Abstract
Mariposite (Cr-rich muscovite) crystals formed the during metasomatic replacement of serpentinite yield 40Ar/39Ar ages demonstrating that gold-quartz-carbonate mineralization in the Sierra Nevada Foothills Metamorphic Belt occurred as early as 152 Ma and continued until at least 122 Ma. The earliest gold mineralization is found in the Grass Valley District; mineralization subsequently migrated southward along the Melones fault zone, forming the Mother Lode Gold Belt. These ages correspond to a lull in Sierra Nevada plutonism, and with periods of ductile deformation along the Melones and related fault zones. Our data suggest that gold-quartz-carbonate mineralization was not associated with Sierra Nevada plutonism, but instead was related to changes in plate dynamics, and generation and migration of CO2-rich ore-forming metamorphic fluids along the continental margin of North America. In this model, shallow gold mineralization was produced by heating of the lower crust within the structural mélange of the Sierra Nevada Foothills Metamorphic Belt as a response to an increase in dip of the subducting slab and oceanward migration of the saw-tooth thermal structure.
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