The geology and geochemistry of gold mineralization and associated alteration at the Rundle gold deposit, Abitibi Subprovince, Ontario

Abstract

The Rundle gold deposit in the Swayze greenstone belt of the Abitibi subprovince is at the contact of a feldspar porphyry stock with an east-southeast-trending zone of sheared mafic volcanic rocks. The deposit consists of five tabular mineralized zones, each composed of fine veins of quartz and ferroan dolomite and fracture-controlled alteration hosted in both porphyry and mafic volcanic rocks. Zones of alteration were mapped for each type of host rock and designated as distal, medial, and proximal alteration facies, reflecting their distance from the mineralized zones. Distal and medial alteration in the mafic volcanic rocks and distal alteration in the feldspar porphyry are characterized by calcite and chlorite and the replacement of actinolite and epidote. The formation of dolomite and sericite and the replacement of calcite and chlorite characterize the proximal alteration facies in the mafic volcanic rocks and the medial and proximal alteration in the feldspar porphyry. Alteration, as determined by mass balance calculations, is dominated by carbonatization, sulfidation, and Na metasomatism and increases in the trace elements Au, W, Te, Ag, and As. Cu and Mo increase with alteration in the feldspar porphyry only, and Sb increases with alteration in the mafic volcanic rocks. Many of the changes in trace element contents of the wall rocks, which could be important in lithogeochemical exploration for Au, were detected by a simple statistical treatment of the data.The extent and geometry of the proximal and medial alteration facies and the geometry of the mineralized zones are controlled by the east-southeast trend of the sheared rocks. The distal alteration occurs at the perimeter of the feldspar porphyry stock and in mafic volcanic rocks surrounding the stock. The distribution of the alteration and its more extensive development in the porphyry stock suggest that the flux of hydrothermal fluids was controlled bypermeable zones created by the competency contrast of the relatively brittle porphyry stock and the more ductile mafic volcanic rocks.