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Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25 wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample. This work highlights the impact of shale–water–O 2 interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study.

The Kerman copper belt in Iran contains a number of important porphyry copper deposits, including Sarkuh and Iju. Molybdenite Re-Os isotope dating of the Sarkuh and Iju porphyry copper deposits shows that mineralization occurred at 15.14 ± 0.08 and 9.8 ± 0.06 Ma, respectively. Compared with the previous Re-Os dating of molybdenite in the region, it is revealed that Cu mineralization was an ongoing process in an arc setting during the Miocene. The available zircon dates of the granitic rocks from the Sarkuh (15.18 ± 0.43 Ma) and Iju (9.27 ± 0.50 Ma) porphyry copper deposits indicate that the mineralization occurred contemporaneously with the emplacement of collision-related ore-hosting porphyries. The Re content (1,715.40 ppm) of molybdenite and the δ^(34)S_(CDT) values (0.05‰) of pyrite-chalcopyrite from Iju are consistent with its origin of sulfur and metals from a dominantly mantle source. However, the lower Re content (302.21 ppm) of molybdenite and higher δ^(34)S_(CDT) values (3.20‰) of pyrite-chalcopyrite from Sarkuh suggest additional contributions from crustal materials. It is likely that the younger porphyry copper deposits in the Kerman copper belt, such as Iju, are related to the greater contribution of postcollisional mantle-derived magmas, while the older deposits (e.g., Sarkuh) were formed during the collisional event at the Oligocene-Miocene interval where the magma had some additions from the lower crustal melts generated during crustal thickening.

This study analyzes Re and Os contents of Au rich sulfides from the Carbon Leader (Tau Lekoa and Mponeng), the VCR (Mponeng and Tau Lekoa), and the Black Reef (Modder East), as well as native Au in the Carbon Leader, to further investigate the detrital (placer), modified detrital (placer), and hydrothermal models for the source of Au in mines from the Witwatersrand. An isochron plot of the results from the four locations yields an imprecise isochron age of 2381 ± 140 Ma (MSWD = 17) and initial 187 Os/ 188 Os = 0.24 ± 0.21. Isochron plots for the individual reefs also yielded ages that are younger than the deposition of the basin Witwatersrand sediments. Samples that possess higher concentrations of Os (> 100 ppb) and low 187 Re/ 188 Os ratios ( To unify all of the data providing a large variation of chronologic information, Os mixing diagrams for both gold and sulfide minerals produce realistic mixing trends when solving Os initial ratios for 2050 Ma. The two most probable end members for the mixing relationships are a high Os concentration non-radiogenic source and a low Os concentration highly radiogenic source. The high concentration non-radiogenic Os source has been measured in multiple studies and has been clearly derived from the subcontinental lithospheric mantle (SCLM). The low concentration, highly radiogenic source is debatable. The measured concentrations of Os in the Witwatersrand ores are much greater than published Os concentrations in other hydrothermal ores. The data could indicate a redistribution of the detrital Os signatures at 2050 Ma; thus a modified detrital model best explains the previously published data and this data.

Copper isotope ratios measured in minerals and shallow groundwater and surface waters provide insight into high-temperature mineralization and active weathering processes at the Pebble porphyry Cu-Au-Mo deposit, Alaska. The West zone of the deposit contains hypogene mineralization with a supergene overprint and a thin oxide leached capping, whereas the contiguous East zone contains only hypogene mineralization. Sulfide-rich rock powders and mineral separates have δ65Cu values that range from 0.78 to 2.28‰ (hypogene West), 0.02 to 1.55‰ (hypogene East), �3.49 to 1.88‰ (oxide West), and �5.04 to 1.27‰ (supergene West). The results from hypogene samples show that there is a systematic increase in δ65Cu values from deeper to shallower portions of the deposit. Furthermore, the δ65Cu values correlate with silicate alteration assemblages; mostly positive values correspond to quartz-illite-pyrite, sericite and quartz-pyrophyllite alteration zones which formed at relatively lower temperatures, whereas negative values characterize the higher temperature potassic and sodic-potassic domains. This empirical evidence could indicate that fractionation of Cu isotopes during hypogene alteration is controlled by pH and/or temperature variations. Shallow surface waters proximal to the deposit, and which likely interacted with underlying concealed mineralization, have heavy δ65Cu values which contrast with lighter values in waters distal from the deposit. Patterns measured in the copper isotope ratios of both solids and surface waters demonstrate the potential use of copper isotope distribution as a vectoring tool in mineral exploration and aid in understanding the sources of copper in the surface and near-surface environments.

During weathering, Fe in primary minerals is solubilized by ligands and/or reduced by bacteria and released into soil porewaters. Such Fe is then removed or reprecipitated in soils. To understand these processes, we analyzed Fe chemistry and isotopic composition in regolith of the Shale Hills watershed, a Critical Zone Observatory in central Pennsylvania overlying iron-rich shale of the Rose Hill Formation. Elemental concentrations were measured in soil from a well-drained catena on a planar hillslope on the south side of the catchment. Based upon X-ray diffraction and bulk elemental data, loss of Fe commences as clay begins to weather ∼15 cm below the depth of auger-refusal. More Fe(III) was present than Fe(II) in all soil samples from the ridge top to the valley floor. Both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock. Loss of ferrous Fe is attributed mostly to abiotic or biotic oxidation. Loss of Fe is most likely due to transport of micron-sized particles that are not sampled by porous-cup lysimeters, but which are sampled in stream and ground waters. The isotopic compositions (δ56Fe, relative to IRMM-014) of bulk Fe and 0.5 N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between −0.3‰ and +0.3‰, with Δ56Febulk-extractable values between ∼0.2‰ and 0.4‰. Throughout the soils along the catena, δ56Fe signatures of both bulk Fe and HCl-extracted Fe become isotopically lighter as the extent of weathering proceeds. The isotopic trends are attributed to one of two proposed mechanisms. One mechanism involves Fe fractionation during mobilization of Fe from the parent material due to either Fe reduction or ligand-promoted dissolution. The other mechanism involves fractionation during immobilization of Fe (oxyhydr)oxides. If the latter mechanism is true, then shale – which comprises one quarter of continental rocks – could be an important source of isotopically heavy Fe for rivers.

A significant proportion of the copper in the Canariaco Norte porphyry copper deposit in northern Peru occurs in chalcocite and covellite-rich veins and disseminations that exist from the surface to depths greater than 1 km. The overall range of Cu isotopic ratios of 42 mineral separates from Canariaco varies from −8.42 to 0.61 ‰, with near-surface chalcocite and Fe oxides having isotopically depleted values compared to chalcocite, covellite, and chalcopyrite from deeper levels. The majority (34 of 36) of measured Cu sulfides have a typical hypogene copper isotope composition of δ65Cu = 0.18 ± 0.38 ‰, with no enriched isotopic signature existing in the Canariaco Norte sulfide data. Thus, the copper isotope data indicate that most of the chalcocite and covellite formed from high-temperature hypogene mineralization processes and that only a minor portion of the deposit is enriched by supergene processes. The nonexistence of an enriched δ65Cu reservoir suggest the presence of an undiscovered lateral/exotic Cu occurrence that enriched 65Cu that remained in solution during weathering. Regardless of the cause, the comparative analysis of the Cu isotope dataset reveals that little exploration potential for an extensive supergene enrichment blanket exists because the weathering history at Canariaco Norte was not conducive to preservation of enriched Cu at depth beneath the leach cap.

The complex weathering processes which govern the production of soil from bedrock have proven difficult to understand for many lithologies. Weathering of black shale is of particular interest because it releases organic carbon and heavy metals as solutes and therefore impacts the health of terrestrial and aquatic ecosystems. To understand black shale weathering, a geochemical survey was initiated for soils developed on shales of the Marcellus Formation at a zero-order catchment at a satellite site of the Susquehanna/Shale Hills Critical Zone Observatory located in Jackson Corner, Pennsylvania. This formation is an organic- and metal-rich, carbonaceous shale that underlies much of New York, Pennsylvania, Ohio and West Virginia. In this paper, we focus on the effects of weathering on variations of Cu isotopes in the shale. Cu concentration data for soil were normalized using Ti concentrations to document the mobility of Cu relative to bedrock. At both the ridgetop and valley floor, depletion profiles for Cu are documented in the soils. The Cu in the soils is depleted in 65 Cu (average δ 65 Cu = − 0.5‰ ± 0.2) compared to the parent material (average δ 65 Cu = 0.03‰ ± 0.15). Consistent with loss of Cu from soils, the pore waters contain 10 ppb Cu on average and are enriched in the heavy isotope (average value δ 65 Cu = 1.14‰ ± 0.44). Rayleigh fractionation models using the concentration and isotope data of the soils are consistent with pyrite weathering and loss of Cu from the ridgetop, but downslope transport and Cu re-precipitation at the valley floor.

Copper isotope compositions of secondary phases from the supergene profile of the Bayugo porphyry copper-gold deposit (Philippines) show significant isotopic fractionation relative to hypogene sulfide phases and reveal systematic patterns of isotopic enrichment in the leached cap iron oxides and supergene copper sulfides. Thirty-nine iron oxide, copper oxide, and copper sulfide minerals yielded δ 65 Cu values between −3.4 and +6.9 per mil. Samples of hypogene sulfides have a narrow range of values near zero per mil (−0.6 to +1.0‰). In contrast, samples of supergene sulfides (chalcocite and djurleite) collected from the exotic and enriched copper zones define a trend of decreasing δ 65 Cu from the enriched source area and proximal exotic zone (>3‰) downgradient to the distal portions ( These findings are significant, because within exotic copper zones the copper isotope values of supergene sulfides can provide an indication both of proximity and direction to the source area. The technique could be applied in the search for unrecognized porphyry centers upgradient from headless exotic copper occurrences, by exploring for the supergene copper phases with the highest δ 65 Cu values. Independently, copper isotope ratios of iron oxides in surface samples or fossil leached caps might be used to rank prospects and to focus drilling in areas with the greatest potential for mature enrichment profiles, by mapping the distribution of isotopically enriched leached cap iron oxides. The leached cap screening and targeting tool could enhance the iron oxide mapping techniques traditionally applied in leached cap interpretation and exploration.

Low-temperature ore deposits exhibit a large variation in δ 65 Cu (~ 12‰), and this range has been attributed, in part, to isotope fractionation during weathering reactions of primary minerals such as chalcocite and chalcopyrite. Here, we examine the fractionation of Cu isotopes during dissolution of another important Cu ore mineral, bornite, using a novel approach that combines time-resolved X-ray diffraction (XRD) and isotope analysis of reaction products. During the initial stages of bornite oxidative dissolution by ferric sulfate ( 65 Cu) relative to the solid, with an average apparent isotope fractionation (Δ aq − min = δ 65 Cu aq − δ 65 Cu min 0 ) of 2.20 ± 0.25‰. When > 20 mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with ∆ aq − min 0 values ranging from − 0.21 ± 0.61‰ to 0.92 ± 0.25‰. XRD analysis allowed us to correlate changes in the atomic structure of bornite with the apparent isotope fractionation as the dissolution reaction progressed. These data revealed that the greatest degree of apparent fractionation is accompanied by a steep contraction in the unit-cell volume, which we identified as a transition from stoichiometric to non-stoichiometric bornite. We propose that the initially high ∆ aq − min values result from isotopically heavy Cu ( 65 Cu) concentrating within Cu 2+ during dissolution. The decrease in the apparent isotope fractionation as the reaction progresses occurs from the distillation of isotopically heavy Cu ( 65 Cu) during dissolution or kinetic isotope effects associated with the depletion of Cu from the surfaces of bornite particles.

A non-metallic flow-through reaction cell is described, designed forin situtime-resolved X-ray diffraction coupled with stable isotope analysis. The experimental setup allows the correlation of Cu isotope fractionation with changes in crystal structure during copper sulfide dissolution. This flow-through cell can be applied to many classes of fluid–mineral reactions that involve dissolution or ion exchange.

Isotopic fractionation of Fe and Mo during weathering could contribute toward the isotopic signatures of river and ocean waters. To investigate weathering processes, batch experiments were carried out at pH 6 under oxygenated conditions to investigate the influence of a nitrogen-fixing soil bacterium and organic ligands on the extent and isotopic signature of Fe and Mo release from two Pennsylvania shales with different mineralogies: an olive to grey shale (Rose Hill) and a black shale (Marcellus). Results of these studies showed that Fe and Mo were mainly released from illite, chlorite/vermiculite, and Fe oxides in the RHS and from pyrite in the MS. Dissolution rates of the clays estimated from our batch experiments are broadly consistent with published estimates. Release of Fe from both shales was only enhanced by the hydroxamate siderophore desferrioxamine B (DFAM). In contrast, none of the treatments enhanced release of Mo. Furthermore, release of this metal was measurable only for the black shale. For both shales, Fe that was released in the presence of DFAM was depleted in 56 Fe as compared to the bulk rock. In contrast, leachate solutions were enriched in the heavier Mo isotope as compared to the bulk Marcellus in all experiments with and without organic ligands and the bacterium. The isotopic fractionations in Fe and Mo are broadly consistent with previously reported fractionation effects during sorption onto precipitating Fe,Mn oxides. The observed Mo isotope fractionation is also consistent with all published studies of rivers, i.e., riverine Mo is enriched in heavy Mo isotopes. The observed Fe isotope fractionation is likewise consistent with published studies for some rivers, i.e., riverine Fe is sometimes depleted in 56 Fe. Our experimental results are consistent with sorption of Fe and Mo onto Fe (oxyhydr)oxides during weathering as a possible explanation for the isotopic signatures of these metals in many rivers.

Cu isotope measurements of minerals from American southwest porphyry Cu deposits reveal a positive relationship with the grade of Cu and display spatial patterns. We measured the Cu isotope composition of 87 minerals contained within veins from drill core and hand specimen samples from three deposits (the Silver Bell, Ray, and Morenci mines). The overall range of Cu isotope ratios varies from −13.5 to +8.3 per mil. The deposits possess three different Cu reservoirs in which each has unique Cu isotope signatures, where δ65Cu for leach cap minerals < hypogene minerals < supergene enrichment minerals. The pattern exists in all three deposits and resulted from oxidative weathering of Cu at surface and reprecipitation of Cu at depth. A Rayleigh distillation model mimics the Cu isotope patterns found in the leach cap and quantifies the degree of leaching. A comparison of the isotopic composition of the three reservoirs among the three deposits reveals distinct differences—the Silver Bell mine possesses leach cap minerals that are more depleted in 65Cu and enrichment minerals that are more enriched in 65Cu than both the Morenci and Ray mines. A possible explanation for the difference among the deposits could indicate that the Silver Bell deposit experienced a greater degree of leaching than the Ray and Morenci deposits. The comparative analysis of the Cu isotope data reveals the exploration potential of using Cu isotope fractionation in the supergene environments.

Copper isotope ratios and Cu concentrations have been measured in oxide and sulphide minerals from leached cap (hematite-goethite-jarosite-malachite), supergene (chalcocite-pyrite-hematite) and hypogene (pyrite-chalcopyrite) zones in the Chahfiroozeh, Darrehzar, Iju and Parkam porphyry Cu deposits from the Kerman porphyry Cu belt of Iran. In general, δ 65 Cu values show distinctive values of 0.23‰ to 0.79‰ in hypogene, 2.52‰ to 4.82‰ in supergene and -1.00‰ to -6.16‰ in leached cap minerals. Leach cap minerals contain less Cu than supergene and hypogene minerals. This study supports previous distinct Cu isotopic patterns for leached cap, supergene and hypogene zones reported from other porphyry Cu deposits. Weathering and oxidative dissolution of Cu sulphide minerals normally generate isotopically heavier fluids and lighter residual minerals. Upon encountering reducing conditions at greater depths, the oxidative solutions precipitate secondary Cu-sulphides in the enriched zone and the new minerals possess heavier Cu isotopic compositions. The degree of isotopic separation and difference in concentrations among the three reservoirs is most likely related to the weathering processes and the isotopic signatures present in the leach cap may have exploration potential.

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut. Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ 56 Fe = 0 to − 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ 56 Fe = − 0.26 to − 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10 − 14 mol m − 2 s − 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.

Measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open-system behavior of Re–Os in molybdenites. Molybdenite samples from six porphyry copper deposits, one epithermal polymetallic vein deposit, four skarns, and three Fe-oxide Cu–Au deposits were analyzed. The δ97Mo‰ (where Open image in new window) for all samples varied from 1.34 ± 0.09‰ to −0.26 ± 0.04‰. This is the largest molybdenum isotopic variation in molybdenite from high-temperature ore deposits recorded to date. δ97Mo‰ of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples. Isotope values for Mo also vary within the individual deposits. In general, molybdenites from porphyry copper deposits have the lightest values averaging 0.07 ± 0.23‰ (1σ). Molybdenites from the other deposit types average 0.49 ± 0.26‰ (1σ). The variations could be related to the fractionation of Mo into different mineral phases during the ore-forming processes. A comparison of the Mo isotope ratios and the Re–Os ages obtained from the same aliquot may possess a geochronological evaluation tool. Samples that yielded robust ages have different Mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages. Another observed relationship between the Re–Os and Mo isotope data reveals a weak correspondence between Re concentration and Mo isotope composition. Molybdenites with higher concentrations of Re correspond to lighter Mo isotope values.

We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ 65 Cu‰ = (( 65 Cu/ 63 Cu sample / 65 Cu/ 63 Cu NIST 976 standard ) − 1) ⁎ 10 3 . Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰. Distinct Cu isotopic reservoirs exist for high temperature hypogene, enrichment, and leach cap minerals. Chalcopyrite from high temperature primary mineralization forms a relatively tight cluster of δ 65 Cu values of 1‰ to − 1‰ whereas secondary minerals formed by low temperature reveal a range of δ 65 Cu values from − 16.96‰ to 9.98‰. Secondary chalcocite is relatively heavy with δ 65 Cu varying from − 0.3‰ to 6.5‰. Leach cap minerals dominated by Fe-oxides (jarosite, hematite and goethite) are relatively light ranging from − 9.9‰ to 0.14‰. Although the dataset is relatively small ( n = 50 total minerals analyzed), a combination of these data with values from previously published reports [Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S. and Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma-source mass spectrometry; implications for use as geochemical tracers. Chemical Geology, 163(1–4): 139–149.; Larson et al., 2003; Mathur, R. et al., 2005 . Cu isotopic fractionation in the supergene environment with and without bacteria. Geochimica et Cosmochimica Acta, 69(22): 5233–5246.; Markl et al., 2006, and Maher et al., 2007], show a distinct pattern of heavier isotopic signature in supergene samples and a lighter isotopic signature exists in the leach cap and oxidation zone minerals. The pattern could be used as a tool for exploration geology by providing the following information: 1) Identification of highly fractionated copper isotope ratios in copper sulfide and Fe-oxide samples that indicate supergene processes and the extent of leaching and enrichment copper 2) Identification of highly fractionated copper isotope ratios in surface and/or groundwaters that indicate the active weathering copper sulfides that experienced significant enrichment .

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ65Cu values (based on 65Cu/63Cu) of enargite (δ65Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ65Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ Δ aq-min o = - 0.57 ± 0.14 ‰ , where mino refers to the starting mineral) and no apparent fractionation for enargite ( Δ aq-min o = 0.14 ± 0.14 ‰ ). Abiotic fractionation is attributed to preferential oxidation of 65Cu+ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of 65Cuaq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ65Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.

Copper isotope fractionation in United States cents traces changes in the source of copper and identifies historical events. Application of copper isotopes as a geochemical tracer requires consistent isotopic signatures of the ores and refined metals. Overlapping isotopic signatures of crushed ores, chalcocite and refined metal extracted from Morenci, Arizona indicate modern mining processes that produce distinguishable single ore deposit geochemical signatures. The coincidence of copper isotope ratios in metals and ore deposits also exists within the United States cents analyzed here. Specifically, historical records confirm two different sources for copper in cents from 1800 through 1867. The copper isotope composition of the 1828, 1830, 1836, 1838 and 1843 cents coincides with the Cornwall ores of England, and cents post 1850 (1859, 1862) correspond with the Michigan ores of the United States. Three of the thirty-six post 1867 cents measured possess fractionated copper isotope ratios and indicate the change in source of copper for the United States cent.

The Aguablanca Ni-Cu ore deposit is hosted by subvertical magmatic breccia bodies in which fragments of pyroxenite are cemented by sulfides. It is located within the Aguablanca Stock, which includes diorite, gabbro, norite and gabbronorite. Re-Os isotope data of magmatic pyrrhotite, chalcopyrite, and pentlandite and hydrothermal pyrite show that the Os has a mixed origin, derived from a juvenile mantle source and a crustal source. The more likely contaminant of the primitive melt is the late Neoproterozoic to early Cambrian sequence that extensively crops out in the area. Data from the disseminated ore are aligned along a reference line yielding 383 ± 59 Ma (MSWD = 3.0; 187Os/ 188Osi = 0.17 ± 0.01). This age is similar to previous 40Ar/39Ar and U-Pb data of the host rocks (ca. 340 Ma) and confirms that both the host rocks and the mineralization are Variscan in age. These results also show that both the magmatic and hydrothermal processes took place in a short time span, with the hydrothermal pyrite being ...

The Miduk porphyry copper deposit is located in Kerman province, 85 km northwest of the Sar Cheshmeh porphyry copper deposit, Iran. The deposit is hosted by Eocene volcanic rocks of andesitic – basaltic composi-tion. The porphyry-type mineralization is associated with two Miocene calc-alkaline intrusive phases (P1 and P2, respectively). Five hypogene alteration zones are distinguished at the Miduk deposit, including magnetite-rich potassic, potassic, potassic – phyllic, phyllic and propylitic. Mineralization occurs as stockwork, dissemination and nine generations (magnetite, quartz – magnetite, barren quartz, quartz-magnetite-chalcopyrite-anhydrite, chalcopyrite – anhydrite, quartz-chalcopyrite-anhydrite-pyrite, quartz-molybdenite-anhydrite ± chalcopyrite ± magnetite, pyrite, and quartz-pyrite-anhydrite ± sericite) of veinlets and veins. Early stages of mineraliza-tion consist of M-type veins in the deepest part of the deposit and the main stage of mineralization contains chalcopyrite, magnetite and anhydrite in the potassic zone. The high intensity of mineralization is associated with P2 porphyry (Miduk porphyry). Based on petrography, mineralogy, alteration halos and geochemistry, the Miduk porphyry copper deposit is similar to those of continental arc setting porphyry copper deposits. The Re-Os molybdenite dates provide the timing of sulfi de mineralization at 12.23 ± 0.07 Ma, coincident with U/Pb zircon ages of the P2 porphyry. This evidence indicates a direct genetic relationship between the Miduk porphyry stock and molybdenite mineralization. The Re-Os age of the Miduk deposit marks the main stage of magmatism and porphyry copper formation in the Central Iranian volcano-plutonic belt. Keywords

The isotopic composition of dissolved Cu and solid Cu-rich minerals [δ65Cu (‰) = (65Cu/63Cusample/65Cu/63Custd) - 1)*1000] were monitored in batch oxidative dissolution experiments with and without Thiobacillus ferrooxidans. Aqueous copper in leach fluids released during abiotic oxidation of both chalcocite and chalcopyrite was isotopically heavier (δ65Cu = 5.34‰ and δ65Cu = 1.90‰, respectively, [±0.16 at 2σ]) than the initial starting material (δ65Cu = 2.60 ± 0.16‰ and δ65Cu = 0.58 ± 0.16‰, respectively). Isotopic mass balance between the starting material, aqueous copper, and secondary minerals precipitated in these experiments explains the heavier isotopic values of aqueous copper. In contrast, aqueous copper from leached chalcocite and chalcopyrite inoculated with Thiobacillus ferrooxidans was isotopically similar to the starting material. The lack of fractionation of the aqueous copper in the biotic experiments can best be explained by assuming a sink for isotopically heavy copper present in the bacteria cells with δ65Cu = 5.59 ± 0.16‰. Consistent with this inference, amorphous Cu-Fe oxide minerals are observed surrounding cell membranes of Thiobacillus grown in the presence of dissolved Cu and Fe. Extrapolating these experiments to natural supergene environments implies that release of isotopically heavy aqueous Cu from oxidative leach caps, especially under abiotic conditions, should result in precipitates in underlying enrichment blankets that are isotopically heavy. Where iron-oxidizing cells are involved, isotopically heavy oxidized Cu entrained in cellular material may become associated with leach caps, causing the released aqueous Cu to be less isotopically enriched in the heavy isotope than predicted for the abiotic system. Rayleigh fractionation trends with fractionation factors calculated from our experiments for both biotic and abiotic conditions are consistent with large numbers of individual abiotic or biotic leaching events, explaining the supergene chalcocites in the Morenci and Silver Bell porphyry copper deposits.

The ores of the Ertsberg district consist of both porphyry stockwork and contact controlled mineralization related to the Grasberg and Ertsberg intrusion centers. High-grade copper mineralization with gold occurs in skarns on the periphery of each of the intrusion centers. The periphery mineralization includes the Kucing Liar skarn-related ores, the massive sulfide lenses on the borders of the Grasberg Intrusive Complex, and the giant skarn bodies of the Deep and Intermediate Ore Zones, together with the ores of the Dom complex on the margin of the Ertsberg diorite. The Ertsberg orebody, the basis of discovery of the district, is a copper–gold, roof-pendant of sedimentary strata in the diorite. In order to clarify genetic and geochronologic links between the peripheral sediment-hosted ore bodies and the intrusion-hosted ore bodies, Re–Os isotopes of the ores and subjacent Precambrian and Phanerozoic crust were analyzed. Re–Os geochronology of molybdenites was used to resolve the age relationships of the ore bodies. The Grasberg and Kucing Liar molybdenites have older molybdenite mineralization ages of 2.88 ± 0.02 Ma, and 3.01 ± 0.02 Ma, respectively, whereas the Ertsberg molybdenite has a younger age of 2.54 ± 0.02 Ma. These data agree with Ar chronologies of the alteration associated with mineralization ( Pollard and Taylor, 2000 [Pollard, P.J., Taylor, R., 2000. Geochronology of intrusive rocks and Cu–Au mineralization in the Ertsberg District. In: Proceedings of Freeport-Institut Teknologi Bandung (ITB) Geological Symposium, Bandung, Java, pp. 17–19.]), and also indicate that the Grasberg and Kucing Liar molybdenites are closely similar in age, and older than the Ertsberg Intrusive System. Re–Os data from pyrite, chalcopyrite, magnetite, gold, and the surrounding sedimentary rocks from the district define a mixing relationship between samples from the initial stockwork mineralization and peripheral ores of each intrusion center. Sulfide and oxide minerals from primary stockwork mineralization are isotopically homogeneous and define trends on isochron diagrams with distinct Os initial ratios, whereas sulfides and oxides taken from peripheral ores are isotopically heterogeneous and define trends on mixing diagrams. One possible end member for the mixing is sedimentary rock surrounding the porphyries. The measured Os ratios of the sedimentary rocks range from 0.52 to 2.21, and overlap the Os initial ratios of the sulfides and oxides sampled from both stockwork and contact mineralization. The data suggest a model in which the continental crust supplied some of the Os (and by inference other metals) during magma genesis in the lower crust and the remainder came from sedimentary rocks near the intrusion centers.

New Re-Os molybdenite ages provide constraints on the timing of Late Archean Cu-Au-Mo mineralization in the northern Carajas Mineral Province. Molybdenite from the Gameleira iron oxide Cu-Au-Mo deposit yielded an age of 2,614±14 Ma. This age overlaps within its analytical error with Re-Os ages of molybdenite from the Serra Verde Cu-Au-Mo vein deposit (2,609±13 Ma) and from the nearby small Garimpo Fernando gold mining operation (2,592±13 and 2,602±13 Ma), which is probably related to the latter. The geochronological data imply that the hydrothermal Cu-Au-Mo mineralization in these three deposits was epigenetic and coincides with a regional tectonic regime changing from dextral transtension and clastic sedimentation at 2.7–2.6 Ga to sinistral transpression and inversion at 2.6 Ga. Previously reported stable isotope and microthermometric data are compatible with a magmatic affiliation of the Cu-Au-Mo ores at Gameleira and Serra Verde. A genetic relationship of mineralization may therefore exist with 2.56–2.76 Ga Archean alkaline granitoids or with 2.6–2.7 calc-alkaline to tholeiitic volcanic-arc type magmatism.

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite-magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The 40Ar/39Ar biotite, illite, and alunite dates (at 2 sigma error), and a Re-Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean 40Ar/39Ar plateau age of 34.4 plus or minus 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded 40Ar/39Ar ages of 34.5 plus or minus 0.5 Ma (plateau age) and 32.6 plus or minus 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 plus or minus 0.2 Ma for molybdenite at Rosario is slightly younger than the 40Ar/39Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 plus or minus 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean 40Ar/39Ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35.2 plus or minus 0.3 Ma. The monzonite is intruded by postmineralization porphyry dikes of similar composition, from which igneous biotite yielded a 40Ar/39Ar age of 34.7 plus or minus 0.3 Ma. Igneous biotite in the Rosario Porphyry cooled through its closure temperature during and after the formation of illite, implying that porphyry-style ore and alteration minerals in the Rosario deposit had formed by 34.3 Ma. The age of alunite at Rosario and La Grande indicate that a second discrete episode of hydrothermal activity was superimposed, 1.8 plus or minus 0.4 m.y. later, onto the earlier-formed porphyry Cu system. Hydrothermal activity at Ujina is constrained by the 40Ar/39Ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0.5 plus or minus 0.4 m.y. The biotite granite at La Profunda, 1.5 km east of Ujina, has a igneous biotite age of 81.2 plus or minus 2.9 Ma, indicating that this intrusion is unrelated to the mineralized Eocene-Oligocene porphyry intrusions at Ujina, Rosario, and Quebrada Blanca. The 40Ar/39Ar data for igneous biotite in the Rosario and Ujina porphyries are cooling ages after multiple hydrothermal events. In most cases, these are minimum hydrothermal ages (i.e., ages of the last high-temperature hydrothermal event to have affected the samples), not magmatic ages. However, the biotite age for the Inca Porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration.

Re-Os isotopes are used to determine the age of iron oxide Cu-Au mineralization at Candelaria, Chile, and to explore possible genetic links with the batholithic intrusions in the area. Re-Os ages calculated from molybdenite are 114.2 ± 0.6 Ma and 115.2 ± 0.6 Ma, and they are interpreted to represent the age of mineralization. These ages are consistent with previously reported ages for biotite alteration that range from 114 to 116 Ma. An isochron calculated by Re/Os ratios from hydrothermal magnetite and sulfides constrains an initial 187 Os/ 188 Os ratio of 0.36 ± 0.10. The initial 187 Os/ 188 Os ratio for sulfide from Bronce mine, a small satellite of the Candelaria orebody, is 0.33 ± 0.01. These values are broadly similar to the calculated initial 187 Os/ 188 Os ratio for magmatic magnetite in nearby batholithic rocks that range from 0.20 to 0.41. The relatively radiogenic initial 187 Os/ 188 Os ratio represents a mixture of mantle and crustal components in the ores and batholitic rocks. The similarity in initial 187 Os/ 188 Os ratio of the ore and magmatic oxides suggest that the granitic plutons could be the source of metals in the Candelaria district. These data are consistent with the existence of a significant magmatic fluid component in the hydrothermal system as suggested by previously published work. In order to establish a regional perspective, we analyzed ore minerals from other Chilean deposits of the iron oxide (Cu-U-Au-rare earth elements [REE]) class. Magnetite from the Manto Verde iron oxide Cu-Au deposit has Os and Re concentrations of 11 to 17 parts per trillion (ppt) and 4 to 6 parts per billion (ppb), respectively. The initial 187 Os/ 188 Os ratio is approximately 0.20 and is similar to that of ore minerals from Candelaria and of the Early Cretaceous batholithic intrusions. These data indicate a similar metal source for the mineralization at Manto Verde and Candelaria. Magnetite from three magnetite-apatite deposits of the Chilean iron belt have Re concentrations between 0.8 and 3 ppb and Os concentrations between 11 and 76 ppt. Calculated initial 187 Os/ 188 Os ratios of these magnetites range from 1.2 to 8.4 and are distinctly different from those of the iron oxide Cu-Au deposits. The cause of the comparatively high radiogenic signatures in the magnetite-apatite ores is probably related to fluid interactions with (i.e., leaching) the surrounding sedimentary rocks during their genesis. In contrast to the iron oxide Cu-Au systems, the Os in magnetite-apatite ores could be derived from sedimentary rocks. This evidence suggests that predominantly basin-derived, nonmagmatic brines formed these magnetite-apatite deposits.

Aims β-amyloid (Aβ) plaques are a key feature of Alzheimer’s disease pathology but correlate poorly with dementia. They are associated with astrocytes which may modulate the effect of Aβ-deposition on the neuropil. This study characterised the astrocyte response to Aβ plaque subtypes, and investigated their association with cognitive impairment. Methods Aβ plaque subtypes were identified in the cingulate gyrus using dual labelling immunohistochemistry to Aβ and GFAP+ astrocytes, and quantitated in two cortical areas: the area of densest plaque burden and the deep cortex near the white matter border (layer VI). Three subtypes were defined for both diffuse and compact plaques (also known as classical or core-plaques): Aβ plaque with (1) no associated astrocytes, (2) focal astrogliosis or (3) circumferential astrogliosis. Results In the area of densest burden, diffuse plaques with no astrogliosis (β = -0.05, p = 0.001) and with focal astrogliosis (β = -0.27, p = 0.009) significantly associated with lower MMSE scores when controlling for sex and age at death. In the deep cortex (layer VI), both diffuse and compact plaques without astrogliosis associated with lower MMSE scores (β = -0.15, p = 0.017 and β = -0.81, p = 0.03, respectively). Diffuse plaques with no astrogliosis in layer VI related to dementia status (OR = 1.05, p = 0.025). In the area of densest burden, diffuse plaques with no astrogliosis or with focal astrogliosis associated with increasing Braak stage (β = 0.01, p

Copper isotope signatures in waters emanating from mineralized watersheds provide evidence for the source aqueous copper in solution. Low-temperature aqueous oxidation of Cu sulfide minerals produces significant copper isotopic fractionation between solutions and residues. Abiotic experimental data of fractionation (defined as Δliquid–solid ‰ = δ65Culiquid − δ65Cusolid) are on the order of 1–3‰ and are unique for copper rich-sulfide minerals. Data presented here from ores and waters within defined boundaries of porphyry copper, massive sulfide, skarn, and epithermal ore deposits mimic abiotic experiments. Thus, the oxidation of sulfide minerals appears to cause the signatures in the waters although significant biological, temperature, and pH variations exist in the fluids. Regardless of the deposit type, water type, concentration of Cu in solution, or location, the data provide a means to trace sources of metals in solutions. This relationship allows for tracking sources and degree of metal migration in low temperature aqueous systems and has direct application to exploration geology and environmental geochemistry.

The Grasberg is a porphyry copper deposit that is crosscut by a second stage mineralization which is greatly enriched in gold. Sulfides from the porphyry-type event yield a 2.9±0.3 Ma Re–Os isochron that agrees with published geochronology. The initial 187Os/188Os ratio of the isochron is 0.56±0.02, and implies a significant crustal component for the source of Os and by inference the other base metals. The samples from the crosscutting secondary event do not form an isochron, but form a mixing line with the older porphyry style mineralization as one of the end members and a very radiogenic end member possibly shale. The initial 187Os/188Os ratios from samples of the second event range from 0.81 to 1.26 and correlate with gold content. The most radiogenic samples have the highest gold. The Re–Os isotope data indicate different crustal sources for the ore-forming elements at the Grasberg Cu–Au deposit and support a model in which gold is derived from sedimentary protoliths, that may have been pre-enriched by sedimentary processes. The gold derived from these sources is concentrated by magmatic/hydrothermal systems. The requirement of a sedimentary source for gold to produce hydrothermal gold deposits has been debated for decades and this study provides strong support for the model.

Re-Os isotopes in pyrite and chalcopyrite from early high-temperature hypogene alteration assemblages in Chilean porphyry copper deposits identify the source of Os and, by inference, Cu in these ore systems. Typical concentrations for Os in both pyrite and chalcopyrite are between 7 and 30 ppt (10 ‐12 g/g), and for Re between 0.200 and 10 ppb (10 ‐9 g/g). Re-Os isochrons yield ages and initial 187 Os/ 188 Os ratios for the sulfides. The isotopic data reflect the relative contributions of copper from the mantle and crust in Chilean porphyry copper deposits. Seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in Chile were studied. The initial osmium ratios of the first stage of mineralization at each of the deposits range from 0.15 to 5. These values are more radiogenic than the present chondritic mantle (~0.13), and indicate significant crustal contributions of Os to the magmatic and/or hydrothermal systems. There is a strong correspondence between the total copper content and initial Os isotopic ratios in base-metal porphyry deposits. The larger deposits have lower initial Os ratios than the smaller, less significant deposits. This relationship implies that larger deposits acquire a greater proportion of Os from the mantle. The initial Os ratio of samples in the central segment of porphyry copper deposits of northern Chile also decreases with decreasing age of the deposit. A plausible interpretation of the Re-Os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources.

Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between 0.05–0.7 and 0.6–66 ppb, respectively. 187Re/188Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial 187Os/188Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial 187Os/188Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the genesis of these deposits.

Hypotheses concerning processes and timing of hydrothermal sulfide mineralization in the central Pennsylvanian Appalachians utilize Mississippi Valley-type (MVT) models with Paleozoic ages. To examine this model, we studied sulfide-bearing veins in the Skytop sulfide occurrence that contain pyrite

Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40Ar/39Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40Ar/39Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40Ar/39Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40Ar/39Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40Ar/39Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted

New Re and Os analyses of pyrites from Aljustrel, Lagoa Salgada and Neves-Corvo deposits yield concentrations that are similar to those previously reported for the Rio Tinto and Tharsis deposits, all at the Iberian Pyrite Belt. Osmium contents in IPB pyrites increase from the deeper stockwork zones towards the upper massive sulphides. Pyrite samples from IPB sulphide orebodies plot on a major 187Re/188Os-187Os/188O linear array, yielding an age of 359±26 Ma and an initial 187Os/188O ratio of ca. 0.57. Five samples from Neves-Corvo sulphide-rich stockwork ores yield an age of 358 ±29 Ma and an initial 187Os/188O ratio of 0.49 ±0.07, in accord with palynological and isotopic age constraints for ore formation at the IPB. Pyrites from Neves-Corvo copper-tin ores have Re/Os ratios and osmium concentrations that are similar to those in pyrites from sulphide-rich stockwork ores; however, their highly radiogenic initial 187Os/188O ratios (4.89–7.85, if preserving original isotope system-atics) preclude derivation from the same sources as the remaining IPB sulphides. The overall Re-Os geochemical data conform to previous Sr, Nd and Pb isotopic studies supporting multiple sources for IPB metallogenesis.

High-precision isotopic analyses by MC-ICP-MS were used to investigate the mass-dependent fractionation of Mo and Fe isotopes during bacterial metal assimilation in experiments with Azotobacter vinelandii. A. vinelandii is a diazotroph with high demand for both Mo and Fe during nitrogen fixation. Our results demonstrate that the growth medium became progressively enriched in heavier isotopes of Mo during bacterial growth, indicating preferential assimilation of lighter isotopes. In contrast, for Fe, the medium become isotopically lighter as Fe was removed from solution. The experimental data can be interpreted in terms of Rayleigh fractionation, yielding fractionation factors of 0.9997 and 1.0011 for Mo and Fe, respectively. Hence, we infer Δ97/95Mocells-medium = –0.3‰ and Δ56/54Fecells-medium = 1.1‰. Fractionation of Mo isotopes could result from simple kinetic effects during assimilation, but may also be affected by complexation with high-affinity metal binding ligands. Kinetic effects cannot easily account for the sense of Fe isotope fractionation, and so equilibrium effects, possibly between different Fe complexes, are implied. Adsorption of Mo and Fe onto cell surfaces may also play a role and requires further examination. Isotope fractionation studies using MC-ICP-MS may provide new constraints on the processes by which microbes extract metals from their surroundings, ultimately yielding insights into the mechanisms of metal assimilation into the metallome.

The Red Hills intrusion hosts the easternmost porphyry copper-molybdenum system in southwestern North America and consists of quartz-sulfide stockwork veins in sericitized porphyritic quartz monzonite. Zircon U-Pb and molybdenite Re-Os analyses yield ages of 64.2 ′ 0.2 Ma and 60.2 ′ 0.3 Ma, respectively, indicating that the Red Hills intrusion and mineralization are distinctly older than all other Tertiary magmatism (48-17 Ma) in the Trans-Pecos region of Texas, including the nearby 32 Ma Chinati Mountains caldera. The Red Hills intrusive system is contemporaneous with and genetically related to other Laramide magmatic systems (75-54 Ma) that host porphyry copper deposits in Arizona, southwestern New Mexico, and northern Mexico. These results significantly extend the Laramide magmatic province eastward and suggest that Laramide subduction-related magmatism and deformation are coextensive over a broad area of southwestern North America.