McCoy prospects, Cotopaxi Pegmatite area, Cotopaxi, Cotopaxi Mining District, Fremont County, Colorado, USAi

Regional Level Types
McCoy prospects	Group of Prospects
Cotopaxi Pegmatite area	Area
Cotopaxi	- not defined -
Cotopaxi Mining District	Mining District
Fremont County	County
Colorado	State
USA	Country

A mica-quartz-feldspar pegmatite occurrence located in sec. 34, T48N, R11E, NMM, 4.4 km (2.8 miles) W of Cotopaxi and about 1 mile E of Coaldale, on the N ridge of the Arkansas River, on Bureau of Land Management administered land. Owned by G. W. Faye and L. G. Young. The claims were located by G. W. Faye and L. G. Young on November 1, 1941, but had been previously worked for quartz. These prospects were visited by E.W. Heinrich in November, 1942, and by J. B. Hanley in October, 1943.

Mineralization is a pegmatite deposit hosted in Neoproterozoic granodiorite (Mineral occurrence model information: Model code: 32; USGS model code: 13; deposit model name: pegmatites). The pegmatites crop out on the side of the ridge that forms the valley wall. The pegmatites form a network of branching dikes in a fine-grained red granite wall rock, in which, are exposed inclusions of gneiss several hundred feet across. The largest pegmatite strikes E and dips gently to the N. It is 120 feet long and ranges from 12 to 25 feet in width. The zonal structure in this pegmatite is ill-defined and only the central core of quartz pegmatite, which is 7 feet thick, is distinct. The quartz-microcline-biotite-plagioclase pegmatite that surrounds the quartz core contains large masses of quartz, crystals of feldspar as much as 2 feet across, and sheets of biotite 2 feet wide. The feldspar crystals commonly have a core of graphic granite and a rim, which may be as much as 6 inches thick, of bluish-green plagioclase. This plagioclase is so highly sericitized that its anorthite content cannot be accurately determined, but the observed optical properties indicate that it is albite. The feldspar crystals have been often mistaken for beryl because of the bluish-green color but can be readily distinguished by their relative softness. Accessory minerals are magnetite, epidote, and hornblende. The second large pegmatite, which is 100 feet to the E of the first, strikes N5W and dips 40NE at 20 feet wide. The contact between the pegmatite and the granite wall rock is irregular and has many sharp rolls. Adjacent to the contacts the pegmatite consists of sharply defined minute bands of fine-grained biotite pegmatite alternating with bands of fine-grained, quartz-feldspar pegmatite. The coarse-grained interior of the pegmatite is composed of graphic granite and biotite with accessory magnetite. No sizable concentrations of feldspar, the only mineral of economic significance in the dikes, have been found. Local rocks include Granitic rocks of 1700-m.y. age group.

Workings include unspecified surface openings. The only workings are open-cuts made by slabbing the natural cliffs formed by the pegmatites.

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Link to USGS MRDS:	10018683

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References