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

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

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

The ubiquity of Au-bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of Au and As in this sulfide occurs under a wide range of physico-chemical conditions. Despite significant advances in the last 20years, fundamental factors controlling Au and As ratios in pyrite from ore deposits remain poorly known. Here we explore these constraints using new and previously published EMPA, LA-ICP-MS, SIMS, and μ-PIXE analyses of As and Au in pyrite from Carlin-type Au, epithermal Au, porphyry Cu, Cu-Au, and orogenic Au deposits, volcanogenic massive sulfide (VHMS), Witwatersrand Au, iron oxide copper gold (IOCG), and coal deposits. Pyrite included in the data compilation formed under temperatures from ~30 to ~600°C and in a wide variety of geological environments. The pyrite Au-As data form a wedge-shaped zone in compositional space, and the fact that most data points plot below the solid solubility limit defined by Reich et al. (2005) indicate that Au1+ is the dominant form of Au in arsenian pyrite and that Au-bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native Au. The analytical data also show that the solid solubility limit of Au in arsenian pyrite defined by an Au/As ratio of 0.02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal-chemical properties of pyrite and post-depositional alteration. Compilation of Au-As concentrations and formation temperatures for pyrite indicates that Au and As solubility in pyrite is retrograde; Au and As contents decrease as a function of increasing temperature from ~200 to ~500°C. Based on these results, two major Au-As trends for Au-bearing arsenian pyrite from ore deposits are defined. One trend is formed by pyrites from Carlin-type and orogenic Au deposits where compositions are largely controlled by fluid-rock interactions and/or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids. The second trend consists of pyrites from porphyry Cu and epithermal Au deposits, which are characterised by compositions that preserve the Au/As signature of mineralizing magmatic-hydrothermal fluids, confirming the role of this sulfide in controlling metal ratios in ore systems

Decoupling of Cu and As in Magmatic-hydrothermal systems Evidence from the Pueblo Viejo Au-Ag deposit, Dominican Republic

Pyrite from the Pueblo Viejo high-sulfidation deposit provides evidence for decoupling of Cu and As in hydrothermal solutions. The pyrite that shows this decoupling is in the late-stage veins that also contain sphalerite and minor enargite. Pyrite in the veins shows growth zoning that varies in composition with depth into the deposit. Deepest veins (>150m below the present surface) contain fine-grained (<211m) pyrite with 0.4% As, 0.5% Pb, 0.2% Cu, 0.1% Ag, 0.09% Te and 0.06% Sb (wt% by EMPA). At depths of 120- 103m below the present surface, pyrite contains alternating growth zones with either Cu (<0.78%) or As (<0.69%), but never both. Farther upward in the deposit concentrations of Cu and As in the two types of pyrite increase and Pb (<1.8%), Sb (<0.33%), Ag (<0.1%) and Te (<0.08%) are also in As-rich zones. At a depth of - 20m, Cu and As reach concentrations of up to 3 wt% in separate, alternating growth zones. EMPA elemental maps of the shallowest pyrites reveal that increased concentrations of As and Cu coincide spatially with decreasing concentrations of Fe and show no relation to S, suggesting that both elements substitute for Fe. Chemical compositions of Cu-pyrite and As-pyrite are: (Fe0.95CUo 06) 101S2 and (Fe09~s0_05) 101 Sb respectively. HRTEM observations on pyrite with highest Cu and As concentrations reveal that the pyrite consists of single crystals that are continuous from Cu-rich to As-rich growth zones. There is no visible (by TEM) grain boundary between Cu-rich and As-rich zones. Cu-rich growth zones contain no Cubearing inclusions, whereas As-rich growth zones contain numerous ordered nano-domains rich in As. The alternating sequence of Cu-rich and As-rich zones appears to reflect separation of Cu from As during evolution of the hydrothermal fluids. Similar decoupling of As and Cu is seen in analyses of fumaroles and fluid inclusions (both vapor and liquid), which are enriched in As and Cu, respectively. This suggests that the decoupling is related to magmatic processeses, probably involving vapor-liquid transitions

Nanoscale "liquid" inclusions of As-Fe-S in arsenian pyrite

A new mode of arsenic incorporation into arsenian pyrite has been discovered. Electron microprobe analyses and elemental maps of arsenian pyrite from Pueblo Viejo, Dominican Republic, show that its chemical composition varies from (Fe0.998As0.003)1.001S2 to (Fe0.963As0.050Cu0.003Ag0.001) 1.017S2 and that arsenic is inversely correlated with Fe. High-resolution transmission electron microscopy (HRTEM) images show that some arsenic in this pyrite is present as nanoscale inclusions of amorphous As- Fe-S in a matrix of arsenian pyrite. The amorphous inclusions display negative facets with a cubic or rectangular morphology, typical of negative inclusions of pyrite and arsenopyrite or marcarsite, respectively, and they are oriented parallel to the lattice fringes (100) of the arsenian pyrite matrix. Elemental maps collected by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that the inclusions have a higher content of arsenic than the surrounding pyrite, and TEM-EDX analyses indicate approximate atomic proportions of 62 at% S, 28 at% Fe, and 10 at% As for the inclusions, which are near the minimum melting temperature in the As-Fe-S ternary. These observations suggest that the inclusions were trapped as liquids during growth of the surrounding arsenian pyrite. Although not a new mineral, this constitutes a third form for arsenian pyrite, which has previously been shown to contain arsenic as either As1- or As3+

Structural and chemical discontinuities in pyrite

Pyrite, FeS2, the most common sulfide mineral in ore deposits, exhibits trace-element rich growth zoning that reflects chemical and/or textural changes during its formation (e.g., Large et al., 2008; Barker et al., 2009; Deditius et al., 2009a). TEM data, coupled with EMPA analyses, allow identification and characterization of structural and chemical discontinuities between growth zones in pyrite at the nanoscale. EMPA analyses and SEM observations of pyrite from high-sulfidation epithermal deposits (Yanacocha and Pueblo Viejo) reveal that highest concentrations of trace elements (Au, Ag, As, Pb, Cu, Sb, Ni, Te) coincide with porous growth zones that vary in thickness from ̴50 nm to hundreds of micrometers. HRTEM and HAADF-STEM observations show that the growth zones consist of nanolayers containing homo- or heterogeneously distributed As and Cu in single crystals of pyrite or randomly distributed (As,Au,Pb)-rich aggregates of nanoparticulate pyrite, with individual grains ranging from 8 to 900 nm in size. �False� growth zoning is formed by densely distributed crystalline or amorphous non-pyrite phases

Decoupled geochemical behavior of As and Cu in hydrothermal systems

Cu-rich and As-rich growth zones in pyrite provide new insights into the composition of late-stage magmatic fluids and their host hydrothermal ore deposits. The pyrite is from the Pueblo Viejo (Dominican Republic) and Yanacocha (Peru) high-sulfidation gold-silver deposits, which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes. Electron microprobe analysis, secondary ion mass spectrometry, and elemental maps show that pyrite, the most common sulfide mineral in both deposits, contains three different types of growth zones: (1) As-rich zones that are enriched in Au, Ag, Sb, Te, and Pb, (2) Cu-rich zones with significantly lower concentrations of these elements, and (3) barren pyrite zones with no other elements. These zones are interpreted to result from mixing between the pyrite-forming fluid and vapors that invaded the main hydrothermal system episodically. Comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic-hydrothermal systems suggests that the As-rich vapor formed at high and possibly magmatic temperatures, whereas the Cu-rich vapor formed at lower temperatures, possibly during migration of the original magmatic vapor. The presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high-sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors, probably from underlying magmas

Trace metal nanoparticles in pyrite

Hydrothermal pyrite contains significant amounts of minor and trace elements including As, Pb, Sb, Bi, Cu, Co, Ni, Zn, Au, Ag, Se and Te, which can be incorporated into nanoparticles (NPs). NP-bearing pyrite is most common in hydrothermal ore deposits that contain a wide range of trace elements, especially deposits that formed at low temperatures. In this study, we have characterized the chemical composition and structure of these NPs and their host pyrite with high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), analytical electron microscopy (AEM), and electron microprobe analysis (EMPA). Pyrite containing the NPs comes from two types of common low-temperature deposits, Carlin-type (Lone Tree, Screamer, Deep Star (Nevada, USA)), and epithermal (Pueblo Viejo (Dominican Republic) and Porgera (Papua New-Guinea)).EMPA analyses of the pyrite show maximum concentrations of As (11.2), Ni (3.04), Cu (2.99), Sb (2.24), Pb (0.99), Co (0.58), Se (0.2), Au (0.19), Hg (0.19), Ag (0.16), Zn (0.04), and Te (0.04) (in wt.%). Three types of pyrite have been investigated: "pure" or "barren" pyrite, Cu-rich pyrite and As-rich pyrite. Arsenic in pyrite from Carlin-type deposits and the Porgera epithermal deposit is negatively correlated with S, whereas some (colloform) pyrite from Pueblo Viejo shows a negative correlation between As. +. Cu and Fe. HRTEM observations and SAED patterns confirm that almost all NPs are crystalline and that their size varies from 5 to 100. nm (except for NPs of galena, which have diameters of up to 500. nm). NPs can be divided into three groups on the basis of their chemical composition: (i) native metals: Au, Ag, Ag-Au (electrum); (ii) sulfides and sulfosalts: PbS (galena), HgS (cinnabar), Pb-Sb-S, Ag-Pb-S, Pb-Ag-Sb-S, Pb-Sb-Bi-Ag-Te-S, Pb-Te-Sb-Au-Ag-Bi-S, Cu-Fe-S NPs, and Au-Ag-As-Ni-S; and (iii) Fe-bearing NPs: Fe-As-Ag-Ni-S, Fe-As-Sb-Pb-Ni-Au-S, all of which are in a matrix of distorted and polycrystalline pyrite. TEM-EDX spectra collected from the NPs and pyrite matrix document preferential partitioning of trace metals including Pb, Bi, Sb, Au, Ag, Ni, Te, and As into the NPs. The NPs formed due to exsolution from the pyrite matrix, most commonly for NPs less than 10. nm in size, and direct precipitation from the hydrothermal fluid and deposition into the growing pyrite, most commonly for those > 20. nm in size. NPs containing numerous heavy metals are likely to be found in pyrite and/or other sulfides in various hydrothermal, diagenetic and groundwater systems dominated by reducing conditions

A proposed new type of arsenian pyrite: Composition, nanostructure and geological significance

This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1--pyrite, in which As1- substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3 + + y Au+ + 1 - y (□) ⇔ 2 Fe2 +; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1--pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems