Geological Survey Professional Paper Volume 474 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1987* Excerpt: ...is moderately clear, and ranges in composition from calcic oligoclase to bytownite. Scapolite occurs as anhedral grains that are intergrown with pyroxene in a mosaic pattern in a few calc-silicate gneisses in the south part of the district and occurs sparsely in the calc-silicate gneiss body on the east slope of Nevada Hill about 1,000 feet west of Central City (pi. 1). The nv of scapolite from five separate localities ranges from 1.586 to 1.589, indicating a composition of approximately Megs Ma35, according to the data of Winchell (1948, p. 294). Microcline and magnetite-ilmenite are variable components, sphene is less common, and calcite occurs in small amounts in some rocks, especially those containing scapolite and sparse quartz. Other minor accessory minerals are unidentified sulfides, apatite, and zircon. The rocks are nearly equigranular and have wholly crystalloblastic textures; these characteristics indicate virtually contemporaneous crystallization. The major minerals are intergrown typically to form mosaic patterns. Garnet and, less commonly, pyroxene and hornblende tend to be poikilitic and include a wide variety of other minerals. ORIGIN OF LAYERED ROCKS The layered rocks of the Central City district are interpreted by us to be of metasedimentary origin. The mineralogic assemblages that formed depended mainly upon the aggregate composition of the original rocks and probably only to a minor extent on the addition and subtraction of materials by fluids of ultrametamorphic and magmatic origin. Probably the microcline gneiss was derived from feldspathic sandstones (arkose?), and the biotite gneisses were derived from graywacke sandstones and shaly sediments. The origin of the other less abundant rocks intercalated with the dominant gneiss units is...