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

Goddard & Glass (1940; Am Min 25:381), Hanson & Pearce (1941; Am Min 26:110) and Gay (1957; Min Mag 31:422) reported mineralogically zoned REE-rich veins (sum REE=56 oxide wt %; Y₂O₃=3 wt %), with dark black allanite borders and grey cerite-rich cores, at the “Rusty Gold” locality near Jamestown, CO. These veins are associated with 1.4 Ga Silver Plume granite, at the contact with small lenticular roof-pendants of 1.7 Ga Idaho Springs group biotite schists. They also contain purple fluorite, brown epidote, bastnäsite, törnebohmite and monazite, as well as minor quartz, uraninite, sulfides, and Fe-oxides. Goddard & Glass determined bulk-rock composition of the cerite-rich central zone of these veins, subtracted estimated modal percentages of minor minerals (quartz, fluorite, uraninite and bastnäsite) associated with this zone, and thereby calculated an approximate cerite composition. They also estimated a Proterozoic age of 0.9 Ga based on a U/Pb whole-rock ratio, and suggested that these veins were genetically related to their Silver Plume granite host, not the younger Tertiary mineralization in the Jamestown district. In order to assess their conclusions with modern analytical tools, we have relocated these outcrops and obtained mineral chemistry by electron microprobe. Our new microprobe analyses suggest that the cerite is actually fluorbritholite, as it contains only negligible Al₂O₃, Fe₂O₃ and MgO, but ~1.0 wt % P₂O₅. We determined an average oxide wt % composition for this fluorbritholite 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; plus HREE. This is only the seventh occurrence worldwide of fluorbritholite, first reported in alkali syenite from Mont Saint Hilaire, Canada, by Gu Jiexiang et al. (1994; Min Record 27:463). Our U-Th-total Pb monazite age of 1.40-1.45 Ga, which is similar to the host Silver Plume granite, confirms the Proterozoic age for this REE-rich mineralization. These veins are hydrothermal in origin, and their REE-rich nature must depend in part on the high F content of the complex fluids they precipitated from, which also contained CO₂, SO₂, and H₂O. They represent an unusual end product of REE enrichment of Proterozoic ‘A-Type’ granites in Colorado, such as the Silver Plume and Pikes Peak batholiths.