Pathfinder signatures in placer gold derived from Au-bearing porphyries.

Porphyry and epithermal mineralization of early Late Cretaceous Age (ca. 76-74Ma) at Casino, Revenue/Nucleus and Sonora Gulch areas in the central and eastern Dawson Range in west-central Yukon is spatially related to numerous placer gold mining areas. Placer- lode relationships have been established through study of gold alloy compositions and associated mineral inclusion assemblages. At Casino, hypogene gold grains are liberated by erosion and pass into the placer system without compositional modification, as evidenced by the common alloy signatures and a mineral association of Au, (Bi-Pb-Te-S) minerals and chalcopyrite. A second signature of higher- Ag, chalcopyrite- poor gold has been identified in placer populations, but this gold type also exhibited the Bi-Pb-Te-S signature. The results suggest that placers contain a mixture of gold derived from the porphyry and peripheral or shallower level epithermal mineralization. The Pb-Bi-Te-S association evident at Casino was also recorded at Revenue/Nucleus and Sonora Gulch. This generic signature of gold in Cu-Mo (-Au) porphyry deposits and their associated distal epithermal manifestations could be applied to exploration in areas where placer-lode relationships are unclear.