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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 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]

This paper addresses the question of whether or not there are fundamental differences in the composition of magmatic-hydrothermal fluids in barren versus variably mineralized systems, and if so, at which stage of magmatic-hydrothermal evolution the differences were generated. Combining microthermometry and laser ablation ICP-MS, we studied high-temperature, premineralization fluid inclusions in three barren granites, one Sn-W-mineralized granite, one porphyry Cu-mineralized intrusion, one porphyry Mo-mineralized intrusion, and one Th-U-REE--mineralized granite, and compared the results with published data from 10 other intrusive systems. Many of the fluid inclusions are paragenetically associated with melt inclusions, providing direct insights into the stage at which metals were transferred from the crystallizing magma into the exsolving aqueous fluids. In 10 out of 14 magma systems in which the physical and compositional properties of the magmatic fluid(s) could be determined, the exsolved aqueous fluid was a single phase of relatively low salinity (2-13 wt % NaCl equiv; avg 5 wt % NaCl equiv), confirming predictions based on indirect evidence and observations made in earlier studies. Furthermore, in magmas that crystallized at low pressure (< 1.3 kbars), the salinity of the magmatic fluid increased with increasing degree of crystallization, whereas in magmas that crystallized at higher pressure (>1.3 kbars) the opposite behavior occurred, as predicted by published numerical models of aqueous fluid-saturated magma crystallization. During the transition from magmatic to subsolidus conditions, both pressure and temperature within and adjacent to intrusions decreased, leading to widespread vapor-brine immiscibility. Due to the low salinity of the single-phase parent fluid the two-phase field generally was entered from the vapor side, resulting in condensation of subordinate amounts of brine from the low-density bulk fluid. If the transition from single- to two-phase aqueous fluid occurred in locally closed systems one can use the compositions of vapor and brine in subsolidus boiling assemblages to calculate the composition of the bulk fluid. This method was applied to those occurrences in which no single-phase parent fluid was found (i.e., where the earliest aqueous fluid found was already in the two-phase field). The metal content of the least fractionated low-salinity fluids at each location correlates positively with the type and amount of mineralization in the associated intrusions, with high Cu concentrations being observed in fluid related to porphyry Cu deposits, high Sn and W concentrations in fluids related to Sn-W mineralization and high Ce concentrations in fluids related to REE mineralization. The earliest fluids in barren intrusions are comparatively metal poor, except for two cases in which certain metal concentrations are as high as in mineralized systems. A significant portion of the geochemical signature of barren versus mineralized intrusions was thus inherited from earlier stages in the development of these magma systems. In the case of Mo, Sn, W, and Ce, the correlation between fluid composition and type and amount of mineralization is more distinct in high-temperature brines than in the least fractionated low-salinity fluids. This, together with other lines of evidence, suggests that brine condensates play a central role ill the formation of Sn, W, Mo, and REE deposits. In the case of porphyry Cu (-Au) systems it appears more likely that the greater proportion of metal precipitated from the vapor phase, although brines may have played a significant role as well. [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]

• Fluid inclusions from juvenile clasts record pre-eruptive pressure variations in the magma reservoir. • Most metals get enriched in the exsolving single-phase fluid compared to the rhyolitic melt. • Rapidly erupted fluid inclusions escaped post-entrapment modifications seen in slowly cooled samples from ore deposits. Pre- to syn-eruptive fluids released by the magmatic system forming the non-welded rhyolitic Kos Plateau Tuff ignimbrite, Greece, were characterized using fluid inclusions entrapped in quartz from granitic clasts entrained from the roof or walls of the feeding magma reservoir. This fluid was initially of intermediate density and relatively low salinity (3–11 wt% NaCl equivalent). Following significant pressure drop(s), it separated into immiscible brine and CO 2 -bearing vapor occurring as texturally less mature inclusions. The transition from single- to two-phase-fluid occurred at magmatic conditions, as supported by high fluid inclusion homogenization temperatures (ca. 700 °C) and simultaneous trapping of fluid inclusions with silicate melt inclusions. The large recorded pressure drop together with texturally immature fluid inclusion shapes suggests a possible direct relation between the fluid phase change and the incipient 161 ka caldera-forming eruption. All inclusion types occur predominantly along (pseudo-) secondary trails in the granitic clasts, indicating efficient entrapment in microfractures formed at high crystallinity, as attested to by elevated concentrations of incompatible elements in the silicate melt inclusions. Fast quenching of the inclusions due to eruption prevented significant post-entrapment chemical modification, providing pristine samples of magmatic fluid compositions. Element concentrations in co-existing fluids and silicate melt were analyzed using laser ablation inductively-coupled plasma mass spectrometry and compared in view of element transporting capabilities and magmatic-hydrothermal ore formation. Economically important metals such as Cu, Zn, Mo, and W were extracted from the silicate melt by the fluid. In particular, Cu concentrations in the single-phase fluid in the range of 100–500 ppm agree well with previous experimental and modeling studies and, therefore, represent values expected for primary magmatic fluids. Most elements got further enriched in the brine upon phase separation, except Li, B, and As, which may have contributed significantly also to the vapor phase. Considering the similarity between vapor and intermediate-density fluid in terms of salinity, however, the metal-enriched brine phase only comprised a minor fraction of the bulk fluid. [ABSTRACT FROM AUTHOR]

The article focuses on the process of gold mineralization and ore formation in the Earth's crust. There are different procedures for extracting valuable metals. Metal ores are formed when the water-rich fluids circulate through large volumes of the Earth's crust by selective enrichment. Gold mineralization is associated with magmatic processes. There are several issues related to magmatic-hydrothermal ore formation. Researchers are analyzing samples of magmatic fluids that contain gold deposits. The fluids were formed at the Ladolam Mine on the Lihir Islands, Papua New Guinea. The Ladolam gold deposit was formed from Luise Volcano.

Abstract: The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated with hydrous magmas of broadly calc-alkaline composition, thought to be generated by contemporaneous subduction zone processes. Molybdenum is generally considered to be derived from the continental crust while Cu and Au are sourced in the mantle wedge above subducting slabs. Here we show that neither contemporaneous subduction nor derivation of Mo from crustal sources is required to explain the genesis of porphyry–Cu–Mo–Au deposits on Proterozoic lithosphere in the eastern Rocky Mountains. Uniform Pb isotope ratios measured by LA-MC-ICP-MS in individual fluid inclusions from distinct Cu–Au and later Mo ore-forming stages at Bingham Canyon, USA, demonstrate a common metal source. Uranogenic Pb isotope ratios are particularly non-radiogenic (17.494< 206Pb/204Pb<17.534; 15.553< 207Pb/204Pb<15.588) and plot to the left of the geochron and above the mantle Pb evolution line. In 207Pb/206Pb vs. 208Pb/206Pb space, the fluid Pb isotope data cluster at the non-radiogenic end of a mixing line described by >80 feldspar data from igneous rocks intimately associated with magmatic-hydrothermal ore formation, which extends to modern depleted mantle or upper crust. Forward Monte Carlo simulations require three events for the U–Th–Pb isotope evolution of the fluid: (1) Late Archean formation of enriched crust is followed by (2) preferential extraction of Pb from this aged crust into a subduction fluid characterized by drastically reduced U/Pb that metasomatized lithospheric mantle at ∼1.8Ga. This mantle reservoir then evolved to produce the retarded uranogenic Pb isotope signatures of the Bingham Canyon Cu–Mo–Au deposit in the Cenozoic (3). Similarly retarded uranogenic Pb isotope data characterize the giant porphyry–Mo and Climax-type Mo deposits of Henderson, Questa, Butte, and SE Arizona that occur in Proterozoic sutures of the central and eastern Rocky Mountains. We propose that Cenozoic melting of subcontinental lithospheric mantle metasomatized by subduction fluids during early Proterozoic amalgamation of terranes to the Wyoming Craton provides the metal endowment and subduction flavour to the giant magmatic-hydrothermal Cu–Mo–Au ore deposits in western North America, which together constitute the world''s major molybdenum ore province. [Copyright &y& Elsevier]

Iron oxide-apatite (IOA) deposits are an important source of iron and other elements (e.g., REE, P, U, Ag and Co) vital to modern society. However, their formation, including the namesake Kiruna-type IOA deposit (Sweden), remains controversial. Working hypotheses include a purely magmatic origin involving separation of an Fe-, P-rich, volatile-rich oxide melt from a Si-rich silicate melt, and precipitation of magnetite from an aqueous ore fluid, which is either of magmatic-hydrothermal or non-magmatic surface or metamorphic origin. In this study, we focus on the geochemistry of magnetite from the Cretaceous Kiruna-type Los Colorados IOA deposit (∼350 Mt Fe) located in the northern Chilean Iron Belt. Los Colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in IOA deposits. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) transects and electron probe micro-analyzer (EPMA) wavelength-dispersive X-ray (WDX) spectrometry mapping demonstrate distinct chemical zoning in magnetite grains, wherein cores are enriched in Ti, Al, Mn and Mg. The concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt, whereas magnetite rims show a pronounced depletion in these elements, consistent with magnetite grown from an Fe-rich magmatic-hydrothermal aqueous fluid. Further, magnetite grains contain polycrystalline inclusions that re-homogenize at magmatic temperatures (>850 °C). Smaller inclusions (<5 μm) contain halite crystals indicating a saline environment during magnetite growth. The combination of these observations are consistent with a formation model for IOA deposits in northern Chile that involves crystallization of magnetite microlites from a silicate melt, nucleation of aqueous fluid bubbles on magnetite surfaces, and formation and ascent of buoyant fluid bubble-magnetite aggregates. Decompression of the fluid-magnetite aggregate during ascent along regional-scale transcurrent faults promotes continued growth of the magmatic magnetite microlites from the Fe-rich magmatic-hydrothermal fluid, which manifests in magnetite rims that have trace element abundances consistent with growth from a magmatic-hydrothermal fluid. Mass balance calculations indicate that this process can leach and transport sufficient Fe from a magmatic source to form large IOA deposits such as Los Colorados. Furthermore, published experimental data demonstrate that a saline magmatic-hydrothermal ore fluid will scavenge significant quantities of metals such as Cu and Au from a silicate melt, and when combined with solubility data for Fe, Cu and Au, it is plausible that the magmatic-hydrothermal ore fluid that continues to ascend from the IOA depositional environment can retain sufficient concentrations of these metals to form iron oxide copper–gold (IOCG) deposits at lateral and/or stratigraphically higher levels in the crust. Notably, this study provides a new discrimination diagram to identify magnetite from Kiruna-type deposits and to distinguish them from IOCG, porphyry and Fe–Ti–V/P deposits, based on low Cr (<100 ppm) and high V (>500 ppm) concentrations. [ABSTRACT FROM AUTHOR]

Subduction-related magmas have higher volatile contents than mid-ocean ridge basalts, which affects the dynamics of associated submarine hydrothermal systems. Interaction of saline magmatic fluids with convecting seawater may enhance ore metal deposition near the seafloor, making active submarine arcs a preferred modern analogue for understanding ancient massive sulfide deposits. We have constructed a quantitative hydrological model for sub-seafloor fluid flow based on observations at Brothers volcano, southern Kermadec arc, New Zealand. Numerical simulations of multi-phase hydrosaline fluid flow were performed on a two-dimensional cross-section cutting through the NW Caldera and the Upper Cone sites, two regions of active venting at the Brothers volcanic edifice, with the former hosting sulfide mineralization. Our aim is to explore the flow paths of saline magmatic fluids released from a crystallizing magma body at depth and their interaction with seawater circulating through the crust. The model includes a 3 × 2 km 2 sized magma chamber emplaced at ∼ 2.5 km beneath the seafloor connected to the permeable cone via a ∼ 200 m wide feeder dike. During the simulation, a magmatic fluid was temporarily injected from the top of the cooling magma chamber into the overlying convection system, assuming hydrostatic conditions and a static permeability distribution. The simulations predict a succession of hydrologic regimes in the subsurface of Brothers volcano, which can explain some of the present-day hydrothermal observations. We find that sub-seafloor phase separation, inferred from observed vent fluid salinities, and the temperatures of venting at Brothers volcano can only be achieved by input of a saline magmatic fluid at depth, consistent with chemical and isotopic data. In general, our simulations show that the transport of heat, water, and salt from magmatic and seawater sources is partly decoupled. Expulsion of magmatic heat and volatiles occurs within the first few hundred years of magma emplacement in the form of rapidly rising low-salinity vapor-rich fluids. About 95% of the magmatically derived salt is temporarily trapped in the crust, either as dense brine or as precipitated halite. This retained salt can only be expelled by later convection of seawater during the waning period of the hydrothermal system (i.e., “brine mining”). While the abundant mineralization of the NW Caldera vent field at Brothers could not be classified as an economic ore deposit, our model has important implications for submarine metal enrichment and the origin of distinct ore types known from exposed systems on land. Sulfide-complexed metals (notably Au) will preferentially ascend during early vapor-dominated fluid expulsion, potentially forming gold ± copper rich vein and replacement deposits in near-seafloor zones of submarine volcanoes. Dense magmatic brine will initially accumulate chloride-complexed base metals (such as Cu, Fe, Pb and Zn) at depth before they are mobilized by seawater convection. The resulting mixed brines can become negatively buoyant when they reach the seafloor and may flow laterally towards depressions, potentially forming layers of base metal sulphides with distinct zonation of metals. [ABSTRACT FROM AUTHOR]

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]

Abstract: Magmatic-hydrothermal ore deposits are a major resource of gold. Volatiles exsolving from magmas rising through the Earth''s crust are the key agent for enrichment of gold in these deposits. We report the first experimental determination of gold solubilities in the magmatic volatile phases at conditions typical of most magmas associated with gold deposits (T=1000°C, P=150MPa). We show that gold hydrosulfide complexes supersede gold chloride complexes, and more importantly, that the stability of gold hydrosulfide complexes is greatly increased by the presence of minute concentrations of KCl or NaCl (0.1–0.5mol/kg H2O). The amplifying effect of alkali chlorides on the solubility of gold in H2S bearing volatiles may explain the preferential association of many giant hydrothermal gold deposits with high-potassium alkaline mafic to intermediate igneous rocks, which exsolve volatiles that simultaneously contain both H2S and alkali chlorides in significant concentrations. [Copyright &y& Elsevier]

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]