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McCoy prospects, Cotopaxi Pegmatite area, Cotopaxi, Cotopaxi District, Fremont Co., Colorado, USAi
Regional Level Types
McCoy prospectsGroup of Prospects
Cotopaxi Pegmatite areaArea
Cotopaxi- not defined -
Cotopaxi DistrictMining District
Fremont Co.County

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Latitude & Longitude (WGS84): 38° 21' 55'' North , 105° 44' 15'' West
Latitude & Longitude (decimal): 38.36528,-105.73750
GeoHash:G#: 9wuf3bhmc
USGS MRDS Record:10018683
Locality type:Group of Prospects
Köppen climate type:BSk : Cold semi-arid (steppe) climate
Nearest Settlements:
Coaldale255 (2011)1.8km
Howard723 (2011)12.6km
Salida5,467 (2017)29.5km
Smeltertown120 (2011)31.5km
Poncha Springs777 (2017)33.8km

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.

Regions containing this locality

North America PlateTectonic Plate
Rocky Mountains, North AmericaMountain Range

Select Mineral List Type

Standard Detailed Strunz Dana Chemical Elements

Mineral List

5 valid minerals.

Rock Types Recorded

Note: this is a very new system on mindat.org and data is currently VERY limited. Please bear with us while we work towards adding this information!

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Alphabetical List Tree Diagram

Detailed Mineral List:

'Albite-Anorthite Series'
Reference: USGS Prof. Paper 227
Reference: USGS Prof. Paper 227
Formula: {Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
Reference: USGS Prof. Paper 227
Reference: USGS Prof. Paper 227
'K Feldspar'
Reference: U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.
Formula: Fe2+Fe3+2O4
Reference: USGS Prof. Paper 227
Formula: K(AlSi3O8)
Reference: USGS Prof. Paper 227
Formula: KAl2(AlSi3O10)(OH)2
Reference: USGS Prof. Paper 227
Muscovite var: Sericite
Formula: KAl2(AlSi3O10)(OH)2
Description: An alteration product of plagioclase.
Reference: Hanley, J.B., Heinrich, E.W., and Page, L.R. (1950), Pegmatite investigations in Colorado, Wyoming, and Utah 1942-1944, A report on resources of beryllium, tantalum, and lithium minerals and of muscovite, USGS Professional Paper 227, 125 pp.: 62.
Formula: SiO2
Reference: U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.; USGS Prof. Paper 227

List of minerals arranged by Strunz 10th Edition classification

Group 4 - Oxides and Hydroxides
Group 9 - Silicates
var: Sericite9.EC.15KAl2(AlSi3O10)(OH)2
Unclassified Minerals, Rocks, etc.
'Albite-Anorthite Series'-
'K Feldspar'-

List of minerals arranged by Dana 8th Edition classification

Group 58 - SOROSILICATES Insular, Mixed, Single, and Larger Tetrahedral Groups
Insular, Mixed, Single, and Larger Tetrahedral Groups with cations in [6] and higher coordination; single and double groups (n = 1, 2)
Group 71 - PHYLLOSILICATES Sheets of Six-Membered Rings
Sheets of 6-membered rings with 2:1 layers
Group 75 - TECTOSILICATES Si Tetrahedral Frameworks
Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
Group 76 - TECTOSILICATES Al-Si Framework
Al-Si Framework with Al-Si frameworks
Unclassified Minerals, Mixtures, etc.
'Albite-Anorthite Series'-
'K Feldspar'-
var: Sericite

List of minerals for each chemical element

H MuscoviteKAl2(AlSi3O10)(OH)2
H Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
H Muscovite (var: Sericite)KAl2(AlSi3O10)(OH)2
O QuartzSiO2
O MicroclineK(AlSi3O8)
O MuscoviteKAl2(AlSi3O10)(OH)2
O MagnetiteFe2+Fe23+O4
O Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
O Muscovite (var: Sericite)KAl2(AlSi3O10)(OH)2
Al MicroclineK(AlSi3O8)
Al MuscoviteKAl2(AlSi3O10)(OH)2
Al Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
Al Muscovite (var: Sericite)KAl2(AlSi3O10)(OH)2
Si QuartzSiO2
Si MicroclineK(AlSi3O8)
Si MuscoviteKAl2(AlSi3O10)(OH)2
Si Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
Si Muscovite (var: Sericite)KAl2(AlSi3O10)(OH)2
K MicroclineK(AlSi3O8)
K MuscoviteKAl2(AlSi3O10)(OH)2
K Muscovite (var: Sericite)KAl2(AlSi3O10)(OH)2
Ca Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)
Fe MagnetiteFe2+Fe23+O4
Fe Epidote{Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

1600 - 2500 Ma

ID: 3186950
Paleoproterozoic crystalline metamorphic rocks

Age: Proterozoic (1600 - 2500 Ma)

Lithology: Orthogneiss/paragneiss

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

1600 - 2500 Ma

ID: 2855906
Granitic rocks of 1700-m.y. age group

Age: Proterozoic (1600 - 2500 Ma)

Stratigraphic Name: Boulder Creek Granite; Cross Creek Granite; Denny Creek Granodiorite; Kroenke Granodiorite; Browns Pass Quartz Monzonite; Powderhorn Granite; Pitts Meadow Granodiorite; Bakers Bridge Granite; Tenmile Granite

Description: Includes Boulder Creek -M.Y. AGE GROUP (AGE 1650-1730 M.Y.)--Includes Boulder Creek, Cross Creek, Denny Creek, Kroenke, Browns Pass, Powderhorn, Pitts Meadow, Bakers Bridge, and Tenmile Granites, Quartz Monzonites, or Granodiorites; also, unnamed granitic rocks

Comments: Original map source: Green, G.N., 1992, The Digital Geologic Map of Colorado in ARC/INFO Format: U.S. Geological Survey Open-File Report 92-0507, 9 p., scale 1:500,000.

Lithology: Major:{granite}, Minor:{granodiorite,quartz monzonite}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License


Sort by

Year (asc) Year (desc) Author (A-Z) Author (Z-A)
Heinrich, E.W. (1948), Fluorite-Rare Earth Mineral Pegmatites of Chaffee and Fremont Counties, Colorado, American Mineralogist: 33(1-2): .
Hanley, J.B., Heinrich, E.W., and Page, L.R. (1950), Pegmatite investigations in Colorado, Wyoming, and Utah 1942-1944, A report on resources of beryllium, tantalum, and lithium minerals and of muscovite, USGS Professional Paper 227, 125 pp.: 62.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10018683.
Conservation Division completion date: 1-64.

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