NEW AGE CONSTRAINTS ON LATE CRETACEOUS THRUSTING AND ASSOCIATED GOLD MINERALIZATION IN SE CALIFORNIA

The rocks of the Cargo Muchacho Mountains in SE California record a complex tectonic history, spanning the Jurassic to present. The open pit of the inactive Oro Cruz gold mine provides exceptional exposure, offering the opportunity to determine relative age relations and unravel the locally-preserved structural record.

We present new U-Pb titanite, Re-Os sulfide, U-Pb zircon, and U-Pb apatite ages. Together with detailed field observations in and around the Oro Cruz mine and data from the literature, these outline the following geological history: during Jurassic arc activity, the Tumco Formation host rocks were deposited as volcaniclastic material at 185 Ma. They were subsequently buried to ~23 km and intruded by a series of dioritic to granitic calc-alkaline plutons at 164-158 Ma. All of these units were affected by ductile thrusting in the Late Cretaceous, concomitant with major metasomatism, gold mineralization, and the intrusion of syn-kinematic pegmatites. Based on U-Pb analysis of titanite in sheared granite, we have constrained this high-T shearing to ~64 Ma. This closely matches the timing of gold mineralization (~65 Ma, Re-Os analysis of associated pyrite and chalcopyrite) and the emplacement of garnet-bearing pegmatites (~67-63 Ma, U-Pb analysis of zircons). This Late Cretaceous tectonic shortening may be related to Laramide flat-slab subduction, during which metamorphic devolatilization of underthrust sediments could have produced the gold-bearing mineralizing fluids.

By ~54-47 Ma, the rocks had cooled to below the U-Pb closure temperature of apatite (~450-500°C). Subsequent thrust faulting reflects ongoing shortening after exhumation to shallow crustal levels. Normal-sense faults likely formed later, possibly during Oligocene to Miocene Basin and Range extension. Dextral strike-slip faults probably formed as a result of Late Miocene to present activity on the San Andreas system. Fluids infiltrated during these brittle faulting events, oxidizing most of the sulfide mineralization and making the gold more readily recoverable.