

include a temperature correction. The latter authors

recommend that the barometer be used only for horn-

blendes with Fetot/(Fetot +Mg) < 0.65, which is ful-

filled for all amphiboles analyzed from ‘‘SR8’’ (their

ratio ranging from 0.17 to 0.45). However, the con-

ditions of: (i) plagioclase composition of An25–35 , (ii)

coexistence with K-feldspar, and (iii) coexistence with

a vapor phase may not always have been fulfilled. The

effect of neglecting these criteria is not well known,

but a test on ‘‘SR8’’-similar experimental run products

of Mt. Pinatubo dacite with the assemblage hornblen-

de +magnetite + plagioclase (An40) + ilmeniteFanhy-

drite (i.e., An-content too high, K-feldspar missing,

and partly vapor-undersaturated) yielded Al-in-horn-

blende pressures that reproduced the run conditions to

within F 0.5 kbar (runs 58 and 67 of Scaillet and

Evans, 1999). An empirical error bar of F 0.5 kbar has

therefore been added to the data points in Fig. 2b.

all values in wt.%)

a2O K2O Total T

(An30)

P

(An30)

T

(An40)

P

(An40)

T

(An50)

P

(An50)

.4 0.7 98.6 742 2.2 769 1.7 817 0.6

.3 0.6 98.8 732 1.9 760 1.4 808 0.4

.3 0.6 99.0 732 2.1 760 1.7 807 0.7

.8 0.7 98.9 760 5.0 784 4.4 825 3.2

.6 0.9 98.7 733 6.4 756 5.9 794 4.9

.7 0.8 98.8 749 5.5 772 5.0 812 3.9

.8 0.9 99.0 725 6.6 748 6.1 788 5.1

.4 0.7 99.3 733 2.0 760 1.6 809 0.5

.7 1.0 98.8 758 3.5 784 3.0 829 1.8

.7 1.1 98.6 763 3.5 789 2.9 834 1.7

.9 1.1 99.8 772 3.5 799 2.9 844 1.6

ns are shown on the right.

Fig. 2. (a) Composition of five amphibole phenocrysts and four

amphibole inclusions in apatite and quartz, plotted in a diagram

according to Leake et al. (1997). Some data points are hornblendes

sensu strictu, while others are edenites, pargasites, and hastingites.

(b) Reconstructed P–T conditions of amphibole crystallization,

based on Al-in-hornblende barometry and plagioclase–hornblende

thermometry. The uncertainty in temperature arises from the

compositional range of coexisting plagioclase, whereas the

uncertainty in pressure accounts for the fact that not all phases

required for the application of the barometer may have been

coexisting. Also included is the stability field of magmatic calcite

and the reaction curve (calcite + quartz X wollastonite +CO2) in the

presence of a pure CO2 fluid (see text for further explanations).


The results suggest that the amphibole phenocrysts

originate from different depths, with one group hav-

ing crystallized at pressures of more than 5 kbar and

one at a pressure of less than 2 kbar (cor-responding

to 14 and 6 km, respectively). Notably, core–rim

variations within individual phenocrysts are much

smaller than variations between them, which is diffi-

cult to explain by ascent of a single magma batch (in

this case, rims of high-pressure phenocrysts should

display the same chemistry as low-pressure phenoc-

rysts). This suggests that several magmas were in-

volved in the generation of ‘‘SR8’’ and that one of

them ascended directly from z 14 km depth without

residing for an extended period of time at lower

pressure.

2.3.5. Magmatic calcite

Several quartz phenocrysts host small inclusions of

calcite (Fig. 3). The following observations support a

primary (i.e., magmatic) origin of this calcite: (i) the

inclusions are not intersected by any cracks or fluid

inclusion trails; (ii) they occur as single crystals

(which would not be the case if they were secondary

fillings or replacements); and (iii) there is no calcite

present in the altered groundmass. Primary calcite is

very rare in siliceous magmas, and its crystallization

from granitic melts has been experimentally con-

strained to pressures above 3 kbar (Swanson, 1979).

The stability of calcite in silicate melts is limited by

the equilibrium:

calciteðCaCO3Þ þ quartzðSiO2Þ

¼ wollastoniteðCaSiO3Þ þ CO2 ð1Þ

which is divariant in the presence of H2O in a

coexisting fluid phase. The stability field of calcite

in granitic magmas is limited by the granite solidus

and the strongly pressure-dependent position of reac-

tion (1) above (Fig. 4a). The lowest pressure at which

melt + calcite can coexist is at 3.8 kbar, with an aCO2

of 0.2 in the coexisting fluid phase (Fig. 4b). Hence,

the presence of primary calcite in quartz phenocrysts

of ‘‘SR8’’ suggests that these quartz crystals formed

at z 3.8 kbar (translating into a depth of z 10.8 km

based on an average crustal rock density of 2.85 g/

cm3), and that the melt contained appreciable amounts

of dissolved CO2.

Additional information can be derived from the

fact that the calcite inclusions are still intact and show

no reaction with the host quartz (the Raman spectrum

in Fig. 3b shows no signs of the prominent wollas-

tonite bands at 971 and 636 cm� 1). Once incorporat-

ed within another mineral, the pressure in the calcite

inclusion is determined by the relative volume change

occurring during decompression and cooling. The

compressibilities of quartz and calcite are 25� 10� 6

and 13.5� 10� 6 MPa� 1, respectively, whereas the

thermal expansion coefficients are 44� 10� 6 and

28.5� 10� 6 K� 1, respectively (Gottschalk, 1997).

Fig. 3. (a) Photomicrograph of a quartz phenocryst with inclusions of magmatic calcite (transmitted light). (b) Raman spectra obtained from the

same calcite aggregate, showing the typical wavelength shifts produced by calcite.


Therefore, isobaric magma cooling leads to an in-

crease of pressure within the calcite inclusion, where-

as isothermal magma decompression causes the

pressure to drop rapidly to nearly zero. In the latter

case, calcite starts to react with the host quartz to

produce wollastonite + CO2 (i.e., reaction (1) is driven

to the right), until the pressure required for calcite

stability is reached (e.g., 3 kbar at aH2O= 0 and 770 jC

in Fig. 4b). Thus, if nearly isothermal (770F 50 jC)decompression from greater than 5 kbar to less than 2

kbar (as suggested by hornblende–plagioclase ther-

mobarometry) was followed by a prolonged residence

time of the magma at lower pressure, it would have

resulted in partial reaction of the calcite to wollaston-

ite and a buildup of 1 kbar overpressure within the

inclusions. Such a high overpressure in inclusions of

>50 Am size inevitably would have resulted in inclu-

sion decrepitation (Roedder, 1984). Calcite inclusions

in the quartz phenocrysts of ‘‘SR8,’’ however, are still

intact and show no signs of partial reaction to wol-

lastonite, suggesting that they are preserved in a

metastable equilibrium. This requires rapid ascent of

Fig. 4. (a) T–XCO2diagram showing the position of the granite

solidus and the reaction (calcite + quartz X wollastonite + CO2) at

pressures of 2, 5, and 10 kbar. The stability field of magmatic calcite

at 10 kbar is indicated by light gray shading, whereas the

corresponding field at 5 kbar is shown in dark gray. At 2 kbar,

the granite solidus and the calcite–wollastonite reaction curve do

not intersect, precluding crystallization of magmatic calcite. (b) P–T

stability field of magmatic calcite. The minimum is located at 3.8

kbar and XCO2c 0.2 in the coexisting fluid. Data source: Jacobs and

Kerrick (1981), Ebadi and Johannes (1991), and Holland and Powell

(1998).

A. Audetat et al. / Lithos156

some quartz phenocrysts from z 10.8 km depth, in

agreement with the findings from hornblende thermo-

barometry.

2.4. Summary of results for Santa Rita

We have shown that the premineralization quartz-

monzodioritic magma at Santa Rita contained 1–2

vol.% anhydrite phenocrysts, which were decom-

posed to an orange-brown powder. This amount of

anhydrite translates into a minimum content of

2500–5000 ppm S in the magma, making it one

of the most sulfur-rich magmas ever recorded. In

comparison, the sulfur-rich magmas erupted from

Mt. Pinatubo and El Chichon contained 1000–4000

and 5000 ppm S, respectively (Baker and Ruther-

ford, 1996). Magmatic oxygen fugacity was high

(zNNO+ 1), as shown by the assemblage quartz +

magnetite + sphene +Mg-rich amphibole and the ab-

sence of sulfides.

The results of plagioclase–hornblende thermome-

try and Al-in-hornblende barometry suggest that at

least two magma batches were involved in the gener-

ation of ‘‘SR8,’’ one of which has ascended directly

from z 14 km depth. Rapid ascent of one magma

batch is suggested also by the metastable presence of

calcite inclusions within quartz phenocrysts.

72 (2004) 147–161

3. Relation to other calc-alkaline magmas

3.1. Sulfur speciation

Sulfur speciation in silicate melts and associated

stability of sulfides vs. sulfates strongly depends on

the oxidation state of the magma. Relatively reduced

magmas (logfO2<NNO+ 0.5) are characterized by the

predominance of S2 � and become saturated with

sulfides (pyrrhotite or immiscible sulfide melts),

whereas oxidized magmas (logfO2>NNO+ 0.5) con-

tain mainly sulfate (SO4)2� and precipitate anhydrite

(Whitney and Stormer, 1983; Carroll and Rutherford,

1987; Luhr, 1990; Baker and Rutherford, 1996; Ducea

et al., 1999). The transition from S2– predominance to

SO42–predominance (both at the 90% level) occurs

between logfO2=NNO� 1 and NNO+ 1.5, indepen-

dent of melt chemistry and absolute values of P and T

(Matthews et al., 1999). At a given pressure and

temperature, sulfur solubility in dacitic melts displays

a minimum at around logfO2=NNO � 0.5 and

increases towards both lower and higher oxygen

fugacities (Carroll and Rutherford, 1988).

ithos 72 (2004) 147–161 157

3.2. Oxidation state

The oxidation state of natural silicic magmas can

be constrained from mineral equilibria between ferro-

magnesian silicates and oxides (Wones, 1981;

Ghiorso and Sack, 1991; Frost and Lindsley, 1992;

Xirouchakis et al., 2001). Felsic (i.e., quartz-saturated)

magmas display a systematic relationship between the

oxygen fugacity and the ferromagnesian silicate

phases present, suggesting that fO2was actually buff-

ered by mafic minerals in these magmas (Carmichael,

1967, 1991). Their oxygen fugacity was found

to increase from olivine-bearing magmas (logfO2=

NNO� 3 to NNO� 1) over orthopyroxene-bearing

magmas (Fbiotite, amphibole, cummingtonite; log-

fO2=NNO� 0.5 to NNO+ 1) to biotiteF amphiboleF

sphene-bearing magmas (logfO2=NNO+1 to NNO+

2.5; olivine and orthopyroxene absent). The T– fO2

field of orthopyroxene vs. biotite + amphibole-bearing

magmas is shown in Fig. 5.

Intermediate-to-felsic rocks hosting porphyry-Cu

deposits are characterized by the occurrence of

amphiboleF biotite (e.g., Lowell, 1974; Gustafson

and Hunt, 1975). They thus belong to the most

oxidized group with an oxygen fugacity of log-

fO =NNO+ 1 to NNO+ 2.5, in which the majority

A. Audetat et al. / L

2

Fig. 5. Oxygen fugacity of natural silicic magmas as a function of

their ferromagnesian mineral assemblage (data from Carmichael,

1991). Also shown is the boundary between SO42� and S2�

predominance of dissolved sulfur (after Carroll and Rutherford,

1988), and the reconstructed crystallization conditions of sulfur-rich

magmas erupted from Mt. Pinatubo (Pin) and El Chichon (EC), as

summarized in Baker and Rutherford (1996).

of sulfur occur as sulfate. The same applies to coarser-

grained intrusions cropping out in the vicinity of (or

below) the porphyry stocks, which are characterized

by the assemblage amphibole +magnetite + sphene

(Keith and Swan, 1995; MacMillan and Panteleyev,

1995) and may represent the source of the mineraliz-

ing fluids. Hence, porphyry-Cu-related magmas seem

to be too oxidized that magmatic sulfides could have

played a major role in the mineralization process. The

following arguments, however, suggest that this may

be a rash conclusion.

Firstly, one has to bear in mind that the change

from oxidized to reduced sulfur does not occur

instantaneously, but follows approximately a function

of the form: log[XSO4(2� )/XS(2 � )] = 1.02�DNNO�

0.45, where Xi is the mole fraction of species i and

DNNO is the deviation of fO2(in log units) from the

NiNiO buffer (Wallace and Carmichael, 1994). It

implies that two log units above NNO, there are still

2.5% of the total sulfur present as sulfide. This amount

is certainly negligible in terms of sulfur budget, but it

can be dramatic with respect to the budget of

chalcophile elements with high sulfide/silicate melt

partition coefficients such as copper and gold

(DCu, pyrrhotite/silicate melt and DAu, sulfide melt/silicate melt

are on the order of 2.6� 103 and 5.7� 103, respec-

tively; Jugo et al., 1999).

Secondly, there is increasing evidence for the

presence of magmatic sulfides in earlier stages of

porphyry-Cu evolution, as revealed by recent discov-

eries in the Bingham and Tintic mining districts, at

Mt. Pinatubo and at Alumbrera (Pallister et al., 1997;

Imai et al., 1997; Keith et al., 1997; Hattori, 1997;

Hattori and Keith, 2001; Maughan et al., 2002; Halter

et al., 2002b).

3.3. Significance of contributions by mafic magmas

At Mt. Pinatubo, it could be shown that the

sulfur-rich eruptions between June 7 and June 15

in 1991 were triggered by intrusion of hot, basaltic

magma into a relatively cool, dacitic magma cham-

ber that was highly oxidized (logfO2=NNO+ 3) and

saturated with anhydrite (Hattori, 1993, 1997; Kress,

1997; Pallister et al., 1997). The intruding basaltic

magma contained globules of an immiscible sulfide

melt, and it has been proposed that the sulfur-rich

nature of the Mt. Pinatubo volcanic system ulti-


mately stems from interaction with these mafic melts

(Hattori, 1993, 1997; Kress, 1997; Pallister et al.,

1997).

Evidence for mixing between mafic and silicic

magmas has been reported also from the Bingham

and Tintic mining districts (Keith et al., 1997; Hattori

and Keith, 2001; Maughan et al., 2002). Magmatic

sulfides are present mainly in intermediate magma

compositions, but they are conspicuously absent in the

most mafic member. The presence of magmatic barite

inclusions in olivine phenocrysts from the most mafic

unit at Bingham suggests that this magma was oxi-

dized and contained high amounts of dissolved sulfur

(Keith, personal communication, 2003).

Most recently, magmatic sulfides have been de-

scribed from intermediate intrusions in the Farallon

Negro Volcanic Complex (Halter et al., 2002b). The

fact that the Cu/Au ratio in the magmatic sulfides

closely matches the Cu/Au ratio of the bulk ore at

Alumbrera suggests that magmatic sulfides form an

important part of the metal enrichment process. In

contrast to the previous two localities, closed-system

magma fractionation, rather than magma mixing, has

been invoked to explain the compositional evolution

of the Farallon Negro Volcanic Complex (Halter et al.,

2002b).

It should be noted that in the case of magma mixing,

the record of an involvement of mafic magmas is likely

to become obscured. The only evidence for the former

presence of mafic magmas may be found in a mafic

enclave, in the trace element signature of the whole

rock, or in the occurrence of inversely zoned phenoc-

rysts. In the case of Santa Rita, magma mixing is

indicated by the presence of compositionally diverse

amphibole phenocrysts and intact calcite inclusions

within quartz phenocrysts. In fact, the presence of

compositionally variable amphibole phenocrysts

seems to be a characteristic feature of porphyry-Cu-

related rocks in general and has even been proposed as

a criterion to distinguish barren from mineralized

systems (Mason, 1978; Mason and McDonald, 1978;

Hendry et al., 1985).

4. Conclusions

The premineralization quartz-monzodioritic mag-

ma at Santa Rita had a high oxygen fugacity and

was highly enriched in sulfur. Magmatic oxygen

fugacity is constrained by the coexistence of

quartz + amphibole + biotite + sphene (suggesting a

logfO2zNNO+1; Wones, 1989; Carmichael, 1991),

and by the absence of magmatic sulfides. The sulfur

content of the magma was on the order of 2500–

5000 ppm S, as suggested by the former presence of

1–2 vol.% anhydrite phenocrysts. Similarly high

sulfur contents were observed in the magmas erup-

ted from Mt. Pinatubo and El Chichon (e.g., Luhr,

1990; Baker and Rutherford, 1996). The presence of

compositionally diverse amphibole phenocrysts

without markedly zoned intermediate members indi-

cates that several magmas were involved in the

formation of the quartz-monzodiorite. Al-in-horn-

blende barometry as well as magmatic calcite inclu-

sions within quartz phenocrysts suggest that one of

these magmas ascended directly from z 14 km

depth.

A comparison with other porphyry-type Cu

deposits reveals that most of them are associated

with oxidized intrusions in which the dominant

sulfur species was sulfate. It might be premature,

however, to conclude that magmatic sulfides were

not important in these systems. Firstly, even small

amounts of magmatic sulfides can dominate the

budget of chalcophile elements, and, secondly, there

is increasing evidence from both active and fossil

porphyry-Cu systems that magmatic sulfides are

present at stages where mafic melts interact with

the oxidized magmas. Future studies will show

whether such an interaction is a common feature of

volcano-plutonic complexes hosting porphyry-Cu ore

deposits.

Acknowledgements

We like to thank Bob Bodnar (Virginia Tech, USA)

and the Swiss National Science Foundation for

financing this study. We are grateful also to Luca

Fedele and Gretchen Benedix for help with the

electron microprobe, and to Jing Leng for performing

the Raman analyses. The paper improved significantly

through discussions with Werner Halter and thorough

reviews by Jeff Keith and Fleurice Parat. Additional

thanks go to Bob North for guiding the senior author in

the Santa Rita mine, and to Lawford Anderson for


providing a spreadsheet for hornblende–plagioclase

thermobarometry.

References

Anderson, J.L., Smith, D.R., 1995. The effects of temperature and fO2

on the Al-in-hornblende barometer. Am. Mineral. 80, 549–559.

Audetat, A., Gunther, D., Heinrich, C.A., 2000. Causes for large-

scale metal zonation around mineralized plutons: fluid inclusion

LA-ICP-MS evidence from the Mole Granite, Australia. Econ.

Geol. 95, 1563–1581.

Baker, L.L., Rutherford, M.J., 1996. Crystallization of anhydrite-

bearing magmas. Trans. R. Soc. Edinb. Earth Sci. 87, 243–250.

Blevin, P.L., Chappell, B.W., 1992. The role of magma sources,

oxidation states and fractionation in determining the granite

metallogeny of eastern Australia. Trans. R. Soc. Edinb. Earth

Sci. 83, 305–316.

Burnham, C.W., Ohmoto, H., 1980. Late-stage processes of felsic

magmatism. In: Ishihara, S., Takenouchi, S. (Eds.), Granitic

Magmatism and Related Mineralization. Mining Geology Spe-

cial Issue. Soc. Res. Geol., Japan, pp. 1–13.

Candela, P.A., 1989. Felsic magmas, volatiles and metallogenesis.

In: Whitney, J.A., Naldrett, A.J. (Eds.), Ore Deposition Asso-

ciated with Magmas. Reviews in Economic Geology, vol. 4.

Soc. Econ. Geol., Socorro, NM, pp. 223–233.

Candela, P.A., 1992. Controls on ore metal ratios in granite-related

ore systems: an experimental and computational approach.

Trans. R. Soc. Edinb. Earth Sci. 83, 317–326.

Candela, P.A, Holland, H.D., 1986. A mass transfer model for

copper and molybdenum in magmatic hydrothermal systems,

the origin of porphyry-type ore deposits. Geochim. Cosmochim.

Acta 81, 1–19.

Carmichael, I.S.E., 1967. The iron– titanium oxides of salic vol-

canic rocks and their associated ferromagnesian silicates. Con-

trib. Mineral. Petrol. 14, 36–64.

Carmichael, I.S.E., 1991. The redox state of basic and silicic mag-

mas: a reflection of their source regions? Contrib. Mineral. Pet-

rol. 106, 129–141.

Carroll, M.R., Rutherford, M.J., 1987. The stability of igneous

anhydrite: experimental results and implications for sulfur be-

havior in the 1982 El Chichon trachyandesite and other evolved

magmas. J. Petrol. 28, 781–801.

Carroll, M.R., Rutherford, M.J., 1988. Sulfur speciation in hydrous

experimental glasses of varying oxidation state: results from

measured wavelength shifts of sulfur X-rays. Am. Mineral.

73, 845–849.

Cygan, G.L., Candela, P.A., 1995. Preliminary study of gold parti-

tioning among pyrrhotite, pyrite, magnetite, and chalcopyrite in

gold-saturated chloride solutions at 600 to 700 jC, 140 MPa

(1400 bars). In: Thompson, J.F.H. (Ed.), Magmas, Fluids and

Ore Deposits, vol. 23. Mineralogical Association of Canada,

Victoria, British Columbia, pp. 129–137. Short Course Series.

Dilles, J.H., 1987. Petrology of the Yerington batholith, Nevada:

evidence for evolution of porphyry copper ore fluids. Econ.

Geol. 82, 1750–1789.

Ducea, M.N., McInnes, B.I.A., Wyllie, P.J., 1999. Experimental

determination of compositional dependence of hydrous silicate

melts on sulfate solubility. Eur. J. Mineral. 11, 33–43.

Ebadi, A., Johannes, W., 1991. Beginning of melting and compo-

sition of first melts in the system Qz–Ab–Or–H2O–CO2. Con-

trib. Mineral. Petrol. 106, 286–295.

Frost, B.R., Lindsley, D.H., 1992. Equilibria among Fe–Ti-oxides,

pyroxenes, olivine and quartz: Part II. Application. Am. Mine-

ral. 77, 1004–1020.

Ghiorso, M.S., Sack, R.O., 1991. Fe–Ti oxide geothermometry,

thermodynamic formulation and the estimation of intensive

variables in silicic magmas. Contrib. Mineral. Petrol. 108,

485–510.

Gottschalk, M., 1997. Internally consistent thermodynamic data for

rock-forming minerals in the system SiO2 –TiO2 –Al2O3 –

Fe2O3 –CaO–MgO–FeO–K2O–Na2O–H2O–CO2. Eur. J.

Mineral. 9, 175–223.

Gunther, D., Heinrich, C.A., 1999a. Comparison of the ablation

behaviour of 266 nm Nd:YAG and 193 nm ArF excimer lasers

for LA-ICP-MS analysis. J. Anal. At. Spectrom. 14, 1363–1368.

Gunther, D., Heinrich, C.A., 1999b. Enhanced sensitivity in laser

ablation-ICP mass spectrometry using helium–argon mixtures

as aerosol carrier. J. Anal. At. Spectrom. 14, 1363–1368.

Gunther, D., Audetat, A., Frischknecht, R., Heinrich, C.A., 1998.

Quantitative analysis of major, minor and trace elements in

fluid inclusions using laser ablation-inductively coupled plas-

ma-mass spectrometry (LA-ICP-MS). J. Anal. At. Spectrom. 13,

263–270.

Gunther, D., Frischknecht, R., Heinrich, C.A., Kahlert, H.J., 1997.

Capabilities of an argon fluoride 193 nm excimer laser for laser

ablation inductively coupled plasma mass spectrometry micro-

analysis of geological materials. J. Anal. At. Spectrom. 12,

939–944.

Gustafson, L.B., Hunt, J.P., 1975. The porphyry copper deposit at

El Salvador, Chile. Econ. Geol. 70, 857–912.

Halter, W.E., Pettke, T., Heinrich, C.A., Roth-Rutishauser, B.,

2002a. Major to trace element analysis of melt inclusions by

laser-ablation ICP-MS: methods of quantification. Chem. Geol.

183, 63–86.

Halter, W.E., Pettke, T., Heinrich, C.A., 2002b. The origin of Cu/Au-

ratios in porphyry-type ore deposits. Science 296, 1844–1846.

Hammarstrom, J.M., Zen, E., 1986. Aluminum in hornblende, an

empirical igneous geobarometer. Am. Mineral. 71, 1297–1313.

Hattori, K., 1993. High-sulfur magma, a product of fluid discharge

from underlying mafic magma: evidence from Mount Pinatubo,

Philippines. Geology 21, 1083–1086.

Hattori, K., 1997. Occurrence and origin of sulfide and sulfate in

the 1991 Mount Pinatubo eruption products. In: Newhall, C.G.,

Punongbayan, R.S. (Eds.), Fire and Mud, Eruptions and Lahars

of Mount Pinatubo, Philippines. University of Washington, Se-

attle, pp. 807–824.

Hattori, K.H., Keith, J.D., 2001. Contribution of mafic melt to

porphyry copper mineralization, evidence from Mount Pinatubo,

Philippines, and Bingham Canyon, Utah, USA. Miner. Depos.

36, 799–806.

Hendry, D.A.F., Chivas, A.R., Long, J.V.P., Reed, S.J.B., 1985.

Chemical differences between minerals from mineralizing and


barren intrusions from some North American porphyry copper

deposits. Contrib. Mineral. Petrol. 89, 317–329.

Hernon, R.M., Jones, W.R., Moore, S.L., 1964. Geology of the

Santa Rita Quadrangle, New Mexico, U.S. Geological Survey

Report GQ-0306.

Holland, T., Blundy, J., 1994. Non-ideal interactions in calcic am-

phiboles and their bearing on amphibole–plagioclase thermo-

metry. Contrib. Mineral. Petrol. 116, 433–447.

Holland, T.J.B., Powell, R., 1998. An internally consistent thermo-

dynamic data set for phases of petrological interest. J. Meta-

morph. Geol. 16, 309–349.

Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., Sisson,

V.B., 1987. Confirmation of the empirical correlation of Al in

hornblende with pressure of solidification of calc-alkaline plu-

tons. Am. Mineral. 72, 231–239.

Imai, A., Listance, E.L., Fujii, T., 1997. Highly oxidized and sulfur-

rich dacitic magma of Mount Pinatubo: implication for metal-

logenesis of porphyry copper mineralization in the Western Lu-

zon Arc. In: Newhall, C.G., Punongbayan, R.S. (Eds.), Fire and

Mud: Eruptions and Lahars of Mount Pinatubo, Philippines.

University of Washington, Seattle, pp. 865–874.

Jacobs, G.K., Kerrick, D.M., 1981. Devolatilization equilibria in

H2O–CO2 and H2O–CO2–NaCl-bearing fluids: an experimen-

tal and thermodynamic evaluation at elevated pressures and

temperatures. Am. Mineral. 66, 1135–1153.

Jones, W.R., Hernon, R.M., Moore, S.L., 1967. General geology of

the Santa Rita quadrangle, grant country New Mexico. U.S.

Geol. Surv. Prof. Paper 555 (144 pp.).

Jugo, P.J., Candela, P.A., Piccoli, P.M., 1999. Magmatic sulfides

and Au:Cu ratios in porphyry deposits: an experimental

study of copper and gold partitioning at 850 jC, 100 MPa

in a haplogranite melt –pyrrhotite – intermediate solid solu-

tion–gold metal assemblage, at gas saturation. Lithos 46,

573–589.

Keith, S.B., Swan, M.M., 1995. The great Laramide porphyry cop-

per cluster of Arizona, Sonora, and New Mexico: the tectonic

setting, petrology, and genesis of a world-class porphyry metal

cluster. In: Coyner, A.R., Fahey, P.L. (Eds.), Geology and Ore

Deposits of the American Cordillera. Geological Society of Ne-

vada, Reno, NV, pp. 1667–1747.

Keith, J.D., Dallemeyer, R.D., Kim, C.S., Kowallis, B.J., 1991. The

volcanic history and magmatic sulfide mineralogy of latites of

the central East Tintic Mountains, Utah. In: Raines, G.L., Lisle,

R.E., Schafer, R.W., Wilkinson, W.H. (Eds.), Geology and Ore

Deposits of the Great Basin. Geological Society of Nevada,

Reno, NV, pp. 461–483.

Keith, J.D., Whitney, J.A., Hattori, K., Ballantyne, G.H., Christian-

sen, E.H., Barr, D.L., Cannan, T.M., Hook, C.J., 1997. The role

of magmatic sulfides and mafic alkaline magmas in the Bing-

ham and Tintic mining districts, Utah. J. Petrol. 38, 1679–1690.

Kress, V.C., 1997. Magma mixing as a source for Pinatubo sulphur.

Nature 389, 591–593.

Larocque, A.C.L., Stimac, J.A., Keith, J.D., Huminicki, M.A.E.,

2000. Evidence for open-system behavior in immiscible Fe–

S–O liquids in silicate magmas: implications for contributions

of metals and sulfur to ore-forming fluids. Can. Mineral. 38,

1233–1249.

Leake, B.E., et al., 1997. Nomenclature of amphiboles: report of the

Subcommittee on Amphiboles of the International Mineralogi-

cal Association, Commission on New Minerals and Mineral

Names. Can. Mineral. 35, 219–246.

Lowell, J.D., 1974. Regional characteristics of porphyry copper

deposits of the southwest. Econ. Geol. 69, 601–617.

Luhr, J.F., 1990. Experimental phase relations of water- and sulfur-

saturated Arc magmas and the 1982 eruptions of El Chichon

volcano. J. Petrol. 31, 1071–1114.

Lynton, S.J., Candela, P.A., Piccoli, P.M., 1993. An experimental

study of the partitioning of copper between pyrrhotite and a

high-silica rhyolitic melt. Econ. Geol. 88, 901–915.

MacMillan, W.J., Panteleyev, A., 1995. Porphyry copper deposits

of the Canadian Cordillera. In: Pierce, F.W., Bolm, J.G. (Eds.),

Porphyry Copper Deposits of the American Cordillera. Arizona

Geological Society Digest, vol. 20, pp. 203–218.

Mason, D.R., 1978. Compositional variations in ferromagnesian

minerals from porphyry copper-generating and barren intrusions

of the Western Highlands, Papua New Guinea. Econ. Geol. 73,

878–890.

Mason, D.R., McDonald, J.R., 1978. Intrusive rocks and porphyry

copper occurrences of the Papua New Guinea–Solomon Islands

region, a reconnaissance study. Econ. Geol. 73, 857–877.

Matthews, S.J., Moncrieff, D.H.S., Carroll, M.R., 1999. Empirical

calibration of the sulfur valence oxygen barometer from natural

and experimental glasses, method and applications. Mineral.

Mag. 63, 421–431.

Maughan, D.T., et al., 2002. Contributions from mafic alkaline

magmas to the Bingham porphyry Cu–Au–Mo deposit, Utah,

USA. Miner. Depos. 37, 14–37.

Pallister, J.S., Hoblitt, R.P., Meeker, G.P., Knight, R.J., Siems, D.F.,

1997. Magma mixing at Mount Pinatubo: petrographic and

chemical evidence from the 1991 deposits. In: Newhall, C.G.,

Punongbayan, R.S. (Eds.), Fire and Mud, Eruptions and Lahars

of the Mount Pinatubo, Philippines. University of Washington

Press, Seattle, pp. 687–731.

Parat, F., Dungan, M., Streck, M.J., 2002. Anhydrite, pyrrhotite and

sulfur-rich apatite: tracing the sulfur evolution of an oligocene

andesite (Eagle Mountain CO, USA). Lithos 64, 63–75.

Peng, G., Luhr, J.F., McGee, J.J., 1997. Factors controlling sulfur

concentrations in volcanic apatite. Am. Mineral. 82, 1210–1224.

Roedder, E., 1984. Fluid inclusions. Rev. Mineral. 12 (644 pp).

Rose, A.W., Baltosser, W.W., 1966. The porphyry copper deposit at

Santa Rita, New Mexico. In: Titley, S.R., Hicks, C.L. (Eds.),

Geology of the Porphyry Copper Deposits in Southwestern

North America. Univ. Arizona, Tucson, AZ, pp. 205–220.

Scaillet, B., Evans, B.W., 1999. The 15 June 1991 eruption of

Mount Pinatubo: I. Phase equilibria and pre-eruption P–T–

fO2– fH2

O conditions of the dacite magma. J. Petrol. 40,

381–411.

Streck, M.J., Dilles, J.H., 1998. Sulfur evolution of oxidized arc

magmas as recorded in apatite from a porphyry copper batholith.

Geology 26, 523–526.

Swanson, S.E., 1979. The effect of CO2 on phase equilibria and

crystal growth in the system KAlSi3O8–NaAlSi3O8–CaAl2-Si2O8 –SiO2 –H2O–CO2 to 8000 bars. Am. J. Sci. 279,

703–720.


Titley, S.R., 1993. Characteristics of porphyry copper occurrence in

the American Southwest. In: Kirkham, R.V., Sinclair, W.D.,

Thorpe, R.I., Duke, J.M. (Eds.), Mineral Deposit Modeling.

Geological Association of Canada Special Paper, vol. 40,

pp. 433–464.

Wallace, P.J., Carmichael, I.S.E., 1994. S speciation in submarine

basaltic glasses as determined by measurements of SKa X-ray

wavelength shifts. Am. Mineral. 79, 161–167.

Whitney, J.A., Stormer Jr., J.C., 1983. Igneous sulfides in the fish

canyon Tuff and the role of sulfur in calc-alkaline magmas.

Geology 11, 99–102.

Wones, D.R., 1981. Mafic silicates as indicators of intensive vari-

ables in granitic magmas. Kozan Chishitsu (Min. Geol.) 31,

191–212.

Wones, D.R., 1989. Significance of the assemblage titanite +mag-

netite + quartz in granitic rocks. Am. Mineral. 74, 744–749.

Xirouchakis, D., Lindsley, D.H., Frost, B.R., 2001. Assemblages

with titanite (CaTiOSiO4), Ca–Mg–Fe–olivine and pyroxenes,

Fe–Mg–Ti-oxides, and quartz: Part II. Application. Am. Min-

eral. 86, 254–264.

Documents

Magmatic Anhydrite