Constraints on the solid solubility of Hg, Tl, and Cd in arsenian pyrite

Arsenic-rich (arsenian) pyrite can contain up to tens of thousands of parts per million (ppm) of toxic heavy metals such as Hg, Tl, and Cd, although few data are available on their solid solubility behavior. When a compilation of Hg, Tl, and Cd analyses from different environments are plotted along with As in a M(Hg, Tl, and Cd)-As log-log space, the resulting wedge-shaped distribution of data points suggests that the solid solubility of the aforementioned metals is strongly dependent on the As concentration of pyrite. The solid solubility limits of Hg in arsenian pyrite—i.e., the upper limit of the wedge-shaped zone in compositional space—are similar to the one previously defined for Au by Reich et al. (2005) (CHg,Au = 0.02CAs + 4 × 10−5), whereas the solubility limit of Tl in arsenian pyrite is approximated by a ratio of Tl/As = 1. In contrast, and despite a wedge-shaped distribution of Cd-As data points for pyrite in Cd-As log-log space, the majority of Cd analyses reflect the presence of mineral particles of Cd-rich sphalerite and/or CdS. Based on these data, we show that arsenian pyrite with M/As ratios above the solubility limit of Hg and Tl contain nanoparticles of HgS, and multimetallic Tl-Hg mineral nanoparticles. These results indicate that the uptake of Hg and Tl in pyrite is strongly dependent on As contents, as it has been previously documented for metals such as Au and Cu. Cadmium, on the other hand, follows a different behavior and its incorporation into the pyrite structure is most likely limited by the precipitation of Cd-rich nanoparticulate sphalerite. The distribution of metal concentrations below the solubility limit suggests that hydrothermal fluids from which pyrite precipitate are dominantly undersaturated with respect to species of Hg and Tl, favoring the incorporation of these metals into the pyrite structure as solid solution. In contrast, the formation of metallic aggregates of Hg and Tl or mineral nanoparticles in the pyrite matrix occurs when Hg and Tl locally oversaturate with respect to their solid phases at constant temperature. This process can be kinetically enhanced by high-to-medium temperature metamorphism and thermal processing or combustion, which demonstrates a retrograde solubility for these metals in pyrite

Alteration of U(VI)-Phases under oxidizing conditions

Uranium-(VI) phases are the primary alteration products of the UO 2 in spent nuclear fuel and the UO 2+x in natural uranium deposits. The U(VI)-phases generally form sheet structures of edge-sharing UO 2 2+ polyhedra. The complexity of these structures offers numerous possibilities for coupled-substitutions of trace metals and radionuclides. The incorporation of radionuclides into U(VI)-structures provides a potential barrier to their release and transport in a geologic repository that experiences oxidizing conditions. In this study, we have used natural samples of UO 2+x , to study the U(VI)-phases that form during alteration and to determine the fate of the associated trace elements

Behaviour of trace elements in arsenian pyrite in ore deposits

As-bearing pyrite is one of the main hosts for Au and other trace elements in epithermal, Carlin and mesothermal (orogenic) Au deposits. A review of our own and published SIMS, EMPA, LA-ICP-MS and PIXE analyses of pyrite from these deposits suggests that the solubility of Ag, Te, Hg, Sb and Pb in arsenian pyrite is controlled by As-content in a manner similar to that previously reported for Au by Reich et al., (2005). The trace elements can be divided into two groups that exhibit different solubility limits: i) Au, Ag, Te, Hg and Bi ii) Sb and Pb. HRTEM and HAADF-STEM observations reveal nanoparticles with compositions of Sb-As-Fe-Ni, Sb-Pb-Te, Pb-Bi, PbS and Ag in arsenian pyrite above the solubility limit. Most nanoparticles are between 5 and 200 nm, with some containing Pb reaching 500 nm. Pyrite from Carlin-type and epithermal deposits contains larger amounts of Sb and/or As than pyrite from higher-temperature orogenic gold/mesothermal deposits. This suggests that the solubility of trace elements in pyrite appears to decrease with increasing temperature

Geochemical and micro-textural fingerprints of boiling in pyrite

The chemical composition, textures and mineral associations of pyrite provide key information that help elucidate the evolution of hydrothermal systems. However, linking the compositional and micro-textural features of pyrite with a specific physico-chemical process, e.g., boiling versus non-boiling, remains elusive and challenging. In this study we examine pyrite geochemical and micro-textural features and relate these results to pyrite-forming processes at the active Cerro Pabellón Geothermal System (CPGS) in the Altiplano of the northern Chile. We integrate electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data with micro-textural observations of pyrite and associated gangue minerals recovered from a ∼500 m long drill core that crosscuts the argillic, sub-propylitic and propylitic alteration zones of the CPGS. Additionally, we carried out a Principal Component Analysis (PCA) in order to inspect and understand the main data structure of the pyrite geochemical dataset. The concentrations of precious metals (Au and Ag), metalloids (As, Sb, Se, Bi and Tl), and base and heavy metals (Cu, Co, Ni and Pb) in pyrite from the CPGS are significant. Among the elements analyzed, As, Cu and Pb are the most abundant with concentrations that vary from a few parts per million (ppm) to wt% levels (up to 4.4 wt% of As, 0.5 wt% of Cu and 0.2 wt% of Pb). Based on contemporaneous gangue mineral associations and textures, the mechanisms of pyrite precipitation in the CPGS were inferred. Pyrite formed during vigorous boiling is characterized by relatively high concentrations of As, Cu, Pb, Ag and Au and lower concentrations of Co and Ni compared to pyrite formed under different conditions. These anhedral to euhedral pyrite grains display zones with a porous texture and abundant mineral micro- to nano-inclusions (mainly galena and chalcopyrite) indicating a formation by rapid crystallization. In contrast, pyrite formed under gentle boiling (more gradual cooling and less abrupt physico-chemical variations than in vigorous boiling) to non-boiling conditions is characterized by a higher concentration of Co and Ni, and relatively low concentrations of As, Cu, Pb, Ag and Au. Texturally, these pyrites form aggregates of euhedral and pristine pyrite crystals with scarce pores and mineral inclusions suggesting formation under steadier physico-chemical conditions. Our results show that pyrite can not only record the chemical evolution of hydrothermal fluids, but can also provide critical information related to physico-chemical process such as boiling and phase separation. Since boiling of aqueous fluids is a common phenomenon occurring in a variety of pyrite-forming environments, e.g., active continental and seafloor hydrothermal systems, and porphyry Cu-epithermal Au-Ag deposits, pyrite compositional and textural features are a valuable complement for discriminating and tracking boiling events in modern and fossil hydrothermal systems

Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru

We report the first occurrence of Hf-rich kosnarite [K(Hf,Zr)2(PO4)3], space group R-3c, Z = 6, in the giant Chaquicocha high-sulfidation epithermal gold deposit in the Yanacocha mining district, Peru. Kosnarite crystals are small (<100 μm) and occur in 2–3-mm-thick veins that cut intensively silicified rocks. The paragenesis includes a first stage of As-free pyrite and quartz (plus gratonite and rutile), followed by trace metal-rich pyrite [(Fe,As,Pb,Au)S2] and secondary Fe sulfates. Kosnarite is associated with quartz and is clearly late within the paragenetic sequence. Electron microprobe analyses (EMPA) of kosnarite show relatively high concentrations of HfO2 and Rb2O (7.61 and 1.05 wt.%, respectively). The re-calculated chemical formulas of kosnarite vary from KΣ1.00(Zr1.93Na0.01Hf0.01Mn0.01)Σ1.96(P3.04O4)Σ3 to (K0.92Rb0.05Na0.03)Σ1.00(Zr1.81Hf0.19)Σ2.00 [(P2.98Si0.02As0.01)Σ3.01O4]Σ3, where Hf and Rb are most likely incorporated according to a coupled substitution of Hf4+ + Rb+ ⇔ Zr4+ + K+. Back-scattered electron (BSE) images and elemental mapping of kosnarite reveal that Hf and Rb are enriched in 2–10-μm-wide oscillatory and/or sector zones. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of such zones reveal a pattern of alternating, 5–50-nm-thick, Hf-rich and Zr-rich nanozones. These high-resolution observations indicate that the incorporation of Hf does not appear to cause significant distortion in the kosnarite structure. Semiquantitative TEM-energy-dispersive X-ray spectrometry (EDS) analyses of the nano-layers show up to 22 wt.% of HfO2, which corresponds to 31 mol% of the hypothetical, KHf2(PO4)3, end-member. The presence of kosnarite in the advanced argillic alteration zone at Yanacocha is indicative of Hf and Zr mobility under highly acidic conditions and points towards an unforeseen role of phosphates as sinks of Zr and Hf in high-sulfidation epithermal environments. Finally, potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset

Kiruna-Type Iron Oxide-Apatite (IOA) and Iron Oxide Copper-Gold (IOCG) Deposits Form by a Combination of Igneous and Magmatic-Hydrothermal Processes: Evidence from the Chilean Iron Belt

Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ 18 O values in magnetite are within the range of 0 to 5‰, and bulk δ 56 Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ 18 O and δD for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and-59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187 Os/ 188 Os ratio (Osi) for magnetite from Los Colorados is 1.2, overlapping Osi values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous mag-netite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies

Trace elements in magnetite from massive iron oxide-apatite deposits indicate a combined formation by igneous and magmatic-hydrothermal processes

Iron oxide-apatite (IOA) deposits are an important source of iron and other elements (e.g., REE, P, U, Ag and Co) vital to modern society. However, their formation, including the namesake Kiruna-type IOA deposit (Sweden), remains controversial. Working hypotheses include a purely magmatic origin involving separation of an Fe-, P-rich, volatile-rich oxide melt from a Si-rich silicate melt, and precipitation of magnetite from an aqueous ore fluid, which is either of magmatic-hydrothermal or non-magmatic surface or metamorphic origin. In this study, we focus on the geochemistry of magnetite from the Cretaceous Kiruna-type Los Colorados IOA deposit (~350. Mt Fe) located in the northern Chilean Iron Belt. Los Colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in IOA deposits. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) transects and electron probe micro-analyzer (EPMA) wavelength-dispersive X-ray (WDX) spectrometry mapping demonstrate distinct chemical zoning in magnetite grains, wherein cores are enriched in Ti, Al, Mn and Mg. The concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt, whereas magnetite rims show a pronounced depletion in these elements, consistent with magnetite grown from an Fe-rich magmatic-hydrothermal aqueous fluid. Further, magnetite grains contain polycrystalline inclusions that re-homogenize at magmatic temperatures (>850. °C). Smaller inclusions (500. ppm) concentrations

Super-sieving effect in phenol adsorption from aqueous solutions on nanoporous carbon beads

Removal of aromatic contaminants, like phenol, from water can be efficiently achieved by preferential adsorption on porous carbons which exhibit molecular sieving properties. Here, we present nanoporous carbon beads exhibiting an outstanding sieving effect in phenol adsorption from aqueous solution at neutral pH, which is evidenced experimentally and theoretically. The molecular sieving with pure phenol adsorbed phase is achieved by tuning the pore size and surface chemistry of the adsorbent. This study elucidates the essential role of hydrophobic interactions in narrow carbon micropores in removal and clean-up of water from organic pollutants. Furthermore, we suggest a new theoretical approach for evaluation of phenol adsorption capacity that is based on the Monte Carlo simulation of phenol adsorption with the relevance to the pore size distribution function determined by the density functional theory method from low temperature nitrogen adsorption

Leaching of brannerite in the ferric sulphate system. Part 2: Mineralogical transformations during leaching

Brannerite, UTi2O6, is the most common refractory uranium mineral and is the most important uranium ore mineral after uraninite and coffinite. In order to develop an effective treatment method for the hydrometallurgical extraction of uranium from ores containing brannerite, it is necessary to understand the chemistry of the leaching process. Part 1 of this series described the results of a study of the chemical reaction mechanisms of brannerite under conditions similar to those used industrially. In this paper, the mineralogical data obtained from samples collected during the leaching work is used to derive further insight into the transformations that take place during leaching. Detailed characterisation of the brannerite feed specimen and leach residues was carried out using surface imaging and X-ray diffraction techniques. It was shown that the brannerite specimen is heterogeneous, consisting of at least two phases. The brannerite phase was metamict and showed signs of natural alteration to fine-grained (10-20 nm) crystalline anatase. Comparisons between the feed and residues showed that the X-ray amorphous materials, in particular lead and silicon enriched areas identified near the anatase were most susceptible to leaching while the crystalline material was relatively resistant to leaching. These results demonstrate that the extent of brannerite alteration and its texture are important considerations to the hydrometallurgical behaviour of a particular ore along with the typical concerns such as grade, liberation size and gangue mineralogy