The nature and partitioning of invisible gold in the pyrite-fluid system

doi:10.1016/j.oregeorev.2019.04.024

Ore Geology Reviews

Volume 109, June 2019, Pages 545-563

https://doi.org/10.1016/j.oregeorev.2019.04.024 Get rights and content

Highlights

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Au in sulfide-sulfate solutions is complexed with the HS⁻, $S_{3}^{\cdot -}$ and $S_{2}^{\cdot -}$ ions.
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In As-poor systems invisible gold in pyrite is present as Au(0) and Au(I).
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Au(I) in pyrite occurs as chemisorbed S-Au-S_n moieties similar to those in the fluid.
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Au(I) uptake by pyrite from fluid is enhanced with decreasing temperature.
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Au(I) pyrite/fluid partition coefficients allow predictions of gold tenors in pyrite ore.

Abstract

The most characteristic feature of hydrothermal deposits of gold is its intimate association with pyrite. Microscopically visible gold occurs in pyrite ore as metal particles of >0.1 µm in size, together with so called “invisible” gold, undetectable by conventional microscopic methods. The chemical, redox and structural state of this invisible gold and the mechanisms of its incorporation into pyrite remain both inconsistent and controversial since the dawn of economic geology. To clarify these issues, we performed laboratory experiments to simulate interactions of gold-bearing sulfur-rich hydrothermal fluids with arsenic-free pyrite at temperatures from 350 to 450 °C and pressures from 400 to 700 bar, typical of the formation conditions of many types of gold deposits. Gold solubility was measured in these fluids as a function of sulfur speciation and acidity. Gold redox and structural state in pyrite was characterized by high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), together with more traditional analytical techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD), electron probe micro analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results show that dissolved Au in sulfide-sulfate solutions forms complexes with hydrogen sulfide, and tri- and di-sulfur radical ions whose amounts depend mostly on the fluid pH and total sulfur concentration. Invisible gold in pyrite occurs as Au metal submicron- to nano-sized particles and chemically bound Au(I) in the form of (poly)sulfide clusters composed on S-Au-S linear units, similar to those in aqueous complexes. Our findings contest the common belief that Au(I) substitutes for Fe and/or S in the structure of As-poor pyrite. The partition coefficient of Au(I) between pyrite and the fluid, D_py/fl, is determined to be 0.15 ± 0.07 at 450 °C in a wide range of Au fluid phase concentrations (10–1000 ppm), but much higher D_py/fl values, between 10 and 50, are found at 350 °C. These Au partitioning trends coupled with the new data on Au molecular environment in pyrite suggest a control of Au(I) incorporation in the mineral by a chemisorption step. Extrapolated to Au contents of hydrothermal fluids of the Earth’s crust which are typically below 1 ppm, our D_py/fl values reproduce fairly well the natural Au tenors in As-poor pyrites (∼0.1–1 ppm Au), which are 100–1000 times lower than those typically observed in arsenian pyrites and arsenopyrites (10–1000 ppm Au at As tenors of 0.01–10 wt%). Our results thus indirectly highlight a key role played by arsenic in gold enrichment in As-bearing iron sulfide ore, a role that yet remains to be fully understood and quantified.

Graphical abstract

Introduction

The most characteristic feature of hydrothermal deposits of gold is its intimate association with iron sulfide minerals, pyrite and arsenopyrite, in which Au concentrations may reach 100–1000 ppm, i.e. almost million times the Au average crustal abundance. Gold affinity for these minerals has been recognized since the first half of the 20th century (e.g., Goldschmidt, 1937). Both ‘visible’ native gold (i.e., optically detectable particles >0.1–1 μm) in physical association with pyrite, and ‘invisible’ (i.e., optically undetectable) gold, possibly incorporated into the mineral structure, occur (e.g., Boyle, 1969, Cathelineau et al., 1989, Cook and Chryssoulis, 1990; references therein). Invisible gold, also termed refractory gold, is not only difficult to identify, but also hard to recover from ore (e.g., Yang et al., 1998, Adams, 2005, Marsden and House, 2009). Such refractory gold poses problems both for accurate estimates of the gold distribution and endowment of a deposit and for efficient extraction of the metal from iron sulfide ore. Resolving these issues requires knowledge of the nature, distribution and mechanisms of incorporation of invisible gold in pyrite, which all yet remain enigmatic. The present study was designed in an attempt to better understand the gold-pyrite relationships, using a novel integrated experimental, spectroscopic and modeling approach.

The literature on Au-bearing pyrite over the four last decades is full of discussions about the chemical and redox state of invisible gold. Based on a variety of analytical techniques such as scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), secondary ion mass spectrometry (SIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), electron probe microanalyzer (EPMA), Mössbauer spectroscopy (MBS), proton induced x-ray emission (PIXE), x-ray photoelectron spectrometry (XPS), x-ray absorption spectroscopy (XAS), atom probe microscopy (APM), atomic force microscopy (AFM), tunneling spectroscopy (STS), coupled with analyses of elemental distribution and inter-element correlations, most studies agree that invisible gold occurs in two major chemical states: an elemental form as nanometer-size particles of Au(0) or alloys with some other trace metals (e.g., Bi, Te, Ag), and a chemically bound form (e.g., Marion et al., 1986, Arehart et al., 1993, Scaini et al., 1998, Friedl et al., 1995, Fleet and Mumin, 1997, Genkin et al., 1998, Den Besten et al., 1999, Simon et al., 1999, Tauson, 1999, Cabri et al., 2000, Palenik et al., 2004, Reich et al., 2005, 2006, Reich et al., 2010, Mikhlin et al., 2011, Deditius et al., 2011, Fougerouse et al., 2016, Trigub et al., 2017a). However, despite significant recent advances in analytical and microscopic methods listed above, the nature of this latter type of gold and factors controlling its incorporation in pyrite remain both inconsistent and controversial. For example, the redox state of chemically bound gold varies from Au³⁺ to Au¹⁻, and its coordination number from 2 to 6, according to different authors (e.g., see Cabri et al., 2000 for review); such large variations make it difficult to attribute to Au an exact structural position in pyrite and to identify the nature of coordinating atoms (e.g., Au, Fe, S). The situation is further complicated by the presence of other minor and trace elements (Cu, Sb, Ni, Co, Ag, Bi, Se, Te) and, in particular, of arsenic with which gold is systematically correlated in arsenian pyrite (e.g., Reich et al., 2005). These elements, often present in far greater concentrations (>0.1–1.0 wt%) than Au (typically < 0.01–0.1 wt%), greatly complicate the assessment, using inter-element correlations, of the Au oxidation state and atomic substitutions in pyrite. The use of direct redox-sensitive spectroscopic methods such as X-ray photoelectron, X-ray absorption or Mössbauer spectroscopy is often limited by low concentrations, poor selectivity for Au in the presence of As and other elements, and a lack of adequate standards.

As a result of these limitations, the mechanisms of Au incorporation in hydrothermal pyrite and arsenopyrite and the role of arsenic and other elements played in this process still remain hypothetical. For example, some earlier studies invoked gold transport by the fluid phase in the form of Au-As-S (Sb, Se, Te) aqueous complexes and their co-precipitation with ferrous iron (Boyle, 1969, Boiron et al., 1989). Other studies suggested chemisorption of dissolved Au as Au-S(As) complexes at As-rich, Fe-deficient sites (Mao, 1991, Fleet and Mumin, 1997, Cepedal et al., 2008), electrochemically driven adsorption of negatively charged

Au {(HS)}_{2}^{-}

complexes on semiconducting As-pyrite and arsenopyrite surfaces (Mironov et al., 1981, Möller and Kersten, 1994, Widler and Seward, 2002), or Au³⁺ and Au⁺ precipitation on sulfide surfaces due to Au reduction (Bancroft and Hyland, 1990, Scaini et al., 1998, Maddox et al., 1998, Mikhlin et al., 2011). However, these hypotheses are mostly based on low-temperature experiments (<100 °C), some of them involving Au aqueous complexes (

{AuCl}_{4}^{-}

Au {(CN)}_{2} {Br}_{2}^{3 -}

), which are unlikely at hydrothermal conditions (e.g., see Pokrovski et al., 2014 for a review of Au speciation in hydrothermal fluids). These limitations complicate the extrapolation of the results of such experiments to the far greater temperatures of Au-bearing pyrite formation in nature (>150–200 °C). Different isomorphic substitution models of Au in the pyrite structure were also proposed, including coupled substitutions of As³⁺ + Au⁺ for 2 Fe²⁺, As³⁺ + Au³⁺ for 3 Fe²⁺ (Deditius et al., 2008), Cu⁺ + Au³⁺ for 2 Fe²⁺ (Chouinard et al., 2005), or Au⁺ + S²⁻ for Fe²⁺ + HS⁻ (Tauson, 1999), but analytical methods capable of unambiguously supporting these models are lacking. Note that the geometry of the available structural sites in pyrite composed of Fe(S₆) octahedrons and S(Fe₃S) distorted tetrahedrons (Le Pape et al., 2018) is not favorable for an energetically stable incorporation of both Au⁺ and Au³⁺ ions that have linear and square planar coordination geometries in most inorganic and organometallic compounds (e.g., Cotton and Wilkinson, 1988). Furthermore, common precipitation mechanisms of As and Au due to fluid reduction and/or cooling, or coupled As-Au redox reactions on the pre-existing (arseno)pyrite have also been considered (e.g., Heinrich and Eadington, 1986, Pokrovski et al., 2002), but they remain qualitative owing to the complexity of the fluid composition and evolution in hydrothermal systems and the interplay of multiple factors controlling mineral solubility such as temperature (T), pressure (P), redox, acidity (pH), and fluid composition (Pokrovski et al., 2014).

It follows from this brief overview that a deeper understanding of the gold-pyrite(-arsenic) relationships in hydrothermal systems awaits experimental studies at pertinent laboratory-controlled conditions, coupled with Au-selective in situ spectroscopic techniques. In the recent years, advances in synchrotron-based high-resolution X-ray absorption/fluorescence spectroscopy (e.g., Proux et al., 2017, Manceau et al., 2018, Le Pape et al., 2018; references therein) have provided an unprecedented gain both in sensitivity (metal detection limits <1 ppm) and selectivity (ability to isolate the trace metal fluorescence signal in a complex mineral/organic matrix), now enabling to directly “see” invisible gold in iron sulfide minerals. In this study, we designed hydrothermal experiments aimed at investigating the Au-pyrite interactions in model S-bearing fluid-pyrite systems at temperatures (350–450 °C) and pressures (400–700 bar) typical of those of various types of gold deposits, as a function of gold and sulfur speciation in the fluid phase. The Au-bearing pyrites resulting from such experiments were investigated by high-resolution X-ray absorption spectroscopy (HR-XAS) coupled with more “traditional” methods (EPMA, SEM, XRD, LA-ICPMS, ICP-AES) to reveal the state of invisible gold in pyrite and to derive pyrite-fluid partition coefficients for chemically bound Au. The results allow predictions of invisible Au contents in ore, and more reliable identification of the factors leading to Au enrichment in iron sulfide minerals. Although the present work deals with As-free systems, the results also help indirectly assess the role of arsenic, thus providing the foundation for future experimental and analytical studies on far more complex As-bearing systems.

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Section snippets

Experimental strategy and conditions

The primary goal of this exploratory experimental study was to investigate the state of gold incorporated into pyrite under conditions pertinent to natural hydrothermal systems forming gold deposits. Gold incorporation in pyrite is expected to occur via i) exchange of an Au-bearing aqueous fluid with already existing sulfide minerals, and/or ii) simultaneous precipitation of gold and pyrite (and other metals) from an aqueous fluid, driven by changes in T, P or fluid composition. Such

Batch-reactor experiments

Gold concentrations measured in quenched solutions from batch-reactor experiments at 450 °C and 700 bar are reported in Table 1. The dissolved Au concentrations vary from ∼100 ppm (in acidic solutions) to almost 3000 ppm (in alkaline solutions). A reasonable quantification limit for aqueous Au in our experimental systems is about 100–150 ppm, which is due to intrinsic uncertainties of the batch-reactor technique such as incomplete recovery of Au precipitated on the reactor walls after the

Interpretation of gold solubility and speciation in sulfur-bearing fluids

The interpretation of Au solubility measured in our experimental sulfate-sulfide systems in terms of Au aqueous complexes is primarily based on knowledge of sulfur speciation itself, which has been a subject of recent systematic research in similar systems at elevated temperatures and pressures using in situ spectroscopic methods (Pokrovski and Dubrovinsky, 2011, Jacquemet et al., 2014, Barré et al., 2017, Pokrovski and Dubessy, 2015, Pokrovski et al., 2015). These direct spectroscopic data,

Concluding remarks

The key points of this study are the following:

A combination of experimental, analytical and modeling approaches used in this work enables new insight into one of the oldest enigmas of Ore Deposit Research, the nature of invisible gold in pyrite. We show that gold is incorporated from hydrothermal fluids into As-poor pyrite both as native metal, Au(0) in the form of nano- and submicron particles, and as chemically bound gold of a nominal redox state of +1, Au(I).

High-resolution XANES

Acknowledgments

This work was funded by the French National Research Agency (grants RadicalS – ANR-16-CE31-0017 and SOUMET – ANR-2011-Blanc SIMI 5-6 009), the Institut des Sciences de l’Univers of the Centre National de la Recherche Scientifique (grants OrPy-CESSUR and AsOrPy-CESSUR), and the Institut Carnot ISIFoR (grant OrPet). We acknowledge the European Synchrotron Radiation Facility (ESRF) for providing access to beamtime and infrastructure. DFT calculations were performed using HPC resources from CALMIP

References (101)

P.R. Amestoy et al.
Hybrid scheduling for the parallel solution of linear systems
Parallel Comput.
(2006)
G. Barré et al.
First evidence of the trisulfur radical ion S₃^- and other sulfur polymers in natural fluid inclusions
Chem. Geol.
(2017)
P.B. Barton
Thermochemical study of the system Fe-As-S
Geochim. Cosmochim. Acta
(1969)
J. Bebie et al.
Surface charge development on transition metal sulfides: an electrokinetic study
Geochim. Cosmochim. Acta
(1998)
I. Berrodier et al.
Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1: X-ray absorption and raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite
Geochim. Cosmochim. Acta
(2004)
B. Bostick et al.
Arsenite sorption on troilite (FeS) and pyrite (FeS₂)
Geochim. Cosmochim. Acta
(2003)
N.J. Cook et al.
Textural control on gold distribution in As-free pyrite from the Dongping, Huangtuliang and Hougou gold deposits, North China craton (Hebei Province, China)
Chem. Geol.
(2009)
E. Curis et al.
Simulating systematic errors in X-ray absorption spectroscopy experiments: Sample and beam effects
Spectrochim. Acta B
(2005)
A.P. Deditius et al.
A proposed new type of arsenian pyrite: composition, nanostructure and geological significance
Geochim. Cosmochim. Acta
(2008)
A.P. Deditius et al.
Trace metal nanoparticles in pyrite
Ore Geol. Rev.
(2011)

A.P. Deditius et al.

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

Geochim. Cosmochim. Acta

(2014)

T. Driesner et al.

The system H₂O–NaCl. Part I: correlation formulae for phase relations in temperature–pressure–composition space from 0 to 1000 °C, 0 to 5000 bar, and 0 to 1 XNaCl: Geochim

Cosmochim. Acta

(2007)

J.W. Johnson et al.

SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000°C

Comput. Geosci.

(1992)

M.A. Kokh et al.

Combined effect of carbon dioxide and sulfur on vapor-liquid partitioning of metals in hydrothermal systems

Geochim. Cosmochim. Acta

(2016)

M.A. Kokh et al.

The role of carbon dioxide in the transport and fractionation of metals by geological fluids

Geochim. Cosmochim. Acta

(2017)

C. Kusebauch et al.

Partitioning of arsenic between hydrothermal fluid and pyrite during experimental siderite replacement

Chem. Geol.

(2018)

Y. Mikhlin et al.

Understanding the initial stages of precious metals precipitation: nanoscale metallic and sulfidic species of gold and silver on pyrite surfaces

Ore Geol. Rev.

(2011)

J.R. Mycroft et al.

Spontaneous deposition of gold on pyrite from solution containing Au(III) and Au(I) chlorides: Part I, a surface study

Geochim. Cosmochim. Acta

(1995)

G.S. Pokrovski et al.

The effect of sulfur on vapor-liquid fractionation of metals in hydrothermal systems

Earth Planet. Sci. Lett.

(2008)

G.S. Pokrovski et al.

Stability and solubility of arsenopyrite, FeAsS, in crustal fluids

Geochim. Cosmochim. Acta

(2002)

G.S. Pokrovski et al.

A new view on gold speciation in sulfur-bearing hydrothermal fluids from in-situ X-ray absorption spectroscopy and quantum-chemical modeling

Geochim Cosmochim Acta

(2009)

G.S. Pokrovski et al.

An in situ X-ray absorption spectroscopy study of gold-chloride complexing in hydrothermal fluids

Chem. Geol.

(2009)

G.S. Pokrovski et al.

Stability and abundance of the trisulfur radical ion S₃^- in hydrothermal fluids

Earth Planet. Sci. Lett.

(2015)

M. Reich et al.

Solubility of gold in arsenian pyrite

Geochim. Cosmochim. Acta

(2005)

M. Reich et al.

“Invisible” silver and gold in supergene digenite (Cu_1.8S)

Geochim. Cosmochim. Acta

(2010)

P.J. Renders et al.

The adsorption of thio gold(I) complexes by amorphous As₂S₃ and Sb₂S₃ at 25 and 90°C

Geochim. Cosmochim. Acta

(1989)

M.A.A. Schoonen et al.

Mechanisms of pyrite and marcasite formation from solution: III Hydrothermal processes

Geochim Cosmochim Acta

(1991)

Y.V. Shvarov

A suite of programs, OptimA, OptimB, OptimC, and OptimS, compatible with the Unitherm database, for deriving the thermodynamic properties of aqueous species from solubility, potentiometry and spectroscopy measurements

Appl. Geochem.

(2015)

A. Stefánsson et al.

Stability of chloridogold(I) complexes in aqueous solutions from 300 to 600°C and from 500 to 1800 bar

Geochim. Cosmochim. Acta

(2003)

J.M. Stromberg et al.

Applications of synchrotron X-ray techniques to orogenic gold studies; examples from the Timmins gold camp

Ore Geol. Rev.

(2019)

D.A. Sverjensky et al.

Water in the deep Earth: The dielectric constant and the solubilities of quartz and corundum to 60 kb and 1200 °C

Geochim. Cosmochim. Acta

(2014)

A.L. Trigub et al.

Experimental determination of gold speciation in sulfide-rich hydrothermal fluids under a wide range of redox conditions

Chem. Geol.

(2017)

A.M. Widler et al.

The adsorption of gold(I) hydrosulfide complexes by iron sulphide surfaces

Geochim. Cosmochim. Acta

(2002)

F. Xia et al.

Mechanism and kinetics of pseudomorphic mineral replacement reactions: a case study of the replacement of pentlandite by violarite

Geochim. Cosmochim. Acta

(2009)

Adams, M.D., ed., 2005. Advances in Gold Ore Processing....

N.N. Akinfiev et al.

Thermodynamic description of aqueous nonelectrolytes at infinite dilution over a wide range of state parameters

Geochim. Cosmochim. Acta

(2003)

G.B. Arehart et al.

Gold and arsenic in iron sulfides from sediment-hosted disseminated gold deposits: implication for depositional processes

Econ. Geol.

(1993)

G.M. Bancroft et al.

Spectroscopic studies of adsorption/reduction reactions of aqueous metal complexes on sulphide surfaces

Rev. Miner. Geochem.

(1990)

P.B. Barton et al.

Phase relations involving sphalerite in the Fe-Zn-S system

Econ. Geol.

(1966)

M.C. Boiron et al.

Conditions of gold-bearing arsenopyrite crystallization in the Villeranges basin, Marche-Combrailles shear zone, France. A mineralogical and fluid inclusion study

Econ. Geol.

(1989)

R.W. Boyle

Hydrothermal transport and deposition of gold

Econ. Geol.

(1969)

O. Bunau et al.

Self-consistent aspects of x-ray absorption calculations

J. Phys.: Condens. Matter

(2009)

L.J. Cabri et al.

Chemical speciation of gold in arsenopyrite

Can. Mineral.

(2000)

L.J. Cabri et al.

The nature of “invisible” gold in arsenopyrite

Can. Mineral.

(1989)

M. Cathelineau et al.

Gold in arsenopyrites: crystal chemistry, location and state, physical and chemical conditions of deposition

Econ. Geol. Monograph

(1989)

A. Cepedal et al.

Gold-bearing As-rich pyrite and arsenopyrite from the El Valle gold deposit, Asturias, northwestern Spain

Can. Miner.

(2008)

Chase, M.W., Jr., 1998. NIST-JANAF Thermochemical Tables, Fourth Edition. J. Phys. Chem. Ref. Data, Monograph No. 9;...

A. Chouinard et al.

Crystallographic controls on trace-element incorporation in auriferous pyrite from the Pascua epithermal high-sulfidation deposit, Chile-Argentina

Can. Min.

(2005)

N.J. Cook et al.

Concentrations of «invisible» gold in the common sulfides

Can. Mineral.

(1990)

A.F. Cotton et al.

Advanced Inorganic Chemistry

(1988)

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The mineralogy and chemical composition of native gold from twenty-seven representative orogenic gold deposits were investigated using optical microscopy, electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Minor elements, such as Ag and Cu, occur in solid solution, while trace elements, such as Fe, As, S, Hg, form impurities or micro inclusions in native gold. Partial least squares-discriminant analysis (PLS-DA) has identified compositional characteristics based on mineral association, gold texture and dominant country rocks. Gold grains are commonly associated with pyrite, arsenopyrite, chalcopyrite, pyrrhotite, and tourmaline in the studied orogenic deposits. Chemical variations in gold with different associated mineral assemblages are related to the partitioning of trace elements between co-crystalizing minerals and gold during precipitation. Gold inclusions in gangue minerals are discriminated from later gold in fractures based on contents of Ag, Fe, Pb, and Bi, which indicates that gold in different paragenetic stages of mineralization can be identified using its trace element signature. Gold hosted in different regional country rocks can be discriminated by Ag, Cu, Pd, Sb, and Hg, likely because of reactions of hydrothermal fluids with the regional country rocks along the fluid flow paths.
Suppression of arsenopyrite oxidation by microencapsulation using ferric-catecholate complexes and phosphate
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Citation Excerpt :
Arsenopyrite (FeAsS), the most common arsenic (As)-bearing sulfide mineral in nature, is often found in auriferous sulfide ores. Due to the fact that arsenopyrite is one of the most common hosts of invisible gold (Au), which is optically undetectable Au particles because of its incorporation into the structure of host minerals, auriferous sulfide ores are typically processed by flotation to recover the bulk of gold-bearing arsenopyrite, and then flotation concentrates are processed via pretreatment (e.g., acidic pressure oxidation (Gudyanga et al., 1999), alkaline pretreatment (Mesa Espitia and Lapidus, 2015), biooxidation (Ciftci and Akcil, 2010), pyrolysis (Dunn and Chamberlain, 1997), ultra-fine grinding (Corrans and Angove, 1991)) to expose Au particles encapsulated in host minerals followed by cyanidation—a conventional hydrometallurgical technique for the selective leaching of gold (Asamoah et al., 2018; Deol et al., 2012; Pokrovski et al., 2019; Tabelin et al., 2020a). Because arsenopyrite is not dissolved during cyanidation, it ends up with the leaching residues typically disposed of into tailings storage facilities (TSF).
Mineral processing, pyro- and hydrometallurgical processes of auriferous sulfide ores and porphyry copper deposits (PCDs) generate arsenopyrite-rich wastes. These wastes are disposed of into the tailings storage facilities (TSF) in which toxic arsenic (As) is leached out and acid mine drainage (AMD) is generated due to the oxidation of arsenopyrite (FeAsS). To suppress arsenopyrite oxidation, this study investigated the passivation of arsenopyrite by forming ferric phosphate (FePO₄) coating on its surface using ferric-catecholate complexes and phosphate simultaneously. Ferric iron (Fe³⁺) and catechol form three types of complexes (mono-, bis-, and triscatecholate complexes) depending on the pH, but mono-catecholate complex (i.e.,[Fe(cat)]⁺) became unstable in the presence of phosphate because the chemical affinity of Fe³⁺—PO₄^3– is most probably stronger than that of Fe³⁺—catechol in [Fe(cat)]⁺. When two or more catechol molecules were coordinated with Fe³⁺ (i.e., [Fe(cat)₂]^– and [Fe(cat)₃]^3–), however, these complexes were stable irrespective of the presence of phosphate. The treatment of arsenopyrite with [Fe(cat)₂]^– and phosphate could suppress its oxidation due to the formation of FePO₄ coating, evidenced by SEM-EDX and XPS analyses. The mechanism of FePO₄ coating formation by [Fe(cat)₂]^– and phosphate was confirmed by linear sweep voltammetry (LSV): (1) [Fe(cat)₂]^– was oxidatively decomposed and (2) the resultant product (i.e., [Fe(cat)]⁺) reacts with phosphate, resulting in the formation of FePO₄.
The state of Au and As in pyrite studied by X-ray absorption spectroscopy of natural minerals and synthetic phases
2020, Ore Geology Reviews
In many hydrothermal deposits arsenian pyrite contains economic concentrations of Au in an “invisible” form, which is structurally bound in pyrite or has grain size <0.1 μm. Here we use X-ray absorption spectroscopy (XAS) of natural minerals and synthetic phases to reveal the forms of Au occurrence in pyrite and determine the effect of physicochemical-compositional parameters on the concentration and state of “invisible” Au. A sample of natural arsenian pyrite with 300 ppm Au and 0.34 wt% As (average value) from the Samolazovskoe Au-sulfide deposit (Yakutia, Russia) was used to study the states of Au and As, and two arsenian pyrite samples from the Vorontsovka Carlin-type deposit (North Urals, Russia) were used to characterize the state of As. The synthesis experiments were performed employing the hydrothermal method at 300 °C/P_sat and 450 °C/1000 bar, at contrasting redox states – in oxidized (H₂S/H₂SO₄ redox buffer) and reduced (H₂S predominates) fluids, in As-free and As-bearing systems. As a result, a series of samples of As-free and arsenian pyrites was obtained with Au concentrations from ca. 100 to 300 ppm. The concentration of “invisible” Au, which was homogeneously distributed within the samples, was independent of As concentration but decreased with increasing synthesis temperature. The concentration of Au dissolved in acidic sulfide hydrothermal fluid was not affected by the presence of As. The X-ray absorption XANES/EXAFS spectra were recorded simultaneously at Au L₃-edge and As K-edge. The use of the high energy resolution fluorescence detection (HERFD-XAS) mode made possible to acquire data for the trace amounts of Au in As-rich samples. According to XANES spectroscopy Au occurs in pyrite in the 1 + oxidation state. Two forms of Au were detected in the pyrite samples: the solid solution Au and Au₂S-like clusters/inclusions. In the solid solution state Au replaces Fe in the structure of pyrite and is coordinated only with sulfur atoms (N_S = 6, R_Au-S = 2.41 ± 0.01 Å) in all pyrite samples independently of the conditions of their formation/synthesis and As content, no As atoms were detected in the local atomic environment of Au. Thus, the Au-S distance in the 1st coordination shell increased by 0.15 Å relatively to the Fe-S distance in pure pyrite. The distant coordination shells of the solid solution Au correspond to the pyrite crystal structure. The presence of the Au₂S-like clusters leads to a significant decrease of the calculated values of coordination number of Au and Au-S interatomic distance. In natural pyrite samples from the Vorontsovka deposit As¹⁻ replaces S in the anionic sites, increasing the 1st shell As-S and As-Fe distances by 0.07–0.1 Å relatively to the pure pyrite structure. A contribution of As³⁺ oxide was detected in pyrite from the Vorontsovka deposit. The synthetic pyrite samples contain As in the form of As¹⁻ solid solution and As³⁺ and As⁵⁺ oxides. Results of our study show that neither the concentration and speciation of As nor the redox state of the system affect the state of “invisible” Au in the studied pyrites.
Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado
2020, Geochimica Et Cosmochimica Acta
Tellurium has a wide variety of applications, most importantly in the solar energy industry and is eco-toxicologically significant; however, the magmatic-hydrothermal processes causing the pronounced Te enrichment together with Au in some epithermal districts are still poorly constrained. Hydrothermal and alkaline magmatic activity in post-subduction environments are suggested to be a critical component in the evolution of this Te-rich sub-class of low-sulfidation epithermal deposits. Cripple Creek represents an example for a world-class low-sulfidation epithermal Au-Te anomaly in the continental crust. This area represents a natural laboratory to investigate the processes of ore-formation and Au-Te enrichment in alkaline igneous rock-hosted epithermal systems.
Here, we present the first micro-analytical approach that combines petrographic observations with in situ LA-ICP-MS analyses and trace element mapping to define the key ore-forming processes of Au and Te in the Cripple Creek epithermal complex. Two main styles of mineralization can be distinguished: (1) low-grade Au disseminated pyrite-rich ores in the permeable brecciated host rocks and (2) high-grade quartz-fluorite veins rich in calaverite (AuTe₂), coloradoite (HgTe), petzite (Ag₃AuTe₂), altaite (PbTe) and native Au.
Pyrite trace element mapping revealed distinct variations and decoupling of elements (Au-Te) and element pairs (Au-As vs. Co-Ni) within and between different sites in the epithermal district. This is reflected by the concentric/growth zoning of these elements in pyrite that were interpreted to be caused by fluid boiling associated with the deposition of the low-grade disseminated host rock ores at temperatures between 220 and 350 °C leading to the precipitation of Au, As, Co and Ni from the liquid phase and the preferential partitioning of Te into the vapor phase. The subsequent condensation of the Te-rich vapors in metal-bearing meteoric waters led to the Te precipitation being decoupled from Au in the low-grade ores.
The high-grade Au-Te mineralization was likely initiated by the influx of magmatically derived oxidized fluids. Subsequently, fluid temperatures dropped due to mixing with meteoric waters resulting in fluid boiling under low pressure conditions at shallower crustal levels (<1000 m) compared to the low-grade ores. Elevated fO₂ minimized the loss of Te to the vapor phase and the strong boiling conditions at lower fluid temperatures (105–200 °C) caused the contemporaneous precipitation of Au and Te in the high-grade veins.
An arsenic-driven pump for invisible gold in hydrothermal systems
2021, Geochemical Perspectives Letters
Revealing the Chemical Form of "invisible" Gold in Natural Arsenian Pyrite and Arsenopyrite with High Energy-Resolution X-ray Absorption Spectroscopy
2019, ACS Earth and Space Chemistry

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The nature and partitioning of invisible gold in the pyrite-fluid system

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Experimental strategy and conditions

Batch-reactor experiments

Interpretation of gold solubility and speciation in sulfur-bearing fluids

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Parallel Comput.

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