Petrology of a REE pegmatite near Wellington Lake, CO: An intermediary between NYF-type and miarolitic REE-poor pegmatites in the Pikes Peak Batholith

Markus B. Raschke, Philip M. Persson, Charles Stern, and Julien Allaz
Geol. Soc. Am. Abstracts 45, 41 (2013).
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Numerous pegmatites in the South Platte district, CO, at the northern end of the 1.0 Ga Pikes Peak batholith, are known for their unusual REE enrichment [Simmons, Col. Geol. Surv., Res. Series 11, 131, (1980); Simmons et al., Geochim. Cosmochim. Acta, 51, 455 (1987)]. These pegmatites are classified as Nb-Y-F type (NYF) and are a globally significant example of these types. We present a petrographic and mineralogical analysis of a yet undescribed REE pegmatite near Wellington Lake, situated on the western periphery of the South Platte district. This pegmatite is a steeply dipping, lenticular body in Pikes Peak granite. In contrast to a nearby (< 2km) mineralogically and radially zoned (wall zone, composite Qtz+Mcl core) and fluorite rich NYF pegmatite (McGuire pegmatite), the Wellington Lake pegmatite is not zoned, with little fluorite, and open up to meter-sized REE-rich pods. Major constituents of the Wellington pegmatite are quartz, microcline-perthite, ‘clevelandite’ albite, and biotite; mineralogy of REE-pods includes fluocerite, bastnasite, columbite, and secondary uranium minerals, associated with smoky quartz crystals and hematite. Most notable are well-developed tabular crystals of fluocerite, which exhibit partial pseudomorphic replacement to bastnasite. Such mineral assemblage was previously described from miarolitic pegmatites near Colorado Springs as ‘Tysonite’ [Comstock & Allan, Am. J. Sc., 19, 390 (1880)], and this is the first occurrence reported in the South Platte district. Large smoky quartz crystals exhibiting strong recrystallization and the fluocerite/bastnasite pseudomorphosis suggest late-stage hydrothermal fluid dissolution. Whereas the concentration of zoned HREE-rich pegmatites in the northern and central part of the South Platte is thought to result from a late-stage, volatile rich mobile magma in the mesozone of the Pikes Peak batholith, the southern and eastern pegmatites are generally LREE-dominant and euhedral crystals are more common [Haynes, GSA Bull., V. 76, p. 441 (1965)]. Our mineralogical analysis suggests that the Wellington Lake pegmatite represents an intermediate between the zoned, HREE-rich pegmatites of the central South Platte and the REE poor miarolitic pegmatites of outlying areas of the Pikes Peak batholith such as Wigwam Creek and Lake George.