Geology of the Rockland Mine Area, Western Nevada: Au-Ag Mineralization Associated with Late Miocene Rhyolite Domes

摘要

The Rockland mine area is located in the southern part of the Pine Grove mining district 40 km south of Yerington, Nevada, in the western part of the Walker Lane. The district produced approximately 400,000 oz Au, mainly during the late 1800s. The Rockland mine produced approximately 50,000 oz AuEq from late Miocene, low sulfidation type, high-grade epithermal quartz veins. The northern Pine Grove Hills consist of a gently west-tilted horst block composed mostly of Jurassic granitic rocks, bounded on the north and east sides by middle to late Miocene sedimentary and lesser volcanic rocks. The dominant structural feature in the area is the Pine Grove fault system (PGFS), a series of NW- to N-striking normal faults which down-drop the Miocene rocks on the east, with cumulative offset of several hundred meters or more. The PGFS in the Rockland mine area appears to be offset by or merge with younger northerly-striking Basin and Range type normal faults along the eastern margin of the range. NNE- and NE-striking steeply dipping faults are the main control for quartz veins and Au-Ag mineralization in the Rockland mine area. Basement rocks consist mainly of Jurassic granodiorite, which is faulted against or unconformably overlain by the Miocene Wassuk Group. The late Miocene Morgan Ranch Formation (≤9 to~8? Ma) is the principal Wassuk Group unit exposed and is a heterogeneous sequence> 250 m thick of poorly consolidated coarse conglomerate and breccia composed of Mesozoic granitic clasts, with lesser amounts of sandstone, siltstone and tuff. A late Miocene rhyolite dome field, ~50 km~2 in area consisting of numerous coalescing domes, plugs, and dikes intrudes the Morgan Ranch Formation and granodiorite. Tuff apron, carapace breccia, and epiclastic deposits are developed around many of the larger rhyolite domes. Subordinate basalt flows occur along the east side of the range and mainly overlie the Morgan Ranch Formation. The rhyolite domes in the area have average K-Ar ages of 7.1 Ma. Alteration is best developed in Miocene rocks and consists of various combinations of quartz, illite, kaolinite, alunite, and pyrite. Chalcedonic silicification and quartz veins and stockworks are associated with stronger alteration in rhyolite dikes and domes. At the Rockland mine, individual veins average 1 m wide, and consist of crustiform and bladed quartz, and banded chalcedonic quartz. Vein breccia and banding textures indicate multiple stages of quartz deposition. The main ore minerals are fine-grained pyrite, pyrargyrite, and argentite, with minor stephanite and covel- lite; fine-grained electrum occurs in higher grade (> 30 g/t Au) samples. The main Rockland vein was mined vertically for~300 m and~300 m along strike. Wide-spaced drilling in the late 1980s encountered significant Au-Ag mineralization in addition to the historically mined ore zones. In the Eastside Corridor area 3 km east of the Rockland mine, thick intervals of "blind" Au-Ag mineralization occur beneath 100 m of relatively unmineralized rock. Drill intercepts up to 100-200 m average 0.5 to 1.0 g/t Au in a NNE-trending zone several hundred meters wide and 800 m long in quartz veins and breccia zones hosted by rhyolite and adjacent sedimentary rocks; local concentrations as much as 20.9 g/t Au and 100 g/t Ag occur over 1.52 m. The Au-Ag mineralization and veins are overlain by a large blanket like halo of strongly anomalous As and Sb. The Eastside Corridor area is interpreted as the uppermost part of an epithermal vein system similar to that exposed at the Rockland mine. Precious metal mineralization formed at~7 Ma. The Rockland mine area bears similarities to bonanza Au-Ag vein deposits associated with rhyolite domes, including Aurora, Hollister (Ivanhoe), and Sleeper in Nevada, and El Pe?ón in northern Chile.