Relationship of mineralogy, petrology, and structural geology to gold mineralization at the Twin Falls Gold Occurrence, Beardmore, Ontario

Abstract

The purpose of this thesis was to study the mineralogy, petrology, and structural geology of the Twin Falls Gold Occurrence and determine whether or not a relationship existed between these features and the gold mineralization. The answers to these problems were sought using methods of geological field mapping, thin and polished section studies, and petrological-chemical analyses. No visible gold was observed in hand samples or polished sections. The gold assays do show a sympathetic relationship with the pyrite concentrations of the veins suggesting that the gold is included within the pyrite as fine-grained crystal lattice impurities. Gold deposition was relatively late in the geological history of the area. It occurred post-volcanism, post-metamorphism, and following at least one phase of deformation. The gold mineralization was structurally controlled as is shown by the infilling of east-west fractured or sheared zones with gold-bearing pyrite and quartz-pyrite veins. The gold-bearing veins are preferentially positioned in or near narrow (25 meter wide) rhyolite tuff units. The rocks adjacent to the veins have been pervasively altered, showing a depletion in calcium and enrichment in silica and potassium. The geological characteristics of the Twin Falls Gold Occurrence have indicated that the environment for the gold deposition was one of epizonal intrusion superimposed upon a sequence of deformed and altered felsic and intermediate volcanic rocks.