Sulphide remobilization in Archean volcano-sedimentary rocks and its significance in Proterozoic silver vein genesis, Cobalt, Ontario: Discussion

Abstract

The authors provide a much-needed description of the interflow sediment that have long been recognized as having a spatial relationship to many of the important silver veins in the Cobalt camp. Comparison of the Archean geology of the other silver mining camps within the Cobalt Embayment casts additional light on the role of these sediments in ore control. Interflow sediments as described by the authors are only well developed in the Cobalt camp, but have been noted on a lesser extent in underground exposures in Casey Township (Thomson 1965). No such occurrences have been reported in close relationship with ore veins from the Archean rocks of South Lorrain or Gowganda, and at Elk Lake there are no Archean rocks exposed (MacKean 1964). In the Miller Lake part of the Gowganda camp, contacts in the Archean rocks influenced development of the silver veins in the underlying Nipissing diabase (Hester 1967,1988), but there are no interflow sediments of the kind described by the authors. The contacts that influenced vein formation are characterized by physical dissimilarity of the igneous rocks involved, such as that between intrusive felsites or diabase dykes of the Matachewan swarm and serpentinites. No sulphide occurrences are known in the Archean. Although interflow sediments are absent, the mineralogical composition of the ore veins is virtually the same as that found at Cobalt. Relative amounts of some of the constituent minerals vary. Both lead and copper occur mainly with quartz and were deposited during a later phase of mineralization than was the silver, but in the same structures. The only chemical difference of note is the increase in iron content at the expense of cobalt in the Gowganda veins. The above evidence suggests that the intefflow sediments at Cobalt played a purely structural role in the development of the silver veins. Had they contributed to the vein content at Cobalt, there would be difficulty in explaining the presence of the almost identical suite of minerals in the veins at Gowganda, where there are no interflow sediments. The source of the Ag-Bi-Co-Ni-As suite of minerals in the veins of the Cobalt embayment has yet to be explained. The Archean contains no deposits of these elements, which can be demonstrated to have formed before the post-Nipissing mineralizing event. Concentrations of silver far in excess of the total produced from the veins at Cobalt occur within the Archean volcanic rocks in the coeval massive sulphide orebodies of the Kidd Creek mine (Walker et al. 1975) near Timmins, about 3 15