Role of surface phenomena in concentrating incompatible elements: Au in pyrite from hydrothermal clays at thermal fields in southern Kamchatka

Abstract

The paper reports data on pyrite obtained using microscopic, physicochemical, and analytical techniques. The major mineral concentrating Au in clays at the Upper Koshelevskoe and East Pauzhetka hydrothermal fields is pyrite, which contains Au evenly distributed in its crystals (0.07–0.25 ppm) and Au bound to the surface of its crystals (0.8–6.8 ppm). The clear correlations between the concentration of equally distributed Au and the topological surface area of single crystals and the absence of correlations with the bulk specific BET surface area rule out purely adsorption mechanisms of Au accommodation at the surface of pyrite crystals, because otherwise the concentration of the minor element would have been proportional to the actual surface area but not the geometric one. In contrast to what is typical of high-temperature hydrothermal systems, at hydrothermal fields Au is an element highly compatible with pyrite. This may be explained by changes in the growth mechanisms of pyrite crystals and the transition to their growth via incorporation of colloid and subcolloid particles. Low-temperature pyrite in hydrothermal clays exhibits certain geochemical features important for exploration for gold deposits related to modern and ancient hydrothermal systems. This pyrite differs from higher temperature pyrite at ancient gold deposits in bearing sulfoxide sulfur species on the surface of its crystals instead of monosulfide species and also in having a less ordered and dense structure, higher porosity, and a globular topography of the surface of crystals.