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New Mexico Mineral Symposium
November 8-9, 2014

Abstract

The Lesser-Known Minerals of the Pikes Peak Batholith

Peter J. Modreski1 and Jack Thompson2

1U.S. Geological Survey, Mail Stop 150, Box 25046 Federal Center, Denver, CO, 80225, pmodreski@usgs.gov

2 1830 Mesita Ct., Colorado Springs, CO, 80905

The best known minerals of the Pikes Peak batholith are, of course, those that occur in the miarolitic pegmatites—pegmatites with open cavities “pockets”, which may contain free-growing, near-euhedral crystals. The major minerals include microcline (and its variety, amazonite), quartz (especially smoky quartz), and albite (often as the lamellar variety, cleavelandite); with relatively common accessory minerals that include biotite, fluorite, goethite, hematite, topaz, zinnwaldite, and zircon. But a wide variety of other minerals also occur; some well-known, some not; some sporadically distributed throughout the entire batholith, and some localized to one or a few districts within it.

The Pikes Peak batholith covers about 1,200 mi2 (3,100 km2) of Colorado—about 1.15% of the total area of the state—and is just over 1 billion years old (1.08 Ga). Structures within the Pikes Peak Granite forming the main mass of the batholith show evidence of three main intrusive centers, from north to south the Buffalo Park, Lost Park, and Pikes Peak centers. Within and around the edges of the batholith, smaller, late stage plutons occur and can be divided into either sodic or potassic types; it is in or around these late plutons that most of the pegmatites occur. The igneous rocks of these late intrusive centers are not limited to granite but include more mafic and quartz-poor rock types such as syenite, monzonite, granodiorite, diorite, and gabbro. The mineral-bearing localities in the batholith can be grouped into several genetic and geographic types:

Miarolitic pegmatites: These are concentrated around the late intrusive centers that include Lake George, West Creek, Sugarloaf Peak, Mount Rosa, Redskin Stock, Tarryall, a large unnamed body in the Rampart Range, and a number of late-stage intrusions around Pikes Peak itself. The best-known pegmatite localities—hunted mainly for their smoky quartz, amazonite, and topaz—include those around Lake George/Crystal Peak, Tarryall Mountains/Pilot Peak, Harris Park, Wigwam Creek, Devils Head, Stove Mountain, Sentinel Rock and Specimen Rock, Cameron Cone/Crystal Park, and Glen Cove. Some of the more notable, unusual minerals—many of which contain barium, beryllium, and the rare earth elements—found in these pegmatites include barite, barylite (BaBe2Si2O7), bastnäsite, bertrandite, beryl, cassiterite, columbite, genthelvite (Zn8Be6Si6O24S2), milarite (KCa2AlBe2Si12O30·0.5H2O), and phenakite. Carbonate minerals (calcite, rhodochrosite, siderite) also occur, mostly pseudomorphed by goethite or hematite; it is usually impossible to tell what the original rhombohedral mineral was.

Mount Rosa–St. Peters Dome-Stove Mountain area: This area contains pegmatites in and around several bodies of alkali- and fluorine-rich granites, including the Mount Rosa Granite, Windy Point Granite, and a fayalite granite. Just west of Colorado Springs, the area is usually accessed via the Gold Camp Road. Distinctive minerals from this area include an abundance of the black, sodic amphibole, riebeckite; the golden-brown, mica-like chain silicate mineral, astrophyllite; zircon, pyrochlore, and rare-earth minerals including bastnäsite-(Ce) and fluocerite-(Ce). Fayalite, the iron end-member of the olivine mineral group, is said to have been found both here and at Crystal Park, sometimes as remarkably large cleavage masses “to 10 pounds”, containing inclusions of a related olivine-group mineral, laihunite. Concentrations of uncommon fluoride minerals, including cryolite, elpasolite, gearksutite, pachnolite, prosopite, ralstonite, thomsenolite, and weberite, are found near the “Eureka Tunnel”, mined for cryolite and also a source of sometimes-gemmy zircon crystals. Fluorite-quartz vein deposits also occur in the St. Peters Dome area, and were once mined for fluorite. Classic papers on the Mount Rosa area are those by Gross and Heinrich (1965, 1966a,b), and a recent excellent paper is that by Zito and Hanson (2014) about Stove Mountain. As Zito and Hanson point out, two distinct types of pegmatites occur in the area; miarolitic cavity-type pegmatites, similar to those in the rest of the Pikes Peak area, and sodic, Mount Rosa-type pegmatites, typically containing riebeckite. Galena plus several of its alteration products, including wulfenite, have been reported from the pegmatites here.

Badger Flats—Boomer mine—Lake George beryllium area: This district is unique within the batholith as it consists of greisen-type mineralization, a type of fluorine- and beryllium-rich hydrothermal alteration associated with the margins and tops of igneous intrusions. Beryllium minerals here include beryl, bertrandite, euclase, and phenakite, associated with fluorite, topaz, cassiterite, scheelite, and wolframite. Metal sulfides also occur, including arsenopyrite, chalcopyrite, galena, molybdenite, and sphalerite. These deposits, northwest of Lake George, are at the edge of the Pikes Peak batholith, along the margin of the Redskin Stock, both within the stock and in the older metamorphic country rock (Hawley 1963, 1969).

South Platte pegmatite district: This district includes a number of large pegmatite bodies in the form of cylindrical, vertical pipes typically between about 50 to 200 ft in diameter within the host Pikes Peak Granite. The largest, the Oregon No. 3, is 600 x 280 ft in horizontal cross-section. Quite different from the amazonite-smoky quartz miarolitic pegmatites, they generally lack any open cavities, are concentrically zoned with core, intermediate, and wall or border zones, and often contain zones of secondary hydrothermal alteration. They tend to be rich in rare earth and uranium-thorium minerals and fluorite, and are classic examples of the NYF (niobium-yttrium-fluorine) type of pegmatite. Much of the fluorite is unusually rich in ytrrium and the other rare-earth elements, is opaque rather than transparent, and has distinctive fluorescence colors. Simmons and Heinrich (1980) provide an excellent description of the district; the book describes and shows internal maps of 32 of the largest of the pegmatites, but at least 50 total are known. Some of the “best known” include the Little Patsy, Luster group, Oregon group, Seerie, Snowflake, and White Cloud pegmatites. They are located mostly in the northern part of the batholith, east of Buffalo Creek, but outliers exists west of Wellington Lake (McGuire pegmatite) and near Divide (Black Cloud pegmatite) and Lake George (Teller pegmatite). William B. “Skip” Simmons of the University of New Orleans and his graduate students and colleagues have produced a considerable series of research papers on the mineralogy and geochemistry of these pegmatites, which has included the discovery of the new mineral species, samarskite-(Yb). Characteristic minerals include fluorite, allanite-(Ce), bastnäsite-(Ce), euxenite-(Y), fergusonite-(Y), fluocerite-(Ce), gadolinite-(Ce), monazite-(Ce), samarskite-(Y), synchysite-(Y), thalenite-(Y), thorite, uraninite, xenotime-(Y), yttrotantalite-(Y), and zircon. Much of the zircon is the opaque, metamict, highly radioactive variety, “cyrtolite”. An exact and accurate count of the total number of minerals that occur within the entire Pikes Peak batholith is difficult to attain because of the number of unconfirmed or semi-confirmed older reports; the complexity of many of the mineral groups (including the micas and the rare-earth minerals) and changes in their nomenclature over the years; differences in the way individual species vs. families or groups of minerals have been referred to; and the number of separate localities and districts or subdistricts within the batholith, and whether some localities along the margins should be counted as part of the batholith. All told, there appear to be about 128 valid reported mineral species from the Pikes Peak batholith. These include 4 type minerals: elpasolite (St. Peters Dome), murataite-(Y) (St. Peters Dome), samarskite-(Yb) (Little Patsy pegmatite, South Platte district), and siderophyllite (from “Pikes Peak”, collected by A.E. Foote circa 1880).

Detailed descriptions of the minerals of the batholith can be found in Muntyan and Muntyan (1985), Eckel (1997), and Raines (2010). Good accounts of the geology of the whole batholith include Smith et al. (1999), Wobus and Anderson (1978), and Wobus (2001).

Figure1
Accompanying map: From Wobus and Anderson (1978), figure 1, p. 82, with acknowledgements to U.S. GeologicalSurvey.

References

  1. Eckel, Edwin B., 1997, Minerals of Colorado, updated and revised: Golden, CO, Fulcrum Press, 665 pp.
  2. Gross, E.B., and E.W. Heinrich, 1965, Petrology and mineralogy of the Mount Rosa area, El Paso and Teller Counties, Colorado, I. The granites: American Mineralogist, v. 51, no. 9, pp. 1273–1295.
  3. Gross, E.B., and E.W. Heinrich, 1966a, Petrology and mineralogy of the Mount Rosa area, El Paso and Teller Counties,Colorado. II. Pegmatites: American Mineralogist, v. 51, no.3–4, p. 299–323.
  4. Gross, E.B., and E.W. Heinrich, 1966b, Petrology and mineralogy of the Mount Rosa area, El Paso and TellerCounties, Colorado, III. Lamprophyres and mineral deposits: American Mineralogist, v. 51, no. 9–10, pp. 1433–1442.
  5. Hawley, C.C., 1963, Geology of the Pikes Peak Granite and associated ore deposits, Lake George beryllium area, ParkCounty, Colorado: U.S. Geological Survey Open-File Report 63–44, 283 pp.
  6. Hawley, C.C., 1969, Geology and beryllium deposits of the Lake George (or Badger Flats) beryllium area, Park andJefferson Counties, Colorado: U.S. Geological Survey Professional Paper 608-A, 44 pp. + 4 plates.
  7. Muntyan, Barbara L., and John R. Muntyan, 1985, Minerals of the Pikes Peak Granite: Mineralogical Record, v. 16,no. 3, pp. 217–230.
  8. Raines, E., 2010, A brief summary of the mineral deposits of the Pikes Peak batholith, Colorado: Rocks & Minerals,v. 76, no. 5, pp. 298–325.
  9. Simmons, W.B., Jr., and E.W. Heinrich, 1980, Rare Earth Pegmatites of the South Platte District, Colorado: ColoradoGeological Survey Resource Series 11, 131 p.
  10. Simmons, W.B., Maxie T. L., and Brewster, R. H., 1987, Geochemistry and evolution of the South Platte granitepegmatite system, Jefferson County, Colorado: Geochimica et Cosmochimica Acta, vol. 51, no. 3, p. 455-471.
  11. Smith, D. R., Noblett, J., Wobus, R. A., Unruh, D., and Chamberlain, K. R., 1999, A review of the Pikes Peak batholith,Front Range, central Colorado: A "type example" of A-type granitic magmatism: Rocky Mountain Geology, v. 34, no.2, pp. 289–312.
  12. Wobus, R. A., 2001, Precambrian igneous and metamorphic rocks in the Florissant region, central Colorado: their topographicinfluence, past and present: Bulletin No. 5, Pikes Peak Research Station, Colorado Outdoor Education Center,Florissant, Colorado, 9 pp.
  13. Wobus, R.A., and R.S. Anderson, 1978, Petrology of the Precambrian intrusive center at Lake George, southern Front Range,Colorado: Journal of Research of the U.S. Geological Survey, v. 6, no. 1, pp. 81–94.
  14. Zito, G., and Hanson, S. L., 2014, Minerals from the miarolitic pegmatites in the Stove Mountain area, Colorado Springs,Colorado: Rocks & Minerals, vol. 89, no. 3, pp. 224–237.
pp. 18-20

35th Annual New Mexico Mineral Symposium
November 8-9, 2014, Socorro, NM