Magmatic-hydrothermal and wall rock alteration at the Lake Shore gold deposit, Kirkland Lake, Ontario.

Abstract

The Lake Shore Au deposit lies in the Kirkland Lake Au camp, the second largest Archean camp in North America. The deposit is hosted in an alkaline to sub-alkaline intrusive complex composed of augite syenite, syenite, feldspar porphyry and quartz-feldspar porphyry. Feldspar and quartz-feldspar porphyry are the latest of the intrusive phases within the Lake Shore property and host over two-thirds of the Au in the camp. Detailed petrography and geochemistry of the feldspar porphyry indicate that it is predominantly a quartz-monzodiorite. The primary mineralogy of this unit consists of plagioclase, orthoclase, biotite, and hornblende, with accessory magnetite, apatite, titanite, monazite, and zircon. Miarolitic cavities within this unit suggest a relatively shallow level of emplacement as well as an H$\sb2$O-rich nature for the magma. Four successive alteration facies are recognized within the feldspar porphyry. The concentric nature of the later three alteration facies as zones about the auriferous veins may be the result of several overprinting, chemically distinct hydrothermal events, each of which penetrated the wall rocks to subsequent decreasing depths. A second possibility is that they are the result of a fluid which evolved as it penetrated the wall rocks. (Abstract shortened by UMI.)