A review of Te and Se systematics in hydrothermal pyrite from precious metal deposits: Insights into ore-forming processes

Pyrite is one of the most common minerals in many precious and base metal hydrothermal ore deposits and is an important host to a range of trace elements including Au and Co and the semi-metals As, Se, Sb, Te and Bi. As such, in many hydrothermal ore deposits, where pyrite is the dominant sulphide phase, it can represent a major repository for these elements. Furthermore, the concentrations and ratios of Au, As and Co in pyrite have been used to infer key ore-forming processes. However, the mechanisms controlling the distribution of Te and Se in pyrite are less well understood. Here we compare the Te and Se contents of pyrite from a global dataset of Carlin type, orogenic Au, and porphyry-epithermal deposits to investigate: (1) the potential of pyrite to be a major repository for these elements; and (2) whether Te and Se provide insights into key ore-forming processes. Pyrite from Carlin-type, low-sulphidation and alkaline igneous rock-hosted epithermal systems is enriched in Te (and Se) compared to pyrite from high-sulphidation epithermal and porphyry Cu deposits. Orogenic Au pyrite is characterised by intermediate Te and Se contents. There is an upper solubility limit for Te as a function of As in pyrite, similar to that established for Au by Reich et al. (2005); and this can be used to identify Te present as telluride inclusions, which are common in some epithermal-porphyry and orogenic Au deposits. Physicochemical fluid parameters, such as pH, redox and temperature, as well as crystal-chemistry control the incorporation and concentration of Se and Te in pyrite. Neutral to alkaline fluids have the ability to effectively mobilise and transport Te. Fluid boiling in porphyry-epithermal systems, as well as wall rock sulphidation and oxidation in Carlin-type (and orogenic Au) deposits can effectively precipitate Te in association with pyrite and Au. In contrast, Se concentrations in pyrite apparently vary systematically in response to changes in fluid temperature, irrespective of pH and A. Hence, we propose that the Se contents of pyrite may be used as a new geo-thermometer for hydrothermal ore deposits. Furthermore, the comparison of bulk ore and pyrite chemistry indicates that pyrite represents the major host for Te and Se in Carlin-type and some epithermal systems, and thus pyrite can be considered to be of economic interest as a potential source for these elements.