EOCENE PALEOPLACER GOLD, SIERRA NEVADA FOOTHILLS, CALIFORNIA: CHARACTERISTICS AND POTENTIAL SOURCE(S)
At the Cleary Hill placer mine, gold occurs in Eocene fluvial sediments dominated by quartz pebbles and cobbles derived from low-grade metasedimentary rocks. Clast diversity is low, and igneous clasts are rare. There has been extensive post-depositional clay alteration and cementation by authigenic pyrite or iron oxides. Gold particles from both a pyrite-cemented layer immediately above the basement unconformity, and from an iron oxide-rich layer 2 m above the basement contact, are rounded with only minor flattening, suggesting relatively short fluvial transport distances. No well-defined thin flakes indicative of extensive flattening during long distance travel have been recognized. In detail, gold particles have rough and convoluted surfaces. Some fine-textured crystalline gold occurs on particle surfaces as well. At least some of the surface gold has textures suggesting secondary addition by the plating on the surface of detrital gold cores. The irregular surfaces are almost 100% Au, with <0.5 wt% Ag. The post-depositional chemical and mineralogical modification has so far limited our ability to identify potential source area(s).