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Application of U-Th-Pb phosphate geochronology to young orogenic gold deposits: New age constraints on the formation of the Grass Valley gold district, Sierra Foothills province, California

June 3, 2015

The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping east-west (E-W) veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping north-south (N-S) veins hosted by the Grass Valley granodiorite; the latter have yielded about 70 percent of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170-140 Ma and 120-80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotopic dating of hydrothermal xenotime yielded the first absolute age of 162±5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of a Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wallrock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153-151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9±2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7±0.6 to 161.9±1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (~800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (<300°C). The age of the granodiorite is indistinguishable from that of the N-S veins, as recorded by the U-Pb age of xenotime in those veins. Consequently, the N-S veins must have formed between 162 and 157 Ma, the maximum permissive age of magma emplacement and the youngest permissive xenotime U-Pb age, respectively, during an E- to ENE-directed compressional regime. The geochemistry of the Grass Valley granodiorite is consistent with it being the product of arc magmatism. It served as a receptive host for mineralization, but it is has no direct genetic relationship to gold mineralization. Initial uplift of the intrusive mass correlates with the initial voluminous fluid flow event and vein formation at depths of no greater than 3 km. The E-W gold-bearing veins hosted within greenschist-facies country rocks adjacent to the intrusion formed during a second hydrothermal event 5-10 million years later than the magmatism and were contemporaneous with a shift to a transtensional deformation denoted by sinistral strike-slip faulting.