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

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, Dpy/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 Dpy/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 Dpy/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.