Platinum-group elements-bearing pyrite from the Aguablanca Ni-Cu sulphide deposit (SW Spain): a LA-ICP-MS study

摘要

Despite the fact that pyrite is a relatively common phase in Ni-Cu-Platinum-Group Elements (PGE) magmatic sulphide deposits, it has been neglected in the studies on PGE distribution in favour of pyrrhotite, pentlandite and chalcopyrite. An example of this is the Aguablanca deposit, where although pyrite is an important phase in the semi-massive ore and an early study has revealed that it hosts traces of PGE, the presence and origin of PGE into the pyrite has not been investigated in detail. With this in mind, we have measured by laser ablation ICP-MS the content of these and other chalcophile elements (Au, Ag, Co, Ni, Cu, Se, Sb, As, Bi and Te) in pyrite exhibiting different textures. The results show that 1) large idiomorphic pyrite is compositionallyzoned with Os-Ir-Ru-Rh-As-rich layers and Se-Co-rich layers; 2) some idiomorphic pyrites contain unusually high PGE contents (up to 32 ppm Rh and 9 ppm Pt); 3) ribbon-like and small-grained pyrites host IPGE (i.e., Iridium-group PGE, Os, Ir, Ru and Rh) in similar contents (100-200 ppb each) to the host pyrrhotite; and 4) pyrites replacing to plagioclase are depleted in most metals (i.e., PGE, Co, Ni and Ag). Overall, the different textural types of pyrite have similar abundances in Pd, Au, Se, Bi, Te, Sb and As. Mineralogical and compositional data suggests that 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 during cooling. During this episode, pyrites inherited the IPGE content of mineral to that was replaced, 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.