DISTRIBUTION AND OCCURRENCE OF FLUORINE, FLUORITE, AND FLUORSPAR IN CENTRAL COLORADO

Central Colorado contains abundant fluorine in rocks, veins, fluorspar deposits, and water. The 4,040 rock samples with fluorine (F) analyses in the USGS geochemical database from this area average 3,420 ppm F. Unmineralized rocks average 1,670 ppm F (crustal average = 650 ppm). Paleoproterozoic metamorphic and plutonic rocks and Mesoproterozoic plutons average 1,310, 1,090, and 1,240 ppm F, respectively. Primary fluorite is common in Mesoproterozoic but not Paleoproterozoic plutons, possibly due to differences in volatile segregation during crystallization or level of exposure. Pikes Peak plutons (~1.1 Ga) average 2,110 ppm F and contain abundant primary fluorite. Cambrian alkalic igneous rocks and veins average 980 ppm F and contain fluorine minerals, and Phanerozoic sedimentary rocks average 1,360 ppm. Cenozoic plutonic and volcanic rocks average 1,190 ppm. Mineralized rocks (481) of all types (altered, vein) average 16,970 ppm F. Of these, 112 samples from fluorspar deposits contain >1% F. The remaining 369 mineralized samples from a variety of deposit types average 1,110 ppm F. Half of these samples contain less F than the crustal average; thus, not all mineralized rocks have elevated fluorine. About 335 uncategorized samples in the database average 800 ppm F.

Most fluorspar was mined from Neogene deposits along the north-trending Rio Grande rift. Fluorspar also was produced from the ~20 Ma Wagon Wheel Gap district, the 45-Ma porphyry deposits at Jamestown, and the Tertiary alkalic igneous-related St. Peters Dome district near Pikes Peak. Many Cenozoic mineral deposits, notably Cu-Mo and Climax-type Mo deposits, contain abundant primary and late-stage hydromagmatic fluorite. Previous isotopic studies indicated that these magmas in part were derived from assimilation of Proterozoic crust, which has above-average F; the original mode of occurrence of the F and the degree of partial melting also may have influenced the incorporation of F into the magmas. High F in some modern hot springs (Poncha Springs, Mt. Princeton, Blue River) indicates recycling of F from fluorspar deposits or high-F source rocks. Erosion of F-rich rocks and mineral deposits has redistributed F into soils and water, which has created a naturally high background F level in this region.