Extreme As partitioning into pyrite during hydrothermal siderite replacement in As poor fluids

Pyrite is among the most abundant sulphide minerals in the Earth’s crust and an important sink for As and valuable elements like Au and Te (1, 2). Hence, arsenian pyrite is of potential economic interest, but also an environmental hazard if As is released to ground- and surface waters. Knowing the limits and mechanisms of As incorporation in pyrite is therefore required to better understand the formation of high-grade ore deposits and to assess the contribution of As to contamination sites.It is suggested that As either substitutes for S or Fe in the bulk crystal or forms localized high-As domains within the pyrite lattice. In various studies, partition coefficients (DAs) were determined that range from DAs= 300 – 1600, with DAs being the ratio of the As concentration in pyrite relative to the corresponding fluid (3). However, previous studies focused on As-rich fluids (1 – 200 ppm), whereas the partitioning behaviour in low-As fluids (<1 ppm) has not been studied to date.We synthesized pyrite under Carlin-style, hydrothermal conditions (200°C, pH 5) by replacing siderite in H2S-rich fluids containing As concentrations between 1 ppb and 10 ppm. Quantitative fluid analysis and high-resolution LA-ICP-MS mappings indicate an order of magnitude stronger partitioning of arsenic from As-poor fluids (1 ppb, DAs= 15000), leading to unexpectedly high enrichment of As in pyrite. This observation suggests a highly efficient mechanism of As fixation into pyrite from As-poor fluids at moderate temperature conditions.

References:[1] Börner et al.(2021). DOI:10.1016/j.oregeorev.2021.104314.[2] Kusebauch et al.(2019). DOI:10.1126/sciadv.aav5891.[3] Kusebauch et al.(2018). DOI:10.1016/j.chemgeo.2018.09.027.