SHEAR-RELATED GOLD BEARING QUARTZ VEINS IN METAMORPHIC ROCK AT MINERAL RIDGE, ESMERALDA COUNTY, NEVADA

Mineral Ridge is a shear-related, quartz-hosted gold deposit hosted in a NW-trending metamorphic core complex in the Silver Peak Range, west-central Nevada. The Silver Peak Range, which contains the Mineral Ridge metamorphic core complex along its southeastern segment, lies within the Walker Lane, a zone of disrupted structures which forms the accommodation zone between the north-northwest trending Sierra Nevada block and the north-northeast trending mountains of the Basin and Range. The Mineral Ridge metamorphic core complex, suggested to be part and parcel of the Silver Peak- Lone Mountain detachment system, may be a reflection of this accommodation.

Ore occurs in ductily deformed and variably metamorphosed schist, gneiss, and marble of the Neoproterozoic Wyman Formation, Cretaceous(?) foliated biotite granite and locally pegmatitic leucogranite. These deformed rocks lie beneath an upper plate composed of unmineralized and largely undeformed Neoproterozoic clastic and carbonate rocks, and Miocene volcanic rock, the two plates separated by a detachment fault which dips easterly between 20 and 40 degrees. Rocks of the lower plate contain abundant quartz veins, lenses, and boudins, which are the main expression of gold mineralization at Mineral Ridge. Subparallel footwall shears, rheologic and compositional contrasts between schist, marble, and granite, and gentle undulations in subsidiary shears appear to control quartz vein density and ultimately, gold deposits. Gold occurs as free grains in quartz veins and lenses in the lower plate, often in association with pyrite, galena, and sphalerite. A late argillization carries abundant pyrite but appears unrelated to main gold event.

Mineral Ridge occurs in an area of intense strain and deformation, and the relationship of intrusions, contraction, and extension to the story of gold mineralization remains enigmatic. Recent field work, petrography, and radiometric dating may help elucidate the timing and nature of the Mineral Ridge system.