The mineralogical variability of hydrothermal native element-arsenide (five-element) associations and the role of physicochemical and kinetic factors concerning sulfur and arsenic

Abstract

Abstract The hydrothermal native element-arsenide mineralization comprises Ni-, Co-, and Fe-arsenides and -sulfarsenides, which typically form characteristic overgrowth textures on skeletal native silver and/or bismuth aggregates. Native arsenic, native antimony, antimony-arsenic-silver alloys, and uraninite are present in some of these deposits. Famous deposits of this type include Cobalt, Ontario; Bou Azzer, Morocco; Kongsberg, Norway; Jachymov, Czech Republic; and Schneeberg, Germany. As the physico-chemical conditions of formation (e.g., p, T, fluid composition) and the host rocks are variable, the only unifying feature for their crystallization is a geologically fast reduction process of a Ni-, Co-, Fe-, As bearing fluid e.g., by methane, graphite or Fe2+-bearing minerals. The present contribution combines a comprehensive literature review with novel calculated stability relations of native elements (Ag, Bi, As), Ni-, Co- and Fe-mono-, di- and sulfarsenides, and sulfides/sulfosalts to understand the details of this formation mechanism and to explain the mineralogical and textural diversity observed in five-element assemblages. The characteristic sequence of Ni-\u202f→\u202fCo-\u202f→\u202fFe-diarsenides is ubiquitous and can be explained by continuous reduction of an arsenic- and metal-bearing aqueous solution. This unique succession is largely independent of the metal ratios in the fluid, as three orders of magnitude differences between Fe, Co, and Ni concentrations are needed to change this sequence at neutral pH. At more basic conditions, the diarsenide sequence changes to Co\u202f→\u202fNi\u202f→\u202fFe, which has been observed at only two localities (Valais and Pirineos). Also, the prevalence of mono- vs. diarsenides is mainly pH-dependent. Furthermore, differences in reduction agent, initial pH, fluid/rock ratio, and the crystallization in contained microenvironments all produce visible differences in mineralogy and/or textures that record the details of the formation processes. The stability of dissolved sulfide plays a crucial role in five-element mineralogy, as Co- and Fe-arsenides would not form in the presence of appreciable amounts of sulfide. The absence of large quantities of sulfide in the arsenide stage can be attributed to a lack of sulfur or a thermodynamic disequilibrium between sulfate and sulfide; both cases occur in nature. While the fluid prior to reduction must have been oxidized, slow reaction kinetics for the sulfate-sulfide compared to the arsenite/arsenate-arsenide conversion favor the formation of arsenides and native metals. Their formation is, hence, kinetically controlled, which is supported by the commonly skeletal textures. Sulfarsenides and/or sulfides of Co, Ni, Fe, Pb, and Cu appear only late in the five-element assemblages, when sulfide reduction progresses, and the system re-attains thermodynamic equilibrium or sufficient influx of sulfide has occurred.