THE COMPOSITION AND ORIGIN OF HYDROTHERMAL FLUIDS IN A NYF-TYPE GRANITIC PEGMATITE, SOUTH PLATTE DISTRICT, COLORADO: EVIDENCE FROM LA ICP MS ANALYSIS OF FLUORITE-AND QUARTZ-HOSTED FLUID INCLUSIONS

Abstract

Hydrothermal fluorite‐rare-earth-element REE mineralization occurs in the zoned Oregon No. 3 (niobium‐yttrium‐fluorine) granitic pegmatite (NYF-type) hosted by the Proterozoic Pikes Peak granite in north-central Colorado. The mineralization comprises secondary albite, fluorite and REE minerals (predominantly samarskite and fergusonite) that replace primary pegmatitic core-zone quartz, core-margin zone fluorite and wall-zone quartz and microcline perthite. Primary fluid inclusions in hydrothermal, purple, white and colorless fluorite and secondary and pseudosecondary inclusions in pegmatitic core-zone quartz were analyzed using microthermometry and laser-ablation ICP‐MS. Such analyses of fluid inclusions in fluorite are problematic owing to compositional complexity, poor absorbance of energy, and the presence of cleavage. However, representative and quantitative analyses were made using a combination of a constricted laser beam, a traversed opening technique, and by correcting for a contribution to the fluid-inclusion signal by the host. Microthermometric and LA‐ICP‐MS analyses indicate that four assemblages of compositionally distinct fluid inclusions occur in core-zone quartz (fluids F1 through F3) and hydrothermal purple, white and colorless fluorite (fluids F2 and F4). Fluids F1, F2, and F3 are represented by secondary and pseudosecondary L‐V and L‐V‐H inclusions in quartz and primary(?) L‐V‐H inclusions in white fluorite. These inclusions have homogenization temperatures of 93 to 149°C and contain moderate- to high-salinity (22 to 29 equiv. wt.% NaCl) fluids that can be characterized as Na + K + Sr + Ba ± Ca solutions. These three fluids (F1, F2, and F3) could not be distinguished from one another using microthermometry. They do have distinct chemical characteristics that are apparent from the LA‐ICP‐MS data; F2 contains detectable concentrations of Ca, and F1 and F3 do not. Fluid F4 is represented by primary L‐V inclusions in purple, white and clear fluorite. These inclusions have homogenization temperatures of 81 to 114°C and comprise a low-salinity (9 to 13 equiv. wt.% NaCl) fluid that can be characterized as a Na + K + Sr + Ba solution. Only F4 appears to have been directly involved in the formation of the fluorite‐REE mineralization, and these inclusions, particularly those hosted by purple fluorite, contain the highest concentrations of REE, Y, Th and U. Some compositional variation shown by F2, however, can be explained by either wall-zone albitization, which accompanies hydrothermal fluorite‐REE mineralization, or by fluorite dissolution. Modeling of the fluid‐rock reaction indicates that all four fluids appear to have been derived from within the pegmatite, during its later stages of crystallization. Fluorite‐REE mineralization appears to have occurred at relatively constant T and pH, largely as a result of mixing of Ca liberated from the pegmatite wall-zone during albitization in the presence of an F-bearing hydrothermal fluid.