New Mexico Mineral Symposium — Abstracts

During five days in August of 2010, the authors visited numerous pegmatites in northern and central Colorado in pursuit of samples of microcline feldspar to add to the worldwide suite of samples that had already been studied by Munoz. The focus of his work has been the study of microstructures—exsolution and twinning—in potassium feldspar, and interpretation of what these can tell about the cooling history of pegmatites. His goal was to collect clean, large, orientable cleavage samples of microcline, free of hydrothermal alteration effects, which would preserve a record of the original crystallization conditions and subsequent cooling history of the feldspar.

Aside from the Tertiary-age pegmatites in the Mount Antero granite, most Colorado pegmatites are related to Precambrian granitic rocks. Best known are the amazonite-bearing miarolitic pegmatites in the ~1.1 Ga (1.1 b.y. old) anorogenic Pikes Peak granite, but in the northern part of the Pikes Peak batholith a swarm of vertical, pipe-like, concentrically zoned pegmatites also occurs that constitute the South Platte pegmatite district. These have an NYF (niobium-yttrium-fluorine) geochemical affinity and are enriched in those elements, often containing large masses of fluorite.

Older pegmatites in Colorado are associated with the synorogenic ~1.7 Ga suite of granite, granodiorite, and other igneous rocks known as the Routt (Boulder Creek) plutonic suite. These pegmatites typically occur as contorted bodies within high-grade metamorphic rocks that surround the plutons. The pegmatites have an LCT (lithium-cesium-tantalum) geochemical affinity; several contain lithium minerals (lepidolite, colored tourmaline, spodumene), and many contain beryl, black tourmaline (schorl), and phosphate minerals, all of which are rare in the Pikes Peak batholith. The actual age of most of the pre-Pikes Peak pegmatites has not been directly determined, and although most of the larger pegmatites are believed to be genetically related to the 1.7 Ga plutons, the affiliation of many is in doubt and it remains uncertain how many pegmatites are instead related to the granitic intrusives of the dominantly anorogenic ~1.4 Ga Berthoud (Silver Plume) plutonic suite. Our goal was to visit and sample as many different types of pegmatites as possible during Munoz' visit to Colorado.

The field work took us to pegmatites near Denver, Golden, and Evergreen (the "Denver Mountain Parks area"), the Crystal Mountain pegmatite district of Larimer County, the Eight Mile Park, Micanite, and Texas Creek pegmatite districts north of the Arkansas River, to Trout Creek Pass, and to the South Platte pegmatite district of the Pikes Peak batholith. Luis had already obtained samples of microcline from amazonite-bearing miarolitic cavity pegmatites on a previous visit to Colorado. Pegmatites we visited, with some of the notable minerals occurring in them, were:

1. Unnamed, simple pegmatite dike in Golden Gate Canyon, near the Golden Gate grange.
2. Unnamed simple pegmatite dike in Golden Gate Canyon, east of Mt. Galbraith Park trailhead.
3. Roscoe pegmatite dike, Clear Creek Canyon (biotite, magnetite, rare-earth minerals).
4. Unnamed garnet-bearing pegmatite dike near the top of Douglas Mountain Road.
5. Evans Ranch (Bald Mountain) lithium-bearing pegmatite (lepidolite, dark blue-green elbaite).
6. Lesser White Cloud pegmatite, South Platte pegmatite district.
7. White Cloud pegmatite, South Platte district (fluorite, gadolinite, synchysite).
8. Unnamed small pegmatite north of Raleigh Peak, South Platte district.
9. Oregon No. 3 pegmatite, South Platte district.
10. Oregon No. 21/2 pegmatite, South Platte district.
11. Oregon No. 11/2 pegmatite, South Platte district.
12. Oregon No. 1 (?) pegmatite, South Platte district.
13. Devils Hole pegmatite, Texas Creek area, Fremont County (beryl, columbite, rose quartz).
14. Chief lithium pegmatite, Texas Creek area, Fremont County (lepidolite, dark green elbaite).
15. Mica Lode pegmatite, Eight Mile Park district, Fremont County.
16. Climax mica mine pegmatite, Micanite district, Park County (abundant muscovite).
17. Rose Dawn pegmatite, Micanite district, Fremont County (beryl, fluorapatite, columbite).
18. Clora May pegmatite, Trout Creek Pass, Chaffee County (aeschynite).
19. Unnamed quartz-microcline pegmatite knob north of Clora May pegmatite, Trout Creek Pass.
20. McGuire (Lone Lode) pegmatite, South Platte district, Park County (ilmenite, fluorite, topaz).
21. Hyatt (Big Beryl) pegmatite, Crystal Mountain district, Larimer County (beryl).
22. Tourmaline Prospect pegmatite, Crystal Mountain district, Larimer County (schorl).
23. Bull Elk Beryl Crystal No. 1 pegmatite, Crystal Mountain district, Larimer County.
24. Big Boulder pegmatite, Crystal Mountain district, Larimer County (spodumene, schorl).

References:

1. Sanchez-Munoz, L., Modreski, P. J., and Frost, R. B., 2011, K-feldspar twin-structures from orogenic and anorogenic granitic pegmatites in central North America; in Contributions to the 5th International Symposium on Granitic Pegmatites, Mendoza, Argentina, Asociaci??n Geol??gica Argentina, Ser. D, Publicaci??n Especial No. 14, pp. 179-183.
2. S??nchez-Mu??oz, L., Crespo, E., Garcia-Guinea, J., J. M. de Moura, 0., and Zagorsky, V. Y., 2009, What is a twin-structure? an answer from microcline minerals from pegmatites: Estudos Geol??gicos, v. 19, no. 2, pp. 240-245.
3. S??nchez-Mu??oz, L., Garcia-Guinea, J., Zagorsky, V. Y., J. M. de Moura, 0., and Modreski, P. J., 2011, K-feldspar minerals defined from their twin-structures: application to a preliminary classification of pegmatites; in Contributions to the 5th International Symposium on Granitic Pegmatites, Mendoza, Argentina, Asociaci??n Geol??gica Argentina, Ser. D, Publicaci??n Especial No. 14, pp. 175-178.