Copper-arsenic decoupling in an active geothermal system: A link between pyrite and fluid composition

Over the past few decades several studies have reported that pyrite hosts appreciable amounts of trace elements which commonly occur forming complex zoning patterns within a single mineral grain. These chemical zonations in pyrite have been recognized in a variety of hydrothermal ore deposit types (e.g., porphyry Cu-Mo-Au, epithermal Au deposits, iron oxide–copper–gold, Carlin-type and Archean lode Au deposits, among others), showing, in some cases, marked oscillatory alternation of metals and metalloids in pyrite growth zones (e.g., of Cu-rich, As-(Au, Ag)-depleted zones and As-(Au, Ag)-rich, Cu-depleted zones). This decoupled geochemical behavior of Cu and As has been interpreted as a result of chemical changes in ore-forming fluids, although direct evidence connecting fluctuations in hydrothermal fluid composition with metal partitioning into pyrite growth zones is still lacking. In this study, we report a comprehensive trace element database of pyrite from the Tolhuaca Geothermal System (TGS) in southern Chile, a young and active hydrothermal system where fewer pyrite growth rims and mineralization events are present and the reservoir fluid (i.e. ore-forming fluid) is accessible. We combined the high-spatial resolution and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling capacity of secondary-ion mass spectrometry (SIMS) in a suite of pyrite samples retrieved from a ∼1 km drill hole that crosses the argillic (20 to 450 m) and propylitic (650 to 1000 m) alteration zones of the geothermal system. We show that the concentrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te), and base and heavy metals (e.g., Cu, Co, Ni, Pb) in pyrite at the TGS are significant. Among the elements analyzed, As and Cu are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., up to ∼5 wt.% As, ∼1.5 wt.% Cu). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs. isotope concentration profiles reveal that pyrites from the TGS are characterized by chemical zoning where the studied elements occur in different mineralogical forms. Arsenic and Co occur as structurally bound elements in pyrite, Cu and Au in pyrite can occur as both solid solution and submicron-sized particles of chalcopyrite and native Au (or Au tellurides), respectively. Pyrites from the deeper propylitic zone do not show significant zonation and high Cu-(Co)-As concentrations correlate with each other. In contrast, well-developed zonations were detected in pyrite from the shallow argillic alteration zone, where Cu(Co)-rich, As-depleted cores alternate with Cu(Co)-depleted, As-rich rims. These microanalytical data were contrasted with chemical data of fluid inclusions in quartz and calcite veins (high Cu/As ratios) and borehole fluid (low Cu/As ratios) reported at the TGS, showing a clear correspondence between Cu and As concentrations in pyrite-forming fluids and chemical zonation in pyrite. These observations provide direct evidence supporting the selective partitioning of metals into pyrite as a result of changes in ore-forming fluid composition, most likely due to separation of a single-phase fluid into a low-density vapor and a denser brine, capable of fractionating Cu and As

Multi-element isotopic evolution of magmatic rocks from Caviahue-Copahue Volcanic Complex (Chile-Argentina): Involvement of mature slab recycled materials

International audienc

Detailed Chemistry Promotes Understanding of Octane Numbers and Gasoline Sensitivity

Detailed kinetic models of pyrolysis and combustion of hydrocarbon fuels are now reliable tools which can aid the design of internal combustion engines required to meet the increasingly stringent pollutant formation and engine efficiency standards. The aim of this paper is to discuss and verify the potential of these kinetic models in analyzing the knock related combustion behavior of hydrocarbon fuels with particular regard to octane numbers and octane sensitivity. Detailed chemistry not only helps to explain the different reactivities of alkanes and alkenes but also the combustion behavior of hydrocarbon mixtures. A two-zone model of a spark ignition engine, coupled with the detailed chemistry of combustion processes, was developed and utilized for the predictions of octane numbers. This model explains the effect of various components on the knocking behavior of the fuel under different operating conditions and is thus a useful tool both in formulating new fuels and designing new engines

Trace element signature of pyrite from the Los Colorados iron oxide-apatite (IOA) deposit, Chile: A missing link between Andean IOA and iron oxide copper-gold systems?

Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust

Trace element signature of pyrite from the Los Colorados iron oxide-apatite (IOA) deposit, Chile: A missing link between Andean IOA and iron oxide copper-gold systems?

Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust

Phase Behavior of DNA-Stabilized Carbon Nanotubes Dispersions: Association with Oppositely-Charged Additives

The formation of liquid crystalline phases or isotropic clusters is observed in carbon nanotubes systems experiencing repulsive and attractive interactions, respectively. ssDNA-stabilized nanotubes act as strongly repulsive charged rods, showing nematic phases in (pseudo)-binary and ternary systems, in the presence of a nonadsorbing polymer. Switching between purely repulsive and attractive regime has not been investigated yet. For this reason, dispersions of ssDNA-stabilized nanotubes were added with an oppositely charged additive (i.e., protein or surfactant), and the resulting systems were investigated. In both phase diagrams a strong associative behavior was observed. At additive charge excess, a redispersion of the complex was obtained. The phenomenon was substantial in the case of surfactant system, where a charge inversion was also observed. A fine-tuning of attractive and repulsive interactions promoted aggregation and redispersion of carbon nanotube complexes. The introduction of weak attractive forces may promote the formation a cluster phase of ssDNA-stabilized nanotubes, with possible application as "multicompartimental" delivery systems