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The Cu-Au deposit of Bor (Serbia) represents a continuum of mineralization styles, from porphyry-style ore occurring in quartz-magnetite-chalcopyrite veins and chalcopyrite disseminations to high-sulfidation epithermal Cu-Au ores in pyrite-chalcopyrite and anhydrite-sulfide veins. Decisive for the great economic importance of Bor is the presence of exceptionally rich high-sulfidation massive sulfide orebodies, composed of pyrite + covellite + chalcocite/digenite and minor anhydrite and enargite. They form irregular bodies measuring 0.5–10 million tons of ore grading up to 7% Cu, hosted by andesites and surrounded by intense argillic alteration. This study focuses on a small but rich underground orebody mined out recently, where limited drillcore is preserved for quantitative geochemical study. This paper documents the vein relationships within the deep porphyry-style orebody of Borska Reka, the transitional porphyry-epithermal veins, and the overlying and laterally surrounding epithermal massive sulfides of the Bor deposit. Geological observations indicate that the formation of massive sulfide orebodies concludes the ore formation. Mass balance calculations, recast into geologically realistic bulk fluid-rock reactions, confirm textural evidence that near-isovolumetric replacement of andesite host rocks is the dominant formation mechanism of massive sulfide orebodies at Bor, whereby all lithophile elements including Si are dissolved and only Ti stays relatively immobile. While net volume changes for individual mineralization styles within the massive sulfide orebody vary from − 16% volume loss to + 127% volume gain, overall volume change for the whole massive sulfide orebody was probably slightly negative. Brecciation is important only as means of creating channelways for reactive fluid that turns the andesite protolith into massive sulfide, whereas net breccia infill occurred only locally. [ABSTRACT FROM AUTHOR]

Geologic field relationships combined with spatially resolved geochemistry and high-precision U-Pb geochronology of zircons allow quantification of magmatic processes leading to porphyry emplacement and Au-Cu ore formation at Ok Tedi, the Earths youngest giant porphyry-skarn deposit. Trace element contents in zircons were obtained by laser ablation-inductively coupled plasma-mass spectrometry before selected grains were removed from the sample mount, chemically abraded, spiked, and dissolved for high-precision U-Pb geochronology by chemical abrasion-isotope dilution-thermal ionization mass spectrometry. U-Pb geochronology by laser ablation-sector field-inductively coupled plasma-mass spectrometry on abundant inherited zircon cores that occur in all intrusions together with Hf isotope analyses by solution multicollector-inductively coupled plasma-mass spectrometry permitted further assessment of the degree of crustal assimilation of the upper crustal magma reservoir. The combined chemical data and precise crystallization ages of zircons indicate closed system fractional crystallization and concurrent cooling in a common magma reservoir, from which the intrusions at Ok Tedi were successively extracted. Inherited zircon populations and Hf isotope analyses of Pleistocene zircons record lower crustal assimilation with Proterozoic basement. Based on cathodoluminescence (CL) imaging a chemically distinct zircon population with low Th/U was identified in the youngest synore intrusive rocks. These zircons indicate injection of a distinct magma into the common magma reservoir, which triggered the emplacement of the youngest porphyry, intrusion of a polymictic breccia with partly igneous matrix, and intimately associated hydrothermal Au-Cu mineralization. Crystallization ages of chemically characterized zircon crystals extend over a total time span of 212 ± 44 k.y., recording the minimum duration of fractional crystallization in the large magma reservoir from the time it reached zircon saturation. The youngest zircons in each intrusion resolve the emplacement of early thorite intrusions, low-grade monzodiorite porphyries, including the Sydney monzodiorite and Fubilan porphyry, into three time-resolved pulses separated by a total of ~160 k.y. The Sydney monzodiorite contains barren quartz veins and produced endoskarn and exoskarn; however, the exact timing of skarn-hosted Au-Cu mineralization is not yet clear. The emplacement of the ore-bearing Fubilan porphyry and the intrusive breccia overlap at 1.187 ± 0.022 Ma, constraining the time and duration of porphyry emplacement with associated Au-Cu mineralization. The injection of a distinct, probably more mafic magma batch into the magma reservoir preceded the emplacement of the Fubilan porphyry and the intrusive breccia by less than the resolution of 30 k.y., indicating that this event was the immediate trigger for Au-Cu mineralization at Ok Tedi. [ABSTRACT FROM AUTHOR]

The Altar porphyry Cu-(Mo-Au) deposit occurs in the Miocene to early Pliocene copper belt of the high Andes of western Argentina, close to the Chilean border. A cluster of late Miocene porphyries intruded a more extensive complex of early Miocene rhyolitic to andesitic volcanic and subvolcanic rocks. Potassic alteration in the Central Porphyry was partially overprinted by feldspar-destructive alteration and finally by argillic to advanced argillic alteration. Early stockwork veins have been cut successively by quartz-pyrite veins, tourmaline veins, and enargite veins. Based on correlation analysis of assay results and mapped abundances of these four vein types, we estimate that approximately 11 to 26% of the copper in the Altar orebody is associated with enargite veins, whereas the remainder is associated with the early stockwork veining and potassic alteration. Gold concentration is low compared with typical Cu-Au porphyries of the Andean back-arc region, but higher than in the giant Miocene deposits of Chile, at an average Au/Cu ratio of 0.14 x 10-4 by weight across the Central Porphyry orebody at Altar. Gold is dominantly associated with chalcopyrite in the domain of stockwork veining and potassic alteration. Small-scale assays in combination with petrographic observations and laser ablation-inductively coupled plasma-mass spectrometry microanalyses show that a significant fraction of the gold in the stockwork veins is enclosed in pyrite as submicroscopic particles attached to inclusions of a Bi-Te-Pb-Ag-enriched Cu-Fe sulfide phase. Microscopic grains of native gold also occur along grain boundaries between pyrite and chalcopyrite, and rarely as larger particles in enargite veins. [ABSTRACT FROM AUTHOR]

The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfi de minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ~10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (~850 °C). During ascent to the level of the porphyry intrusion (0.5-1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the goldonly enrichment in this globally recurring but rare type of gold ore. [ABSTRACT FROM AUTHOR]

The formation of porphyry copper deposits requires a focused flux of magmatic fluid, expelled from a large reservoir of water-, metal-, and sulfur-rich magma. The dimensions of this usually hidden magma reservoir are difficult to determine but can be constrained by combining geophysical observations with thermal constraints and the mass balance imposed by the chemical enrichment of elements in the deposit. Here we show that an internally consistent scenario can be derived for the world-class Cu-Mo-Au deposit at Bingham Canyon (Utah, United States), which quantifies the essential characteristics, approximate dimension, and temporal evolution of a large pluton that generated the deposit. The mineralized district shows a distinct WSW-ENE-striking magnetic anomaly indicating a large intrusive body underlying the sedimentary host rocks of the Oquirrh Mountains. Modeling the deep body by geomagnetic methods is possible because of the high contrast in magnetic susceptibility between sedimentary host rocks and intrusive rocks and because a former volcanic edifice is largely eroded. Additional constraints from drilled geology and district-wide outcropping rocks, including partial demagnetization by hydrothermal alteration on the mine scale, restrict the range of possible solutions to a broadly laccolith-shaped intrusion with a volume of approximately 1,400 to 3,000 km³. From the roof of the laccolith, several smaller subvolcanic stocks and dikes protrude to the present surface, of which a major one is hosting the Bingham Canyon deposit. The roof of the laccolith probably lies between 2 and 3.5 km below the bottom of the present open-pit mine, and the average thickness of the laccolith is constrained between 2 and 3.5 km. Thermal modeling, using pluton dimensions derived from the geologic and geomagnetic modeling, indicates that a single laccolith with a magma volume of -2,000 km³ beneath Bingham would have solidified within about 230,000 years or less. Comparison of the thermal models with published high-precision geochronologic data and petrologic constraints suggests a scenario in which about 1,000 km³ of magma was encapsulated by inward crystallization of the pluton after the preore equigranular monzonite stocks solidified and extrusive volcanism was probably terminated. This encapsulated reservoir was close to water saturation and contained approximately 150 billion metric tons (Gt) of magmatic water for subsequent closed-system fractionation and eventual fluid expulsion driving porphyry copper mineralization. Chemical mass balance shows that the known metal endowment and mapped mass of vein quartz within the deposit can be advected and precipitated by a fluid mass that is slightly smaller than the available 150 Gt of water. A conservative estimate indicates that 115 Gt of water is sufficient to precipitate all the quartz associated with successive Cu-Au- and Mo-stage veins as well as their barren precursors. According to our thermal model, approximately 250 km³ of quartz monzonite magma with a temperature of about 690°C remained partially liquid some 215,000 years after initial intrusion of the laccolith. At that point, it expelled almost simultaneously the quartz monzonite porphyry and the main mass of accumulated fluid, generating most of the vein quartz in the quenched porphyry and the adjacent older rocks. Petrographic evidence indicates that the ore metals precipitated near the end of individual pulses of quartz veining that followed recurrent but waning pulses of porphyry intrusion. Considering published experimental solubility data as well as ore metal contents in fluid inclusions, a small fraction of the available fluid mass is sufficient to transport and precipitate all the ore metals after an initial fluid pulse precipitated most of the quartz. However, the total amount of sulfur present in the deposit, which includes Cu and Mo sulfides as well as a major addition of pyrite, would be facilitated by addition of a mafic magma input into the residual magma chamber that contained the evolved felsic magma. This magmatic injection probably triggered the emplacement of the mineralized porphyries, consistent with the more mafic composition of some of the latest porphyry dikes and the CO2-rich nature of ore-related fluid inclusions. [ABSTRACT FROM AUTHOR]

The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene sub volcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90-260 bars and 320°-430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about post entrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 µg/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive (actor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large sub volcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals. [ABSTRACT FROM AUTHOR]

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in combination with near-infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore-precipitating fluids. We applied the two techniques to well-constrained fluid inclusion assemblages hosted by pyrite, enargite, and quartz to trace the sortie and evolution of the fluids in high-sulfidation epithermal veins overprinting a porphyry copper deposit at Rosin Poieni, Romania. Despite some analytical limitations caused by the sulfide host minerals, the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different. Systematic changes in major and trace element ratios between liquid-vapor, vapor-rich, and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition. Regarding their Cs/(Na + K) ratios, most of epithermal quartz-hosted fluid inclusion assemblages form a well-defined cluster, which coincides with the narrow range of the porphyry-stage fluids trapped in early quartz of the porphyry stockwork veins. Their Cu/(Na + K) ratios are 10 to 100 times lower compared to the pyrite-hosted inclusions and correspond to the lowest Cu/(Na + K) ratios recorded for the porphyry-stage fluids. By contrast, pyrite-bested, vapor-rich fluid inclusions have the highest Cu/(Na + K) similar to the highest Cu/(Na + K) ratios measured in the porphyry-stage fluid inclusions. The results led to the conclusion that the gangue and ore minerals in the high-sulfidation epithermal veins at Rosin Poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystanizing magma at greater depth. These results are consistent with detailed textural observations, but petrography alone could not have led to this unambiguous conclusion. [ABSTRACT FROM AUTHOR]

Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ∼1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (∼0.6 g cm-3) and a salinity of ∼7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CQ2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (-0.2 g cm-3) coexisting with brine inclusions containing ∼45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ∼9/1. The vapor contained Cu as its dominant cation (∼1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (∑36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. Tiffs interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor. [ABSTRACT FROM AUTHOR]

Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same litho-logic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes mad a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry (likes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. [ABSTRACT FROM AUTHOR]

Fluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low- to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems. [ABSTRACT FROM AUTHOR]

Abstract: Essential resources of many rare metals including copper, zinc, molybdenum, silver and gold occur in natural sulfide mineral deposits. Understanding the origin of these metal resources has been limited by a lack of data about the geochemistry of sulfur, the most important and abundant element of ore deposits. We report the first directly measured sulfur concentrations in high-temperature fluids, together with their ore-metal contents, using a new method for sulfur quantification in fluid inclusions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Co-genetic brine and vapor inclusions from magmatic–hydrothermal ore deposits and granitic intrusions show an excess of sulfur over ore metals, as required for efficient ore-mineral precipitation. The results demonstrate that S, Cu and Au are highly enriched in vapor-like magmatic fluids, implying that such low-salinity fluids are the key agent for the formation of porphyry copper and epithermal gold deposits. [Copyright &y& Elsevier]

Magmatic-hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion, vein opening, and hydrothermal ore deposition. It is driven by larger subjacent magma reservoirs, acting as the source of fluid and ore-forming components. High-precision U-Pb ages of individual zircon crystals from porphyries immediately predating and postdating Cu-Au mineralization at Bingham Canyon (Utah, United States) and Bajo de la Alumbrera (northwestern Argentina) show a significant spread of reliably concordant ages. This demonstrates zircon crystal formation over a protracted period of ~1 m.y., which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted. The youngest zircons in all pre-ore and post-ore intrusions overlap within a much shorter time interval of 0.32 m.y. at Bingham Canyon and 0.090 m.y. at Alumbrera; these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods, consistent with thermal constraints. This study illustrates that age brackets based on individual magmatic zircon grains are geologically more informative than the calculation of means and standard deviations based on apparently normal age distributions in zircon populations. [ABSTRACT FROM AUTHOR]

The geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the Upper Miocene Farallón Negro Volcanic Complex (NW Argentina) were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3.5 my. This paper is part of an integrated study of the processes that generate magmatic-hydrothermal fluids responsible for the formation of porphyry-Cu-Au deposits. Here we present new data on bulk rock chemical and Nd, Sr and Pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks, for which geological relations and ages are constrained by field-based stratigraphy and extensive Ar–Ar geochronology.The combined results indicate that voluminous volcanism started at ca. 9.7 Ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits. This basic to intermediate volcanism persisted until 7.5 Ma and was followed at 7.35 Ma by the eruption of dacites as the last recorded major extrusive event. The oldest andesitic intrusions, emplaced at 9.0 Ma, were followed by hypabyssal intrusions of basaltic andesite, andesite and dacite composition, which intruded the volcanic sequence between 8.3 and 6.1 Ma. Intrusions at 6.8±0.2 Ma led to the formation of over 800 Mt of porphyry-Cu-Au ore at Bajo de la Alumbrera. It was formed by a large flux of magmatic-hydrothermal fluids channeled through several phases of fractured dacitic porphyry, which together with immediately following barren andesitic intrusions make up a composite stock. After this magmatic-hydrothermal event, only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6.1 Ma.Geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material, as indicated by linear compositional correlations between compatible and incompatible elements. Initial strontium isotopic ratios vary between 0.7047 and 0.7118 while 143Nd/144Nd range from 0.5127 to 0.5124, and both isotopic variations correlate with each other and with SiO2 content of the rocks. After 8.5 Ma, a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber. A substantial magma reservoir (>7 km3) must have been established by the time of porphyry-Cu-Au mineralization, to allow rapid release of a large volume of ore-forming magmatic brine as a result of wholesale crystallization and de-volatilization of the magma chamber. [Copyright &y& Elsevier]