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The geology and mineralogy of the Brown McDade mine Lamb, John

Abstract

The Brown McDade Mine is a recent gold-silver discovery in the Yukon Territory, halfway between Whitehorse and Dawson. It lies in the area unglaciated during the Pleistocene Period. Diamond drilling and stripping in 1946 indicated commercial values across a width of 10 feet and over a length of 1000 ft. The geologic structure is that of a Late Tertiary, dyke-like body of quartz feldspar porphyry cutting quartz diorite of Jurassic or later age. This body has a Northwest strike and dips steeply west. The ore zone lies in the porphyry and is believed genetically related to it. The mineralization consists of a cherry-like fine grained blue quartz, with disseminated sulphides comprising less than 5% of the mass. Metallic minerals are pyrite, arsenopyrite, and sphalerite, with lesser amounts of galena, chalcopyrite, tetrahedrite, stibnite, bournonite, jamesonite and gold. The gold is associated with the pyrite in fine particles, the majority less than 150 microns in size. The gold-silver ratio is about one to ten. Sericitization and carbonatization are the chief forms of hydrothermal alteration, while considerable limonite, and around the ore zone, jarosite, have been produced by weathering. On the basis of comparisons with known deposits, the Brown McDade is considered to belong to the deeper epithermal type. It should continue to reasonable depths although the ore shoots are likely to be erratic. The area south of the main ore zone, underlain by schistose rocks will probably be unfavorable for the occurrence of ore.

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