Reconnaissance rock geochemistry of Aishihik Lake, Snag and Stewart River map-areas in the Yukon Crystalline Terrane

This is a report on the abundance of zinc, copper, lead, molybdenum, tungsten, mercury, silver and arsenic in 3061 rock samples from part of the Yukon Crystalline Terrane. The data are examined in terms of the geology and it is shown that unique metal abundances and associations exist in some units although most map assemblages are not geochemically distinctive. Of the metamorphic rocks the Nasina quartzite, Klondike schist and Kluane schist have interestingly high concentrations of different metals. For the first two these are thought to indicate possibilities for mineralization. Among the plutonic rocks, nearly all of which have low metal backgrounds, only the Eocene Nisling Range alaskite stands out geochemically. This plutonic suite, together with its dyke swarms and explosive volcanic rocks (Mount Nansen Group), is geochemically active. The alaskite contains molybdenum-copper porphyry occurrences and the acid volcanics locally host gold-silver veins. This volcano-plutonic suite also has possibilities for associated tin and/or uranium mineralization. The upper Triassic Lewes River Group is geochemically rich in copper. The basalt, the time equivalent of the Nicola and Takla Groups of British Columbia, is also host to a number of copper occurrences in Yukon. This report characterizes the frequency distributions of the various metals emphasizing that the shapes of the cumulative frequency curves are as distinctive as the concentrations and are independent of the way the sample population is broken down. Thus, using probability log scales, cumulative curves for zinc are concave, those for arsenic convex, copper curves are straight while those of lead and silver are concave-convex. Comparison of the bedrock geochemistry with a stream sediment survey of the same region predictably shows that zinc and molybdenum are more mobile than copper in streams. Although the geochemistry provides background data on elemental association and abundances in the various suites thus corroborating some of the geological prejudices, it does not lead to new insights into the relations, genesis and history of the rocks. Nor does it specify exploration targets unequivocally. A geochemical study such as this is therefore a luxury justifiable only when the geology is already understood, but unwarranted as an initial examination of a region.