New Mexico Mineral Symposium — Abstracts

The Cobalt Area silver mining area, in northern Ontario, Canada, was discovered in 1903 by workers of the Timiskaming and Northern Ontario Railway and is renowned as the source of 600 million troy ounces of silver, mostly mined as native silver in association with Co-Ni-Fe-Sb sulpharsenide and sulphosalt minerals in carbonate veins. The near-vertical veins were emplaced in fault-controlled fractures in various rock types, all within a few hundred meters of the Nipissing Diabase Sill. The sill intruded local rocks 2.162 billion years ago and deposition of the silver veins occurred after that. According to fluid inclusion studies, the minerals in the veins were precipitated from hypersaline aqueous solutions into fault-controlled fractures at pressures of 600 -1360 bars (60-136Mpa) over the temperature range of 300-350 degrees Celsius. The source of the vein minerals has never been definitively determined but theories share the commonality that the heat from the 333m thick diabase and, perhaps, its parent magma body caused metals to be concentrated from a local source(s) and moved by the saline solutions to eventual emplacement in fractures, in all surrounding rock types, including the Nipissing diabase sill. The elemental make-up of the veins has resulted in a unique and interesting suite of minerals and mineralogical textures, including nine type-locality minerals, which will be covered in detail in this presentation.