Stability and solubility of arsenopyrite, FeAsS, in crustal fluids.

The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)30(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite. Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H2 and H2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH)30(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed. Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>not, vert, similar300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits

Thrust tectonics, crustal thickening, hydrocarbon and ore deposits in northern Central Andes

Thanks to numerous studies realized in cooperation with Peruvian institutions, we propose for the first time in the northern Peruvian Andes a crustal-scale balanced cross-section through the entire orogen to better understand structural architecture, crustal thickening and hydrocarbon-ore deposits genesis. Abundant industrial seismic data provided by Perupetro S.A. allowed to properly constrain the geometry of the forearc and retro-foreland basins (Calderon et al., 2017; Prudhomme et al., in press). Deep crustal structures and Moho geometries are constrained by a recent teleseismic receiver function study (Condori et al., 2017). The restoration, calibrated from new geochronological data and basins analysis, highlight an intermediate stage between the Incaic (late Cretaceous-early Eocene) and Andean (Neogene) orogenies corresponding to a phase of tectonic relaxation and extension. Shortening budgets established from surface and sub-surface data in the upper crust, and from crustal thickening in the middle-lower crust, make it possible to discriminate between the importance and role of each orogeny in the mountain building. The present stage of the balanced cross-section highlights a double-verging orogen, which could result from a total amount of shortening of 180 km fairly distributed between the Incaic and Andean orogenies. Important hydrocarbon and ore deposits located along the balanced cross-section are related to the geodynamic evolution of the successive Incaic and Andean thrust systems. In the forearc (Tumbes-Salaverry) and retro-foreland (Huallaga-Marañon) basins, 2D petroleum modellings have been done using sequential restorations in order to better target exploration. In the Western and Eastern cordilleras and the Subandean zone, significant ore deposits (Cu, Pb, Zn, Au, Ag…) are concentrated in sedimentary reservoirs of Incaic and/or Andean thrust anticlines. We explore and develop an innovative hypothesis, i.e., that there are strong interactions between mineralizing fluids (of both magmatic and sedimentary origin) and petroleum systems (oil shales and reservoirs). Indeed, both ore and oil types of deposits can be found in the same basins, with similar fluid migration and storage processes in sedimentary reservoirs

Metal zoning and precipitation mechanisms in porphyry systems

A great majority ofporphyry districts around the world show aregular pattern of zonal metal distribution –both vertically and laterally. Metal zonation commonly consists ofa Cu±Mo±Au core ringed by successive Cu-Zn, Zn-Pb-Ag, Pb-Ag (±Mn), and As-Sb-Hg±Auzones. If ahigh-sulfidation epithermal system is developed above the porphyry center, a vertical Cu±Au±Agzone may be present in the central upflow conduits. Comparing metal content in natural fluid inclusions from porphyry and associated skarn and epithermal deposits with thermodynamic predictions provides useful constraints on the mechanisms and processes controlling metal transport and deposition in porphyry systems. Results showthat decompression and cooling of a magmatic fluid, accompanied by boiling and water-rock interaction, are likely to be the major causes of zonal metal deposition in magmatic-hydrothermal systems. Large-scale fluid mixing is a relatively uncommon phenomenon during metal precipitation and is not expected to be a major cause of ore formation in the porphyry environment

Stability and abundance of the trisulfur radical ion S-3(-) in hydrothermal fluids

International audienceThe interpretation of sulfur behavior in geological fluids and melts is based on a long-standing paradigm that sulfate, sulfide, and sulfur dioxide are the major sulfur compounds. This paradigm was recently challenged by the discovery of the trisulfur ion S-3(-) in aqueous S-bearing fluids from laboratory experiments at elevated temperatures. However, the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions. Here we used in situ Raman spectroscopy on model thiosulfate, sulfide, and sulfate aqueous solutions across a wide range of sulfur concentration (0.5-10.0 wt%), acidity (pH 3-8), temperature (200-500 degrees C), and pressure (15-1500 bar) to identify the different sulfur species and determine their concentrations. Results show that in the low-density (<0.2 g/cm(3)) vapor phase, H2S is the only detectable sulfur form. By contrast, in the denser liquid and supercritical fluid phase, together with sulfide and sulfate, the trisulfur radical ion S-3(-) is a ubiquitous and thermodynamically stable species from 200 degrees C to at least 500 degrees C. In addition, the disulfur radical ion S is detected at 450-500 degrees C in most solutions, and polymeric molecular sulfur with a maximum abundance around 300 degrees C in S-rich solutions. These results, combined with revised literature data, allow the thermodynamic properties of S-3(-) to be constrained, enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids. These predictions suggest that S-3(-) may account for up to 10% of total dissolved sulfur (S-tot) at 300-500 degrees C in fluids from arc-related magmatic-hydrothermal systems, and more than 50% S-tot at 600-700 degrees C in S-rich fluids produced via prograde metamorphism of pyrite-bearing rocks. The trisulfur ion may favor the mobility of sulfur itself and associated metals (Au, Cu, Pt, Mo) in geological fluids over a large range of depth and provide the source of these elements for orogenic Au and porphyry-epithermal Cu-Au-Mo deposits. Furthermore, the ubiquity of S-3(-) in aqueous sulfate-sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass-dependent and mass-independent fractionation of sulfur isotopes

The S<SUB>3</SUB><SUP>-</SUP> Ion Is Stable in Geological Fluids at Elevated Temperatures and Pressures

International audienceThe chemical speciation of sulfur in geological fluids is a controlling factor in a number of processes on Earth. The two major chemical forms of sulfur in crustal fluids over a wide range of temperature and pressure are believed to be sulfate and sulfide; however, we use in situ Raman spectroscopy to show that the dominant stable form of sulfur in aqueous solution above 250°C and 0.5 gigapascal is the trisulfur ion S3-. The large stability range of S3- enables efficient transport and concentration of sulfur and gold by geological fluids in deep metamorphic and subduction-zone settings. Furthermore, the formation of S3- requires a revision of sulfur isotope-fractionation models between sulfides and sulfates in natural fluids

Fluid density control on vapor-liquid partitioning of metals in hydrothermal systems

International audienceHot aqueous fluids, both vapor and saline liquid, are primary transporting media for metals in hydrothermal-magmatic systems. Despite the growing geological evidence that the vapor phase, formed through boiling of magmatic ore-bearing fluids, can selectively concentrate and transport metals, the physical-chemical mechanisms that control the metal vapor-liquid fractionation remain poorly understood. We performed systematic experiments to investigate the metal vapor-liquid partitioning in model water-salt-gas systems H2O-NaCl-KCl-HCl at hydrothermal conditions. Measurements show that equilibrium vapor-liquid fractionation patterns of many metals are directly related to the densities of the coexisting vapor and liquid phases. Despite differences in the vapor-phase chemistry of various metals that form hydroxide, chloride, or sulfide gaseous molecules of contrasting volatile properties, water-solute interaction is a key factor that controls the metal transfer by vapor-like fluids in Earth's crust. These findings allow quantitative prediction of the vapor-liquid distribution patterns and vapor-phase metal transport in a wide range of conditions. Our density model accounts well for the vapor-brine distribution patterns of Na, Si, Fe, Zn, As, Sb, and Ag observed in fluid inclusions from magmatic-hydrothermal deposits. For Au and Cu, the partitioning in favor of the liquid phase, predicted in a sulfur-free system, contrasts with the copper and gold enrichment observed in natural vapor-like inclusions. The formation of stable complexes of Cu and Au with reduced sulfur may allow for their enhanced transport by sulfur-enriched magmatic-hydrothermal vapors

The effect of the trisulfur radical ion on molybdenum transport by hydrothermal fluids

International audienceKnowledge of molybdenum (Mo) speciation under hydrothermal conditions is a key for understanding the formation of porphyry deposits which are the primary source of Mo. Existing experimental and theoretical studies have revealed a complex speciation, solubility and partitioning behavior of Mo in fluid-vapor-melt systems, depending on conditions, with the (hydrogen)molybdate (HMoO4-, MoO42-) ions and their ion pairs with alkalis in S and Cl-poor fluids [1-3], mixed oxy-chloride species in strongly acidic saline fluids [4, 5], and (hydrogen)sulfide complexes (especially, MoS42-) in reduced H2S-bearing fluids and vapors [6-8]. However, these available data yet remain discrepant and are unable to account for the observed massive transport of Mo in porphyry-related fluids revealed by fluid inclusion analyses demonstrating 100s ppm of Mo (e.g., [9]). A potential missing ligand for Mo may be the recently discovered trisulfur radical ion (S3•-), which is predicted to be abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids [10]. We performed exploratory experiments of MoS2 solubility in model sulfate-sulfide-S3•--bearing aqueous solutions at 300°C and 450 bar. We demonstrate that Mo can be efficiently transported by S3•--bearing fluids at concentrations ranging from several 10s ppm to 100s ppm, depending on the fluid pH and redox, whereas the available data on OH-Cl-S complexes cited above predict negligibly small (<100 ppb) Mo concentrations at our conditions. Work is in progress to extend the experiments to wider T-P-composition range of porphyry fluids and to quantitatively assess the role of S3•- in Mo transport by geological fluids.1. Kudrin A.V. (1989) Geochem. Int. 26, 87-99. 2. Minubayeva Z. and Seward T.M. (2010) Geochim. Cosmochim. Acta 74, 4365-4374. 3. Shang L.B. et al. (2020) Econ. Geol. 115, 661-669. 4. Ulrich T. and Mavrogenes J. (2008) Geochim. Cosmochim. Acta 72, 2316-2330. 5. Borg S. et al. (2012) Geochim. Cosmochim. Acta 92, 292-307. 6. Zhang L. et al. (2012) Geochim. Cosmochim. Acta 77, 175-185. 7. Kokh M.A. et al. (2016) Geochim. Cosmochim. Acta 187, 311-333. 8. Liu W. et al. (2020) Geochim. Cosmochim. Acta 290, 162-179. 9. Kouzmanov K. and Pokrovski G.S. (2012) Soc. Econ. Geol. Spec. Pub. 16, 573-618. 10. Pokrovski G.S. and Dubessy J. (2015) Earth Planet. Sci. Lett. 411, 298-309