PROTEROZOIC FLUORBRITHOLITE-BEARING REE-RICH HYDROTHERMAL PODS AND VEINS FROM NEAR JAMESTOWN, COLORADO

The anorogenic 1.4 Ga Longs Peak St-Vrain monzo- to syenogranite (“Silver Plume” type intrusion) is strongly enriched in light REE. Associated pods and veins in pegmatite near Jamestown, CO (locality known as “Rusty Gold”) exhibit an unusual REE mineralization. Early investigations by Goddard & Glass (1940; Am. Min. 25:381) and Hanson & Pearce (1941; Am. Min. 26:110) indicated a Proterozoic age (~0.9 Ga) for this occurrence based on U/Pb whole-rock ratio. The complexly zoned mineral assemblage, with predominantly cerite, fluorite, bastnäsite, törnebohmite, and monazite in the core, surrounded by an allanite-rich rim, was tentatively called “cerite” rock. Despite the unusual mineralogy, no further investigation has yet provided accurate quantitative mineral chemistry and age determination.

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.