Showing papers in "Economic Geology in 2005"

Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls

Abstract: More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina, Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable for the formation of giant porphyry copper deposits, although several of the largest systems are associated with high K calc-alkalic intrusions. The 25 largest gold-rich porphyry deposits are concentrated in the southwest Pacific and South America, with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries. In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and oceanic plateaus beneath oceanic island and continental arcs. In several examples, these tectonic perturbations have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene). Topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.

Sediment-hosted lead-zinc deposits: A global perspective

Abstract: Sediment-hosted Pb-Zn deposits contain the world's greatest lead and zinc resources and dominate world production of these metals. They are a chverse group of ore deposits hosted by a wide variety of carbonate and siliciclastic roch that have no obviolls genetic association with igneous activity. A nmge of ore-fortl1ing processes in a vmiety of geologic and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subt)1Jes: Mississippi Valley-type (MVT) and sedimentmy exhalative (SEDEX), Despite the "exhalative" component inherent in the term "SEDEX," in this manusclipt, direct evidence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The chstinction between some SEDEX and MVT depOSits can be quite subjective because some SEDEX ores replaced carbonate, whereas some MVT depOSits formed in an early diagenetic environment and display laminated ore textures. Geologic and resource information are presented for 248 depositS that provide a framework to describe ,mel compare these deposits. Nine of tlle 10 largest sediment-hosted Pb-Zn deposits are SEDEX, Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits and 15 MVT (excluding Iris-type) deposits. Despite the skewed distribution of the deposit size, the two deposits types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn), with a fairly consistent ratio of about lO/l, regardless of the size of the deposit or district. Zinc grades are approximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerozoic, MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their host rocks; however, a few are close <~5 m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constnuned by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. Fluid inclusion, isotopic studies, and deposit modeling suggest that MVf and SEDEX deposits formed from basin brines with similar temperatures of mainly 90° to 200°C and lO to 30 wt percent NaCI equiv. Lead isotope compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide critelia that distinguish MVT from SEDEX subtypes. However, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVf sulfur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVf ores extend over a wider range and include more data with negative isotopic values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deposits is their depositional environment, which is determined by their respective tectonic settings. The contrasting tectonic setting also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology, deposit morphology, and ore textures. Blief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no Significant differences in grade tonnage values between MVT deposits and the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the hish ores commonly ascribed as uniquely hish type is in fact no different from several MVT deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain signiflcantly higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits are almost three times more ellliched in Ag and one and a half times more enriched in Pb compared to other SEDEX deposits. Metamorphism is a charactelisoc feature but not a prerequisite for inclusion in the Broken Hill-type category, and IGlown Broken Hill-type examples appear to occur in Paleo- to Mesoproterozoic terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category of SEDEX deposits.

100th Anniversary Special Paper: Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits

Abstract: In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vapor-derived fluid. Analyses of vapor (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperatures (400°–650°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeCl m . n H2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species. Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, by showing a range of possible vapor evolution paths for the conditions of porphyry-epithermal systems. At the world-class Bingham Canyon porphyry Cu-Au deposit, evidence from fluid inclusions supports a model in which a single-phase fluid of intermediate to vapor-like density ascends from a magma chamber. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapor side of the critical curve, without significantly changing the composition of the expanding vapor. Vapor and brine reach Cu-Fe sulfide saturation as both phases cool below 425°C. Vapor, which is the dominant fluid in terms of the total mass of H2O, Cu, and probably even Cl, is interpreted to be the main agent of metal transport. The evolution of fluids leading to high-grade epithermal gold mineralization is initiated by an H2S-, SO2-, Au-, and variably Cu- and As-rich vapor, which separates from an FeCl2-rich brine in a subjacent porphyry environment. In the early stages of the hydrothermal system, vapor expands rapidly and on reaching the epithermal environment, condenses, producing hypogene advanced argillic alteration and residual vuggy quartz and, in some cases, coeval high-sulfidation precious metal mineralization (e.g., Pascua). More commonly, the introduction of precious metals occurs somewhat later, after the site of magmatic fluid exsolution has receded to greater depth. Because of the relatively high pressure, the vapor separating from brine at this stage cools along a pressure-temperature path above the critical curve of the system, causing it to contract to a liquid capable of transporting several parts per million Au to temperatures as low as 150°C.

100th Anniversary Special Paper: Metal Concentrations in Crustal Fluids and Their Relationship to OreFormation

Abstract: A database of saline fluid compositions, including deep shield ground waters, sedimentary formation waters, geothermal brines, and fluids from metamorphic and igneous rocks and veins, is used to explore the controls on metal concentrations in crustal fluids. There are no systematic differences between analyses of fluids sampled by drilling and analyses of fluid inclusions. Over the wide range studied, temperature emerges as a dominant control on the concentrations of Fe, Mn, Zn, and Pb in solution, although the more limited data for Cu are equivocal. Chloride concentration is also important, with the mole ratio metal/chloride (Me/Cl) remaining reasonably constant at constant temperature over a wide range of chlorinities for all four metals considered in detail. There is no evidence for significant differences in transition-metal speciation with increasing chloride nor between low- and high-temperature fluids, although in the case of Zn, complexes with additional Cl may be important at low temperature. Plots of log Me/Cl versus 1/T for the transition metals considered each yield a linear correlation, with about five orders of magnitude variation in Me/Cl between diagenetic and magmatic temperatures. There is approximately two orders of magnitude variability at each temperature, which probably arises in large part from variations in pH. Limited data for low-salinity, CO2-rich fluids indicate that they lie on the same trends, with transition-metal concentrations controlled by fluid salinity and temperature. Order of magnitude concentrations of Fe, Mn, Zn, and Pb in any chloride-dominated crustal fluid can be predicted with the following equations (T in K, concentration ratios are molar): log (Fe/Cl) = 1.4 – (1,943/T) ± 1; log (Mn/Cl) = 0.55 – (1,871/T) ± 1; log (Zn/Cl) =– (1,781/T) ± 1; log (Pb/Cl) = –1.2 – (1,533/T) ± 1. The results demonstrate that crustal fluids are strongly buffered through interactions with the rocks (or melts) that host them. Thus, of the major variables influencing metal concentrations in solution, only temperature and chloride concentration can be considered as truly independent. The plots show that metal-rich fluids may arise through equilibration of chloride-rich waters with normal silicate rocks. Saline magmatic fluids, which may attain extremely high concentrations of transition metals, have clear ore-forming potential, as do formation brines from deep, hot basins; cooler basins do not permit such high concentrations of base metals to be attained. The results of this study emphasize the importance of the distribution and cycling of chloride in the crust for the distribution of base metal deposits; it is often the salinity of ore fluids that is the primary anomaly.

Evolution of a Submarine Magmatic-Hydrothermal System: Brothers Volcano, Southern Kermadec Arc, New Zealand

Abstract: Brothers volcano, which is part of the active Kermadec arc, northeast of New Zealand, forms an elongate edifice 13 km long by 8 km across that strikes northwest-southeast. The volcano has a caldera with a basal diameter of ~3 km and a floor at 1,850 m below sea level, surrounded by 290- to 530-m-high walls. A volcanic cone of dacite rises 350 m from the caldera floor and partially coalesces with the southern caldera wall. Three hydrothermal sites have been located: on the northwest caldera wall, on the southeast caldera wall, and on the dacite cone. Multiple hydrothermal plumes rise ~750 m through the water column upward from the caldera floor, originating from the northwest caldera walls and atop the cone, itself host to three separate vent fields (summit, upper flank, northeast flank). In 1999, the cone site had plumes with relatively high concentrations of gas with a ΔpH of −0.27 relative to seawater (proxy for CO2 + S gases), dissolved H2S up to 4,250 nM, high concentrations of particulate Cu (up to 3.4 nM), total dissolvable Fe (up to 4,720 nM), total dissolvable Mn (up to 260 nM) and Fe/Mn values of 4.4 to 18.2. By 2002, plumes from the summit vent field had much lower particulate Cu (0.3 nM), total dissolvable Fe (175 nM), and Fe/Mn values of 0.8 but similar ΔpH (−0.22) and higher H2S (7,000 nM). The 1999 plume results are consistent with a magmatic fluid component with the concentration of Fe suggesting direct exsolution of a liquid brine, whereas the much lower concentrations of metals but higher overall gas contents in the 2002 plumes likely reflect subsea-floor phase separation. Plumes above the northwest caldera site are chemically distinct, and their compositions have not changed over the same 3-year interval. They have less CO2 (ΔpH of −0.09), no detectable H2S, total dissolved Fe of 955 nM, total dissolved Mn of 150 nM, and Fe/Mn of 6.4. An overall increase in 3He/4He values in the plumes from R/RA = 6.1 in 1999 to 7.2 in 2002 is further consistent with a magmatic pulse perturbing the system. The northwest caldera site is host to at least two large areas (~600 m by at least 50 m) of chimneys and sub-cropping massive sulfide. One deposit is partially buried by sediment near the caldera rim at ~1,450 m, whereas the other crops out along narrow, fault-bounded ledges between ~1,600 and 1,650 m. Camera tows imaged active 1- to 2-m-high black smoker chimneys in the deeper zone together with numerous 1- to 5-m-high inactive spires, abundant sulfide talus, partially buried massive sulfides, and hydrothermally altered volcanic rocks. 210Pb/226Ra dating of one chimney gives an age of 27 ± 6 years; 226Ra/Ba dating of other mineralization indicates ages up to 1,200 years. Formation temperatures derived from Δ34Ssulfate-sulfide mineral pairs are 245° to 295° for the northwest caldera site, 225° to 260°C for the southeast caldera and ~260° to 305°C for the cone. Fluid inclusion gas data suggest subsea-floor phase separation occurred at the northwest caldera site. Alteration minerals identified include silicates, silica polymorphs, sulfates, sulfides, Fe and Mn oxide and/or oxyhydroxides, and native sulfur, which are consistent with precipitation at a range of temperatures from fluids of different compositions. An advanced argillic assemblage of illite + amorphous silica + natroalunite + pyrite + native S at the cone site, the occurrence of chalcocite + covellite + bornite + iss + chalcopyrite + pyrite in sulfide samples from the southeast caldera site, and veins of enargite in a rhyodacitic sample from the northwest caldera site are indicative of high-sulfidation conditions similar to those of subaerial magmatic-hydrothermal systems. The northwest caldera vent site is a long-lived hydrothermal system that is today dominated by evolved sea-water but has had episodic injections of magmatic fluid. The southeast caldera site represents the main upflow of a relatively well established magmatic-hydrothermal system on the sea floor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each of these different vent sites represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.

Siderophile and Chalcophile Metal Variations in Flood Basalts from the Siberian Trap, Noril’sk Region: Implications for the Origin of the Ni-Cu-PGE Sulfide Ores

Abstract: The Talnakh, Kharaelakh, and Noril’sk I intrusions of the Noril’sk region are coeval with the Permo-Triassic Siberian trap flood basalt and contain one of the largest known resources of Ni-, Cu-, and platinum group element (PGE)-enriched sulfide mineralization. The ~3.5-km-thick stratigraphy of the basalts consists of a Lower sequence of high Ti alkalic, subalkalic, and picritic basalts, and an Upper sequence of low Ti basalts. The stratigraphy of the Upper sequence consists of a group of picritic and tholeiitic basalts assigned to the Tuklonsky Formation, overlain by the Nadezhdinsky Formation which is represented by a sequence of contaminated tholeiites that show an upward decline in the degree of contamination into the overlying Morongovsky and Mokulaevasky Formations. The Nadezhdinsky Formation is depleted in Ni, Cu, and PGE, with the most depleted basalts having concentrations below the analytical determination limits of 0.06 ppb for Pd and 0.1 ppb for Pt and a factor of at least 100 less than the Pd and Pt concentrations of the rest of the Upper sequence. The greatest PGE depletion occurs a short distance above the base of the Nadezhdinsky, and these flows are overlain by basalts that exhibit a gradual recovery in PGE concentration toward values typical of the Mokulaevsky Formation and the overlying basalts (~6.8–12.1 ppb Pt and ~7.2–16.5 ppb Pd). Within the Nadezhdinsky Formation, there is a strong correlation between Pd/Zr and La/Sm ratios that indicates that the magmas with the largest contribution from crustal material are also the most PGE depleted. Moreover, the contribution of crust to the magma and the degree of metal depletion decreased through time. The PGE-undepleted Upper sequence tholeiites have Pd/Pt of 0.45 to 2.0, and the ore-forming sulfides (Pd/Pt ~3–4) were probably formed from magmas with Pd/Pt ratios at the upper limit of this range; the metal-depleted basalts have lower values (Pd/Pt ~0.3). The high Pd/Pt ratio of the ores and the low Pd/Pt ratio of the basalts indicate that the processes of ore formation and flood basalt magmatism were important because they indicate that the basalts have the Pd/Pt ratio of a magma that has segregated sulfide with the same Pd/Pt ratio as the Noril’sk ores. Moreover, the ore deposits are located in the region where the metal-depleted Nadezhdinsky Formation is ~500 m thick and forms a >5,000-km3 volcanic center. The continuous changes in basalt chemistry through the stratigraphy of the Upper sequence are consistent with processes that took place in a very large staging chamber rather than within multiple ~5- to 10-km3 discrete high-level magma chambers on the scale of the mineralized intrusions. In the staging chamber, Tuklonksy-type magmas interacted with crust to produce the contaminated basalts of the Nadezhdinsky Formation. The staging chamber containing the crustally contaminated magma was then replenished with PGE-undepleted magma. Initially, the replenishing magma became sulfide saturated, thereby producing the ore-forming sulfides, and the magma in the staging chamber remained sulfide-saturated. As further injections of PGE-undepleted magma entered the staging chamber, the melt became sulfide undersaturated and magmas tapped from it were progressively less PGE depleted. The deposits of the Noril’sk, Talnakh, and Kharaelakh intrusions were formed by injection of olivine-bearing melts containing immiscible sulfide that were produced in the staging chamber. The ores inherited a heavy sulfide isotope composition possibly by reaction with evaporite-laden country rocks. The unradiogenic composition of the Os in the ores is consistent with interaction between the crustally contaminated magma (which had become depleted in the PGE and radiogenic Os) in the staging chamber with later pulses of uncontaminated mantle-derived magma. The high PGE contents of PGE-undepleted Siberian trap are due to interaction of plume-generated picritic magmas with lithosphere that had been depleted in sulfide by previous melt extraction.

A New Look at the Geology of the Zambian Copperbelt

Abstract: The Zambian Copperbelt accounts for approximately 46 percent of the production and reserves of the Central African Copperbelt, the largest and highest grade sediment-hosted stratiform copper province known on Earth. Deposits in the Zambian Copperbelt are hosted by the Neoproterozoic Katangan Supergroup, a relatively thin (~5 km) basinal succession of predominantly marginal marine and terrestrial metasedimentary rocks that lacks significant volumes of igneous rocks. The stratigraphic architecture of the Katangan Supergroup in the Zambian Copperbelt is comparable to that of Phanerozoic rift systems. The basal portion of the sequence (Lower Roan Group) contains continental sandstones and conglomerates deposited in a series of restricted subbasins controlled by extensional normal faults. These largely terrestrial sediments are abruptly overlain by a regionally extensive, variably organic rich marginal marine siltstone/shale (Copperbelt Orebody Member, or 'Ore Shale') that contains the majority of ore deposits. This horizon is overlain by laterally extensive marine carbonates and finer grained clastic rocks that evolved through time into a platformal sequence of mixed carbonate and clastic (Upper Roan Group) rocks with abundant evaporitic textures, including widespread breccias thought to record the former presence of salt, now dissolved. Rocks of the overlying Mwashia and Kundelungu Groups are dominantly shallow marine in origin. Three significant tectonic events affected the basin. Extension associated with early rifting led to the development of isolated fault-controlled basins and subsequent linkage of these basins along master faults at the time of Copperbelt Orebody Member deposition. A later period of extension occurred during late Mwashia to early Kundelungu time (~765-735 Ma) and is associated with limited mafic magmatism. Basin inversion and later compressive deformation (~595-490 Ma) culminated in upper greenschist-grade metamorphism (~530 Ma) in the Zambian Copperbelt. The majority of ore deposits in the Zambian Copperbelt occur within a 200-m stratigraphic interval centered on the rocks of the Copperbelt Orebody Member. Deposits are broadly stratiform and are grouped into argillite- (70% of ore) and arenite-hosted (30% of ore) types. The distribution, geometry, and size of deposits are fundamentally controlled by early subbasin fault architecture and the availability of both in situ and mobile reductants, the distribution of which is linked to basin structures. Argillite-hosted deposits occur within relatively dark and locally carbonaceous siltstones and shales, suggesting the former presence of an in situ organic reductant. These deposits are laterally extensive with strike lengths up to 17 km. Arenite-hosted deposits occur in both the footwall and hanging wall of the Ore Shale and have maximum strike lengths of 5 km. They occur at sites that were geometrically favorable for mobile hydrocarbon or sour gas accumulation. Both argillite- and arenite-hosted deposits contain so-called barren gaps of weakly to unmineralized strata that are typically associated with the fault-bounded shoulders of early subbasins. Two mineralization assemblages occur in the Zambian Copperbelt. The volumetrically dominant type consists of prefolding disseminated and lesser vein-hosted Cu-Co sulfides. The most typical sulfide assemblage in the deposits is chalcopyrite-bornite with subsidiary chalcocite and pyrite. The Zambian Copperbelt is unusual among sediment-hosted stratiform copper districts in having abundant Co and low Ag, Zn, and Pb. The Cu-Co sulfide carrollite is widespread in the district, although cobalt is present in economic quantities in only some deposits on the western side of the district. The Zambian Copperbelt also contains ubiquitous, but volumetrically minor, Cu-U-Mo-(Au) mineralization in postfolding veins. Cu-Co sulfides display complex textural relationships that are best explained by multistage ore formation. Diagenetic to late diagenetic mineralization is indicated by the typically nonfracture-controlled distribution of both sulfide and gangue phases, replacive textures of Cu-Co sulfides after diagenetic cements and pyrite, and an approximate 815 Ma Re-Os isochron age for sulfide precipitation at the Konkola deposit. Brines capable of mobilizing metals were most likely generated during development of evaporitic environments in units of the Upper Roan Group, and/or subsequent dissolution of these evaporites to form the Upper Roan Group breccias. Late diagenetic to early orogenic mineralization is recorded by prefolding bedding-parallel veinlets and texturally and compositionally comparable disseminated Cu-Co sulfides. An Re-Os isochron age on Cu-Co sulfides from two arenite- and one argillite-hosted deposits of 576 plus or minus 41 Ma is consistent with early orogenic hydrocarbon or sour gas production. The minor Cu-U-Mo-(Au) mineralization event occurred following ostpeak metamorphism, at approximately 500 Ma. The Zambian Copperbelt ore province is characterized by stratigraphically and laterally widespread metasomatism that records a protracted history of basinal brine migration. Although the alteration history is complex, it can be broadly categorized into an early Ca-Mg-SO4, anhydrite- and dolomite-dominant stage involving brine reflux below the level of Upper Roan Group evaporites; a second, K-dominant stage characterized by widespread and commonly intense development of K-feldspar and locally sericite, best developed in rocks of the Lower Roan Group and associated with ore; and a third, Na-dominant stage characterized by development of albite, commonly at the expense of earlier-formed K-feldspar. Albite dominates in Upper Roan Group breccias and Mwashia-Lower Kundelungu strata. It is also locally associated with a late Cu-U-Mo-(Au) vein event. Although none of these alteration types are direct guides to ore, they demonstrate widespread brine circulation within the lower parts of the Katangan Supergroup.

100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits

Abstract: Mineral deposit types commonly have a distinctive temporal distribution with peaks at specific periods of Earth history. Deposits of less redox-sensitive metals, such as gold, show long-term temporal patterns that relate to first-order changes in an evolving Earth, as a result of progressively declining heat production and attendant changes in global tectonic processes. Despite abundant evidence for plate tectonics in the early Precambrian, it is evident that plume events were more abundant in a hotter Earth. Episodic growth of juvenile continental crust appears to have been related to short-lived (<100 m.y.) catastrophic mantle-plume events and formation of supercontinents, whereas shielding mantle-plume events correlated with their breakup. Different mineral deposit types are associated with this cycle of supercontinent formation and breakup. Broadly synchronous with juvenile continental crust formation was the development of subcontinental lithospheric mantle, which evolved due to progressively declining heat flow and decreasing plume activity. Archean subcontinental lithospheric mantle has a distinct mineralogical composition and is buoyant, whereas later lithosphere was progressively more dense. Changes in the buoyancy of both oceanic lithosphere and subcontinental lithospheric mantle led to evolution of tectonic scenarios in which buoyant, roughly equidimensional, early Precambrian cratons were rimmed by Proterozoic or Phanerozoic linear elongate belts of neutral to negative buoyancy. Orogenic gold deposits, which formed over at least 3.4 b.y., had the highest preservation potential of any gold deposit type. The pattern of formation and preservation, from episodic to more cyclic, broadly mirrors that of crustal growth. Early Precambrian (mostly ca. 2.7 and 2.0–1.8 Ga) deposits, protected from uplift and erosion in the centers of buoyant cratons, are rare between ca. 1.7 Ga and 600 Ma due to the change to more modern-style plate tectonic processes, with nonpreservation of deposits of this age due to uplift and erosion of more vulnerable orogenic belts. Volcanic-hosted massive sulfide (VHMS) deposits were accreted into the convergent margin terranes in which orogenic gold deposits were forming. Their temporal distribution, from strongly episodic to more cyclic peaks, also supports a model of selective preservation. The first appearance of iron-oxide copper-gold (IOCG) deposits at ~2.55 Ga closely follows development of early Precambrian subcontinental lithosphere mantle. Their genesis involved melting of metasomatized subcontinental lithosphere mantle, so they could not form until such metasomatized mantle evolved below cratons with buoyant lithosphere. Giant Precambrian paleoplacer gold deposits probably formed by effective fluvial sorting under extreme climatic conditions but were largely preserved due to early buoyant subcontinental lithospheric mantle below hosting foreland basins. Unequivocal intrusion-related gold deposits are related to complex felsic intrusions with a mixed mantle-crustal signature, which intruded deformed shelf sedimentary sequences close to but outside craton margins. Given that post-Paleoproterozoic uplift and erosion is likely in vulnerable orogenic belts with negatively buoyant lithosphere, this deposit type is likely to be rare in Paleozoic and older terranes. Gold-bearing deposit types thus display distinctive temporal distributions related to change from a more buoyant plate tectonic style in the early hotter Earth to a modern plate tectonic style typical of the Phanerozoic. Late Archean formation of buoyant subcontinental lithospheric mantle was particularly important in the anomalous preservation of some earlier formed deposit types located inboard of craton margins and in providing critical conditions for the formation of others. Development of negatively buoyant subcontinental lithospheric mantle can explain the lack of preservation of some deposit types that formed in the later Proterozoic. A single fundamental concept of coupled episodic crustal growth and preservation in the Archean and Paleoproterozoic, evolving to decoupled episodes of growth and preservation from the Mesoproterozoic onward, can thus explain the temporal distribution of a number of gold-bearing mineral deposit types.

The sediment-hosted stratiform copper ore system

Abstract: Sediment-hosted stratiform copper deposits comprise disseminated to veinlet Cu and Cu-Fe sulfides in siliciclastic or dolomitic sedimentary rocks. Sediment-hosted stratiform copper deposits are extremely common though economically significant deposits are rare. They account for approximately 23 percent of the world's Cu production and known reserves in addition to being significant sources of Co and Ag. Three sedimentary basins (the Paleoproterozoic Kodaro-Udokan in Siberia, the Neoproterozoic Katangan in central Africa, and the Permian basin of central Europe) contain supergiant (>24 million metric tons (Mt) contained Cu) deposits. Sediment-hosted stratiform copper deposits are the products of evolving basin-scale fluid-flow systems that include source(s) of metal and S, source(s) of metal- and S-transporting fluids, the transport paths of these fluids, a thermal and/or hydraulic pump to collect and drive the fluids, and the chemical and physical processes which result in precipitation of the sulfides. Metal sources are undoubtedly red-bed sedimentary rocks containing Fe oxyhydroxides capable of weakly binding metals. Sulfur may be derived from marine or lacustrine evaporites, reduced seawater, or hydrogen sulfide-bearing petroleum. Metals appear to have been transported at low to moderate temperatures in moderately to highly saline aqueous fluids, with the temperature of the fluid largely dependent on the time of fluid migration in the basin's burial history. These basinal fluids were focused to potential metal precipitation sites by thinning of the red-bed sequence at basin margins, by faults, by differentially permeable sedimentary units, by paleotopography within the basin, or along the margins of salt diapirs. Fluid movement produced widespread, basin-scale alteration that has commonly been overlooked but can form an important exploration guide. Sulfide precipitation occurred due to reduction, typically caused by reaction with carbonaceous rocks or petroleum. The amount of sulfides present at any deposit may be either metals or sulfide limited or could have been controlled by the amount of available reductant (e.g., petroleum). While understanding of sediment-hosted stratiform copper ore genesis at the deposit scale is relatively robust, there are still significant questions in regards its position in terms of basin evolution. A wide variety of basin architectures and processes can lead to the formation of sediment-hosted stratiform copper deposits. Despite general agreement that sulfides postdate sedimentation, the absolute age of mineralization in many deposits has been difficult to document and the available evidence suggests that deposits can form throughout a basin's evolution from early diagenesis of ore host sediments to basin inversion and metamorphism. Supergiant and giant deposits formed in basins which underwent prolonged periods of fluid flow and in which unique conditions allowed for the accumulation of large amounts of metal-bearing fluid, sufficient reduced S, and large amounts of reductants.

Mixing of Sodic and Calcic Brines and Uranium Deposition at McArthur River, Saskatchewan, Canada: A Raman and Laser-Induced Breakdown Spectroscopic Study of Fluid Inclusions

Abstract: The richest U deposit in Saskatchewan, Canada, occurs in the McArthur River area, in the vicinity of the unconformity between the Athabasaca sandstones and an Archean to lower Proterozoic basement. Paleofluids related to the silicification of the sandstones and the formation of pre- and postore cements in breccias were studied using microthermometry, Raman microspectroscopy, and laser induced breakdown spectroscopy (LIBS) on individual fluid inclusions. A detailed reconstruction of the fluid composition in the system Na-Ca-Mg-Cl shows that two types of brines are responsible for the main quartz cements: an NaCl-rich brine (25 wt % NaCl, up to 14 wt % CaCl 2 , and up to 1 wt % MgCl 2 ), which is interpreted as a primary formation water that was expelled from bedded evaporites; and a CaCl 2 -rich brine (5–8 wt % NaCl, 20 wt % CaCl 2, and up to 11 wt % MgCl 2 ), which is considered to have formed during the interaction between the NaCl-rich brine and Ca-rich minerals in the basement and was introduced into the fault system and mixed with the NaCl-rich brine during the critical stage of U deposition. The pressure-temperature conditions of formation of the quartz cements are estimated to be 1,200 to1,400 bars and 190° to 235°C for the silicification events during the preore stage, and 500 to 900 bars after a pressure decrease from lithostatic conditions and slightly lower temperatures due to the mixing of the NaCl-rich brine with the cooler (approx 140°C) CaCl 2 -rich brine during the main stage of breccia sealing. Temperature and pressure drops combined with the effects of brine mixing appear to be key factors for the main stages of quartz cementation and U deposition at the McArthur deposit.

The Geomicrobiology of Ore Deposits

Abstract: Bacterial metabolism, involving redox reactions with carbon, sulfur, and metals, appears to have been important since the dawn of life on Earth. In the Archean, anaerobic bacteria thrived before the Proterozoic oxidation of the atmosphere and the oceans, and these organisms continue to prosper in niches removed from molecular oxygen. Both aerobes and anaerobes have profound effects on the geochemistry of dissolved metals and metal-bearing minerals. Aerobes can oxidize dissolved metals and reduced sulfur, as well as sulfur and metals in sulfide minerals can contribute to the supergene enrichment of sulfide ores, and can catalyze the formation of acid mine drainage. Heterotrophic anaerobes, which require organic carbon for their metabolism, catalyze a number of thermodynamically favorable reactions such as Fe-Mn oxyhydroxide reductive dissolution (and the release of sorbed metals to solution) and sulfate reduction. Bacterial sulfate reduction to H S can be very rapid if reactive organic carbon is present and can lead to precipitation of metal sulfides and perhaps increase the solubility of elements such as silver, gold, and arsenic that form stable Me-H S aqueous complexes. Similarly, the bacterial degradation of complex organic compounds such as cellulose and hemicellulose to simpler molecules, such as acetate, oxalate, and citrate, can enhance metal solubility by forming Me organic complexes and cause dissolution of silicate minerals. Bacterially induced mineralization is being used for the bioremediation of metal-contaminated environments. Through similar processes, bacteria may have been important contributors in some sedimentary ore-forming environments and could be important along the low-temperature edges of high-temperature systems such as those that form volcanogenic massive sulfides.

Zinc isotope variation in hydrothermal systems: preliminary evidence from the irish midlands ore field

Abstract: Little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore-forming systems. We have obtained zinc isotope data from 19 sphalerite samples, formed over a range of well-constrained precipitation conditions, from the Irish Zn-Pb ore field. The results reveal variation in δ 66Zn (where δ 66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)standard– 1] × 1000), from –0.17 to 1.33 per mil relative to the Lyon JMC 3-0749L zinc standard. This variation is significant compared to the external reproducibility (±0.12‰, 2 σ ), and the data show very good mass-dependency with δ 67Zn and δ 68Zn values. Thus, natural variations in the zinc isotope composition of these ore minerals can be resolved. Our results span the entire range of δ 66Zn values measured on terrestrial geologic samples to date. The data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field. We suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions. However, we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources. The significant variation in zinc isotope compositions we have observed in the Irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the Earth’s crust.

Stratiform and Strata-Bound Zn-Pb-Ag Deposits in Proterozoic Sedimentary Basins, Northern Australia

Abstract: In terms of zinc, lead, and silver metal endowment, the Proterozoic sedimentary basins of northern Australia rank number one in the world. The Mt Isa-McArthur basin system hosts five supergiant, stratiform, sedimentary rock-hosted Zn-Pb-Ag deposits (McArthur River, Century, Mt Isa, Hilton, and George Fisher), and one supergiant stratabound Ag-Pb-Zn deposit (Cannington). The major stratiform Zn-Pb-Ag deposits exhibit many similar geological and geochemical features that include: 1) location close to regionally extensive normal and strike-slip synsedimentary faults, 2) organic-rich black shale and siltstone host rocks, 3) laminated, bedding-parallel synsedimentary sulfide minerals, 4) stacked ore lenses separated by pyritic and Fe-Mn carbonate-bearing siltstones, 5) lateral zonation exhibiting an increasing Zn/Pb ratio away from the feeder fault, 6) vertical zonation exhibiting decreasing Zn/Pb ratio up-stratigraphy, 7) an extensive stratabound halo of iron- and manganese-rich alteration in the sedimentary rocks surrounding and along strike from ore, 8) a broad range of 34S values for sulfide minerals, from about 0 to 20 per mil, with pyrite exhibiting a greater spread than base metal sulfides, and 9) lead isotope ratios that indicate derivation of lead from intrabasinal sources with interpreted lead model ages being similar to the measured zircon U-Pb ages of the host rocks. These common features demonstrate that the stratiform Zn-Pb-Ag ores formed approximately contemporaneously with sedimentation and (or) diagenesis. Numerical modelling of fluid flow and heat transport in sedimentary basins suggests that free convection, driven by density changes in sedimentary brines, may be the most important process in forming giant SEDEX deposits. In response to periods of active tectonism during the sag phase of sedimentation, extensional faults are reactivated such that they penetrate the shale and/ or carbonate cap rocks to depths of several kilometres and access fluids contained in the rift phase clastic reservoir. The deep metalliferous brines ascend the faults and discharge on, or close-to, the basin floor. Metal precipitation occurs in the anoxic, organic-rich sub-basins developing adjacent to the syn-sedimentary feeder faults.

Fluid Chemistry, Structural Setting, and Emplacement History of the Rosario Cu-Mo Porphyry and Cu-Ag-Au Epithermal Veins, Collahuasi District, Northern Chile

Abstract: The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern Chile. It comprises high-grade Cu-Ag-(Au) epithermal veins, superimposed on the core of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 million metric tons (Mt) at 1.03 percent copper. An additional 1,022 Mt at 0.93 percent copper occurs in the district at the nearby Ujina and Quebrada Blanca porphyry deposits. The Rosario reserve contains over 95 percent hypogene ore, whereas supergene-sulfide ores dominate at Ujina and Quebrada Blanca. Mineralized veins are hosted within Lower Permian volcanic and sedimentary rocks, Lower Triassic granodiorite and late Eocene porphyritic quartz-monzonite. The Rosario fault system, a series of moderate southwest- dipping faults, has localized high-grade Cu-Ag-(Au) veins. At Cerro La Grande, similar high-grade Cu- Ag-(Au) veins are hosted in north-northeast-trending, sinistral wrench faults. Normal movement in the Rosario fault system is interpreted to have been synchronous with sinistral strike-slip deformation at La Grande. Hydrothermal alteration at Rosario is characterized by a K-feldspar core, focused in the Rosario Porphyry that grades out to a secondary biotite-albite-magnetite assemblage. Paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite-albite. Vein crosscutting relationships indicate that K-feldspar formed during and after biotite-albite alteration. Chalcopyrite and bornite were deposited in quartz veins associated with both K-feldspar and biotite-albite assemblages. The early hydrothermal fluid was a hypersaline brine (40-45 wt % NaCl) that coexisted with vapor between 400 degrees and >600 degrees C. Weakly mineralized illite-chlorite (intermediate argillic) alteration of the early K and Na silicate assemblages was caused by moderate temperature (250 degrees-350 degrees C), moderate-salinity brines (10-15 wt % NaCl). Molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events. These fluids were 350 degrees to 400 degrees C with salinities between 10 and 15 wt percent NaCl. Porphyry-style ore and alteration minerals were overprinted by structurally controlled quartz-alunite-pyrite, pyrophyllite-dickite, and muscovite-quartz (phyllic) alteration assemblages. The quartz-alunite-pyrite alteration formed at 300 degrees to 400 degrees C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllite-dickite assemblage formed between 250 degrees and 320 degrees C from dilute (5 wt % NaCl) fluids. An upward-flared zone of muscovite- quartz-pyrite altered rocks surrounds the fault-controlled domain of advanced argillic alteration. Thick veins (0.5-2 m wide) of fault-hosted massive pyrite, chalcopyrite, and bornite precipitated brines with a salinity of 30 wt percent NaCl at temperatures of 250 degrees to 300 degrees C. Pressure-depth estimates indicate that at least 1 km of rock was eroded at Rosario between formation of the K-Na silicate and advanced argillic assemblages. This erosion was rapid, occurring over a period of 1.8 m.y. The Rosario Porphyry intruded immediately after the Incaic tectonic phase, implying that it was emplaced as the Domeyko Cordillera underwent gravitational collapse, expressed as normal faults in the upper crust. Gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high-sulfidation environment onto the Rosario Porphyry. The hydrothermal system responsible for porphyry Cu mineralization at Rosario was partially exhumed prior to the formation of high-sulfidation ore and alteration assemblages. This implies that emplacement of a second blind intrusion occurred somewhere beneath the Rosario and Cerro La Grande high-sulfidation vein systems and is supported by the fault geometry and zoning of precious metals and sulfosalts at the district scale.

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidence from Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the Lisheen Mine

Abstract: Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

Geology, Mineralization, Alteration, and Structural Evolution of the El Teniente Porphyry Cu-Mo Deposit

Abstract: Sir: The El Teniente Cu-Mo deposit in central Chile is an extraordinary accumulation of Cu-Mo ore (Fig. 1⇓), and we welcome the opportunity provided by Skewes and Stern (2007) to discuss and clarify several important issues regarding its classification and interpretation, and to address several incorrect assertions that they make regarding our manuscript (Cannell et al., 2005). We have interpreted El Teniente to be a typical porphyry deposit, in terms of its alteration assemblages, vein and breccia styles, and spatial and temporal relationships between Cu-Mo mineralization and felsic porphyries. Skewes and Stern (2007) dispute our claims that mineralization occurred primarily in veins and was related to felsic intrusive activity. They also take issue with our interpretations of the various geochronological datasets from El Teniente and our classification of El Teniente as a porphyry Cu-Mo deposit. We address each of these in turn below. In order to facilitate the following discussion, Table 1⇓ correlates the geologic nomenclature of Skewes and Stern (2007) with that of Cannell et al. (2005). Figures 1⇓ and 2⇓ illustrate the location of the main geologic units on underground mine level 6 at El Teniente. Also shown on Figure 1⇓ are the approximate locations of the hypogene sulfide mineral zones. Figure 2⇓ shows the block model of Mo grades in the mine. For more details of geologic relationships at El Teniente, refer to figure 4 of Cannell et al. (2005) and figures 4 and 5 of Skewes et al. (2002). Although the dacite porphyry dike is shown as a single unit on Figure 1⇓, note that Rojas (2002, 2003) mapped multiple phases within this composite intrusive body. Skewes and Stern (2007) criticized our use of what they assert to be an “outdated map of hypogene mineralization” …

Geology and SHRIMP U-Pb Geochronology of the Igarapé Bahia Deposit, Carajás Copper-Gold Belt, Brazil: An Archean (2.57 Ga) Example of Iron-Oxide Cu-Au-(U-REE) Mineralization

Abstract: A striking feature of the Carajas region, Brazil, is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor ore mineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarape Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks. SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 ± 34 Ma. This suggests a correlation between the Igarape Bahia volcano-sedimentary sequence and the Grao Para volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 ± 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 ± 12 Ma SHRIMP age of hydrothermal monazite from the Igarape Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajas belt, indicating that mineralization processes at Igarape Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of carbonates from the Igarape Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores. The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarape Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 ± 12 Ma for hydrothermal monazite indicates that Igarape Bahia is an Archean example of this deposit group.

Alteration Mineralogy and Stable Isotope Geochemistry of Paleoproterozoic Basement-Hosted Unconformity-Type Uranium Deposits in the Athabasca Basin, Canada

Abstract: Unconformity-type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks. Mineralization may extend up to 400 m into the underlying basement rocks. Whereas sandstone-hosted unconformity-type deposits have been well studied, deposits hosted primarily in the basement have not. This study examines the deposits at Rabbit Lake, Dawn Lake, and McArthur River, in the Athabasca basin of Canada, which are hosted by the metamorphic Archean and Early Paleoproterozoic rocks forming the basement to younger Late Paleoproterozoic sandstones. Alteration is similar in the three deposits and is characterized by three distinct paragenetic stages: (1) preore alteration involving illitization of plagioclase and amphibole, followed by chloritization of biotite and illite, which formed at ca. 230°C; (2) ore-stage alteration, characterized by uraninite and coarse-grained illite, which formed at ca. 240°C; (3) postore alteration comprising spherulitic dravite, vein chlorite, quartz, calcite, and Fe, Cu, Co, and Pb sulfides, which formed at ca. 135°C. Fluid circulation associated with emplacement of later Mackenzie dikes initiated partial recrystallization of uraninite. A later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca. 50°C. Stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids (δD = ‐43 to ‐21‰., δ18O = 3‐8‰) were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the Athabasca Formation and transported it into the basement via infiltration along fracture zones associated with reverse faults. Graphitic units in the basement and preore alteration served as both physical (fractured zones) and chemical (reductants) traps for the uranium mineralization. The basinal fluids were responsible for the preore illite-chlorite, synore uraninite-illite, and the early postore alteration events; this differs from many other sandstone-hosted deposits, where both oxidized basinal and reduced basement-derived fluids were responsible for uranium precipitation.

Self-Destructive Sulfide Segregation Systems and the Formation of High-Grade Magmatic Ore Deposits

Abstract: The metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals. Some high-grade magmatic Ni-Cu (±PGE) deposits and many magmatic platinum group element (PGE) (±Ni-Cu) deposits demand very high magma/sulfide ratios that seem improbable in the light of physical and kinetic constraints. Recent models for some high-grade deposits invoke multistage upgrading processes, in which an early-formed sulfide liquid reacts with multiple later batches of silicate magma. Quantitative models of this open-system process demonstrate that it is indeed more efficient than a closed system. However, it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids, further increasing metal concentrations in the remaining sulfide liquids. This combined process is termed "multistage-dissolution upgrading," and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude. Furthermore, if sulfide liquids are extensively dissolved during enrichment, their base metal concentrations will generally stabilize at a limiting value, whereas their PGE concentrations will generally increase without limit. This divergence could account for unusually high PGE concentrations and high PGE/base metal ratios in many PGE-dominated deposits. The multistage-dissolution upgrading model is tested, using reasonable degrees of dissolution, in the context of natural deposits in the Noril’sk area and the Bushveld intrusion. The models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits. Thus, dynamic sulfide-forming magmatic systems may be intrinsically self destructive, but this apparently undesirable attribute could play an important role in forming high-grade deposits. However, dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches. In this case, no sulfide deposit would remain, but metal-depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas. Caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits.

Geochronology of the Sequence Hosting the Broken Hill Pb-Zn-Ag Orebody,Australia

Abstract: Robust, stratigraphic control and constraints on the timing of events have been established for the Paleoproterozoic metasedimentary and metavolcanic succession that hosts the Broken Hill Pb-Zn-Ag orebody in western New South Wales. Geologically consistent depositional, intrusive, and metamorphic ages are elucidated by means of SHRIMP U-Pb analyses of zircon and integrated field studies. Comparisons of the new SHRIMP results with those of previous studies clarify some previously untenable age interpretations. A range of zircon provenance ages is found in albitic metasedimentary rocks from one of the oldest recognized parts of the Willyama Supergroup (Thackaringa Group). The zircons indicate maximum depositional ages of 1720 to 1730 Ma, and a minimum stratigraphic age for the lower Thackaringa Group is constrained by the 1704 ± 3 Ma intrusive Alma Gneiss. The latter is the same age as gneissic granitoid rocks in the Redan zone (southern Broken Hill Domain: 1703 ± 3, 1705± 3 Ma). Felsic magmatism of this age has long been known in the contiguous Olary Domain (South Australia), but has not been recognized previously at Broken Hill. Depositional ages in the Broken Hill Group, the upper part of which hosts the Pb-Zn-Ag orebody, indicate that the entire package was deposited within a period of ca. 10 m.y. An age of 1685 ± 3 Ma for felsic metavolcaniclastic rocks in the Hores Gneiss (spatially and stratigraphically associated with the ore horizon) corroborates previous work. This age is within error of that determined for the hanging-wall and footwall unit (Rasp Ridge Gneiss) of the orebody (1683 ± 3 Ma). It suggests that felsic magmatism in the Rasp Ridge Gneiss was coeval with that of the Hores Gneiss. Previous interpretations, which suggested that the Hores Gneiss is not a single stratigraphic entity but comprises younger intrusions having ages from 1690 to1640 Ma, are not supported by our data and conclusions. The stratigraphic integrity of the Broken Hill Group (excluding some mafic intrusions) is further supported by results from an older formation near its base, where tuffaceous metasedimentary rocks from the Ettlewood Calc-Silicate Member (Allendale Metasediments) define an age of 1693 ± 4 Ma. This is in good agreement with the age of 1693 ± 5 Ma for the immediately overlying Parnell Formation. Provenance ages obtained from detrital zircons in Broken Hill Group psammites indicate a remarkable uniformity of Paleoproterozoic and late-Archean source(s). Generally matching provenance ages are found in the overlying Sundown Group psammopelites, suggesting a stable flux of sediment fill as the basin deepened and matured. Similar provenance is indicated for upper Paragon Group sedimentary rocks, and inherited zircons in most of the younger granitoids have age spectra similar to those of the detrital zircons. The perpetuation of these age patterns, in both clastic detritus and magmatic xenocrysts, thus broadly fingerprints exposed and buried older crustal elements that provided material to produce the Willyama Supergroup basin fill. Tuffaceous metasiltstones in the middle Paragon Group (Bijerkerno Metasediments) are interpreted to have been derived partly from air-fall volcaniclastic material and give consistent depositional ages of 1655 ± 4 and 1657 ± 4 Ma in the northern Broken Hill and Euriowie blocks, respectively. Younger tuffaceous metasiltstones in the upper Paragon Group (Dalnit Bore Metasediments) provide a stratigraphically consistent, maximum depositional age of 1642 ± 5 Ma. Zircon ages from granitoid intrusions that bracket the deformation events suggest that the D 2 and D 3 deformations are inseparable within the interval 1597 ± 3 to 1591 ± 5 Ma. Low Th/U zircon overgrowths are ubiquitous and, irrespective of stratigraphic position, record metamorphic ages of ~1600 Ma. Metamorphic rutile has the same age, confirming that high-grade metamorphic event(s) and D 2 and D 3 structural events were approximately synchronous. We find no isotopic support for any pre-1600 Ma, high-grade metamorphic events. Using newly acquired local age control for the galena Pb isotope growth curve, the new data support a stratigraphic (syngenetic or diagenetic) model of primary ore formation for the Broken Hill orebody at ca. 1690 Ma, although the present distribution of the lodes likely reflects displacement as a result of later structural remobilization.

100th Anniversary Special Paper:>On Hydrothermal Convection Systems and the Emergence of Life

Abstract: Not only have submarine hydrothermal systems been responsible for a variety of mineral deposits, they may also have contributed to the emergence of life in the Hadean. Sulfide deposits can be precipitated where metalbearing hydrothermal solutions invade bacteriogenic H2S-bearing wet sediments and the overlying seawater or brine. Similarly, life might be viewed as a complex organic product that emerged when and where hydrothermal H2 and other reduced chemical species reacted with CO2 and other mildly oxidized molecules dissolved in the Hadean ocean. Semipermeable and semiconducting FeS barriers or membranes would have precipitated spontaneously where H2 and HS ‐ -bearing alkaline waters at ≤110°C seeped into the cool mildly acidic Fe-bearing Hadean ocean at a submarine hydrothermal mound on a ridge flank or on the deep ocean floor. The mound, consisting of Mg-rich clays, ephemeral carbonates, green rust, as well as the sulfides, acted as a natural, selfrestoring hydrothermal reactor. In particular, the sulfide barriers, composed of mackinawite and greigite, prevented the immediate titration of the two fluids and controlled their interactions. Metal sulfides were, and in tiny amounts are still, vital to all cells. Among other reactions they help catalyze the reduction of CO2 in autotrophic bacteria, including photosynthetic organisms. And the structure of greigite (NiFe5S8) is remarkably similar to the active sites (e.g., NiFe4S5) of the enzyme promoting the early metabolic pathway that generates acetate (CH3COO ‐ ) and H2O from CO2, H2, and a methyl group (-CH3). So clusters of greigite, sequestered in a simple organic envelope, could have acted as a protoenzyme, catalyzing the synthesis of acetate in the hydrothermal mound in the same way. Although, like “spent” ore fluid, most of the acetate and all of the water would have been lost to the Hadean ocean, an acetate fraction retained in microcavities within the mound could have combined to form the simple organic building blocks of life. Hydrothermal ammonia and minor cyanide also would have contributed to the synthesis of amino and nucleic acids. Traces of phosphorylated organic molecules, such as RNA (ribonucleic acid), would have adhered to mineral surfaces in the membranous barriers. Once their phosphates were bonded to such a surface, short RNA strands could have polymerized and provided a crude code for the assembly of variable sequences of amino acids (incipient proteins) generated in the same milieu. Alternatively, they could have replicated further RNA. Amino-acid sequences were a significant component of the first membranes and would have influenced membrane and cell survival. Once RNA codes for successful amino-acid sequences were passed on to daughter cells then life could be said to have emerged and evolution to have begun. Bacteria have flourished around hot springs ever since and on occasion have been responsible for the deposition of giant base metal sulfide deposits at or below the sea floor. Hence the study of life may be best begun by the study of those physico-chemical phenomena which result from the contact of two different liquids. Stephane Leduc 1911, p. xiv.

Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization

Abstract: The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machali Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower epsilon Nd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernandez Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals.

100th anniversary special paper: Sedimentary mineral deposits and the evolution of earth's near-surface environments

Abstract: The nature of sedimentary mineral deposits has evolved during Earth's history in concert with changes in the oxidation (redo) state of the ocean-atmosphere system, biological evolution, and the growing importance of geologically young accumulations of ore-grade material. There is now strong evidence that the atmosphere and the oceans were anoxic, or essentially anoxic, before 2.4 Ga. Banded iron formations (BIF) and the detrital uranium ores formed prior to 2.4 Ga are consistent with such a state. The period between 2.4 and 2.0 Ga is called the Great Oxidation Event by some. Its ores bear unmistakable marks of the presence of atmospheric O{sub 2}. Between 1.8 and 0.8 Ga the Earth system seems to have been remarkably stable. Sedimentary ore deposits of this period were influenced by the presence of O{sub 2}. BIF, sedimentary manganese, and phosphorites disappeared ca. 1.8 Ga, but sedimentary exhalative (SEDEX) deposits and unconformity-type uranium deposits flourished, and nonsulfide zinc deposits put in an appearance. The period between 0.8 Ga and the end of the Proterozoic at 0.54 Ga was as turbulent or more so than the Paleoproterozoic. BIF returned, as did sedimentary manganese deposits and phosphorites. A further rise in the O{sub 2} content of themore » atmosphere and an increase in the sulfate concentration of seawater during this period brought the composition of the atmosphere and of seawater close to their present redox state. The last 540 m.y. of Earth's history have seen the system pass through two supercycles of roughly equal length. Climate, the redox stratification of the oceans ocean mixing, and the nature of sedimentary ores were influenced by tectonically and volcanically driven changes during these supercycles. The evolution of the higher land plants gave rise to coal deposits and sandstone-type uranium ores and was important for the formation of bauxites.« less

Magmatic and Hydrothermal Chronology of the Giant Río Blanco Porphyry Copper Deposit, Central Chile: Implications of an Integrated U-Pb and 40Ar/39Ar Database

Abstract: The history of hypabyssal intrusion and hydrothermal activity in the northeastern and central parts of the be-hemothian (sensu Clark, 1993) Rio Blanco-Los Bronces porphyry copper-molybdenum deposit is clarified on the basis of integrated U-Pb and 40Ar/39Ar geochronology. Isotope dilution thermal ion mass spectrometry (ID-TIMS) U-Pb dates for zircon separates and ID-TIMS and sensitive high resolution ion microprobe (SHRIMP) dates for single zircon grains in pre-, syn- and late-mineralization volcanic and intrusive host rocks in the Rio Blanco, Don Luis, and Sur-Sur mining sectors provide a temporal framework for interpretation of incremental-heating and spot-fusion 40Ar/39Ar dates for, respectively, magmatic biotite and hydrothermal biotite, muscovite, and orthoclase. The ore deposit is hosted in part by 16.77 ± 0.25 to 17.20 ± 0.05 (2σ) Ma andesitic volcanic strata of the Farellones Formation, but the major host rocks are units of the San Francisco batholith, including the 11.96 ± 0.40 Ma Rio Blanco granodiorite (mine terminology), the 8.40 ± 0.23 Ma Cascada granodiorite, and the 8.16 ± 0.45 Ma diorite. Hypabyssal dacitic intrusions (late porphyries) emplaced into the batholith yield 206Pb/238U ID-TIMS dates ranging from 6.32 ± 0.09 Ma (quartz monzonite porphyry), through 5.84 ± 0.03 Ma (feldspar porphyry) to 5.23 ± 0.07 Ma (Don Luis porphyry). The late-mineralization Rio Blanco dacite plug yields a SHRIMP zircon age of 4.92 ± 0.09 Ma. The 40Ar/39Ar plateau ages for phenocrystic biotites in quartz monzonite porphyry, feldspar porphyry, and Don Luis porphyry, as well as the preore diorite, range only from 5.12 ± 0.07 to 4.57 ± 0.06 Ma. All are significantly younger than the corresponding zircons and exhibit no correlation with intrusive sequence. The 40Ar/39Ar ages for hydrothermal biotite and orthoclase veins within the San Francisco batholith units fall in a narrow interval from 5.32 ± 0.27 to 4.59 ± 0.11 Ma. Hydrothermal sericites (muscovite), one associated with chalcopyrite, yielded spot-fusion ages of 4.40 ± 0.15 Ma (Rio Blanco granodiorite hosted) and 4.37 ± 0.06 Ma (Don Luis porphyry hosted). Comparison with the ID-TIMS and SHRIMP zircon ages indicates that most of the 40Ar/39Ar ages, even 95 percent plateaus, do not record initial magmatic cooling or hydrothermal alteration-mineralization events, evidence for quasipervasive reheating to at least 300°C by successive intrusions. Published Re-Os ages for two molybdenite samples range from 5.4 to 6.3 Ma and overlap extensively with the zircon U-Pb ages for the late porphyries. They imply that Cu-Mo mineralization overlapped temporally with the emplacement of, at least, quartz monzonite porphyry and feldspar porphyry units of the late porphyry suite and was, therefore, contemporaneous with the rise of dacitic melts to subvolcanic levels. Hydrothermal activity is inferred to have continued until 4.37 ± 0.06 Ma, following intrusion of the Don Luis porphyry and the early stages of emplacement of the Rio Blanco dacite plug complex. Hypogene Cu-Mo mineralization therefore probably persisted for 2 m.y. The geochronologic data do not resolve whether ore formation was continuous or episodic, but the observed crosscutting relationships between intensely altered and mineralized country rocks and less altered and mineralized late porphyry bodies support a model in which the ascent of metal-rich brines from an unexposed zone of the parental magma chamber was periodically stimulated by magma perturbation and hypabyssal intrusion.

Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids

Abstract: Alteration zones at the gold-rich Bajo de la Alumbrera porphyry copper deposit in northwestern Argentina are centered on several porphyritic intrusions. They are zoned from a central copper-iron sulfide and gold-mineralized potassic (biotite-K-feldspar +/- quartz) core outward to propylitic (chlorite-illite-epidote-calcite) assemblages. A mineralized intermediate argillic alteration assemblage (chlorite-illite +/- pyrite) has overprinted the potassic alteration zone across the top and sides of the deposit and is itself zoned outward into phyllic (quartzinuscovite-illite +/- pyrite) alteration. This study contributes new data to previously reported delta(18)O and delta D compositions of fluids responsible for the alteration at Bajo de la Alumbrera, and the data are used to infer likely ore-forming processes. Measured and calculated delta(18)O and delta D values of fluids (+8.3 to +10.2 and -33 to -81 parts per thousand, respectively) confirm a primary magmatic origin for the earliest potassic alteration phase. Lower temperature potassic alteration formed from magmatic fluids with lower delta D values (down to -123 parts per thousand). These depleted compositions are distinct from meteoric water and consistent with degassing and volatile exsolution of magmatic fluids derived from an underlying magma. Variability in the calculated composition of fluid associated with potassic alteration is explained in terms of phase separation (or boiling). if copper-iron sulfide deposition occurred during cooling (as proposed elsewhere), this cooling was largely a result of phase separation. Magmatic water was directly involved in the formation of overprinting intermediate argillic alteration assemblages at Bajo de la Alumbrera. Calculated delta(18)O and delta D values of fluids associated with this alteration range from +4.8 to +8.1 and -31 to -71 per mil, respectively Compositions determined for fluids associated with phyllic alteration (-0.8 to +10.2 and -31 to -119 parts per thousand) overlap with the values determined for the intermediate argillic alteration. We infer that phyllic alteration assemblages developed during two stages; the first was a high-temperature (400 degrees-300 degrees C) stage with D-depleted water (delta D = -66 to -119 parts per thousand). This compositional range may have resulted from magma degassing and/or the injection of new magmatic water into a compositionally evolved hydrothermal system. The isotopic variations also can be explained by increased fluid-rock interaction. The second stage of phyllic alteration occurred at a lower temperature (similar to 200 degrees C), and variations in the modeled isotopic compositions imply mixing of magmatic and meteoric waters. Ore deposition that occurred late in the evolution of the hydrothermal system was probably associated with further cooling of the magmatic fluid, in part caused by fluid-rock interaction and phase separation. Changing pH and/or oxygen fuoracity may have caused additional ore deposition. The ingress of meteoric water appears to postdate the bulk of mineralization and occurred as the system at Bajo de la Alumbrera waned.

Geology and Genesis of the Multistage High-Sulfidation Epithermal Pascua Au-Ag-Cu Deposit, Chile and Argentina

Abstract: The giant Pascua epithermal Au-Ag-Cu deposit is located in the El Indio belt of north-central Chile and Argentina and was the product of a high-sulfidation hydrothermal system. The host rocks consist mainly of Triassic granite and heterolithic Miocene breccia pipes. Granitic rocks host ~60 percent of the mineralization (or >80% if granitic breccia fragments are included) but were not the cause of hydrothermal activity. The economic mineralization forms a large orebody centered on Brecha Central, the largest of the breccia pipes, and several smaller satellite bodies that are separated by zones of subeconomic mineralization. Hydrothermal activity produced two main stages of advanced argillic and vuggy silica alteration. These were separated by an intermediate stage of argillic alteration, silicification and hypogene jarosite, which occurred penecontemporaneously with the emplacement of Brecha Central. Main-stage Au-Ag-Cu mineralization occurred toward the end of the second stage of alteration and involved precipitation of native gold with pyrite and enargite and incorporation of Au in the structure of these minerals. Four types of mineralization are recognized based on the occurrence of economic concentrations of Au in rocks containing significant amounts of the following minerals: (1) alunite, pyrite, and enargite; (2) pyrite; (3) pyrite and szomolnokite; and (4) native gold. Main-stage gold mineralization was followed by a sulfate stage represented by barite and anglesite at lower elevations and barite, anglesite, plus primary szomolnokite, the first reported occurrence of this phase as a hydrothermal ore mineral, at higher elevations. Late-stage silver mineralization, characterized by the occurrence of microscopic Cl-, I-, and Hg-bearing phases in voids and fractures, enriched the upper parts of the deposit, forming an extensive subhorizontal zone that overprints previous alteration and mineralization. Advanced argillic alteration and high-sulfidation mineralization at Pascua are interpreted to have resulted from a high-level hydrothermal system developed above a porphyry stock located in Argentina. Extensive vuggy silica and advanced argillic alteration reflect interaction of the wall rocks with acidic magmatic vapors. Gold, copper, and arsenic are interpreted to have been transported by these vapors and to have deposited as a result of their cooling and subsequent condensation. Ore-forming hydrothermal activity terminated with silver enrichment in response to the condensation of residual magmatic vapors during the waning stages of the system.

Rhenium-Osmium Geochronology of Arsenopyrite in Meguma Group Gold Deposits, Meguma Terrane, Nova Scotia, Canada: Evidence for Multiple Gold-Mineralizing Events

Abstract: Rhenium-osmium geochronology using arsenopyrite was undertaken for three gold deposits in the Meguma terrane, Nova Scotia, Canada, in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally. Analyses of arsenopyrite from bedding-concordant veins from The Ovens locality, southwest Meguma terrane, yield a precise Re-Os isochron age of 409 ± 5 Ma. Arsenopyrite analyses from a bedding-discordant vein at this locality indicate an identical Re-Os age of 407 ± 4 Ma. Saddle-reef veins from the Dufferin deposit, northeast Meguma terrane, contain arsenopyrite with a precise Re-Os isochron age of 380 ± 3 Ma. The lack of common Os in some arsenopyrite samples from both The Ovens and Dufferin permit calculation of single mineral model ages for each deposit, which are identical to those determined using the isochron method. Initial Os compositions for the two vein types at The Ovens suggest a predominately crustal source of Os in the mineralizing fluids, whereas a less radiogenic initial Os composition for arsenopyrite from Dufferin does not as clearly define a crustal metal source. At a third locality, the Touquoy deposit, the Re-Os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age, possibly as a result of mixing of Re and Os derived from the mineralizing fluid and the shale host rock. The Re-Os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the Meguma terrane coinciding with widespread tectonothermal events: regional deformation and metamorphism associated with Acadian orogenesis, and widespread generation of meta- and peraluminous granites and high-grade metamorphism within the basement rocks under the Meguma terrane. The ca. 407 Ma age for The Ovens arsenopyrite provides the best estimate for the timing of regional Acadian deformation in the Meguma terrane and is slightly older than previous estimates based solely on 40Ar/39Ar dating.