POST-MINERALIZATION EXTENSION: EUREKA MINING DISTRICT, NEVADA, USA

Eureka mining district is centered on the Late Cretaceous Prospect Mountain duplex which extends 10 km north-south and affects the approximately 12,000 meters of clastic and carbonate strata of the Cordilleran miogeocline and the Antler foredeep basin. Structural relief on the duplex is on the order of two kilometers. Mineral deposits formed at 111 Ma (base metals, Au, Ag), 85 Ma (fluorite), and in the Eocene (Au, Ag) were affected by this structure and at least three ensuing extensional events that overprint it.

In the Latest Cretaceous to Eocene, the Sheep Pass Formation was deposited in an (extensional?) basin immediately south of Eureka in Little Smokey valley. Subsequently, large gravitationally driven masses slid radially to the south and west from the flanks of the duplex. In the Mountain Boy Range and Mahogany Hills, these masses extend westward for at least 14 km with a width of 10 km, having the appearance of a mega-landslide. Normal faults within the hanging wall are recognized by the presence of a rose color, brecciated limestone, massive calcite veins, and barite, manganese and Si02 mineralization. The base of the broken mass is the upper contact of the Ordovician Eureka Quartzite, interpreted to have had a westward dip of 10^o to 20^o at the time of movement. Silurian to Mississippian carbonate strata and rhyolite tuffs (32-38 Ma) form the hanging wall and the basal detachment cuts the 34 Ma Wood Cone stock. Restoration of the slide material eastward would bring base metal deposits in the blocks into a peripheral location relative to the potential source, the Late Cretaceous Ruby Hill and Graveyard Flats stocks within the duplex. A hanging wall basin separates the two ranges.

North-trending Basin and Range faults isolate the duplex as an accommodation zone between Diamond Valley half graben to the north and Little Smokey graben to the south. The west flank of the duplex, and the gravity slide, are down faulted a total of 3,500 m along the Spring Valley, Sharp, Cave Canyon and dugout Tunnel faults which further disrupts the mineralized zones.