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

Mineral Ridge is a shear-hosted gold deposit hosted in a NW-trending metamorphic core complex in the Silver Peak Range, west-central Nevada. Ore occurs in ductily deformed and variably metamorphosed schist, gneiss, and marble of the Neoproterozoic Wyman Formation, and Cretaceous(?) foliated biotite granite. 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 meters-thick 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. Non-foliated leucogranite and pegmatite sills, which appear gradational with foliated biotite granite, are also mineralized albeit generally at lower grade. Gold occurs as free grains in quartz veins and lenses in the lower plate, often in association with pyrite, galena, and sphalerite. The most common alteration associated with quartz veining and gold mineralization is sericitization resulting in strong bleaching of the immediate wall rocks. Late argillization carries abundant pyrite but appears unrelated to main gold event.

This study’s goals are to constrain the timing, distribution, and character of gold mineralization at Mineral Ridge. Although sporadically mined since the 1860’s, the timing of mineralization is poorly constrained, resulting in numerous conflicting genetic models for the deposits. The age of the mineralized biotite granite remains contentious; a U/Pb zircon emplacement age would give a maximum age of mineralization, and U/Pb zircon ages on non-foliated leucogranite may provide a minimum age of gold introduction. Dating of mafic dikes in the lower plate, which are not deformed or mineralized, would give a lower-bound to mineralization and deformation. Age data coupled with field mapping of lithology, structure, and alteration, ore petrography, and geochemistry, will better characterize the style and timing of the Mineral Ridge gold deposits.