Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Despite the fact that pyrite is a relatively common phase in Ni-Cu-Platinum-Group Elements (PGE) magmatic sulphide deposits, the trace element content of the pyrite has been neglected in the studies of these deposits with most attention being paid to the PGE concentrations of pyrrhotite, pentlandite and chalcopyrite. Pyrite in these deposits exhibits a range of textures, from euhedral to xenomorphic. The origin of the different pyrites is not always clear; they could have formed by exsolution from monosulphide solid solution (mss), by replacement of the existing minerals during cooling or metamorphism or directly from hydrothermal fluids. In order to provide data on trace element contents of pyrite in a magmatic sulphide deposit and to investigate the origin of the pyrite, we have measured the content of PGE and other chalcophile elements (Au, Ag, Co, Ni, Cu, Se, Sb, As, Bi and Te) by laser ablation ICP-MS in pyrite exhibiting different textures from the Aguablanca Ni-Cu deposit (Spain). The results show that 1) large idiomorphic pyrite is compositionally-zoned with Os-Ir-Ru-Rh-As-rich layers and Se-Co-rich layers; 2) some idiomorphic pyrite grains contain unusually high PGE contents (up to 32 ppm Rh and 9 ppm Pt); 3) ribbon-like and small-grained pyrite hosts IPGE (i.e., Iridium-group PGE, Os, Ir, Ru and Rh) in similar contents (100–200 ppb each) than the host pyrrhotite; and 4) pyrite replacing plagioclase is depleted in most metals (i.e., PGE, Co, Ni and Ag). Overall, the different textural types of pyrite have similar abundances of Pd, Au, Se, Bi, Te, Sb and As. Mineralogical and compositional data suggest that the formation of pyrite is the result of the activity of late magmatic/hydrothermal fluids that triggered the partial replacement of pyrrhotite and plagioclase by pyrite, probably due to an increase in the sulphur fugacity on cooling. During this episode, pyrite inherited the IPGE content of the replaced mineral, whereas other elements such as Pd, Au and semi-metals were likely partially introduced into pyrite via altering fluids. These results highlight that pyrite can host appreciable amounts of PGE and therefore it should not be overlooked as a potential carrier of these metals in Ni-Cu-(PGE) sulphide deposits.