Abstract

Volcanogenic massive sulfide deposits within the Buttle Lake mining camp are associated with andesitic and felsic rocks of the Price and Myra formations in the Paleozoic Sicker Group. The Sicker Group forms the oldest known unit within the Wrangellia terrane.Lead isotope data for galena from the Buttle Lake mining camp indicate that lead evolved in an orogen or island-arc environment. There is a pronounced linear trend in the lead isotope data that can be explained as a mixing line. Positions of data along this trend do not relate to age differences among the ore lenses or to variable selective leaching of lead isotope components from significantly older footwall source rocks. However, the line appears to represent one or a combination of (1) variable mixing of lead from felsic and andesitic rocks (or related magmatic fluids), each with distinctly different proportions of upper crustal and mantle lead isotopes, or (2) mixing of lead isotopes from relatively radiogenic oceanic sediment with relatively primitive lead isotopes from the volcanic rocks (or related magmatic fluids).More specifically, the less radiogenic end member appears to be spatially related to rhyolitic host rocks. The rhyolitic rocks may have contributed a magmatic component or represent leached source rocks of the same age as the mineralization. The more radiogenic deposits generally occur immediately above major discharge stockworks in andesite. The andesites may have a distinctive, relatively radiogenic magmatic source, or contain interbeds of more radiogenic oceanic sediment.Markedly radiogenic lead characterizes most deposits within the H-W horizon, defined in part, by the H-W Main and Battle Main lenses. These lenses, immediately above the Price formation contact, are among the largest; consequently, relatively radiogenic lead isotope compositions may identify the most favorable exploration targets.

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