STRUCTURAL CONTROLS AND TIMING OF GOLD IN SE CALIFORNIA

Gold in SE California has been variably attributed to Jurassic arc magmatism, Late Cretaceous compression, Oligocene-Miocene detachment faulting, or Late Miocene-Recent dextral transform faulting. We test these competing models at the Oro Cruz deposit, where mineralization occurs in gently-dipping zones in Jurassic metavolcaniclastic gneiss, subparallel to the regional foliation. Most of the mineralization is oxidized, and gold occurs with supergene iron oxides, chrysocolla, and malachite, spatially associated with foliation-parallel brittle faults. This resembles detachment-related mineralization, such as in the nearby Picacho and Copperstone deposits. However, a second, volumetrically-minor style of mineralization is also observed, comprising gold with pyrite and chalcopyrite, disseminated in sheared gneiss and associated with narrow, syn-shear quartz-magnetite veins.

We use field observations, microscopy, whole-rock geochemistry, and geochronology to constrain the timing of the various structures and alteration assemblages, and determine with which of these the gold is associated. Together, these data suggest that gold was introduced with the sulfides, which were preferentially precipitated where the mineralizing fluids encountered reactive magnetite, within a bend in a broad ductile shear zone. This is supported by the age constraints: U-Pb analysis of titanite in nearby deformed granite, and of zircon in the abundant syn-kinematic pegmatites, constrain ductile shearing to ~65 Ma, within error of the sulfide Re-Os age. Subsequent brittle faulting, constrained to after ~54 Ma by apatite U-Pb cooling ages, locally reactivated the ductile foliation. Oxidizing fluids introduced during or after this brittle faulting resulted in extensive supergene alteration, locally remobilizing and upgrading the gold.

Thus, gold at Oro Cruz was initially introduced during ductile Laramide thrusting, much like in the thrust-hosted deposits of the Caborca Orogenic Gold Belt of NW Mexico. However, the Laramide-age fabric was reactivated during subsequent brittle faulting, and the gold-sulfide mineralization oxidized, causing it to resemble the detachment-hosted deposits of especially Arizona. This may suggest that some (or all) of the gold in detachment- and transform-hosted deposits of the region may have been originally introduced during Laramide deformation, and thus form part of the Caborca Orogenic Gold Belt.