PYROSMALITE IN CANADIAN PRECAMBRIAN SULFIDE DEPOSITS - MINERAL CHEMISTRY, PETROGENESIS AND SIGNIFICANCE

Pyrosmalite-series minerals [(Fe,Mn)(8)Si6O15(OH,Cl)(10)] are reported from metamorphosed sulfide deposits in three Canadian Precambrian min ing camps: Matagami, Quebec (Archean, Zn-Cu-Ag-Au), Manitouwadge, Onta rio (Archean, Cu-Zn-Ag-Au) and Thompson Nickel Belt, Manitoba (Protero zoic, Ni-Au-PGE). As a result of electron-microprobe data, pyrosmalite is found to form a continuous solid-solution series from a value of M n/(Fe + Mn) of 0.08 to 0.85, and is a sink for Cl. Textural characteri stics and element partitioning indicate that pyrosmalite crystallized during bath regional metamorphism and late-stage hydrothermal activity . However, the initial Cl-enrichment in the host lithologies may have occurred during seafloor hydrothermal alteration as an integral part o f the syngenetic ore-forming processes for the associated base-metal m ineralization. Identification of pyrosmalite in volcanogenic massive s ulfide (VMS) deposits provides further support for a brine-pool model for ore genesis and suggests that the precious-metal mineralization co mmonly associated with these sulfide deposits may have been related to Cl-rich fluids.