Petrology of a REE Pegmatite near Wellington Lake, CO: An Intermediary between NYF-type and Miarolitic REE-poor Pegmatites in the Pikes Peak Batholith

By: Markus B. Raschke, Philip M. Persson, Charles Stern and Julien Allaz

Geological Society of America Abstracts with Programs, 45, 5, 41 (2013)

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.