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Magmatic-vapor expansion and the formation of high-suldation gold deposits:Chemical controls on alteration and mineralization

Richard W. Henley a, Byron R. Berger b,⁎a Research School of Earth Sciences, Australian National University, Canberra, Australiab U.S. Geological Survey, Federal Center MS 964, Denver, CO 80225-0046, United States

a b s t r a c ta r t i c l e i n f o

Article history:

Received 14 July 2010Received in revised form 8 November 2010

Accepted 8 November 2010

Available online 23 November 2010

Keywords:

High-suldation

Gold

Magmatic vapor

Enargite

Silica-alunite

Sulfosalt melt

Tennantite

Solfatara

Large bulk-tonnage high-suldation gold deposits, such as Yanacocha, Peru, are the surface expression of

structurally-controlled lode gold deposits, such as El Indio, Chile. Both formed in active andesite–dacite

volcanic terranes. Fluid inclusion, stable isotope and geologic data show that lode deposits formed within

1500 m of the paleo-surface as a consequence of the expansion of low-salinity, low-density magmatic vapor

with very limited, if any, groundwater mixing. They are characterized by an initial ‘Sulfate’ Stage of advanced

argillic wallrock alteration± alunite commonly with intense silicication followed by a ‘Sulde’ Stage — a

succession of discrete sulde–sulfosalt veins that may be ore grade in gold and silver. Fluid inclusions in

quartz formed during wallrock alteration have homogenization temperatures between 100 and over 500 °C

and preserve a record of a vapor-rich environment.

Recent data for El Indio and similar deposits show that at the commencement of the Sulde Stage,

‘condensation’ of Cu–As–S sulfosalt melts with trace concentrations of Sb, Te, Bi, Ag and Au occurred at

N600 °C following pyrite deposition. Euhedral quartz crystals were simultaneously deposited from the vapor

phase during crystallization of the vapor-saturated melt occurs to Fe-tennantite with progressive

non-equilibrium fractionation of heavy metals between melt-vapor and solid. Vugs containing a range of

suldes, sulfosalts and gold record the changing composition of the vapor.

Published uid inclusion and mineralogical data are reviewed in the context of geological relationships to

establish boundary conditions through which to trace the expansion of magmatic vapor from source tosurface and consequentalteration andmineralization. Initially heat loss from the vapor is high resulting in the

formation of acid condensate permeating through the wallrock. This Sulfate Stage alteration effectively

isolates the expansion of magmatic vapor in subsurface fracture arrays from any external contemporary

hydrothermal activity. Subsequent fracturing is localized by the embrittled wallrock to provide high-

permeability fracture arrays that constrain vapor expansion with minimization of heat loss. The Sulde Stage

vein sequence is then a consequence of destabilization of metal-vapor species in response to depressurization

and decrease in vapor density.

The geology, mineralogy, uid inclusion and stable isotope data and geothermometry for high-suldation,

bulk-tonnage and lode deposits are quite different from those for epithermal gold–silver deposits such as

McLaughlin, California that formed near-surface in groundwater-dominated hydrothermal systems where

magmatic uid has been diluted to less than about 30%. High sul dation gold deposits are better termed

‘Solfataric Gold Deposits’ to emphasize this distinction. The magmatic-vapor expansion hypothesis also

applies to the phenomenology of acidic geothermal systems in active volcanic systems and equivalent

magmatic-vapor discharges on the anks of submarine volcanoes.

Published by Elsevier B.V.

1. Introduction

High-suldation gold deposits range in scale from small, high-grade

lode deposits to very large,low-grade deposits.Theyhave in commonan

envelope of pre-ore advanced argillic alteration assemblages – Sulfate

Stage – that are commonly, but not exclusively, characterized by

oxidized sulfuras sulfate in alunite(KAl3(SO4)2(OH)6),anda subsequent

contrasting sequence of ore-stage sulde assemblages – Sulde Stage –

that typically include pyrite, a range of sulfosalt minerals (including

enargite and tennantite), gold, and silver (Arribas, 1995; Berger and

Henley, 2011).

Since the late 1990s, a number of studies of high-suldation gold–

silver deposits have consistently shown that the temperatures recorded

by uid inclusions in quartz in the Sulfate Stage alteration assemblages

Ore Geology Reviews 39 (2011) 63–74

⁎ Corresponding author.

E-mail address: [email protected] (Byron Berger).

0169-1368/$ – see front matter. Published by Elsevier B.V.

doi:10.1016/j.oregeorev.2010.11.003

Contents lists available at ScienceDirect

Ore Geology Reviews

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o r e g e o r ev

http://dx.doi.org/10.1016/j.oregeorev.2010.11.003http://dx.doi.org/10.1016/j.oregeorev.2010.11.003http://dx.doi.org/10.1016/j.oregeorev.2010.11.003mailto:[email protected]://dx.doi.org/10.1016/j.oregeorev.2010.11.003http://www.sciencedirect.com/science/journal/01691368http://www.sciencedirect.com/science/journal/01691368http://dx.doi.org/10.1016/j.oregeorev.2010.11.003mailto:[email protected]://dx.doi.org/10.1016/j.oregeorev.2010.11.003


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range from about 100 to over 500 °C and that vapor-rich inclusions are

abundant (see Section 2.1.1). In turn recent FESEM data for sulfosalt

assemblages from El Indio, Summitville and Lepanto (Mavrogenes et al.,

2010) strongly suggest near-magmatic temperatures for the initial

copper-rich ore stage of high-suldation deposits (see Section 2.1.2).

These data show that, following initial pyrite deposition, arsenic-rich

sulfosalt melts withtrace contentsof other heavy metals (tin,silver,gold,

bismuth, selenium, and tellurium) were formed from a vapor phase. The

high-temperature melts partially crystallized rst to enargite and

Fe-tennantite and distinctive euhedral quartz crystals (Fig. 1a–c)

whose oxygen isotope composition (Mavrogenes and Henley, unpub-

lished data) is consistent withthat demonstratedfor magmaticuid atEl

Fig. 1. Euhedral quartz and Fe-tennantite (Cu10.6, Fe1.4As3.9Sb0.1S13; Tn 0–5%, Fe 0.8 to 2.2 at.%) inll a void and associated fractures in earlier pyrite, b) euhedral quartz and

Fe-tennantite (Cu10.6Fe1.1As4S13) inll a void in concentrically banded and fractured pyrite, c) euhedral quartz lls in a void in earlier pyrite and with fractional crystallization of

Fe-tennantite aroundquartzfrom Tn 4.0 to 77% (bright zones), d) curved interfaces between low-antimony Fe-tennantite (Tn 3.4%)and fractionated Fe-tennantite (Tn 4.0–77%).Note

the vug-rich rims of the low Tn phase, the distribution of vugs (black) and euhedral quartz, and the minor Sb-enrichment adjacent to one vug, e) dissolution of early pyrite by

Fe-tennantite (Cu10FeAs4S13) with transient development of a metal-decient chalcopyrite corona and isolated bornite and with quartz inlling void space, f) emplectite (CuBiS2)

growing in an isolated vug within enargite. Abbreviations: Qz, quartz, Fe–Tn, Fe-tennantite (numbers are atomic percent Sb/(As+Sb)), ccp, chalcopyrite, Py, pyrite, v, vug. (Sample

location data: El Indio; EI1300, Enargite vein, DDH1300, 490.5 m, EI4216F; DDH4216, 27.7 m; Lepanto; L4=3.29.3 of Mancano (1994, Fig. 8); Red Mountain, CO6, Underground

sample, Longfellow Mine, Red Mountain, Silverton, Colorado). Images: a), b) and e) are reected light images, c) and d) are ASB FESEM images and f) is a BSE in-lens detector FESEM

image.

64 R.W. Henley, B.R. Berger / Ore Geology Reviews 39 (2011) 63–74


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Indio ( Jannas,1995).Fractionation of heavymetals (Sb, Bi,Ag, Te,and Au)

occurred as the residual melt crystallized in the presence of vapor (Fig. 1

c–d) anddissolved earlier pyrite(Fig. 1e) andenargite. Metal transportin

the vapor phase is recorded through the preservation of micron-scale

vugs containing a wide range of discrete suldes and sulfosalts including

tennantite and tetrahedrite, chalcocite, bismuthinite and emplectite

(CuBiS2) (Fig. 1f) as well as quartz, uorapatite and gold (Mavrogenes et

al., 2010).

By contrast the homogenization data for

uid inclusions inenargite at Lepanto, Philippines, suggested a much lower temperature

range (less than 310 °C) for the ore stage (Mancano, 1994; Mancano

and Campbell, 1995). Although, as determined by Hedenquist et al.

(1998, p. 386), no primary uid inclusions were found in the Lepanto

enargite or quartz samples, the acceptance of these uid inclusion

homogenization data (b310 °C) as recording the temperature of

mineralization forced the conclusion that substantial system-scale

cooling had occurred between the alteration and ore stages

(Hedenquist et al. (1998, Fig. 19). In the absence of independent

temperature estimates for the Sulde Stage, this hypothesis was

subsequently adsorbed into a number of generic models for the

formation of high-suldation gold deposits (e.g. Heinrich, 2005;

Muntean and Einaudi, 2001). However a number of fundamental

dif culties with the geology, physics and chemistry attend these

models, not least of which is accounting for how the uid in such large

scale hydrothermal systems lost some 75% of its heat between its

evolution at magmatic temperatures (NN750 °C) and the assumed

temperature of mineralization (b310 °C). Each of these models

imposes critical assumptions about pressure as well as temperature

in the depositionalregime of high suldationmineralsystems that are

not sustained by critical review of available geochemical and,

crucially, geologic data. Neither can these models accommodate the

new high temperature data for the Sulde Stage so that a fresh

approach is necessary.

In this paper, in order to set boundary conditions, werst reviewthe

interpretation of available uid inclusion data in the context of the

geology and geochemistry of high-suldationdeposits.We then provide

a new hypothesis for the origin of these important gold deposits based

on the expansion of a magmatic-vapor phase under low-pressureconditions (see Section 3) that is fully consistent with the geological

relationships (Berger and Henley, 2011) and stable isotope data and

with the active volcanic context in which these deposits formed.

2. The environment of formation of high-suldation lode-gold

deposits

The advanced argillic alteration and ore stages in high-suldation

lode deposits track active faults and branch fractures through

intrinsically low-permeability host rocks such as marine sediment

sequences or ash-ow tuffs. These fracture arrays crosscut host rocks

that commonly contain evidence of previous or concurrent weak,

near-neutral pH groundwater-related alteration indicating that largerscale groundwater-dominated geothermal systems may have operated

contemporaneously with the formation of the high-suldation

mineralization. Based on eld data, Berger and Henley (in press)

suggested that the aggressive silica–alunite1 alteration localized by

syn-hydrothermalfault arrays effectively isolated magmaticuidowin

fractures from any external hydrothermal system—a discretization that

characterizes acidic geothermal systems in active volcanic settings today

(e.g., Ruaya et al., 1992) (see Section 5.2).

Sulfur isotope data consistently emphasize the predominance of

magma-sourced uid in both the alteration and mineralization stages

(e.g., Rye, 1993, 2005) of these deposits. Other stable isotope and uid

inclusion data are alsoconsistent witha high-level volcanicenvironment

and thepredominance of a low-salinity (b5 wt.% NaCleq) magmaticuid

(Bethkeet al., 2005;Jannas et al., 1999; Rye et al., 1992). Theinvolvement

of groundwater has been investigated in a number of

18

O–

D studies, butits proportion in the ore setting is generally much less than 25% and, in

some cases, is close to zero (Bethke et al., 2005; Deyell et al., 2004, 2005;

Hedenquist et al., 1994,1998;Jannas et al., 1999;Rye, 1993; Vennemann

et al., 1993). This is the reverse of the relative proportions of magmatic

uid and groundwater in high-level adularia–sericite (so-called ‘low

suldation’ gold–silver) and analogous submarine massive sulde

systems. In high-suldation deposits, evidence for the involvement of

groundwater (and seawater in coastal settings) is strongest for

distinctive alunite veins and barite that post-date the sulfosalt-gold

stage in many deposits and record waning thermal activity.

2.1. Constraints on temperature

In order to set boundary constraints on, for example, uidtemperature and pressure, for the modeling of the physics and chemistry

of the common and characteristic silica–alunite advanced argillic and

sulde stages of high-suldation gold deposits, it is rst necessary to

carefully review the interpretation of uid inclusion and other data

within their geologic context. The dif culties in interpreting uid

inclusion data for deposits formed at shallow paleo-depths (see

Section 2.2) are well known and primarily relate to uncertainties with

respect to the timing of trapping relative to primary mineral deposition,

the extent of cooling and involvement of groundwater (e.g., Foley et al.,

1989) and as discussed below,phase changes, in thehost mineral,during

the cooling stages of alteration and mineralization.

2.1.1. Silica–alunite stage temperatures

There is general agreement that the silica–alunite stage of intenseleaching and alteration results from the in situ formation of acidic

condensate derived from magmatic vapor (Arribas, 1995; Chouinard

et al., 2005; Hedenquist et al., 1994, 1998; Sillitoe and Hedenquist,

2003; Stoffregen, 1987; Vennemann et al., 1993). At Chinkuashih,

Wang et al. (1999) noted an abundance of vapor-rich inclusions in

Sulfate Stage quartz samples. At Furtei, Sardinia, Ruggieri et al. (1997)

also comment on the abundance of vapor-rich inclusions, and report a

population of distinctive high-salinity, high-temperature inclusions

(390°–500 °C) as wellas lower temperature populations. At Summitville,

Bethke et al. (2005) similarly report an abundance of vapor-rich

inclusions with homogenization temperatures greater than 500 °C.

Hedenquist et al. (1994) also reported an abundance of vapor-rich

inclusions in alteration assemblages in the Nansatsudistrict, Japan, as did

Jannas et al. (1999) in alunite in the Campaña fracture-vein system at ElIndio. Nevertheless, homogenization temperature data for vapor-rich

inclusions2 have been largely ignored in the interpretation of these

deposits, even though their abundance conrms a vapor-rich

environment and a wide range of temperatures and temperature

gradients within the alteration zones during their development.

The mineralogy of the Sulfate Stage has been extensively

documented (e.g., Heald et al., 1987; Hedenquist et al., 1994, 1998;

Jannas, 1995; Stoffregen, 1987). As well as alunite and a range of silica

polymorphs including quartz and chalcedony, it comprises the

minerals that are typical of advanced argillic alteration; namely,1 Silica–alunite. We are concerned in this paper with the reactions between acidiccondensate and rock during alteration and subsequent cooling in the Sulfate Stage. As

a convenience, we shall use the generic term “silica–alunite” to refer to the range of

advanced argillic alteration assemblages (Meyer and Hemley, 1967) that are

encountered in the high-suldation mineral districts whether or not alunite is a

product.

2 Homogenization temperature data for vapor-rich inclusions provide minimum

trapping temperatures for the inclusions.

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kaolinite, pyrophyllite, diaspore, andalusite and disseminated pyrite

and in rare instances, corundum (e.g., Famatina, Argentina, Losada-

Calderon and Bloom, 1990). Alunite itself is stable up to at least 450 °C

(Stoffregen et al., 1994) in accord with temperatures as high as 500 °C

determined through uid inclusion measurements in silica–alunite

alteration zones in many deposits despite the frequent assumption

that the upper stability limit of alunite is 300 °C (e.g., Muntean and

Einaudi, 2001). The alteration assemblage or mineralogical proportions

can change with depth. At El Indio, for example, pyrophyllitepredominates over alunite below 4000 m asl ( Jannas et al., 1999) and

at Chinkuashih, alunite simply disappears at deep levels (Sakazaki et al.,

1964).

Hemley et al. (1980) experimentally determined the phase

relationships of kaolinite, pyrophyllite, diaspore, andalusite, and

corundum as a function of temperature, pressure and the activity of

silica in solution (Fig. 2). During advanced argillic alteration, these

phases may be substituted by alunite (Hemley et al., 1969) in the

presence of alkalis derived from volcanic glass or feldspar in the host

rock, and high sulfate and hydrogen ion concentrations derived from

ef cient partitioning of highly soluble SO2 and HCl from the vapor into

the condensate fraction. When a strongly acidic condensate reacts

with volcaniclastic or sedimentary rocks, it results in high silica

activity in the liquid phase (Africano et al., 2002; Fournier, 1985) such

that metastable polymorphs of silica precipitate with alunite or

pyrophyllite, or with kaolinite (Fig. 2). In addition silica is added to the

wallrock during alteration since the solubility of quartz in the vapor

phase in the main fracture is, at 500 bars and 600 °C, about 1000 mg/kg

(Morey and Hesselgesser, 1951; Fournier and Potter, 1982). When a

small condensate fraction is formed in response to heat loss, partitioning

of silica to the liquid condensate leads immediately to very high silica

concentrations that drive very rapid silica deposition (Fig. 2). During

cooling of the alteration assemblage in the presence of condensate

trapped in pore space, as well as later with the inltration of

groundwater, progressive recrystallization of metastable silica occurs

through a sequence of polymorphs and ultimately to quartz (Africano

and Bernard, 2000; Okamoto et al., 2010). This sequence of processes

results in massive, highly silicied rocks, including “vuggy silica”

textured rocks of the type described by Stoffregen (1987) at Summit-

ville, Jannas et al. (1999) at El Indio and Wang (2010) at Chinkuashih.

Consequently, the trapping of inclusion populations takes place over an

extended period so thata wide range of temperatures arerecorded(e.g.,

Bethke et al., 2005, p. 290). Furthermore, since alteration progressively

reduces permeability, the altered rock becomes both a barrier to the

ingress of groundwater and,at thesame time, an effectiveinsulator with

respect to the dispersion of heat away from the

uid phase that ischanneled through the principal controlling fractures (Berger and

Henley, 2011). The temperature gradients within such insulating

wallrock environments are consequently very large, an attribute that is

reected in the wide range of uid inclusion homogenization data

(N500 to 100 °C), from alteration assemblages in which silica

recrystallized.

During cooling, redistribution and recrystallization of silica in the

altered wallrock occur where available water penetrates or wets the

surface of the primary and successive silica precipitates.

Fluid inclusions in quartz in this evolving assemblage do not therefore

necessarily record the presence of a continuum of liquid water or a

boiling-point depth relationship through which to convert

homogenization and freezing data to an estimate of depth. Rather

the pore pressure during silica recrystallization is more reliably

related to lithostatic pressure (see Section 2.2). Importantly, since

most of the homogenization temperature data record trapping during

cooling fom N500 °C the primary deposition temperature of the host

assemblage was necessarily hotter.

2.1.2. Sul de Stage temperatures

It is demonstrable that it is Sulfate Stage embrittlement of the

wallrock that localizes subsequent fracturing and consequent Sulde

Stage lode mineralization (Berger and Henley, 2011). The latter is

associated with little recognizable wallrock alteration other than, for

example, isolated replacement of early alunite by illite–sericite at El

Indio ( Jannas, 1995). In most deposits, pyrite (±solid solution gold

and silver; e.g., Deditius et al., 2009) is the rst sulde to precipitate in

lodes and is followed by enargite and Fe-tennantite (±minor

chalcopyrite and base-metal suldes) assemblages that also containdistinctive euhedral quartz crystals (Fig. 1) (Mavrogenes et al., 2010;

Mavrogenes and Henley, in preparation). Later stages of sulde–

sulfosalt deposition are characterized by more complex textures and

mineral suites that include some tellurides, selenides and bismuth

minerals as well as ore-grade silver and gold (e.g., Claveria, 2001;

Jannas, 1995). The textural and compositional data (Fig. 1) also show

that high-suldation sulde–sulfosalt assemblages do not show

equilibrium relationships nor are they analogous to the sulde

assemblages encountered in epithermal adularia–sericite type

deposits.

Sulde stage mineral assemblages are not amenable to standard

methods of transmitted light uid inclusion analysis so that infrared

techniques have been progressively developed for the measurement

of homogenization temperatures and salinities in enargite andsometimes pyrite. Moritz (2006) has cautioned that many infrared

determinations of inclusion uid salinity and homogenization

temperatures may be in error and has since published new data for

enargite sampled from a number of deposits in Europe and Peru

(Moritz and Benkhelfa, 2009). These liquid-phase inclusion data

dene a range of salinities from very low to less than 5 wt.% NaCleqand have a range of homogenization temperatures from about

150–300 °C. Kouzmanov et al. (2010) have reported a similar range

of uid inclusion homogenization temperatures for well-formed

enargite crystals in drusy late-stage cavities in minor post-porphyry

veins from the Rosia Poieni porphyry copper deposit, Romania.

The recrystallization of sulfosalts to complex sulfosalt assemblages

together with a wide variety of tellurides (e.g., goldeldite) and native

gold, during cooling to below about 300 °C, has been previously

Fig. 2. Stability relations in the system Al2O3–SiO2–H2O at PH2O=250 bars with the

addition of silica activity-temperature relations for the common polymorphs of quartz

to temperatures well above 300 °C (Okamoto et al., 2010). Deposition of silica occurs

initially as metastable silica polymorphs that subsequently recrystallize to quartz

during cooling of the alteration assemblage with development of pseudo-secondary

uid inclusions with a wide range of homogenization temperatures from ~100 to

N500 °C. Point MH shows, for illustration, the concentration of silica when a liquid

condensate fraction of 10% separatesfrom thevaporat 550 °C based on thevaporphase

solubility data of Morey and Hesselgesser (1951). Constructed from Geochemists

Workbench data kindly provided by James Cleverley.


http://localhost/var/www/apps/conversion/tmp/scratch_7/image%20of%20Fig.%E0%B2%80


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stressed by Sack and Goodell (2002)for theJulcaniand other deposits.

Fig. 3 shows phase relationships in the Cu–Sb–As–S system; enargite

(orthorhombic, wurtzite-type, hexagonal close-packed structure) and

luzonite (tetragonal, sphalerite-type cubic close-packed structure)

are, sensu stricto, polytypes of Cu3AsS4 (Posfai and Sundberg, 1998).

Enargite is the high-temperature form of luzonite above 280–300 °C

and an immiscibility gap is dened between enargite and the luzonite–

famatinite solid solution series (Skinner, 1960) where these minerals

display a wide range of ordered to disordered structures and

intergrowths at the atomic scale. Posfai and Buseck (1998) noted that

the occurrence of enargite is an indicator of its formation well above the

transitionbetween luzonite andenargite(280 °C)and that luzonite may

form through solid-state recrystallization of enargite below thetransition temperature. In describing the enargite inclusion samples

from Lepanto, Mancano (1994) specically noted heterogeneities and

internal structural defects,3 as well as the dif culties caused by varying

degrees of IR transparency in establishing the primary or secondary

nature of inclusions that enabled only 29% of the 97 enargite inclusions

in 17 samples to be interpreted as potentially primary (and two as

secondaries) while all quartz inclusions were classied as secondaries.

We suggest that these data record post-primary phase changes and

fracturing during the cooling of the enargite assemblages in the

presence of groundwater.

2.1.3. Summary of temperatures for alteration and mineralization

The Sulfate Stage uid inclusion and stable isotope data reliably

identify a) a wide range of temperatures (N500 to 100 °C) consistent with

high thermal gradients and the recrystallization of metastable silica

polymorphs to quartz during cooling, and b) derivation from partial

condensation of a magmatic vapor with very limited involvement of

groundwater. Since condensation of vapor occurs through heat loss, it is

implicit that the primary uid4 had a higher enthalpy and temperature

andwastherefore, necessarily,also a vapor . Temperature estimatesfor theSulde Stage cannotbe reliably based onuidinclusiondata in enargite or

other suldes. Micro-analytical data (see Section 1) from El Indio,

Summitville and Lepanto provide independent evidence that sulde–

sulfosalt deposition along with quartz in the earliest part of the Sulde

Stage were at least 550 °C, consistent with the high temperatures

indicatedfor thevapor condensateresponsiblefor Sulfate Stage alteration.

2.2. Constraints on uid pressure

Since many of the well-known high-suldation deposits are

young, pressure estimates for alteration and mineralization can be

made fromeld data. For example at Chinkuashih, Taiwan, the rugged

topography and geological setting of the ≈1 Ma Chinkuashih deposit

suggest that, at most, only one or two hundred meters of rock havebeen removed by erosion. Mining extended to about 850 m below the

present surface with decreasing enargite and increasing Cu/Au ratio.

As discussed above (see Section 2.1.1), uid inclusion formation is

likely to have occurred during recrystallization of silica in the

alteration zones so that interpreted trapping pressures record pore

uid pressures consistent with the reconstructed paleo-depth at

which samples were obtained. Jannas et al. (1999) reconstructed the

original topography at El Indio to suggest a paleo-depth range for

mineralization of −500 to −1100 m (or less if post-mineral fault

movement occurred). Hedenquist et al. (1998) suggested shallow

depth for the Lepanto mineralization and at Goldeld, Nevada, debris

ow sediments (Berger, 2007) containing alunite pebbles occur at the

paleosurface in the eastern part of the district indicating that

mineralization and alteration developed at ≈200 m depth.

2.2.1. Summary of pressure constraints on alteration and mineralization

Since alteration and mineralization at both Chinkuashih and El

Indio were formed from a vapor phase (see Section 2.1) to depths

below the paleosurface of about 1100 m and stable isotope data

indicate limited, if any, incursion of groundwater, theuid pressure in

the developing vein system was evidently greater than that due to an

external groundwater system (≤~110 bars at this depth). At this

depth, the lithostatic pressure would have been about 290 bars and

similar pressure estimates apply for other high-suldation vein

deposits. The pressure constraints provided by geologic and uid

Fig. 3. Phase relations in the system Cu–As–Sb–S redrawn from Posfai and Buseck

(1998). The range of compositions of coexisting phases in the enargite–famatinite

system at Chinkuashih, Goldeld and El Indio is indicated (Huang and Chou, 1975;

Huang and Tsai, 1984; Mavrogenes et al., 2010; Sung, 2005 ). An estimate of the range

for Lepanto ‘enargite–luzonite’ is provided based on a smelter concentrate assay

(Filippou et al., 2007) and our unpublished FESEM data in order to compare with the

maximum and minimum ranges of uid inclusion homogenization data from enargite

(Mancano, 1994). For comparison the compositional range of high temperature

volcanic gas condensates from fumaroles with discharge temperatures N600 °C is

shown based on the data compiled by Williams-Jones et al. (2002). The average crustal

antimony and arsenic concentrations of granite and basalt are shown for reference.

Abbreviations; En, enargite, luz, luzonite, Fam, Famatinite, hcp, hexagonal close packed,

ccp, cubic close packed, int, interlayered polytype.

3 An IR image of enargite in Mancano (1994, Fig. 3a), for example, clearly shows a

relation between an inclusion and a subsolidus structural, not compositional, attribute

of the crystal.

4 Fluid-phase terminology. The term “hydrothermal” has a commonly assumed connota-

tion of referring to a liquid phase. Here we simply use it in reference to the context of the

phenomenology of H2O-dominated uid phases within an epizonal or sub-volcanic context.The term “magmatic vapor” has been widely used to refer to the low-density uid phase

released from a melt so that the term “vapor” has a connotation with respect to both phase

state and provenance. Strictly the term “vapor” refers to a gas phase in equilibrium with a

condensed phase – solid or liquid – as for example in the term “vapor pressure”. Thus a

“magmatic vapor” once released from direct interaction with a melt should be termed a

“magmatic gas” or simply a “gas” or indeed more precisely a “gas mixture” because of the

wide rangeof componentsthatit contains. Sincethis gasmixture is dominated by water it is

condensibleand therefore on expansionmay intersect thetwo-phaseboundary of thesystem

resulting in phase separation to fractions of liquid “condensate” and equilibrium “vapor” (a

gas). Whilst the distinctions may appear semantic, it is important to be aware of the precise

implications of theseterms, particularly with respect to the physicalproperties and behavior

of “uid” phases with respect to uid properties like density and viscosity in geological

environments. Here, the term “vapor” is used generically where the argument has not

reached a point of dening physical properties and then ceases to have value in much the

same way as Verhoogen (1949) advocated the avoidance of the non-specic term

“supercritical”. We have used the term uid as a non-specic term prior to dening its

phase state in line with the recommendations of Bodnar (pers.comm., 2010).




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inclusion data do not therefore sustain the high pressure conditionsrequired by Heinrich et al. (2004) to maintain a liquid phase condition

for high-suldation mineralization.

As discussed later (see Section 5.2), where limited erosion has

occurred, high-suldation lode mineralization is expressed as low grade

bulk-tonnage deposits associated with extensive advanced argillic

alteration, indicating that vapor ow occurred freely to and through

the paleo-surface. Ignoring small changes in vapor density due to

fractional heat loss, thepressure-depth curve fora vapor is much steeper

than that of liquid water (the boiling point-depth curve or cold

groundwater hydrostatic gradient) or of rock (lithostatic gradient)

(Fig. 4). The steep vapor gradient intersects the lithostatic gradient at a

shallow level so thatpressurerelease after choking by mineraldeposition

could produce shallow breccias bodies as is observed in high level

deposits such as Yanacocha, Peru.5 Below this crossover, lithostaticpressuregreatlyexceedsthe vaporpressure inan open fracture. This is an

unstable situation with respect to the relative uid pressure within the

open fracture system which must therefore progressively close by

wallrock fragmentation due to rock stress as well as mineral deposition.

Maintenance of fracture permeability is thus dependent on renewed

fracturing dueto theactive stresseldand thecondition (Cox,2010) that

PuidNProck+T, where T is the tensile strength of the rock(about 10 bars

maximum).

A review of data for alteration and ore mineralization tempera-

tures (see Section 2.1) indicates that the Sulfate Stage uid transiting

to the surface through faults and associated fractures was a

condensible vapor (i.e., condensate liquid formed as uid transited

pressure and temperature gradients in the wallrock) at a temperature

of N550 °C with a density of about 0.03 g/cm3.

3. Magmatic-vapor expansion

The recognition, through stable isotope data, that the uid phase

responsible for bothalteration and ore mineralization in high-suldation

gold deposits was predominantly of magmatic origin forces

consideration of the systematics of magmatic-uid expansion from its

source through a fracture array in the upper few kilometers (b1.5 to

2 km) of theEarth'scrust to whereit discharges at thesurface. Pressure is

the criticalcontrol onuidowand the phase state oftheuid, thelatter

crucially controlling chemical processes such as mineral deposition.

Critical examination of available uid inclusion and other data (see

Section 2) conrms that the uid responsible for alteration and

ore mineralization was a low-salinity, high-temperature (N550 °C),

low-pressure (bb500 bars) vapor phase released from a magmatic

reservoir comprising multiple sub-volcanic intrusions with alteration

and ore mineralization occurring within 1500 m of the surface at

pressures below about 110 bars.

Sodium chloride (b5 wt.% NaCleq) dominates other non-volatile

componentsin bothvolcanic gases anduidinclusions in high-suldation

deposits. By analogy with volcanic gases, CO2 and other insoluble gases

occur at less than 5 mol% such that their contribution on an ideal gasbasis

to the total pressure of thevapor phase is lessthan5%. The phase behavior

of the magmatic uid can appropriately be approximated with respect to

pressure, enthalpy (heat content), temperature and solute content on the

basis of data for the simple NaCl–H2O system (Driesner, 2007). Fig. 5a

shows the extent of the immiscibility gap between liquid and vapor up to

1000 °C in thissystem and Fig. 5b is a projectionof the two-phase surface

and isopleths of NaCl concentration for a selection of illustrative

temperatures. In the high-enthalpy, low-pressure, single-phase region

and below 5 wt.%. NaCleq, the properties of the vapor phase can be

approximated using values for pure water under the same conditions inorder to provide quantication of, for example, the density of the vapor

phase.

The melting relations of water-saturated granite (Fig. 5b) with

respect to pressure and temperature dene the enthalpy range of the

low-salinity uid exsolved from a granitic magma. For example, a

magmatic uid exsolving from melt at ~850 °C in relatively shallow

volcanic settings hasan enthalpy in excess of 4000 kJ/kg and a density

of about 0.2 g/cm3 or less. It follows that, for the shallow (b1.5 km)

volcanic setting of enargite–gold deposits, the low-salinity uid

identiedby uid inclusionand stableisotope analyses is of magmatic

origin and has the properties of a gas; it expands to ll all available

volume.

It is essentialin considering thebehaviorof hydrothermalsystems to

track the fate of all the conservative components in the uid phaseincluding salt content, stable isotopes and heat energy. The thermal

dynamics of a number of common depressurization paths for magmatic

vapor are shown in Fig. 5c in order to contrast that for high-suldation

deposits with others in more permeable environments where

groundwater interactions dominate ore-forming processes. In settings

such as geothermal systems (M–G) where high permeability is

maintained by extensional fracturing, mixing between cold

groundwater and expanding magmatic vapor occurs across the extent

of the system and gives rise to the development of magmatic vapor

plumes as described by Henley and McNabb (1978). In these settings,

uid enthalpy (and therefore temperature), isotope ratios and solute

concentration are determined by the progressive change in the ratio of

magmatic uid to groundwater. Intersection of the two-phase region

may occur on particular enthalpy–

pressure mixing paths depending on

Fig. 4. The relationship between pressure and depthin a vaporexpanding to the surface

through a fracture array. MV represents the ow through low permeability fractured

rock from a high pressure magma source, M, to a low pressure sink, V, provided by the

highly transmissive fracture array. VF is a simplied pressure-depth prole where the

vapor pressure gradient is greater than hydrostatic (see Section 2.2.1). Hydrothermal

brecciation occurs above point S as shown. At surface rapid open-system cooling occurs

due to atmospheric interactions and mixing with shallow groundwater or perched

layers of condensate providing local regions with hydrostatic gradients that may be

penetrated by fumarolic discharges. For further discussion see text and Berger and

Henley(in press). A usefulmodern example of thecontrastof vapor andliquid pressure

proles is detailed by Moore et al. (2008) for the Karaha-Telaga Bodas volcanic

geothermal system, Indonesia.

5 Many of the so-called hydrothermal breccias in shallow deposits can however be

shown to have formed during the Sulfate Stage, for example at Tambo, Chile ( Jannas et

al., 1999), in the presence of aggressively reactive acid condensate indicating

dissolution and steam explosion as their formation process.




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structural-permeability and in these settings porphyry-style

mineralisation (ϕ) may eventuate (see Section 5.1). In the single-phase

region near surface, phase separation occurs as the mixed uid

intersects the two-phase region and, in favorable structural settings,

may generate adularia–sericite styles of epithermal precious and base

metalmineralization (Henleyand Berger, 2000; Henleyand Ellis,1983).

This mixing scenario differs from path M–FHE where the isenthalpic

expansion6 of the magmatic vapor is conned to discrete high-

permeability fracture arrays through impermeable rocks and heat lossis limited by theconductivity of the wallrock (Toulmin and Clark, 1967).

It is this setting that corresponds to the formation of high-suldation

gold deposits and high-enthalpy fumaroles (FHE) in actively degassing

volcanoes. Where expandingvapor loses a signicant proportionof heat

by conduction (M–V), a small fraction of vapor condenses as pressure

drops below M–V –C (Fig. 5c). Through partitioning of acid gases,

primarily SO2, to the condensate in wallrock fractures, aggressive

alteration ensues and gives rise eventually to the characteristic silica–

alunite alteration of high-suldation deposits (see Section 2.1.1). Path

S–S′ is illustrative of conductive cooling where a limited ow of vapor

loses all of its heat to wallrock; such total condensation retains the

composition of the primary vapor phase and the liquid condensate may

deposit sulde minerals in isolated cavity llings.7 A high ΔH/ΔP

trajectory also occurs where vapor condenses directly into a perchedgroundwater or condensate liquid aquifer very close to the surface and

Fig. 5. (a) Three dimensionalperspective of the 2-phase regionin the systemNaCl–H2O

based on the data compilation of Driesner (2007, Supplementary Material). (b)

Enthalpy–pressure–temperature relations for phases in the system NaCl–H2O calcu-

lated using SOWAT code kindly provided by Thomas Driesner.). The gure comprises a

projection of a selection of isopleths and the two-phase boundary at 400°, 750° and

1000 °C to illustrate the expansion of the two-phase eld as temperature increases, the

effect of salt-content on the enthalpy of liquid (L) and gas phases (V) outside the two-

phase eld and the relative location of the halite and halite (H)+vapor (V) elds at

these temperatures. For discussion purposes, density isochors for pure water are also

provided. The water-saturated minimum and 2% water content melting curves for

granite are shown in order to identify the enthalpy–pressure eld for magmatic vapor

derived from epizonal intrusions. c) Illustrative paths for mixing and heat loss ow in

hydrothermal systems; for detail see Section 3.

Fig. 6. Halite solubility in the halite-vapor eld of the NaCl–H2O system, redrawn fromthe data of Sourirajan and Kennedy (1962). An illustrative quasi-isenthalpic

depressurization and expansion path is shown for a magmatic vapor emphasizing the

decrease in water vapor pressure and the rapid decrease in halite solubility in the halite

plus vapor eld both of which lead to rapid sulde and sulfosalt deposition as melt or

solid phase.

6 Heat-balance terminology. We here use the term “isenthalpic” to specify

expansion processes where there is no exchange of heat energy with the surroundings.

The alternative term “irreversible adiabatic expansion” distinguishes the non-

equilibrium aspect of such processes (for example, rapid expansion through a series

of throttles or dilations in a fracture array) and their rate relative to “reversible

adiabatic expansion” processes. The latter refer to the slow constrained expansion of a

uid where there is an inter-conversion of internal energy and work energy. In such a

constrained isentropic process, a far more substantial temperature decrease occurs

relative to that due to irreversible isentropic expansion over the same pressure drop

(Anderson and Crerar, 1993). Both these processes may control the range of fumarole

temperatures within the La Fossa volcano on Vulcano, Italy ( Nuccio et al., 1999).7 An example (see Section 2.1.2) is the enargite and pyrite (TH ~317 °C) within late

drusy cavities in isolated veins cutting the Rosia Poieni porphyry copper deposit

(Kouzmanov et al., 2010).


http://localhost/var/www/apps/conversion/tmp/scratch_7/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_7/image%20of%20Fig.%E0%B5%80


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results in the well-known solfataric environment of active volcanoes

associated with low-enthalpy fumaroles (FLE).

Fig. 6 provides the detail of the low-pressure environment where

quasi-isenthalpic expansion of magmatic vapor enters the halite plus

vapor eld in which the vapor-phase solubility of halite decreases

rapidly with pressure and temperature (or more precisely enthalpy).

As will be discussed later, it is this rapid decrease in concentration of

components such as chloride that, in association with the decrease in

water pressure during expansion, is likely to be the principal controlon the deposition of metal and metal suldes and sulfosalts as well as

gold (Mavrogenes et al., 2010).

4. Chemical processes during alteration and mineralization

In the magmatic-vapor expansion hypothesis proposed in this

paper, it is assumed, in commonwith modelsproposedby Hedenquist

et al. (1998), Henley and McNabb (1978), Williams-Jones and

Heinrich (2005) and earlier by Krauskopf (1964), that metals are

transferred from a magma source as vapor-phase species. The ability

of a vapor phase to transport signicant quantities of metals does,

however, remain one of the most controversial areas in the discussion

of the formation of magmatic-hydrothermal ore deposits despite

prima facie evidence such as the transport and deposition of a widerange of metals including gold, iron, and arsenic in high-temperature

fumarole gases (e.g., Africano et al., 2002; Churakov et al., 2000;

Fulignati and Sbrana, 1998; Larocque et al., 2008; Taran et al., 2000,

2001; Williams-Jones et al., 2002; Yudovskaya et al., 2006). Further

support is provided by uid inclusions in porphyry copper–gold

systems (e.g., Ullrich et al., 1999) and heavy metal sulfosalts and

suldesin vugs in high-suldation deposits (Mavrogenes et al., 2010).

Chemical-vapor transport and deposition at low pressures are also

very well established in the semiconductor and solar technology

industries for a wide spectrum of metals (cf. Hastie, 1975; Kodas and

Hampden-Smith, 1994). However, as stressed by Pokrovski et al. (2008),

there is a frustrating data vacuum for the high-temperature speciation of

metals in a high-temperature gas phase under geological conditions, and

this reects how dif cult such experimental determinationsare given theproblems of containment, sampling and analysis of highly corrosive,

low-density uids.

Pokrovski et al. (2008, p. 357) note that the formation of solvation

shells around core metal species “is the essential factor controlling

vapor-phase transport and vapor–liquid partitioning” in the vapor

phase at high temperatures. Species such as Cu(HS)g, nH2O have high

hydration numbers8 that respond rapidly to decreasing water pressure

as shown by vapor–liquid partition coef cients: in a sulfur-free system

for example, base-metal partition coef cients at 500 °C decrease by two

orders of magnitude between 580 and 480 bars, while partition

coef cients for gold and copper decrease even more rapidly in systems

containing reduced sulfur (Pokrovski et al., 2008). The solubility of

quartz as SiO2·2H2O is strongly pressure dependent (Fournier and

Potter, 1982; Morey and Hesselgesser, 1951) as is that of halite asNaCl·2H2O (Hastie, 1975, p. 82).

Particularly pertinent for high-suldation systems, Pokrovski

et al. (2005) have shown that the dominant arsenic species in low-

density magmatic vapor at 500 °C is As(OH)3 the concentration of

which is therefore strongly dependent on PH2O. For illustrative

purposes the solubility of enargite, as a solid or melt, may then be

written,

3CuðHSÞ0

g þ AsðOHÞ0

3g þ H2Sg ¼ Cu3AsS4 þ 3H2Og þ H2g ð1aÞ

so that, with a=activity, the solubility equilibrium constant, K s,gis,

K s;g ¼ ½a3

H2Og • a H2;g =½a

3

CuðHSÞog • a AsðOHÞo3g •

a H2Sg ð1bÞ

The solubility of enargite in vapor therefore at least a cubic

dependence on the activity of water or, to a reasonable approximation,

the pressure of water in the expanding vapor. Its solubility in a

magmatic vapor, initially at say 1000 bars consequently decreases by a

factor of 1000 in expanding to 100 bars pressure simply due to the

changing pressure of the water-dominated vapor phase.Similarly, there

is an overall solubility decrease of 106 in expandingto a surface pressure

of 1 bar. Inclusion of solvation water molecules (Eq. (1c)) shows a

higher order dependence on the activity or pressure of water; here H2O

distinguishes the properties of solvation water from free water.

3CuðHSÞ0

g ;nH2Og þ AsðOHÞ0

3;g þ H2Sg ¼ Cu3AsS4 þ 3H2Og þ H2;g þ nH2Og

ð1cÞ

The release of water molecules from the solvation sphere due to

vapor expansion and decrease in the dielectric constant has a multiplier

effecton solubility decrease. Copper, andothermetals in thevaporphase

may also be present as chloride species so that when vapor expands into

the halite plus vapor eld (Fig. 6) the removal of available chloride to

halite signicantly reduces their solubility; this is potentially most

signicant for a ‘hard’ metal such as iron in controlling pyritedeposition.

The solubility of enargite in a vapor is also, through Eq. (1b),

dependent upon the rate of change of aH2S/aH2. While wallrock

reactions are commonly assumed to control the redox state (aH2) of

the uid phase in many ore-forming systems, the chemical processes

in a vapor expanding within a fracture array through impermeable

silica–alunite altered wallrocks are dominated by homogenous gas

reactions. The activity of hydrogen, aH2,g, is controlled by a matrix of

reactions but primarily those involving sulfur (Giggenbach, 1987).

SO2ðgÞ þ 3H2ðgÞ ¼ H2SðgÞ þ 2H2OðgÞ ð2aÞ

K 2a ¼ aH2Sd a 2

H2O =aSO2d a

3H2

ð2bÞ

SO2(g) dominates the gaseous sulfur species at magmatic tempera-

turesand, through thenegativevolume changeof Reaction 2a as written,

is also favored by decreasing pressure. Equilibrium calculations show

that aH2S/aH2 does not vary by more than a factor of 10 over the

temperature range 350 to 900 °C or between 10 and 1000 bar pressures

at 600 °Cand is accordingly farlesssignicant than thevariation of water

pressure. Giggenbach(1987), Martin et al. (2009) and Montegrossi et al.

(2008) have drawn attention to the non-equilibrium behavior of the

H2S–

SO2 reaction which in turn implies unfavorable kinetics as a redoxcontrol reaction.

The rapid change in water fugacity and density consequent on

vapor expansion leads to major changes in the solubility of metals –

and silica – in the vapor phase. As a consequence, decompression

results in an immediate very high supersaturation of metals that, in

turn, triggers a cascade of far-from-equilibrium reactions (Henley and

Berger, 2000).As shown in Fig. 1, at El Indio, following pyrite and local

enargite deposition, Cu–Fe–As–S melt condensed from the vapor

phasewith a range of trace heavy metals. As progressive crystallization

proceeded the sulfosalt melts dissolved earlier pyrite9 and enargite and8 ) n, the number of solvation sphere water molecules, is a statistical value extracted

fromexperimental data forthe effective numberof water or other molecules surrounding

a coremolecular species.The bindingof eachsolvation molecule through,forexample, Van

der Waals bonding involves a solvation free energy that contributes to the overall

solubility equilibrium constant for the dissolution reaction in the vapor phase.

9 The sulfosalt melt+pyrite+vapor reaction occurs via chalcopyritesp (Fig. 1e) with

ultimate resorption of the chalcopyrite (Fig. 1e) but leaving islands of pyrite within the

Fe-tennantite to produce the disease texture described by Imai (1999) at Lepanto. The

complex redox reaction process is described elsewhere (Mavrogenes et al., in prep).



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fractionation of antimony, bismuth, tin, selenium, tellurium, silver (and

gold) occurred between the melt and vapor phase to produce

chemically zoned Fe-tennantite and other sulfosalts, tellurides and

bismuth minerals (Mavrogenes and Henley, in preparation) in an

analogous manner to that described by Tomkins (2010) for sulfosalts in

magmatic sulde ore deposits. The simultaneous growth of euhedral

quartz during melt crystallization and fractionation (Fig. 1c and d) is a

consequence of high initial vapor phase silica solubility and the limited

solubilityof water insul

de–

andby analogy sulfosalt–

melts (Wykes andMavrogenes, 2005). The end product of the sulfosalt–sulde crystalliza-

tion sequence is a vapor phase withrelativelyhigh gold and silvercontent

which ultimately separates to form silica-rich high grade gold veins.

Elsewhere vapor deposition may have occurred directly to solid state

sulfosalt phases with partitioning of heavy metals between solid and

vapor.

5. Discussion

It is well established that during thecrystallization of melts, metals

partition into magmatic vapor (e.g., Candela and Holland, 1984;

Simon et al., 2008). This process occurs early during crystallization to

develop an aqueous uid phase within sub-volcanic intrusions at

lithostatic pressure. At 4 km depth, for example, the maximum

pressure is about 1000 bars. Where leakage occurs directly, or more

likely indirectly via a magmatic-vapor reservoir, into active fracture

systems open to thesurface (1 to 110 bars, see Section 2.2) the dropin

uid pressure is more than 890 bars. This disparity in uid pressures

drives the expansion and discharge of magmatic vapor and the

consequent vapor-phase solubility disequilibria that result in the

sequence of sulfosalt–sulde stages typical of high-suldation

deposits. Since rapid vapor expansion constrained by active fracture

arrays within 1500 m or so of the paleosurface involves only minor

heat losses and temperature decrease the Sulde Stage is initiated by

high temperature sulde–sulfosalt assemblages as described by

Mavrogenes et al. (2010). Crystalline enargite and pyrite may form

directly from the vapor phase at temperatures above about 650 °C. At

El Indio and elsewhere these phases are subsequently dissolved by

a Cu–As–S melt that condensed from the expanding vapor phaseand which progressively crystallizes with fractionation of heavy

metals between melt and vapor giving rise, through the possible

mixing of groundwater in open fracture arrays, to the later complex

telluride–bismuth–tin–tungsten enriched vein assemblages with gold

and silver. The recognition of the sulfosalt melt stage is strongly

supported by the combination of several lines of evidence of evidence

as is illustrated in Fig. 1. Key evidence includes the following: (a)

euhedral quartzcrystalsare wholly enclosed within cavities and veinlets

with the sulfosalt (Fig. 1a–c), (b) symmetrical zonation of arsenic,

antimony and tellurium in Fe-tennantite around quartz crystals, (c)

Fe-tennantite demonstrably dissolves early pyrite (Fig. 1e),

(d) preservation of vugs and vug-lling suldes and sulfosalts,

(e) uid inclusions in quartz in vugs in Fe-tennantite crystals

homogenize to a vapor at N450 °C indicating a minimum trappingtemperature (Mavrogenes et al., 2010).

5.1. The volcanic context of high-sul dation gold deposits

It is a matter of common observation that, between eruptive stages,

magmas beneath composite volcanoes continueto release magmatic-gas

mixtures that expand via fracture arrays to form high-temperature

fumaroles and extensive near-surface alteration. Field data for enargite–

gold lode deposits dene an analogous magmatic-vapor expansion

context punctuated by eruption and dome extrusion. Following his

detailed study of Goldeld, Nevada, Ransome (1909, 1910) suggested

that the ores there may have been formed by the ascension of hot, acidic

solutions emanating from a deep magma reservoir. The role of volcanic

gases in the origin of gold deposits was subsequently debated

intermittently until substantially brought into focus in a review of

volcanic-gas chemistry by Hedenquist (1995). This review showed that

despitethedif cultiesin samplingvolcanic gases during eruptionor from

active high-temperature fumaroles between eruptive stages, the

observed ux of metals could be argued as suf cient to account for ore

formation within geologically reasonable time periods (104–105 years)

depending of course on the ef ciency of uid focusing and metal

deposition in the superstructure of the volcanic system.

Whether or not there is a direct link between the formation of enargite–gold lode deposits and porphyry copper–gold deposits is

immaterial to our hypothesis for the fracture-controlled expansion of

magmatic vapor in the formation of high-suldation gold deposits. The

one is not necessarily tied to the other. At Lepanto, for example, the

ranges of the K–Ar data for porphyry and high-suldation mineraliza-

tion (Arribas et al., 1995, GSA Data Repository Item 9517) do not

demonstrate contemporaneity of mineralization but simply allow that

each developed as discrete events of approximate duration 50,000–

100,000 years during the history of the Mankayan volcanic complex

over 700,000 years. The relative structural timing of mineralization and

alteration events is discussed by Berger and Henley (in press). At

Famatina, Pudack et al. (2009) carefully detailed the development of a

magmatic-vapor phaseassociated with earlier high-levelintrusionsthat

are spatially related to weakly-mineralized but high-grade, enargite–

famatinite–gold–silver veins associated with alunite. However, the

chain of evidence between the degassing of the earlier epizonal

intrusions and the development of the enargite–famatinite veins is

broken due to extensive erosion of at least 800 m of the host-rock

sequence prior to the sulfosalt mineralization stage, and the rarity of

usable uid inclusions in the sulfosalt stage samples.

Porphyry copper–gold deposits occur in large but still locally

constrained areas of extensionalfracturing in contrastto high-suldation

deposits that are restricted to much more isolated and irregular fracture

arrays through impermeable rocks. For both, isotope data consistently

identify the role of magmatic vapor sensu latu, but it is more likely in a

composite volcanic system that they record a “magmatic-vapor

reservoir” containing a mixture of magmatic gases derived from suites

of discrete melt reservoirs that may also include basalts (e.g., Roberge

et al., 2009) rather than any single or specic reservoir. The dynamics of theuidphase in such a reservoirare determined by crustal stress which,

in conjunction with pore-uid pressure, leads at some intermediate

depth to theaccumulation of magmatic vapor from availablesourcesand

controls higher-level fracture formation and reactivation (Cox et al.,

2001) assuggested by seismic data at Yellowstone(Husen et al., 2004).In

turn this suggests that high-suldation and porphyry Cu–Au deposits

occupy different structural regimes at different times as stress changes

progress during the history of a large volcanic complex, directly

determining the available structures within which mixing between

magmatic vapor and groundwater can occur (Fig. 5c, MG); the ΔP/ΔH

trajectory determines whether the two-phaseuid regime is intersected

with the potential for the formation of porphyry copper–gold style

mineralisation.

5.2. The near-surface discharge regime

At and very near the surface in “degassing” volcanic environments,

condensation of high-enthalpy volcanic vapor occurs very ef ciently

through heat transfers due to interaction with the atmosphere, contact

with highly permeable, epiclastic sediments, or by condensation and

mixing into groundwater regimes and crater lakes (Brantley et al.,

1987).10 These processes inevitably lead to the formation of localized

10 Crater lakes form through the complete condensation of magmatic vapor and

admixture of meteoric water. Their mineralogy and geochemistry are therefore a

composite of sulfate and sulde geochemistry with clays, alunite, enargite, pyrite, etc.

carried in suspension, ejecta and sediment (e.g., Kawah Ijen, Indonesia, Delmelle and

Bernard, 1994; Kawah Putih, Indonesia, Sriwana et al., 2000).

71R.W. Henley, B.R. Berger / Ore Geology Reviews 39 (2011) 63 –74


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bodies of advanced argillic and associated more extensive zones of

advanced argillic and argillic alteration as well as perched layers of

acidic “steam-heated” water near the surface that mimic topography

and which are the basic elements of solfatara as observed in active or

morestrictly quiescentvolcanic environments today.Thesephenomena

constitute a highly ef cient trap for the remaining metals exiting

the array of ‘feeder’ fractures and result in the shallow formation

of bulk-tonnage, but low-grade, gold deposits. The bulk-tonnage

Pascua–Lama deposit, for example, has a depth extent of about 500 m

and is distinguished by the extraordinary dynamic complexity of therelationships between an active structural array and relationships

between silica–alunite andpyrite–enargite deposition (Chouinard et al.,

2005). The occurrence of both alunite and sulfosalts in some veins is a

consequence of rapidly changing pressures and temperatures in the

near surface part of the system due to complex interplay between

condensate and magmatic vapor on multiple ow paths in the

extensive, active fracture system (Berger and Henley, 2011). In

more permeable systems, magmatic vapor expanding on isolated

fracture arrays may become trapped in groundwater ow systems

with consequent dilution and dispersion of metals, as is commonly

observed for arsenic, for example, in groundwater around active

volcanoes.

6. Summary

High-suldation gold deposits occur in andesite–dacite volcanic

complexes and relate closely to the discharge of magmatic-vapor gas

as it expands at depths of only a few hundred meters beneath

high-temperature fumaroles and solfatara. Their association with

surcial volcanic rocks however led Lindgren (1913) to classify these

depositsas epithermal implying formationtemperatures on theorderof

180–300 °C. Recent stable isotope, uid inclusion and micro-analytical

data however have demonstrated a clearer relationship to much higher

temperature fumarolic discharges as indeed was suggested by Cross

(1891, 1896), and a magmatic uid component of at least 75%mass.

These deposits are therefore more appropriately termed “Solfataric

gold deposits” in order to distinguish them from epithermal (sensu

stricto) gold deposits (e.g., McLaughlin, California) whose geological

context and alteration and mineralization processes relate to the

shallow ‘hot-springs’ discharge regime of paleo-geothermal systems

where, due to mixing, the magmaticuid component is less than about

30%. The time-integrated elements of Solfataric gold deposits are

summarized in Fig. 7 in relation to the pressure and temperature within

an expandingcolumnof magmatic vapor,the spatialcontrol imposed by

active fracture arrays and the formation of solfataras with extensive

advanced argillic alteration and low-grade mineralization solfatara at

the paleosurface. The narrowness of the isotherms localized by owing

fractures and the high thermal gradients are important to note, alongwith the expansion of the advanced argillic alteration due to heat loss

and ‘ponding’ of mixed acid condensate–groundwater aquifers below

the surface. Recognition of the paleo-environment and controls on

vapor expansionis of course signicant in exploration forthesedeposits.

Thissynopsis alsoappliesto solfatarasin activevolcanoesand associated

acidic geothermal systems (Ruaya et al., 1992) and equivalent sea oor

volcanic gas discharges, such as at Brothers Volcano in the southern

Kermadec Arc, New Zealand (de Ronde et al., 2005).

Acknowledgments

We thank Richard Arculus, Paul Barton, Phil Bethke, James Cleverley,

Graham Hughes, Gary Landis, Sue Kieffer, Laurie Madden, John

Mavrogenes, Agnes Reyes, Cornel de Ronde, and Bob Rye for the helpfulcomments and discussion. Dana Bove, Jeff Hedenquist, Kalin Kouzmanov

and Francois Robert kindly provided some of the samples illustrated in

Fig. 1. We thank Steve Kesler and Bob Bodnar for providing constructive

reviews on the original manuscript and Thomas Driesner for providing

access to the programs used in construction of the NaCl–H2O phase

diagrams.

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