Abstract

The Archean volcano-sedimentary succession at Cobalt, Ontario, consists mainly of submarine andesitic flows and turbiditic interflow sedimentary rocks. Steeply dipping faults and large-scale, tight to isoclinar folds have developed in these basement rocks. The faults commonly host silver-vein-bearing structures that extend upward into overlying Proterozoic units. Synvolcanic and synsedimentary base metal sulphide deposits are ubiquitous in the Archean rocks. Epigenetic sulphide mineralization consists of remobilized and replacement sulphides and minor sulpharsenides. Epigenetic sulphides occur in permeable rocks and in fault and fracture zones, commonly in juxtaposition with silver veins.The coincidence of silver veins and structures containing remobilized sulphides suggests that sulphide remobilization from Archean basement rocks into Proterozoic vein-bearing structures predated or accompanied vein development. Sulphide remobilization may have also facilitated the mobilization (leaching) of metals from basement sulphides into the vein-forming hydrothermal fluids.

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