PROTEROZOIC FLUORBRITHOLITE-BEARING REE-RICH HYDROTHERMAL PODS AND VEINS FROM NEAR JAMESTOWN, COLORADO
We have relocated and reinvestigated these veins. Electron microprobe analyses indicate that the cerite is actually fluorbritholite, with an average wt % oxide composition of SiO2=22.8; CaO=11.7; La2O3=6.9; Ce2O3=26.5; Nd2O3=16.8; Pr2O3=3.7; Sm2O3=2.4; Gd2O3=1.5; P2O5 =0.9; Y2O3=2.9; F=~3 (±HREE). The sub-mm scale zoning appears more complex than described in the original work: the allanite-rich outer rim transitions into a fluorbritholite-rich core (±monazite, quartz, fluorite). An intermediate zone composed of törnebohmite and REE-carbonate is sometimes observed, and could represent an altered domain or a second stage growth after allanite. The REE content increases from the rim (∑ ~21% REE) to the REE-rich core (∑ ~40-46% REE) with a relatively greater increase of HREE (to ~1.5%) and decreasing LREE/HREE. A U-Th-total Pb monazite microprobe age of 1420 ± 25 Ma confirms the Proterozoic age for these veins and their co-genetic relationship with the host Silver Plume granite. This is only one of 7 known occurrence worldwide of fluorbritholite, first reported in alkali syenite from Mont Saint-Hilaire, Canada by Jiexiang et al. (1994; Min Record 27:463). We suggest a hydrothermal origin for these veins, with their REE-rich nature due to a high F-content of the complex fluids they precipitated from, which also contained CO2, SO2, P2O5, and H2O. These veins represent a rare and unusual end product of REE enrichment of Proterozoic A-Type granites in Colorado, such as the Silver Plume and Pikes Peak batholiths.