Graduate Thesis Or Dissertation
 

Geology and mineral deposits of the Brattain District, Lake County, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vt150m443

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  • The Brattain district is in the Paisley Mountains about four miles south of Paisley in central Lake County, Oregon. Rocks in the area mapped range from pre-early Oligocene to Pleistocene in age. Lithologies include primarily volcanic flows with minor sedimentary and pryoclastic rocks and a small plutonic complex. The oldest unit in the area is a sequence of dacite flows 2000 to 2600 feet thick, that is pre-early Oligocene in age. Unconformably overlying the dacite flows are 2500 feet of andesitic sedimentary and pyroclastic rocks with intercalated andesite flows of the Cedarville Series of early Oligocene age. The Cedarville Series is the most extensively exposed unit in the area. Dacite flows and andesites of the Cedarville Series have been intruded by a plutonic complex composed of diorite, granodiorite and quartz monzonite. In the western part of the district younger andesite flows overlie older rocks with angular unconformity. The unit is 400 to 500 feet thick and is believed to be middle to late Miocene in age. Silicic flows that range from rhyolite to dacite in composition overlie the andesite flows. The silicic flows are approximately 300 feet thick and are late Miocene or early Pliocene in age. High alumina basalt flows and dikes of Plio­cene age overlie and intrude all older rocks. The flows are approxi­mately 200 feet thick and some dikes exceed 50 feet in width. Two rhyolite plugs and a series of secondarily silicified rhyolite flows of Pliocene to Pleistocene age overlie and intrude the older rocks. The youngest rocks in the area are 100 to 200 feet of vesicular, glassy basalt flows of Pleistocene age. The eastern margin of the area con­tains Pleistocene lacustrine deposits and local landslide deposits. The Paisley Mountains are a small range in the Basin and Range Province. The main mass of the Paisley Mountains is a horst bounded by two approximately parallel northwesterly striking faults. The dacite flows and Cedarville Series have been domed by the intrusion of the plutonic complex. Fracture controlled veins in the Cedarville Series display an imperfect radial distribution and are believed to be associated with formation of the dome. Mineralization and associated hydrothermal alteration are chiefly limited to narrow veins and small disseminated deposits. Pyrite is the most common sulfide. The ore minerals are imperfectly zoned with respect to the plutonic complex. Chalcopyrite is locally abuyndant in and adjacent to the plutonic complex. Sphalerite and minor galena were found only in volcanic host rocks that encircle the complex. Quartz, sericite and kaolinite are the most abundant alteration products. An imperfect zonation of the less common alteration minerals also exists. Tourmaline is found in and adjacent to the plutonic complex. Epidote and calcite occupy an intermediate zone between tourmaline and zeolites that are restricted to volcanic rocks on the periphyry of the district. The mineralizing fluids are believed to have been derived from a source at depth within the plutonic complex.
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