AGE CONSTRAINTS ON THE EMPLACEMENT OF THE GRASS VALLEY GRANODIORITE, CALIFORNIA, AND RELATION TO LODE GOLD FORMATION

The Sierra Nevada foothills (SNFH) orogenic gold province of California is the richest part of the Cordilleran orogen, with an historical lode production of >40 Moz Au. An understanding of the gold-forming events relative to the geologic evolution of this region requires more robust and comprehensive geochronological constraints on the timing of deformation/metamorphism, mineralization, and plutonism. The Grass Valley gold district (GV) of the SNFH province contains numerous deposits hosted within metamorphic rocks and granodiorite. However, little data have been published with regard to the gold-hosting Grass Valley granodiorite (GVGD).

Existing age data for the GVGD are conflicting, with the three previously published age determinations ranging from 164 to 110 Ma. These inconsistencies impede proper interpretation of the tectonic history, as well as correlation with other orogenic gold provinces within the western Cordillera of North America.

To constrain the timing of ore formation and the geological evolution of the GV gold district, least altered samples of GVGD were collected from various locations to produce new, high precision Ar-Ar ages. Samples were selected adjacent (GVGD-4 and GVGD-5) and distant (GVGD-1) to gold orebodies. Biotite from sample GVGD-1 yielded an age of 161.9±0.7 Ma, hornblende from sample GVGD-4 yielded ages of 159.7±0.6, 157.7±1.5, and 156.2±2.0 Ma, and hornblende from sample GVGD-5 yielded ages of 161.9±0.7, 155±3, and 153±2 Ma. Biotite from samples GVGD-4 and -5 were chloritized and unsuitable for analysis. Hornblende from sample GVGD-1 did not produce an interpretable plateau age.

The gold-forming event for the GVGD-hosted veins has yet to be dated, but is inferred to be of similar age to a published mariposite date of 152 Ma from the nearby metamorphic rock-hosted Idaho-Maryland deposit. Our results suggest that the plutonism (~160 Ma) sutured the previously accreted exotic terranes, and was soon followed by the Grass Valley gold-forming event during cooling and uplift. Correlating events along the structural corridor that defines the spatially overlapping northern SNFH-eastern Klamath gold formation event will benefit future exploration and studies of the tectonic evolution of North America.