EOCENE PALEOPLACER GOLD, SIERRA NEVADA FOOTHILLS, CALIFORNIA: CHARACTERISTICS AND POTENTIAL SOURCE(S)

The placer goldfields of the Sierra Nevada foothills, California, constitute one of the largest and most famous Phanerozoic placer accumulations in the world. A key feature of these goldfields is a set of partially-preserved Eocene auriferous paleochannels that are now perched high above the modern drainage system, and have acted as gold sources for the modern rivers. Hypotheses on primary sources for this paleoplacer gold range from the nearby orogenic gold deposits in the Sierra Nevada foothills to the distant epithermal and porphyry deposits of western Nevada. Previous modern work has established the paleoelevation and shown that some of the paleochannels drained to the east. We have quantified the geochemistry and textural characteristics of detrital minerals within the placer profile, using SEM, EMPA, and LA-ICP-MS on quartz, pyrite, clay minerals, and gold.

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).