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The chemical and physical evolution of magmatic to hydrothermal processes in the porphyry Cu-Au depositof Bajo de la Alumbrera (northwestern Argentina) has been reconstructed with a quantitative fluid inclusionstudy. Fluid inclusion petrography, microthermometry, and single inclusion microanalysis by Excimer laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are combined to determine the evolutionof pressure, temperature, and ore metal concentrations (including Cu and Au) in the fluids. Complementaryhydrogen and oxygen isotope analyses are used to further constrain the water sources in the evolving system.The combined data provide a new level of insight into the mechanisms of metal sourcing and ore mineral precipitationin a porphyry-style magmatic-hydrothermal system.
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0361-0128/01/3206/1743-32 $6.00 1743
IntroductionHYDROTHERMAL fluid transport associated with shallowcrustal magmatism is a key process contributing to the for-mation of major metal resources. Among the magmatic-hy-drothermal ore deposits, porphyry Cu-Au-Mo deposits arethe economically most important today and have been the
subject of intensive exploration and research over the lastdecades. The formation of porphyry-style and other mag-matic-hydrothermal orebodies requires a subtle interplay be-tween large-scale tectonics, magma generation, and emplace-ment in the upper crust, advective flow of hydrothermal fluidof diverse provenance, and complex chemical reactions be-tween fluids, precipitating ore minerals, and host rocks un-dergoing hydrothermal alteration. The complexity of theseprocesses demands an integrated approach of geologic, geo-chemical, and experimental investigations (cf. Hedenquistand Richards, 1998).
The Evolution of a Porphyry Cu-Au Deposit, Based on LA-ICP-MS Analysis of Fluid Inclusions: Bajo de la Alumbrera, Argentina
THOMAS ULRICH,,* DETLEF GNTHER,**ETH Zrich, Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zentrum NO, 8092 Zrich, Switzerland
AND CHRISTOPH A. HEINRICHETH Zrich, Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zentrum NO, 8092 Zrich, Switzerland,
and Faculty of Mathematics and Natural Sciences, University of Zrich, Ramistrasse 17, 8006 Zrich, Switzerland
AbstractThe chemical and physical evolution of magmatic to hydrothermal processes in the porphyry Cu-Au deposit
of Bajo de la Alumbrera (northwestern Argentina) has been reconstructed with a quantitative fluid inclusionstudy. Fluid inclusion petrography, microthermometry, and single inclusion microanalysis by Excimer laser ab-lation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are combined to determine the evolutionof pressure, temperature, and ore metal concentrations (including Cu and Au) in the fluids. Complementaryhydrogen and oxygen isotope analyses are used to further constrain the water sources in the evolving system.The combined data provide a new level of insight into the mechanisms of metal sourcing and ore mineral pre-cipitation in a porphyry-style magmatic-hydrothermal system.
Based on previously reported observations of the igneous geology, alteration geochemistry, and veining his-tory of the subvolcanic porphyries at Alumbrera, the distribution of fluid inclusion types in space and time isdocumented. Six major inclusion types are distinguished. The highest temperature brine inclusions (up to750C; P >1 kbar) are mainly recorded in barren quartz magnetite veins in the core of the alteration system.These polyphase brine inclusions (halite sylvite + multiple opaque and transparent daughter crystals) are in-terpreted as the most primitive magmatic fluid recorded at the level of the deposit. They are of moderatelyhigh salinity (5060 wt % NaCl equiv) dominated by NaCl, KCl, and FeCl2, and contain on average 0.33 wtpercent Cu and 0.55 ppm Au. Upon cooling and decompression, these saline liquids exsolve a vapor phase,which is preferentially enriched in Cu relative to its main salt components but probably plays a minor role inthe formation of this particular deposit because of the inferred small mass fraction of vapor. Cooling and de-compression from the highest initial P-T conditions down to about 450C causes magnetite K silicate alter-ation but no saturation in Au or Cu sulfides, as recorded by continually high ore metal concentrations in thefluid inclusions. Coprecipitation of Cu and Au as chalcopyrite and native gold ( some early bornite) occursover a narrow range of decreasing fluid temperature. With cooling from ~400 to 305C, the Cu concentrationin the brine drops by about one order of magnitude to less than ~0.07 wt percent, without a proportional de-crease in major salt components. Ore mineral precipitation extracts ~85 percent of the Cu and Au from thefluid. It is associated with potassic alteration, as shown by a concomitant decrease in the K/Na ratio of the cool-ing magmatic brine and by an increase in its Ba and Sr concentrations (elements which are probably liberatedin the destruction of calcic igneous minerals).
The fluid chemical data demonstrate that the metal ratios in this and probably many other porphyry-styleore deposits are primarily controlled by the magmatic source of the ore brines. On the other hand, the final hy-pogene ore grade of the deposit is controlled by the efficiency of ore mineral precipitation. At Alumbrera,metal extraction is governed by the efficiency of cooling a high flux of magmatic fluid within a small rock vol-ume. Dilution of residual magmatic fluids, as recorded by aqueous fluid inclusions of decreasing salinities andtemperatures below 295C, follows after the main stage of copper introduction and is associated with feldspar-destructive (phyllic) alteration. Geometric relationships, fluid analyses, and stable isotope data together indi-cate that phyllic alteration results from postmineralization hydrothermal activity involving minor mixing be-tween meteoric water, residual brine, and a waning input of magmatic vapor.
Economic GeologyVol. 96, 2001, pp. 17431774
Corresponding author: e-mail, [email protected] *Present address: Geology Department, The National University of Aus-
tralia, 0200 Canberra, Australia.**Present address: Laboratory for Inorganic Chemistry, ETH Zentrum
CAB, 8092 Zrich, Switzerland.
The key role of hydrothermal fluids in the formation of oredeposits associated with shallow intrusions was recognizedvery early (e.g., Lindgren, 1905), and numerous studies onfluid inclusions have demonstrated many common featuresamong porphyry copper deposits worldwide (e.g., Roedder,1971; Nash, 1976; Chivas and Wilkins, 1977; Etminan, 1977;Eastoe, 1978; Wilson et al., 1980; Weisbrod, 1981; Preece andBeane, 1982; Reynolds and Beane, 1985; Quan et al., 1987;John, 1989; Bodnar and Cline, 1991; Cline and Bodnar, 1994;Beane and Bodnar, 1995). High-temperature fluid inclusionswith very high salinity are generally associated with low-den-sity vapor inclusions and indicate a magmatic contribution tothe metal-transporting fluids (Bodnar, 1995). Stable isotopedata of igneous phases, alteration minerals, and fluid inclu-sions have shown that magmatic as well as meteoric watersare involved in porphyry copper systems (e.g., Taylor, 1974;Batchelder, 1977; Solomon, 1983; Bowman et al., 1987; Dilleset al., 1992; Oreskes and Einaudi, 1992; Rye, 1993; Sheets etal., 1996; Hezarkhani and Williams-Jones, 1998). Fluids fromboth sources may be essential for ore formation (Henley andMcNabb, 1978), but the present weight of geologic, experi-mental, and fluid chemical evidence indicates that the metalsare introduced dominantly by magmatic fluids (Candela andHolland, 1986; Heinrich et al., 1992; Bodnar, 1995; Cline,1995; Shepherd and Chenery, 1995; Hedenquist andRichards, 1998; Ulrich et al., 1999). However, the sources andthe geochemical role especially of low- to intermediate-salin-ity fluids of variable density remain ambiguous (Shinoharaand Hedenquist, 1997; Heinrich et al., 1999).
Quantitative information about the major and trace ele-ment composition of the metal-transporting hydrothermalfluids has been a limiting factor in reconstructing the mag-matic-hydrothermal enrichment process in time and space.Petrographic observations of ore mineral daughter crystalsprecipitated inside fluid inclusions (Roedder, 1984, see refer-
ences therein) and more recent microanalytical data (Dia-mond et al., 1990; Boiron et al., 1991; Heinrich et al., 1992;Mavrogenes et al., 1995; Damman et al., 1996) show thatmagmatic fluid inclusions can contain very high ore metalconcentrations. With the recent development of laser abla-tion-inductively coupled plasma-mass spectrometry (LA-ICP-MS; Gnther et al., 1998; Audtat et al., 2000a, b), a sin-gle inclusion analytical technique of sufficient sensitivity,quantitative accuracy, and routine applicability has becomeavailable for extensive studies of texturally complex samples(Audtat et al., 1998). In this paper we report the first studyof a world-class magmatic-hydrothermal ore deposit, whereLA-ICP-MS analyses of some 300 individual fluid inclusionsin 27 samples are integrated with conventional fluid inclusiontechniques and other geochemical data within a good geo-logic framework, as reported in Ulrich and Heinrich (2001).The aim of this study is to quantify the behavior of ore metalsand other elements involved in hydrothermal mass transferby high-temperature brines, vaporlike fluids, and low-salinityaqueous solutions. By combination with stable isotope datawe determine the sources of fluids and metals independentlyand define the mass transfer and the dominant causes formetal precipitation in the orebody.
Geologic SettingThe Cu-Au deposit of Bajo de la Alumbrera in northwest-
ern Argentina (lat 2730' S, long 6660' W) is hosted by sev-eral high K dacite porphyries that intruded into andesiticflows and pyroclastic rocks of the Faralln Negro VolcanicComplex. From crosscutting relationships and petrographicobservations, several porphyries can be distinguished (Table1) that form a composite intrusive complex (Ulrich and Hein-rich, 2001; J. M. Proffett, writ. commun.). The Ar/Ar dates ofbiotite from the earliest mineralized P2 Porphyry (7.10
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TABLE 1. Relative Timing Relationships between the Porphyry Intrusions at Bajo de la Alumbrera
Porphyry Timing relationships Alteration/mineralization
Los Amarillos Porphyry Cut by Early P3 and Northwest Porphyries Intensely feldspar destructive; barren to weakly mineralized
NE Porphyry Cut by Early P3; no other timing relationships Moderate potassic, chlorite epidote; weakly to moderately mineralized
P2 Porphyry Cut by Early P3 Porphyry and all younger porphyries; Quartz-magnetite, strong potassic; strongly mineralized associated with purple quartz veins
Early P3 Porphyry Cuts veins of P2 Porphyry; is cut by Late P3 and Partly quartz-magnetite, strong potassic; moderately to younger porphyries strongly mineralized
Quartz-Eye Porphyry Cut by Late P3; contains most of the purple quartz veins Intensely feldspar destructive; weakly to moderatelymineralized
Late P3 Porphyries Cuts Early P3 Porphyry; is intruded by later porphyries Weak-moderate potassic, weak feldspar destructive; weakly(Campamiento and mineralized North Porphyries)
Northwest Porphyry dikes Intrudes P2 and Early P3 Porphyries Weak potassic; barren
Post-Mineral Porphyry Cuts Late P3 Porphyries Commonly unaltered, slightly feldspar destructive; barren
Summary of the geologic relationships among the different porphyry intrusions; for a complete description of the geology see Ulrich and Heinrich (2001)and J.M. Proffett (unpub. data)
0.13) and a postore dike (6.83 0.07 Ma) constrain the lateMiocene age of mineralization (Sasso and Clark, 1998). TheP2 and the Early P3 Porphyries are the two distinct intrusionshosting the bulk of the economic copper-gold mineralization,which was introduced by at least two separate magmatic-hy-drothermal fluid pulses into a dense network of quartz mag-netite-chalcopyrite veinlets. The slightly later Quartz-EyePorphyry is overprinted by a stockwork of characteristic pinkquartz veins that are associated with intense feldspar-destruc-tive alteration in outcrops. These veins contain only minorbornite and chalcopyrite. At least four subsequent stages ofminor intrusion are weakly altered and essentially unmineral-ized, and all have much lower vein densities than the P2, theEarly P3, and the Quartz Eye Porphyries (Ulrich and Hein-rich, 2001; J. M. Proffett, writ. commun.; Table 1).
The alteration sequence is characterized by an early quartz-magnetite assemblage in the center of the deposit. This alter-ation is concentrated on the P2 Porphyry, but a secondweaker phase of this alteration also affects parts of the EarlyP3 Porphyry. The adjacent potassic alteration (K feldspar +biotite magnetite) is most intensely developed in the min-eralized porphyries but affects in a less intense form (sec-ondary biotite) some of the later intrusions, such as theNorthwest Porphyry dikes. This alteration is surrounded by apropylitic zone at the fringe of the deposit, overprintingmainly the andesitic volcanic rocks and some porphyry dikes.Concentric zones of quartz-magnetite, potassic, and propy-litic alteration of the mineralized stock and all later intrusionsare overprinted by feldspar-destructive alteration, which ad-vanced from the top of the deposit downward along steepcrosscutting structures and lobes. Mineralization is domi-nated by chalcopyrite and intimately intergrown gold. Oremineral precipitation is associated with potassic alteration andoccurs as disseminations in the porphyries and proximal an-desitic volcanic rocks and as texturally late phases in quartz magnetite veinlets. Economic Cu and Au concentrationsoccur in a high-grade ore shell above and around a barrencore of highly quartz-magnetite-altered but sulfide-poorporphyries.
Sample Material for Fluid Inclusion and Isotope StudiesSamples with veins and phenocrysts for petrographic, fluid
inclusion, and stable isotope studies were taken from originalsurface outcrops and the developing mine, but most sampleswere collected from diamond drill core (Appendices 1 and 2).The selection of samples was based on core logging and sur-face mapping of the main alteration zones (quartz-magnetite,potassic, propylitic, and feldspar-destructive alteration) andthe distribution of high Cu-Au grades. The vertical samplingextends to a depth of 550 m below the premining landsurface.
Vein densities and crosscutting relationships were mostclearly observed in the P2, Early P3, and the Quartz-Eye Por-phyries. Fluid inclusion and isotope data were derived fromsamples of the major vein and alteration types in these intru-sions. The Early P3 Porphyry is intimately associated with thesecond major pulse of mineralization and potassic and quartz-magnetite alteration. It provided the preferred and some ofthe best fluid inclusion material. Exceptionally well preservedfluid inclusions were also found in a 5- to 8-cm-thick vuggy
vein of coarse-grained purple quartz from a block of Quartz-Eye Porphyry in the southeastern part of the open pit (sam-ple BLA97). Euhedral crystals up to 1 cm contain inclusionassemblages in a particularly clear time sequence. Biotite in-clusions indicate that this quartz crystallized during potassicalteration. Pyrite and possibly anhydrite (now gypsum) weredeposited later in the vugs, associated with feldspar-destruc-tive overprint of the wall rock.
Fluid inclusion assemblages in the small quartz veinlets re-lated to feldspar-destructive and chlorite-epidote alterationwere inadequate for analysis. Suitable inclusions probably rep-resenting the feldspar-destructive stage were observed on sec-ondary trails in strongly feldspar destructively overprintedsamples, even though the quartz host in these samples belongsto veins of the potassic stage. Secondary trails are very com-mon in these samples and cut trails of primary or pseudosec-ondary inclusions preserved from the potassic veining stage.
For a reconnaissance stable isotope study, selected miner-als were separated from veins and whole-rock samples to ob-tain an enriched mineral fraction of 90 percent purity ormore. Fluid inclusion samples were also used for isotopicanalysis, as far as possible. Quartz phenocryst separates of theleast altered intrusion of the Northwest Porphyry were ana-lyzed to obtain an isotopic signature representative for themagmas. Coexisting quartz and magnetite from the quartz-magnetite alteration zone were used to estimate both the18O composition and the temperature conditions of the ear-liest alteration stage at Alumbrera. To estimate the oxygenisotope composition of fluids associated with the main alter-ation zones, we analyzed the quartz veins related to potassicalteration in three samples of the Early P3 and Quartz-EyePorphyries and quartz separates from late veinlets and matrixquartz associated with feldspar-destructive alteration. Sec-ondary biotite was analyzed for 18O in three wall-rock an-desites with intense potassic alteration, and two albite sam-ples were analyzed to represent the chlorite-epidote zone.
Hydrogen isotopes were determined on biotite of potassi-cally altered rocks, on chlorite of the chlorite-epidote zone,and sericite and/or clay fractions from samples with feldspar-destructive overprint. Hydrogen isotopes of fluid inclusionswere analyzed after thermal extraction of inclusion waterfrom veins related to the quartz-magnetite, the potassic, andthe feldspar-destructive alteration.
To obtain an estimate of the local meteoric water at thetime of hydrothermal activity, we analyzed quartz and fluidinclusion extracts from the post-sulfide stage of an epithermalvein of the Faralln Negro gold mine, located 15 km north-west from Alumbrera along the same structural trend(Malvicini and Llambias, 1963).
Petrography and Distribution of Fluid Inclusions
Fluid inclusion types
Petrography and all subsequent single inclusion measure-ments strictly concentrated on fluid inclusion assemblages,i.e., closely associated groups or trails of inclusions with visu-ally identical phase ratios and similar shape (Goldstein andReynolds, 1994). Six types of fluid inclusions were distin-guished according to the number, the nature, and the volumeproportions of phases present at room temperature (Fig. 1).
EVOLUTION OF PORPHYRY Cu-Au DEPOSIT, BAJO DE LA ALUMBRERA, ARGENTINA 1745
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FIG. 1. Photomicrographs of inclusion types and relative timing among assemblages. A. Polyphase brine inclusion withhematite and three unidentified daughter phases. B. Opaque-bearing brine inclusion characterized by one or two opaquedaughter crystals. C. Simple brine inclusions containing only a halite daughter crystal, which has a similar or slightly largersize than the vapor bubble. D. Two-phase aqueous inclusion (325 m) with a larger or similar proportion of liquid thanvapor. E. High-density vapor inclusions, commonly with a chalcopyrite crystal (1545 m). F. Low-density vapor inclusionscontaining more than 80 percent vapor. G. A low-density vapor trail is cut by a similar trail with small high-density inclusions(sample BLA97). H. Fluid inclusion relationship in a phenocryst that is crosscut by a quartz veinlet (stippled line shows thecontact). The small polyphase and opaque-bearing brine inclusions (I) occur exclusively in the phenocryst (see also Fig. 2A).J. and K. Trails of small polyphase brine inclusions are cut by later low-density vapor trails.
polyphase brine opaque-bearing brine brine
high-density low-density
aqueous
hematite
halite halite
halite
Low-densityvapor inclusions
High-densityvapor inclusions
chalcopyrite
Vein quartz
Low-densityvapor inclusions
Low-densityvapor inclusions
Polyphasebrine inclusions
Polyphasebrine inclusions
Quartz phenocryst
20m 50m
20m50m
20m 20m20m
20m 20m20m A B C
FED
G H
I
J K
Three inclusion types are halite saturated, generally haverounded isometric or negative crystal shapes, and are typically20 to 30 m in size. The polyphase brine inclusions contain adense packing of halite and up to five other transparent andopaque daughter phases (Fig. 1A). Sylvite crystals, alwayssmaller than the halite crystal, were only identified with cer-tainty in a small number of inclusions, as the only isotropiccrystal other than halite. Metallic phases including hematite(red, platy) and chalcopyrite (opaque, triangular) are invari-ably present in polyphase brine inclusions. The size of mea-sured inclusions ranges from 10 to 50 m. Some of thesehighly saline inclusions show minor variations in occurrenceand nature of daughter crystals even among clearly coeval in-clusions in an assemblage, probably due to non-nucleation ofinternal precipitates (non-nucleation; Roedder, 1984). Theopaque-bearing brine inclusions have only a halite crystal plusone or two opaque phases, one of which can be hematite. In-clusion size ranges from 5 to 40 m (Fig. 1B). The simplebrine inclusions are between 5 and 35 m large and contain ahalite cube as the only daughter crystal (Fig. 1C).
Aqueous inclusions have a high filling ratio (avg 80% liquidand 20% vapor) and occur as irregular-shaped to rounded in-clusions (Fig. 1D) in texturally late quartz crystals or as sec-ondary inclusions in earlier quartz. They rarely are larger than15 m.
Vapor-rich inclusions were divided, according to their fill-ing ratio, into high-density vapor inclusions (6090% vapor;Fig. 1E) and low-density vapor inclusions (generally >90%vapor; Fig 1F). Both types commonly reach 25 m in size andhave slightly rounded to perfect negative crystal shapes. Liq-uid occupying less than 20 percent is generally invisible dueto internal reflection of the inclusion walls, except in particu-larly large inclusions near protrusions. High-density vapor in-clusions often contain a small chalcopyrite crystal (Fig. 1E).No daughter crystals have been observed in low-densityinclusions.
Relative timing of fluid entrapment
The sequence of fluid entrapment was inferred from thedistribution of inclusion assemblages in veins of different par-agenetic stages (Ulrich and Heinrich, 2001), from petro-graphic observations of crosscutting trails of inclusions alongindividual fractures (Fig. 1G, J, K), or rare primary inclusionsor pseudosecondary trails that can be attributed to specificgrowth zones in euhedral vein crystals (Fig. 1H, I; see Roed-der, 1984; Goldstein and Reynolds, 1994). However, the com-monly random distribution and superimposition of numerousinclusions in most samples made the determination of relativetiming between inclusion assemblages very difficult.
Veins related to quartz-magnetite and potassic alterationphases of the P2 and Early P3 Porphyries have indistinguish-able inclusion populations, and we infer (but cannot unam-biguously prove) that the fluid history of the two main miner-alizing pulses was similar. Rare crosscutting relationshipsbetween inclusion trails indicate that the polyphase brine in-clusions are earlier than the other brine inclusion types, andthat trails containing only polyphase brine inclusions are ear-lier than trails containing simultaneously trapped polyphaseand vapor inclusions. No direct crosscutting relationships in-volving simple brine inclusions were observed, but these are
considered later than the polyphase and opaque-bearingbrine inclusions, because of their generally lower homoge-nization temperature in individual samples.
Vapor inclusions occur as trails and clusters and low-, aswell as high-density, types were found together withpolyphase brine inclusions in boiling assemblages. These aresingle trails containing vapor- as well as liquid-rich inclusions,trapped from a two-phase fluid in one microfracture. The for-mer coexistence of brine and vapor is unequivocally demon-strated by such boiling trails, but polyphase brine and vaporinclusions are more commonly randomly distributed in largenumbers in the core of quartz crystals. The possible coevaltrapping of such assemblages is difficult to prove. Some as-semblages of high-density vapor inclusions in phenocrystsand veins show early and probably primary character, whereaslow-density vapor inclusions generally occur on sharp sec-ondary trails cutting through entire grains, or in parageneti-cally late veins, indicating a later stage of entrapment.
Some samples from the Quartz-Eye Porphyry have pre-served a particularly clear record of fluid entrapment, and in-clusion assemblages from sample BLA97 (Fig. 2B) havetherefore been singled out in the subsequent data presenta-tion. Quartz phenocrysts contain a generation of small (10m) polyphase brine inclusions with one or more transparentdaughter crystals and a prominent red hematite flake. Thesame type of inclusion was also found together with rare high-density vapor inclusions (Fig. 1E) in a primary growth zone inthe core of the euhedral quartz crystal in the vuggy vein. Thecrystal records a complex fluid history starting with primaryor early pseudosecondary polyphase brine inclusions withoutcoexisting vapor, followed by later polyphase brine inclusionswith high-density vapor inclusions. Subsequent pseudosec-ondary polyphase brine plus high- and low-density vapor in-clusions are associated with biotite precipitation, before thecrystal is rimmed by a thin final growth zone containing ir-regular aqueous and low-density vapor inclusions. The latterfluids slightly predate or are coeval with the precipitation ofpyrite in the residual vug and probably with feldspar-destruc-tive alteration of the wall rock.
Aqueous inclusions are generally late relative to all otherliquid-rich inclusions and to all phases of quartz magnetitestockwork veining. They were found in late anhydrite veinsand as secondary inclusions in earlier quartz veins occurringin samples with an intense feldspar-destructive overprint.Here they commonly occur together with low-density vaporinclusions but never in clear boiling assemblages.
Deposit-scale distribution of fluid inclusions
The distribution of the different inclusion types on thescale of the deposit was studied in an attempt to delineate therelationships between fluids, alteration, and ore deposition.The relative abundance of inclusions was visually estimatedfor each type of inclusion in a total of 38 polished sections(Table 2). The schematic section (Fig. 3) shows the ore-gradedistribution and alteration zones in section 47, with additionalsample and alteration data projected from neighboringsections (see Ulrich and Heinrich, 2001; J.M. Proffett, writ.commun.).
Halite-saturated inclusions generally exceed 50 percent ofthe inclusions in samples from the central part of the deposit.
EVOLUTION OF PORPHYRY Cu-Au DEPOSIT, BAJO DE LA ALUMBRERA, ARGENTINA 1747
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The samples with the largest numbers of polyphase andopaque-bearing brine inclusions are associated with the low-grade core and the quartz-magnetite, as well as the potassic,alteration. Simple brine inclusions are not very abundant andshow no association with the alteration zones nor any correla-tion with ore grades.
Toward the fringe of the deposit, polyphase and opaque-bearing brine inclusions diminish and vapor-rich inclusions,invariably associated with aqueous inclusions, become pre-dominant. This zoning is correlated with the feldspar-de-structive overprint of some of these samples. Aqueous inclu-sions throughout the deposit are secondary and clearly belongto the incipient feldspar-destructive overprint in many sam-ples. Petrography and abundance distribution of the differentinclusion types possibly indicate a trend of decreasing salinityfrom the center to the periphery of the deposit and from earlyto late fluid stages in any one sample. The salinity and tem-perature distribution is further quantified in the followingsection on the basis of microthermometric measurements.
Microthermometry
Experimental approach and evaluation of measurements
Heating and freezing experiments were conducted on aLinkam THMS600 stage for homogenization temperature upto 600C and on a Linkam TS1500 stage for temperatures
above 600C. The precision for freezing runs is about 0.1Cand for heating runs 2C at 500C. The uncertainty for high-temperature measurements with the Linkam TS1500 stagemay be up to 20C. Apparent salinities are reported in weightpercent NaCl equivalent, based on the halite solubility equa-tion for halite-saturated inclusions and on the final ice melt-ing for halite-undersaturated inclusions (Bodnar and Vityk,1994). Minor CO2 detected by laser Raman microprobe insome of the high-density vapor inclusions may cause errors inthe calculation of salinities, but clathrates were generally ab-sent or minor in abundance at the temperature of final icemelting (see Table 3 for abbreviations).
Microthermometric results reported in Tables 4, 5, 6, and allfigures strictly refer to fluid inclusion assemblages. The salinitywithin a single assemblage is quite uniform (typically 3 wt %NaCl equiv) but varies greatly between different assemblages.
Freezing measurements and heating experiments up to thetemperature of halite dissolution were carried out before LA-ICP-MS analysis to obtain the apparent salinities needed forinternal standardization of elemental analyses (see below).Final homogenization above 600C led to stretching and de-crepitation of some inclusions. Limited high-temperaturehomogenization data were therefore measured with inclu-sions remaining in the same assemblage after LA-ICP-MS oron similar assemblages from other parts of the same section.Average homogenization temperatures of these were then
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biotite
pyrite
opaque-bearingbrine inclusions
primary and pseudo-secondary? polyphasebrine inclusions
high-densityvapor inclusions
polyphasebrine inclusions
high-densityvapor inclusions
small polyphase andopaque-bearing brineinclusions,somehigh-densityvapor inclusions
growth zone with irregularsmall aqueous andvapor-rich inclusions
low-and high-densityvapor inclusions
low-densityvapor inclusions
1 mm
vein quartz
phenocryst
magnetite
aqueousinclusions
high-densityvapor inclusions
secondary polyphasebrine inclusions
polyphase brine andhigh-density vaporinclusions
low-densityvapor inclusions andpolyphase brine inclusions
primary? polyphase andopaque-bearing brineinclusions with hematite
polyphase andhigh-densityvapor inclusions
high-densityvapor inclusions and opaque-bearing brine inclusions
heterogeneoustrapping of vapor-richand polyphasebrine inclusions
high-densityvapor inclusions
polyphase brineinclusions andlow-densityvapor inclusions
1 mm
Sample 45-61.3 S Sample BLA 1/97 A, II
FIG. 2. A. Sketch of a phenocryst cut by a veinlet (sample 45-61.3 S). Small, hematite-bearing inclusions (smallest dots)are restricted to the phenocryst, whereas later generations of polyphase and opaque-bearing brine inclusions also occur inthe vein. B. Simplified drawing of crystals from sample BLA97 showing the relative timing relationships of saline and vaporinclusions. The polyphase and opaque-bearing brine inclusions occur in the core, whereas the vapor inclusions are on trailsand commonly along the outer zone of the crystal.
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Tabl
e 2.
Dis
trib
utio
n of
Flu
id I
nclu
sion
s in
Sam
ples
Use
d in
Thi
s St
udy
Vapo
r in
clus
ions
Sam
ple
Dep
thO
re g
rade
sO
paqu
eH
igh-
Low
-ID
(m
bel
ow
Cu
Au
Poly
phas
ebe
arin
gB
rine
Aqu
eous
dens
ityde
nsity
(Fig
. 3)
Dri
ll ho
lesu
rfac
e)(w
t %)
(ppm
)Po
rphy
ry ty
pe a
nd a
ltera
tion
(%)
(%)
(%)
(%)
(%)
(%)
AK
51-6
1.1/
652
40.
020.
03Q
uart
z-m
agne
tite,
P2
Porp
hyry
1025
3510
20A
M51
-61.
1/11
547
0.01
0.02
Qua
rtz-
mag
netit
e, P
2 Po
rphy
ry10
2560
5J
47-5
5.3/
1014
71.
401.
95Q
uart
z-m
agne
tite,
P2
Porp
hyry
405
510
40A
J51
-61.
1/5
523
0.02
0.03
Qua
rtz-
mag
netit
e w
ith c
alci
te fi
lling
, P2
Porp
hyry
2525
1010
30A
45-6
1.3/
1342
81.
402.
43Po
tass
ic w
ith q
uart
z-m
agne
tite,
P2
Porp
hyry
B35
105
4010
H47
-55.
3/7
110
0.50
0.78
Pota
ssic
, P2
Porp
hyry
5015
35I
47-5
5.3/
914
61.
401.
95Po
tass
ic, P
2 Po
rphy
ry25
105
1050
AF
51-5
2.2
/920
40.
010.
02Po
tass
ic, P
2 Po
rphy
ry60
55
525
AG
51-5
2.2
/11
211
0.04
0.09
Pota
ssic
, P2
Porp
hyry
505
1515
510
AH
51-5
2.2/
1321
20.
040.
09Po
tass
ic, P
2 Po
rphy
ryB
5020
1515
AI
51-5
2.2/
1422
40.
110.
11Po
tass
ic, P
2 Po
rphy
ry45
1510
2010
D47
-53/
4822
60.
560.
40Po
tass
ic, P
2 Po
rphy
ry10
255
305
25M
49-5
0.1/
435
70.
090.
04Po
tass
ic, P
2 Po
rphy
ryB
5515
1515
N49
-50.
1/8
371
0.05
0.05
Pota
ssic
, P2
Porp
hyry
4010
2030
AL
51-6
1.1/
853
70.
010.
07Po
tass
ic, P
2 Po
rphy
ry25
1045
20
AE
50.3
-59.
4/8
146
0.74
1.28
Qua
rtz-
mag
netit
e, E
arly
P3
55
525
60F
47-5
3/72
413
0.51
0.75
Qua
rtz-
mag
netit
e, E
arly
P3
1030
1050
R49
-52/
322
60.
050.
14Q
uart
z-m
agne
tite,
Ear
ly P
3B
1530
1510
1020
V49
-52/
1933
40.
100.
06Po
tass
ic w
ith q
uart
z-m
agne
tite,
Ear
ly P
35
1015
6010
B47
-53/
2511
80.
060.
10Po
tass
ic, E
arly
P3
B40
2025
510
C47
-53/
3012
80.
100.
17Po
tass
ic, E
arly
P3
555
205
15E
47-5
3/62
311
0.30
0.23
Pota
ssic
, Ear
ly P
310
4030
155
T49
-52/
1026
00.
080.
06Po
tass
ic, E
arly
P3
1050
515
515
W49
-52/
2133
70.
050.
05Po
tass
ic, E
arly
P3
B15
1045
525
L49
-50.
1/1
354
0.05
0.02
Pota
ssic
, Ear
ly P
3B
1525
355
515
Y49
-61.
4/16
277
0.58
0.80
Fel
dspa
r de
stru
ctiv
ely
over
prin
ted,
Ear
ly P
330
510
55Z
49-6
1.4/
1928
31.
401.
94F
elds
par
dest
ruct
ivel
y ov
erpr
inte
d, E
arly
P3
515
1010
60A
D50
.3-5
9.4/
689
0.72
2.12
Fel
dspa
r de
stru
ctiv
ely
over
prin
ted,
Ear
ly P
310
55
525
50A
A49
.2-4
6.3/
427
80.
800.
53F
elds
par
dest
ruct
ivel
y ov
erpr
inte
d, E
arly
P3
2020
55
2525
AB
49.2
-46.
3/5
279
0.80
0.53
Fel
dspa
r de
stru
ctiv
ely
over
prin
ted,
Ear
ly P
320
2060
AC
49.2
-46.
3/8
292
0.77
0.71
Fel
dspa
r de
stru
ctiv
ely
over
prin
ted,
Ear
ly P
315
1020
55S
49-5
2/9
242
0.06
0.05
Slig
htly
feld
spar
des
truc
tivel
y ov
erpr
inte
d, E
arly
P3
B
3025
105
1020
U49
-52/
1431
50.
140.
16Sl
ight
ly fe
ldsp
ar d
estr
uctiv
ely
over
prin
ted,
Ear
ly P
35
1015
505
15O
49-5
1/1
182
0.94
1.55
Slig
htly
feld
spar
des
truc
tivel
y ov
erpr
inte
d, E
arly
P3
6010
30P
49-5
1/3
184
0.85
1.50
Slig
htly
feld
spar
des
truc
tivel
y ov
erpr
inte
d, E
arly
P3
1015
530
40Q
49-5
1/5
192
0.91
1.48
Slig
htly
feld
spar
des
truc
tivel
y ov
erpr
inte
d, E
arly
P3
355
2040
AO
BL
A 9
7Su
rfac
eF
elds
par
dest
ruct
ivel
y ov
erpr
inte
d,
Qua
rtz-
Eye
Por
phyr
yB
5015
520
10A
NB
LA
18
Surf
ace
Qua
rtz
phen
os in
feld
spar
des
truc
tivel
y ov
erpr
inte
d,
Qua
rtz-
Eye
Por
phyr
y10
55
550
25
Sele
cted
sam
ples
that
wer
e us
ed to
del
inea
te th
e di
stri
butio
n of
the
diff
eren
t inc
lusi
on ty
pes
in th
e de
posi
t; th
e ab
unda
nce
of e
ach
incl
usio
n ty
pe w
as e
stim
ated
opt
ical
ly fr
om d
oubl
e-po
lishe
d se
c-tio
ns; s
ampl
e ID
ref
ers
to th
e fr
eque
ncy
diag
ram
s of
Fig
ure
3; B
= s
ampl
e w
ith b
oilin
g as
sem
blag
e
assumed for the analyzed inclusions. Measuring several as-semblages of identical petrographic appearance within onesection generally showed consistent homogenization temper-atures, indicating that this assumption is valid.
Results
The highest temperature was measured in an assemblage ofopaque-bearing brine inclusions that homogenize to the liq-uid at 771 16C (assemblage 92, Table 4). In general, high-est homogenization temperatures of polyphase brine and
opaque-bearing brine inclusions are around 710 to 730C(Figs. 4A, 5F, G). Among the simple brine inclusions, only theassemblage of sample BLA97 homogenized above 600C, at671 6C (assemblage 104). Two high-temperature boilingassemblages (assemblages 114, 115) showed consistent ho-mogenization of the polyphase brine inclusions at 720C, andthis value was adopted for assemblages 113 and 117, whichdid not homogenize below the upper temperature limit of theheating stage (600C). Homogenization of the polyphase,opaque-bearing, and brine inclusion assemblages occurs ei-ther by halite melting or to liquid (Tm(halite) > Th(L) or Th(L)).
Polyphase brine inclusions: The data for all polyphase brineinclusions in Figure 5A and F show a wide range in homoge-nization temperature and salinity values with a main mode at400 to 500C and a consistent high-temperature mode above700C, which mainly represents the assemblages of sampleBLA97. The assemblage with the lowest Th(L) homogenizes at432 6C and has a salinity of 47 1 wt percent NaCl equiv,whereas the assemblage (62) with the highest salinity (71 1wt % NaCl equiv) homogenizes by halite dissolution (Tm(halite)> Th(L) = 580 3C). Within some assemblages, inclusions
1750 ULRICH ET AL.
0361-0128/98/000/000-00 $6.00 1750
TABLE 3. Nomenclature of Fluid Inclusion Measurements
Th(L) Final homogenization temperature to liquidTm(halite) > Th Final homogenization by halite dissolutionTh(V) Final homogenization to vaporTm(ice) Last ice melting temperatureTm(CO2) Melting temperature of CO2Tm(halite) Melting temperature of halite
TABLE 4. Microthermometry Data and Pressure Estimates
Salinity (wt % Th(L) and Th(halite) Basis for P Assemblage Sample NaCl equiv) (C) P (bars) estimate
21 49-52 19A 3 0.3 240 0.3 50 LV curve4 46-59QZE 7.1 4.2 242 2.6 50 LV curve
20 49-52 14 1.4 0.3 271 20.7 80 LV curve25 51-61.1 II,5/1 17.7 0.1 292 10.0 80 LV curve
110 49-52 3/3 35 0.5 305 5.0 80 Boiling curve100 49-52, 19B/3 41.4 3.9 368 39.6 2810/1501 Halite disappearance101 49-52, 19b/3 IV 42.1 3.7 387 43.2 150 LV curve106 45-61.3, 13/1 43.9 2.8 397 6.5 180 Boiling curve108 49-50,1, 1/8 41 4.7 400 14.1 200 Boiling curve111 49-52 3/3 48.4 4.0 419 26.2 250 Boiling curve109 49-50,1, 4/6 46 3.8 421 1.0 220 Boiling curve73 47-53, 62/1 44 0.3 423 22.5 220 LV curve
112 49-52,21/3 48.4 6.0 448 3.5 270 Boiling curve54 49-51, 5/5 50.9 4.3 463 19.0 320 LV curve
116 BLA1/97A I,2 52.7 3.0 470 14.1 320 Boiling curve58 51-61.1, 5/ 3 49.9 1.8 471 18.4 340 LV curve59 51-61.1, 5 IV 50 0.6 499 12.6 380 LV curve
107 47-53/25B 61.7 2.5 530 0.3 370 Boiling curve57 51-61.1 5 I 45.1 3.3 540 69.1 530 LV curve62 BLA1/97B,2 71.1 0.2 580 3.0 1135/3801 Halite disappearance47 47-53 25B 61.4 2.9 6652 620 LV curve
104 BLA1/97B I,1 36.9 2.8 671 6.0 1100 LV curve113 51-52,2, 13 52 0.4 7203 1050 Boiling curve117 BLA1/97B,2 65.5 2.1 7203 680 Boiling curve115 BLA1/97A,1 63.8 1.9 7202 750 Boiling curve114 BLA1/97A,1 58 1.1 7202 900 Boiling curve61 BLA1/97B2 66.1 3.6 7222 680 LV curve67 BLA IV 97, 3/1 64 1.6 7223 750 LV curve66 Avg BLA IV 97, 3 62 1.2 7223 800 LV curve68 BLA IV 97, 2/6 56 4.0 7223 950 LV curve93 BLA2/97 B2 53 1.5 7302 1080 LV curve91 BLA 1/97B II,1 36.9 2.8 7303 1280 LV curve92 BLA2/97B2 46.5 3.0 771 16.3 1320 LV curve
Microthermometric data and pressure estimate of boiling assemblages and single-phase assemblages that are shown in Figure 6 and which were subse-quently analyzed by LA-ICP-MS; sorting of the table is based on increasing homogenization temperatures (roughly corresponding to a sequences of late toearly assemblages); pressures for nonboiling assemblages are minimum pressures
1 Pressure calculated according to Bodnar (1994) for halite homogenizing inclusions and the assumed pressure, respectively (see text for explanation)2 Single analysis due to decrepitation of inclusions from different assemblages3 Extrapolated values from measurements of similar but different assemblages (see text for explanation)
EVOLUTION OF PORPHYRY Cu-Au DEPOSIT, BAJO DE LA ALUMBRERA, ARGENTINA 1751
0361-0128/98/000/000-00 $6.00 1751
47-51.2
47-5347-55
47-55.347-62
ENEWSW49-.2-46.3 49-50.1
49-52
49-51
51-52.2
50.3-59.4
49-61.4
51-61.1
45-61.4
Deposit-scale distribution of fluid inclusion types
m a.s.l. GypsumFault
Ste
ves
Fault
potassic high-gradeore zone
low-gradecore
coexisting polyphasebrine andvapor inclusions
polyphasebrine incl.
opaque-bearingbrine incl.
aqueous incl. brine incl.
low-densityvapor incl.
high-densityvapor incl.
2600
2400
2300
2200
strong K-feldspar-biotite alterationmoderate andweak K-feldspar-biotite alterationsecondarybiotite alterationweak secondarybiotite alteration inlate porphyries
late feldspar-destructivealteration
quartz-magnetitealteration
chlorite-epidotealteration
Cu-grade contour0.5 wt%approximateboundaries ofmain porphyriesagainst surroundingandesites
average abundance
AA
AB
AC
O
P
Q
AOAN
AJ
B
C
HAD
AE
Z
I
J
Y
AF
AI
D
N
EW
U
S
A
F
AM
AL
R
AH
AG
L
AK
M
qtz-magalteration
13 samples22 samples 16 samples9 samples 16 samples
?
?
?
feldspar-destructive
2500
?
?
FIG. 3. Schematic illustration of section 47through the deposit, with alteration and ore-gradezones from Ulrich and Heinrich (2001) and pie di-agrams showing the relative abundance of the in-clusion types in single samples. Samples with boil-ing assemblages are indicated by white centers,and ticks show the coexistence of low- or high-den-sity vapor inclusions with polyphase brine inclu-sions. Key shows average overall abundance of thedifferent inclusion types in the low-grade core (22samples), quartz-magnetite alteration (9), potassicalteration (16), the high-grade ore zone (16), andthe feldspar destructively altered parts of the de-posit (13). Letters in the pie diagrams correlatewith sample ID of Table 2.
100
200
300
400
500
800100 200 300 400 500 600 700
Boilingassemblages
105
57
78
93
115/117
61/6465
66/67
6970
7172
114
104
92
89
84
90
56
5859 96
48 50
107
91
113
5376102
108/11197
75110
98
88
77
74
100
87
82
83
106
103
6051
101112
109/52
94
73
55
85
62
79
116/54
68
47
49
8663
95
99
80
B
900
81
Tm h
alite
(C
)
Th L (C)
100 200 300 400 500 600 700 800 900
Aqueous inclusions
High density vapor inclusions
Brine inclusions
Opaque-bearingbrine inclusions
Polyphase brine inclusions
calcite
anhydrite
0
10
20
30
40
50
60
70
80
anhydrite
7172
6970
113
91105 104
5662
64
65-68
92
7858
57
79
60
89
4847
50
49
93114
115/117
90
84
43-46
116
5376
59 96
80
88102
108/11197
75110
98
103 85106
101 52
81-83/871165155
86
95/54
107
39
37
34
30
3828/29
41/423640
33
6
31/32
12
11/18
725
26
27
5
5a
21714
161
3 8
4
20
22
2413
21
9
10/15/19
23
63
7477
94
99
100 109
A
61
Sal
inity
(wt%
NaC
l equ
iv.)
Th L or by halite melting (C)
112
FIG. 4. A. Salinity vs. final homogenization temperature (to liquidor by halite melting) for all inclusion types of Alumbrera. Enclosinglines represent single assemblages from different samples. Not alltemperatures shown above 600C have been explicitly measured,some are adopted from similar assemblages (see Table 4) and slightlyshifted for clearer presentation. Stippled lines connect coexistingvapor and liquid inclusions of boiling assemblages. The gap in salin-ity around 25 to 35 wt percent NaCl equiv may result from non-nu-cleation of halite (see Roedder, 1984) or may be the artifact of devi-ations of the real fluids from the NaCl-H2O model system used forthe salinity estimate (Vityk and Bodnar, 1998). B. Halite dissolutiontemperatures as a function of vapor-liquid homogenization temper-ature to liquid for the halite-saturated inclusion types. Inclusionsplotting above the line undergo final homogenization by halite dis-solution, whereas inclusions below the line homogenize to liquid.Numbers next to the labels refer to the measured assemblages.
homogenize both to liquid and by halite disappearance over asmall temperature interval (Fig. 4B). The exceptionally highhomogenization temperatures could indicate some H2O lossdue to postentrapment reequilibration (Sterner et al., 1995;Audtat and Gnther, 1999), but no textural evidence for sucha process was observed in this sample. Even these extremevalues are probably real fluid temperatures, comparable tothose reported by Eastoe (1978) for the Panguna porphyrydeposit, Bougainville.
Melting temperatures of the daughter crystals other thanhalite were observed at 100 to 150C for the first unknownphase, which is rounded, weakly birefringent, and has a rela-tively low relief. After initial melting it reprecipitates oncooling to 90C as an irregular aggregate with strong bire-fringence. The second daughter phase is orthorhombic,weakly birefringent with a moderate relief, and it melts be-tween 200 to 250C. The third phase is small, isometric,round, and birefringent (erythrosiderite?; Wilson et al., 1980)and it melts at 320 to 370C. A small cubic daughter crystal
(sylvite?) melts at 460 to 490C always before halite (e.g., as-semblage 67). Such high melting temperatures are rarely re-ported for sylvite but were previously found in the Endeavour26 North porphyry Cu-Au deposit at Parkes, Australia (Hei-thersay and Walshe, 1995), implying very high K/Na ratios upto 1.1 by weight (Roedder, 1971; Cloke and Kesler, 1979).
In polyphase brine and opaque-bearing brine inclusions,the opaque phases never dissolve even up to 800C. The com-mon presence of hematite daughter crystals in hematite-freerocks indicates H2 diffusion out of the inclusions (Mavro-genes and Bodnar, 1994). Rare, birefringent rectangulardaughter crystals (anhydrite?) also did not dissolve to at least600C, which also may be related to postentrapment oxida-tion of sulfur.
Opaque-bearing brine inclusion: These assemblages aresimilar in salinity and homogenization temperatures topolyphase brine inclusions (Figs. 4A and 5B, G). Apart fromcommon assemblages homogenizing to liquid, there are alsoassemblages in which inclusions homogenize by halite disso-
1752 ULRICH ET AL.
0361-0128/98/000/000-00 $6.00 1752
TABLE 5. Summary of the Microthermometric Data and Estimated Pressures of the Different Inclusion Types
Description of assemblages Microthermometry data Pressure estimate Deposit-scale distribution
Polyphase brine assemblages: Highest avg Th (L) ~722C Highest P(min) = 950 bars Abundant in low-grade core, Randomly distributed inclusions Lowest Th(L) = 430C Lowest P(min) = 350 bars quartz-magnetite and potassic of primary character; secondary Salinity range 40 to 71 wt alteration trails in phenocrystals and veins % NaCl equiv are cut by boiling assemblages; assemblages homogenizing by halite melting are later than those homogenized to liquid
Boiling assemblages: Commonly Highest avg Th(L) ~720C Highest P(min) = 1,050 bars Low-grade core and high-grade ore secondary trails; coexistence of Lowest Th(L) = 300C Lowest P(min) = 80 bars zone, from present surface to at least polyphase and vapor-rich Salinity range 35 to 65 wt 430 m deeper inclusions % NaCl equiv
Opaque-bearing brine Highest avg Th(L) ~771C Highest P(min) = 1,320 bars Dominant in low-grade zone inclusions: Randomly distributed (one assemblage 92) Lowest P(min) = 220 bars high-grade ore zone inclusions of primary character in Lowest Th(L) = 360Cphenocrystals; assemblages Salinity range 37 to 63 wt homogenizing by halite % NaCl equiv melting are later
Brine assemblages: Highest avg Th(L) ~671C Highest P(min) = 1,100 bars Subordinate in low-grade as well as Only small assemblages, (one assemblage 104) Lowest P(min) = 150 bars in high-grade zone trails or randomly distributed Lowest Th (Tm (halite)) = 336C
Salinity range 36 to 46 wt % NaCl equiv
Aqueous assemblages: Highest avg Th(L) = 328C P(min) 600C Highest P(min) = 760 bars High-grade ore zone, feldspar-Secondary trails cutting polyphase Lowest Th(V) ~370C Lowest P(min) = 280 bars destructive overprint, minor in brine trails; randomly distributed Salinity range 1 to 19 wt low-grade zone inclusion of primary character % NaCl equiv
Low-density vapor assemblages: Feldspar-destructive overprint, Primary on late growth zones peripheral areas of the depositrelated to feldspar-destructive alteration; secondary trails cut polyphase brine trails
EVOLUTION OF PORPHYRY Cu-Au DEPOSIT, BAJO DE LA ALUMBRERA, ARGENTINA 1753
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TAB
LE
6. E
lem
ent C
once
ntra
tions
of S
elec
ted
Ele
men
ts in
Flu
id I
nclu
sion
s (a
naly
zed
by L
A-I
CP-
MS)
Ass
em-
Type
of
Tm
(hal
ite)
Th(
L)
Salin
ity w
t %
Li
Na
K
Mn
Fe
Cu
Zn
As
Rb
Sr
Mo
Ag
Sn
Cs
Ba
La
Ce
W
Tl
Pb
Au
blag
ein
clus
ion
Petr
ogra
phic
des
crip
tion
or T
m(ic
e)(
C)
NaC
l equ
iv(p
pm)
(wt%
)(w
t%)
(wt%
)(w
t%)
(wt%
)(w
t%)
(ppm
)(p
pm)
(ppm
)(p
pm)
(ppm
)(p
pm)
(ppm
)(p
pm)
(ppm
)(p
pm)
(ppm
)(p
pm)
(ppm
)(p
pm)
92O
paqu
e-be
arin
g Q
uart
z-E
ye P
orph
yry,
str
ong
390.
577
1.5
46.5
120
10.6
5.30
1.10
8.60
0.01
0.60
5038
060
140
4058
020
9015
02
6
0037
.980
5.00
7.10
1.30
13.2
00.
030.
60n.
a.24
060
n.a.
10n.
a.20
490
6
0053
.39.
200.
0240
1754 ULRICH ET AL.
0361-0128/98/000/000-00 $6.00 1754
57Po
lyph
ase
P2 P
orph
yry,
cle
ar q
uart
z in
vug
37
6.5
540.
845
.111
06.
5010
.20
3.20
10.5
00.
320.
6010
390
5080
n.a.
n.a.
20
60n.
a.5
4n.
a.15
00
brin
e in
clus
ions
with
late
cal
cite
, sec
onda
ry tr
ail
31
.4
69.1
3.
3
20
0.56
0.
69
0.45
0.
96
0.16
0.
10
3
60
9
30
5
10
4
2
100
59Po
lyph
ase
P2 P
orph
yry,
cle
ar q
uart
z in
vug
43
9.0
499.
051
.919
.80
0.10
14
0
EVOLUTION OF PORPHYRY Cu-Au DEPOSIT, BAJO DE LA ALUMBRERA, ARGENTINA 1755
0361-0128/98/000/000-00 $6.00 1755
35H
igh-
dens
ity
Ear
ly P
3 Po
rphy
ry, m
oder
ate
-6.3
39
5.0
9.5
720
3.00
1.60
0.15
2.9
3.00
0.
68n.
a.70
lution or by near-simultaneous vapor and halite disappear-ance (Fig. 4B). Groups of inclusions (e.g., assemblage 91-93)homogenizing above 600C were found exclusively in sampleBLA97. The assemblage with the lowest Th(L) (363 42C)has a salinity of 38 1 wt percent NaCl equiv. The assem-blage with the highest salinity (63 4 wt % NaCl equiv) ho-mogenizes by halite dissolution at 524 25C.
Brine inclusion: Most assemblages homogenize to liquid,and their salinity is near the lower end of the salinity range ofopaque-bearing brine inclusions (Figs. 4A and 5C). Homoge-nization temperature is distinctly lower compared to thepolyphase and opaque-bearing brine inclusions, mostlyaround 340 to 400C. Assemblage 100 shows halite meltingat temperatures much higher than vapor to liquid homoge-nization temperatures, which may indicate trapping at highpressure (Bodnar, 1994; Cline and Bodnar, 1994) or someotherwise unrecognizable postentrapment modification. Theassemblage (104) with the lowest salinity (37 3 wt % NaClequiv) has the highest homogenization temperature of 671 6C, whereas the assemblage (101) with the highest salinity(42 4 wt % NaCl equiv) homogenizes at 387 43C.
Aqueous inclusions: Within a few groups of simple two-phase inclusions, hydrohalite was observed as the last meltingphase in individual inclusions (assemblages 5, 5a, 22, Fig. 4A).
This phase showed metastable behavior as described byRoedder (1984), melting above 0C (in one case up to 9.3C).The brown color of the ice in some of the aqueous inclusionsis typical for a CaCl2-bearing fluid (Shepherd et al., 1985, p109). In assemblages 4, 5, 11, and 18 of feldspar destructivelyaltered Quartz-Eye Porphyry as well as potassically altered P2Porphyry and Early P3 Porphyry, unusually large salinity vari-ations (e.g., up to 8 wt % NaCl equiv) can occur within oneassemblage of aqueous inclusions with constant filling ratiosand homogenization temperatures, suggesting mixing be-tween fluids of different salinity.
Limited microthermometry was carried out on aqueous in-clusions in calcite and anhydrite (Fig 4A, assemblages 1-3).The homogenization temperature of assemblages in anhydriteis 231 22C. The salinity of the aqueous inclusions is veryvariable and ranges from 7 5 wt percent NaCl equiv in oneassemblage to 15 9 wt percent NaCl equiv in another as-semblage. Aqueous inclusions in calcite are very rare and onlya few were suitable for heating and freezing experiments.They have very low salinity with a melting point depression of0.2 to 0.3C, which corresponds to a salinity of 0.4 to 0.5wt percent NaCl equiv. Inclusions in calcite have the lowesthomogenization temperatures measured at Alumbrera (Th(L)= 204 17C). These data are in agreement with the occur-rence of calcite in the latest stage of the vein paragenesis. Thelarger variations in homogenization temperature and salinityof aqueous inclusion assemblages in anhydrite than assem-blages in quartz is possibly due to the lower stability of anhy-drite and/or stretching of the inclusion during heating andfreezing runs.
High-density vapor inclusions: Due to the small amount ofliquid and the difficulty of seeing any phase transitions inhigh-density vapor inclusions, the determination of the ho-mogenization temperature may vary several tens or even hun-dreds (150300C) of degrees (Bodnar et al., 1985). There-fore, homogenization temperatures shown in Figures 4A and5J have a larger range and may be severely underestimated.Similarly, ice melting as a measure of apparent salinity is dif-ficult, and results vary considerably, probably because ofminor coentrapment of coexisting brine. The lowest salinitywithin one assemblage was measured at 1.6 0.7 wt percentNaCl equiv and the highest apparent salinity was 19 2 wtpercent NaCl equiv. For most boiling assemblages, the salin-ity of low- and high-density vapor inclusions had to be in-ferred from the coexisting polyphase brine inclusions, usingthe NaCl-H2O phase diagram (Sourirajan and Kennedy,1962; Urusova, 1975; Bodnar et al., 1985). High-density vaporassemblages, containing inclusions with and without opaquedaughter crystals, show the same salinity and homogenizationtemperature as assemblages where only opaque-bearingvapor inclusions occur.
In a few assemblages (31-34), clathrates were observedduring freezing runs, which indicate the presence of someCO2. Melting of CO2 occurred at Tm(CO2) = 56.1C, andclathrate melting could only be measured in one assemblageat +2.4C. The total CO2 content is probably well below 3 molpercent, the lower limit for visible liquid CO2 (Nash, 1976).
Low-density vapor inclusions: These inclusions providedno information during microthermometric experiments. Nophase changes were observed during freezing.
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05
10
15
20
25
0
5
10
15
20
0
10
20
30
40
05
10
15
20
25
05
1015202530
05
1015
20
25
02468
1012
Th L and Tm halite>Th L (C)Salinity (wt% NaCl equiv.)
Opaque-bearingbrine inclusions
Brine inclusions
High-density vapor inclusions
0
5
10
15
20Polyphase brine inclusions
0 10 20 30 40 50 60 70
05
1015202530
Aqueous inclusions
A
B
C
E
F
G
H
J
ID
102030405060
800200 300 400 600 700500100
0
FIG. 5. Frequency diagrams of salinity and homogenization temperature.The modes at high homogenization temperatures of the polyphase brine andopaque-bearing brine inclusions are real and mainly from high-temperaturefluids trapped in the vein from the Quartz-Eye Porphyry (sample BLA97).
Pressure Estimation and P-T-XNaCl Evolution
Given appropriate experimental data, a unique estimate oftemperature (T) and pressure (P) conditions can be deter-mined from a boiling assemblage (e.g., Roedder and Bodnar,1980). All other inclusion data provide information aboutminimum pressures and temperatures only. The occurrenceof boiling assemblages as well as brine assemblages homoge-nizing by halite dissolution requires significant variations in Pand T along and outside the two-phase region of the multi-component fluid system. Approximate constraints on P-T-XNaCl conditions can be derived by reference to experimentsin the NaCl-H2O model system (Sourirajan and Kennedy,1962; Urusova, 1975; Bodnar et al., 1985; Fig. 6). Table 4 andFigure 6 give the pressure estimates of all boiling assem-blages and the inclusion populations that were later analyzedwith LA-ICP-MS.
The homogenization temperature and salinity of the liquidmember of the boiling assemblages are plotted as filled sym-bols in the phase diagram of the system NaCl-H2O (Fig. 6;dots for sample BLA97, squares for all other samples). Theassemblages (113115, 117) with the highest homogenizationtemperatures (~720C) indicate pressures between 1,050 and680 bars. Lower pressures (300400 bars) are represented bytwo boiling assemblages (107 and 116), and several boilingassemblages (106, 108112) indicate very low pressures of100 to 250 bars at temperatures of 300 to 450C.
Minimum pressures for liquid assemblages homogenizingby vapor disappearance to the liquid were estimated in thesingle-phase liquid field of the system NaCl-H2O and areshown in Figure 6, using open symbols with a bar extendingto higher pressure. The highest minimum pressures of >1.08to >1.32 kbars are indicated by a high-temperature brine andthree opaque-bearing brine assemblages of sample BLA97
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1 1 0 50 100wt% NaClequiv
pre
ssu
re(b
ars)
700C
600C
550C
500C
450Ccritical curve
"liquid""vapor"
V+NaCl
100
0
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
62
92
91
93104
61
115
116
59
57
47
10758
L+V+NaC
l
54
112
73106
108109
111
100101
25420 21
114
110
11368
sample BLA 97
other samples
31
32
33
3540
41/42
45/4644 43
400C
67
66
117
800C
boilingassemblages
91: fluid inclusionassemblage
117: boilingassemblage
minimum P estimate
FIG. 6. Phase diagram of the system NaCl-H2O (Sourirajan and Kennedy, 1962; Urusova, 1975; Bodnar et al., 1985),showing all boiling assemblages and single-phase assemblages analyzed by LA-ICP-MS. Circles describe the P-T-X variationof assemblages from sample BLA97, and squares represent other samples from the main mineralized porphyries. Filled la-bels indicate boiling assemblages (numbers underlined) and open symbols show lower limits for pressure estimates based onbrine-only assemblages. The salinity and homogenization temperature data of the vapor-rich inclusion assemblages havelarger errors due to the underestimation of homogenization temperatures and possible overestimation of the salinity due tocoentrapment of minor liquid. For example, assemblage 35 must have been trapped at pressures greater than 480 bars in thevapor field.
pre
ssur
e (b
ars)
wt% NaClequiv
(9193, 104). Slightly lower minimum pressures are esti-mated for high-temperature polyphase brine inclusions thatfall close to the 700C isotherm on which the high-tempera-ture boiling assemblages plot. In contrast, low-temperatureassemblages ( 500C) of the different inclusion types giveminimum pressures over the range of 400 to below 100 bars.
Several assemblages consistently show homogenization byhalite melting, and in some assemblages, individual inclusionshomogenize by liquid or by halite melting. Homogenizationby halite dissolution has been interpreted to indicate entrap-ment of a halite-saturated hydrothermal brine (Eastoe, 1978;Cloke and Kesler, 1979; Wilson et al., 1980), homogeneoustrapping at high pressures (Bodnar, 1994; Cline and Bodnar,1994), or postentrapment H2O loss or volume shrinkage(Sterner et al., 1988). Halite-homogenizing inclusions atAlumbrera never occur on boiling trails, which would be clearevidence for postentrapment modification (Audtat and Gn-ther, 1999). The analyzed fluid inclusion assemblages showedno petrographic evidence of postentrapment disturbance(with the possible exception of assemblage 100). We there-fore suggest that the halite-homogenizing inclusion assem-blages record the entrapment of a single-phase brine, proba-bly not halite saturated, at pressures well above theliquid-vapor phase boundary (Bodnar, 1994; Cline and Bod-nar, 1994). Minimum pressures can be derived for this type ofassemblage if the salinity is around 40 wt percent NaCl equiv,where experimental isochore data are available (Bodnar,1994). Two assemblages that homogenize by halite melting(62, 100) give unreasonably high pressures using the calcula-tion approach of Bodnar (1994). Assemblage 100, althoughwithin the salinity range of the experimental data, gives pres-sures of around 2.8 kbars due to the large difference betweenvapor homogenization and halite dissolution, but the mi-crothermometric data vary over a large range, probably indi-cating postentrapment disturbance in this case. In the follow-ing plots, this assemblage is grouped next to assemblage (101)of otherwise similar inclusions in the same sample. Assem-blage 62 indicates a minimum pressure of 1,135 bars, but theextrapolation of the experimental data to such high salinities(70 wt % NaCl equiv) and the known deviation from theNaCl-H2O model system may lead to errors in the pressureestimate. When the final homogenization temperature andthe salinity are plotted, this assemblage falls inside the halite-saturated field at ~380 bars (Fig. 6).
Table 5 summarizes the main observations of the deposit-scale distribution and the microthermometric characteristicsof the fluid inclusion assemblages. Even though it is not pos-sible to document the entire fluid history in time at any pointin the deposit, a generalized fluid evolution can be deducedon the assumption that each intrusion and its fluid pulse (in-cluding the Quartz-Eye Porphyry) followed a similar range ofP-T-XNaCl paths. Significant salinity and density variations offluids present during quartz-magnetite and potassic alterationand probably the initial stages of copper deposition can be ex-plained by a P-T path of general cooling and decompressionof one magmatic fluid. The highest pressure fluids trappedabove 600C are interpreted as the earliest inclusion assem-blages. Among these, there is a trend from nonboiling brineswith ~50 wt percent NaCl equiv toward even higher salinity,essentially following a high-temperature isotherm of the
two-phase boundary (Fig. 6). Several of these assemblagesare clear boiling trails, and the salinity variation at high pres-sure and temperature is therefore interpreted to reflectminor water loss of an initial input brine to a high-densityvapor phase. These boiling assemblages exclusively containopaque-rich polyphase brine inclusions, giving a first indica-tion of high ore metal concentrations. At the lower pressureend of this evolution, halite saturation may have occurred atleast locally, as indicated by a reversal of decreasing salinitywith decreasing pressures below 400 bars. Throughoutquartz-magnetite alteration, potassic alteration, and the asso-ciated copper ore deposition, salinities seem to be controlleddominantly by variations in P and T, in a fluid system domi-nated by magmatic liquid + vapor halite. Below 400C,many of the liquid-rich inclusions contain only one or noopaque daughter crystals, and microanalytical data presentedin the following chapter show that this later part of near-iso-baric cooling is associated with the loss of metallic compo-nents by ore metal precipitation. Large variations in salinityonly occur in late-stage, low-temperature aqueous assem-blages, at temperatures between 200 and 300C. This part ofthe fluid evolution is associated with feldspar-destructive al-teration and may involve an influx of a low-salinity fluid of ex-ternal origin mixing with diminishing amounts of magmaticvapor.
LA-ICP-MS Analysis of Fluid Compositions
Methods
After petrographic characterization of the fluid inclusiontypes and the determination of salinity and some of the ho-mogenization temperatures, the composition of selected in-clusions was analyzed by LA-ICP-MS. Representative sam-pling of the bulk composition of the now heterogeneousinclusions is achieved by stepwise increase in the diameter ofthe ablation crater, using an Excimer laser ablation systemwith imaging optics (Gnther et al., 1997, 1998). The entirecontent of the inclusion, daughter crystals as well as liquid, istransported as an aerosol into the plasma, using He as a car-rier gas. Absolute quantification of single inclusion LA-ICP-MS signals was carried out by integration of all elements in-cluding Na, correction for host mineral contributions (mostcommonly Al, Ti, Li, Ca), comparison of intensity ratios withan external NIST glass standard, and referencing the ratios tothe absolute concentration of Na estimated from microther-mometry (Gnther et al., 1997, 1998; Audtat et al., 1998;Gnther and Heinrich, 1999).
Some elements are commonly below detection or were notmeasured for every inclusion assemblage (Mg, Ca, Al, Cl, As,Ag, Mo, Sn, Sb, La, Tl, Bi, U, Au). Where available, upperconcentration limits for these elements are indicated in Table6 and Figures 7 and 8. Analysis of gold concentrations in sin-gle fluid inclusions was a major achievement of this newmethod and was conducted under optimized conditions ofthe ICP-MS, for maximum response on 197Au and a reducedelement menu (Si, Na, Au, Cu, As; Ulrich et al., 1999). Thetransient signals consistently show overlapping short peaks forCu and Au, demonstrating without any doubt that the gold iscontained within the inclusions and that it is adsorbed onto(or intergrown with) a copper sulfide daughter crystal (Ulrich
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concentration in liquid (ppm/wt%)
Ti
Ag Bi
La
Cs
TI
Sr
Rb Ba
Cu
Zn
Mn
K Fe
NaNa
Fe K
MnCu
ZnRb
Li Ba
SrCs
TlCeBi
Ce
Tl
Ag
Sr
Mo
CsBa
Bi
Rb
Pb
Zn
Cu
Mn
K
Fe
Ba
Li
Sr
Mo
Ti
RbPb
Zn
CuMn
AgTl
LaCe
Cs
K Na
Fe
100100.11001 1100.1
1
10
100
10
0.1
100
0.1
1
TlCs
Ag
BaSr
Rb
Cu
Pb
Zn
Mn
Na
K Fe
Ce
Bi
La
Tl
Rb
Sn
Pb
Zn
Cu
Mn
K
Na Fe
B
MoCs
Li
W
Bi
SbAg
LaCe
Ti
Pb
Na
F=27
F=7
F=1
F=2
F=10
A B
C D
E F
116
Assemblage 117 115
114
112106
100100.11001 1100.1
100100.11001 1100.1 100100.11001 1100.1
100100.11001 1100.1 100100.11001 1100.1
1
10
0.1
100
10
0.1
100
1
1
10
0.1
100
10
0.1
100
1
F=8
Th L= 720CP= 900 bars
Th L= 720CP= 680 bars
Th L= 720CP= 750 bars
Th L= 470CP= 320 bars
Th L= 397CP= 180 bars
Th L= 448CP= 270 bars
Sr
Ba
Pb
FIG. 7. Element concentrations in liquid (polyphase brine inclusions) vs. concentrations in coexisting vapor inclusions.The black line represents equal concentration in the two phases. The dashed lines connect elements with similar tendenciesto be fractionated in the liquid. F is the minimum degree of Cu fractionation based on the average value of elements clearlyenriched in the liquid (eq. 1 in text). Error bars indicate 1 of the variability within one assemblage.
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1600
1400
1200
1000
800
400
200
0
600
0
10
20
30
40
50
60
70
80
BLA 97: opaque-bearing and brine inclusions
BLA 97: polyphase brine inclusions Other samples
Th
P
salinity
A
1
0.1
10
100
0.1
1
10
100
1
0.1
10
100
0.1
1
10
100
Mn
Na
PbZn
Rb
Cs
Tl
NaCl eq. B
Fe
MoSr
Cu
K
Au
Ba
Fluid inclusion assemblages
C
FIG. 8. Interpreted P-T-salinity evolution (A) and LA-ICP-MS data (B, C) of the chemical composition of fluid inclusions.Evolution sequence (from left to right) is interpreted on the basis of petrographic timing evidence, decreasing Th, and de-creasing pressures for assemblages with overlapping homogenization temperatures. The high-temperature assemblages ofsample BLA97 are further separated on the left side, based on petrographic timing evidence in a sample that probably rep-resents a separate magma and fluid pulse. Inclusion types are schematically indicated at the top. A. P-T-XNaCl data separatedaccording to relative timing relationships (BLA97) and decreasing homogenization temperature (all other samples). B. andC. The concentration variation for nonreactive and reactive elements, respectively. Stippled lines interpolate elements omit-ted from analytical runs with a reduced element menu optimized for Au. Error bars extending from the bottom of the fig-ure indicate upper limits for analyses where element concentrations are below detection. Error bars show 1 variabilitywithin an assemblage of similar inclusions.
analyses of individual inclusions (open circles) in comparisonto the bulk ore ratio of the deposit (square) and ore sampleassays from section 47 (small dots, see also Ulrich and Hein-rich, 2001, fig. 10). The encircled fluid analyses have anAu/Cu ratio (0.8 ppm Au/0.7 wt % Cu = 1.2 104 by wt)identical to the bulk ore deposit (0.65 ppm Au/0.51 wt % Cu= 1.3 104; Mndez, 1997; Wall, 1997, updated 0.64 ppmAu/0.54 wt % Cu = 1.2 104; David Keough, writ. commun.,2001) and are interpreted as the closest approximation to theinput ore fluid (Ulrich et al., 1999). Many of the polyphasebrine inclusions contain similar Au contents but significantlylower Cu concentrations. Fractionation of some Cu into thevapor phase (possibly on a local scale) is the most likely ex-planation, consistent with the high Cu concentrations in manyanalyzed vapor inclusions and with their poorly determinedAu/Cu ratio of less than 5 104 by weight (calculated fromaverage limits of detection). On the other hand, some Cufractionation may have occurred into magmatic sulfide phasesin some of these samples (C. A. Heinrich, pers. commun.,2001).
Oxygen and Hydrogen IsotopesTo further constrain the fluid sources that were involved in
the formation of the Bajo de la Alumbrera deposit, a selectionof samples was analyzed for their oxygen and hydrogen iso-tope compositions. The D and 18O values of igneous biotiteand quartz of an unaltered Northwest Porphyry intrusion, and
secondary minerals including biotite, albite, sericite + minorclay minerals, quartz, chlorite, and magnetite, were analyzedto deduce the isotopic composition of the fluids associatedwith various stages of alteration. Hydrogen analyses of fluidinclusion extracts of the quartz-magnetite, potassic, andfeldspar-destructive alteration were analyzed for comparisonto the isotopic fluid signatures calculated from fluid-mineralequilibrium.
Procedure and experimental fractionation factors
The oxygen and hydrogen isotope data are expressed rela-tive to standard mean ocean water (SMOW) in per mil (Table7). The absolute errors of the measured data are about 0.2per mil for the oxygen isotopes and about 5 per mil for thehydrogen. The mineral-fluid equilibrium temperatures forthe calculation of the isotopic fluid composition were esti-mated from the fluid inclusion homogenization temperatures(Table 7). The fractionation factors used for the oxygen iso-topes of quartz and albite were according to Bottinga andJavoy (1973) and Matsuhisa et al. (1979) for the respectivetemperatures, and for biotite the fractionation factor was es-timated by subtracting quartz-water from quartz-biotite (Javoy,1977). The hydrogen fractionation factors for biotite andsericite (muscovite) are from Suzuoki and Epstein (1976),and the chlorite fractionation factor is from Graham et al.(1987). However, the temperature estimates of around 300and 350C (from fluid inclusion data) for some of the alter-ation stages are below the temperature range of experimen-tally based fractionation data (e.g., 400800C, Suzuoki andEpstein, 1976). At low temperature (below 400C) the hy-drogen fractionation curves are not well established exceptfor kaolinite (e.g., Gilg and Sheppard, 1996). For that reason,the fractionation factors (1000 ln mineral-water) at low tempera-tures for sericite-kaolinite, chlorite, and biotite were extrapo-lated from Taylor (1997, see diagram on p. 234), and are asfollows: 23 per mil (muscovite), 15 per mil (kaolinite), 35per mil (chlorite), 35 per mil (biotite). The analyzed samplesof the feldspar-destructive alteration consist of a mixture offine-grained sericite and clay minerals, therefore, an averagefractionation factor between the value for kaolinite andsericite of 19 per mil was used. The fractionation factor forbiotite depends on Fe content (Suzuoki and Epstein, 1976).An Mg-rich composition (XFe = 0.4) was assumed for the sec-ondary biotite of the potassic alteration, and a more Fe-rich(XFe = 0.7) composition was used for the igneous biotite in theunaltered sample of the Northwest Porphyry, based on thegeneral compositional trends shown by Ague and Brimhall(1988).
Results and isotopic composition of hydrothermal fluids
The measured isotopic ratios of the minerals and the calcu-lated isotopic signature of the fluid are given in Table 7. Thevariation of the measured D values among the differentalteration minerals of the porphyry samples is small, except-ing the D values for the secondary biotite of the andesiticwall rock and particularly the igneous biotite of the North-west Porphyry that are depleted (Fig. 10A). This low Dvalue of the igneous biotite from the Northwest Porphyry istypical for a melt reservoir that has already degassed much ofits fluid (Taylor, 1986), which is in agreement with the late
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0.01
0.1
1
10
0.01 0.1 1 10Au (ppm)
Au/Cu=1.2*10-4
polyphasebrine inclusions
ore samples bulk ore
Cu loss
inferredinput fluid
FIG. 9. Au vs. Cu concentration for single liquid inclusion analyses (opencircles) representing some of the earliest and highest temperature inclusionassemblages in the orebody (assemblages 6668). Small dots are whole-rockassay from section 47 through the ore deposit and the white square repre-sents the bulk ore-grade ratio of the deposit. Encircled liquid-rich inclusionanalyses match the bulk ore, whereas analyses with lower Cu concentrationsmay have lost some Cu to a vapor phase or early sulfide mineral inclusions(C. A. Heinrich, pers. commun., 2001, modified from Ulrich et al., 1999).
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TABLE 7. Oxygen and Hydrogen Isotope Composition of Minerals, Whole-Rock Samples, and Fluid Inclusions and the Calculated D and 18O Values of the Fluid in Exchange Equilibrium with These Samples
Measured T(C) Calculated fluid1D 18O D 18O
( SMOW) ( SMOW) ( SMOW) ( SMOW)
Igneous minerals
Northwest PorphyryLP Bio -109.8 (0.8) Qtz 9.2 (0.1) 700 -65.9 (2.4) 8.5 (0.4)
Hydrothermal minerals and fluid inclusions
Porphyry quartz-magnetite zone49-60/93m Fluid incl. -65.4 (9) Qtz 10.6 (0.1) 700 -65.4 (9) 9.9 (0.9)
Mag 3.1 8.3 (0.3)
49-61.4/357m Qtz 10.5 (0.04) 700 9.9 (0.9)Mag 2.8 8.0 (0.3)
Porphyry potassic zone51-52.2/308m WR (bio) -68.3 (2.7) Qtz 8.5 (0.2) 500 -29.8 (4.7) 5.3 (0.9)
49.2-46.3/452m WR (bio) -80.1 (3.8) Qtz 9.7 (0.05) 500 -41.7 (4.7) 6.5 (0.9)
BLA 97 Qtz 11.3 (0.2) 500 8.1 (1.2)
43-47.1/195m Fluid incl. -55 (22.8) Qtz 9.2 (0.2) 500 -55 6.1 (0.9)
Porphyry feldspar-destructive zone49.2-46.3/265m WR (ser) -71.1 Qtz 10.3 (0.1) 300 -52.1 (10) 3.4 (1.8)
57.5-60/51m WR (ser) -73.3 (0.2) Qtz 10.7 (0.2) 300 -54.3 (10) 3.9 (1.8)
55.5-60/192m Fluid incl. -52 Qtz 8.3 (0.08) 300 -52 1.4 (1.8)
38-49/97m Fluid incl. -79 (19.5) Qtz 10.2 (0.01) 300 -79 3.3 (1.8)
Porphyry chlorite-epidote zoneBLA55 Chl -76.6 (0.8) Ab 10.6 (0.1) 300 -41.6 (5) 4.7 (1.7)
BLA64 Chl -78.3 Ab 9.9 (0.1) 300 -43.3 (5) 4.0 (1.7)Andesite potassic49-61.4/94m Bio -79.4 (6.7) Bio 7.4 (0.04) 350 -44.4 (5) 9.4 (0.1)
37-49/275m Bio -90.1 (1.6) Bio 6.5 (0.04) 350 -55.7 (5) 8.5 (0.1)
37-49/275m WR (bio) -83.1 (1.8) WR (bio) 6.5 (0.04) 350 -48.1 (5) 8.5 (0.1)
Andesite feldspar-destructive zone46-42.5/46m WR (ser) -69.3 (15.3) Qtz 10.7 (0.2) 300 -50.3 3.9 (1.8)
Present-day meteoric water2 -30 -5
Estimate of meteoric Fluid incl. -70 (3) Qtz 11.70 200 -70 (3) 0.1 (3.2)water of Miocenc agefrom the Faralln Negro mine
Abbreviations: bio = biotite, chl = chlorite, mag = magnetite, qtz = quartz, ser = sericite, WR = whole rockMeasured 18O and D of alteration minerals and the calculated isotopic signature of the fluids in equilibrium with the rock; the numbers in brackets of
the measured isotopic signature correspond to 1 and the numbers in brackets for the calculated signature indicates the variation of the isotopic signature fortemperatures of 50C above and below the cited temperature
1Fractionation factors used for the calculation of the isotopic fluid signature in the respective temperature interval are from Bottinga and Javoy (1973),Matsuhisa et al. (1979), and Javoy (1977) for the oxygen isotopes and from Suzuoki and Epstein (1976) and Graham et al. (1987) for the hydrogen isotopes;since at low temperatures the experimental data are rare, the fractionation factors were taken from the extrapolated curves from the diagram of Taylor (1997)
2Average of present-day measurements of meteoric water at Salta, Argentina; see text for explanation
intrusive stage of the Northwest Porphyry. The oxygen valuesfor quartz and albite range from 8 to 11 per mil for the por-phyries and are lower for the secondary biotite of the an-desites (6.57.5%). This may be due to the different closingtemperatures of biotite and quartz and the higher resistanceof quartz to later isotopic exchange during the feldspar-de-structive overprint of the system. The D value for thefeldspar-destructive overprint of the andesite has a largervariation, including poor reproducibility of the duplicateanalysis. Despite the different diffusion rates and closing
temperatures for O and H of a given mineral (Giletti, 1985,1986), we used temperature data of the fluid inclusion study,because the homogenization temperatures of the fluid inclu-sions are close to or below the estimated closing temperaturesof the analyzed minerals (Zaluski et al., 1994), except for the700C of the magmatic stage. Various processes influence theisotopic composition of minerals and the fluid in a magmatic-hydrothermal system, such as fluid exsolution (Dobson et al.,1989; Taylor, 1992), phase separation (e.g., Shmulovich et al.,1999), open- and closed-system fractionation, fluid mixing,
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-120
-100
-80
-60
-40
-20
0
6 8 10 12
quartz/biotite
biotite/biotite
quartz/biotite
albite/chlorite
quartz/sericite
chlorite-epidote
feldspar-destructive
potassic (wall rock)
Northwest Porphyry
18O
Measured Dand 18O values
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
-10 -5 0 5 10 15
Calculated isotopic composition of the fluid
primarymagmatic water
"present-day
meteoric water"
potassic(wall rock)350C
potassic(porphyry)500C
chlorite-epidote300C
feldspar-destructive300C
NorthwestPorphyry700C
A
B
quartz-magnetite
quartz-magnetite700C
Faralln Negro mine200C
potassic (porphyry)
Fig 10. Isotopic signatures of the fluids associated with the magmatic and the alteration stages. A. Analyses of mineralpairs that were analyzed for hydrogen and oxygen, respectively. B. Calculated isotopic signature of fluids at the indicatedtemperatures for the different alteration zones and the Northwest Porphyry. Closed symbols are calculated v