GEOLOGIC AND GEOCHEMICAL STUDIES IN THE HUNTERS HILL-ITALIAN MOUNTAIN AREA, GUNNISON COUNTY, COLORADO

Conventional K-Ar ages confirm an Oligocene age for the northern pluton of the Italian Mountain intrusive complex (33.5 +- 0.8 Ma), but dates from two plagioclase mineral separates (0.84 % K; 61.6 +- 1.5 Ma and 2.88 % K; 63.4 +- 2.1 Ma) from the same outcrop establish that the southern pluton is Laramide in age. Based on published field observations, the central pluton of the Italian Mountain complex is also Laramide. Cunningham's 1976 fluid inclusion data from the southern pluton indicate lower pressures with higher elevations, showing that the pluton was emplaced after the wallrocks had been rotated into their present, near-vertical attitude. Laramide deformation in this area occurred between 72 and 62 Ma, based on these data and published dates for latest Cretaceous volcanic ash layers in the region. Along the western margin of the northern pluton, Belden shales and limestones were respectively converted to hornfelses and scapolite-diopside-albite skarns. Scapolite compositions are Me 17-28 and Cl/Br ratios are 2.7 to 3.7 times the seawater ratio, suggesting that the parental brine was generated by halite dissolution. Pennsylvanian evaporites in the Eagle Basin are the logical source for the brine. In the scapolite-bearing skarns, abundances of a broad suite of trace element are between those expected for limestones and shales, suggesting that these trace elements were locally derived from the pre-skarn wallrocks and not transported as solutes in the brine. Prior to arriving at the skarn-forming site, the brine was probably cool and had not undergone significant fluid-wallrock interaction. In the subsurface, localized heating and a CO2-rich gas phase generated in the skarn-forming zone probably lowered the bulk density of the upward-moving discharging fluid enough to push a brine from the southwestern Eagle Basin southward Italian Mountain. The flow path probably followed the highly deformed Sawatch frontal zone between Italian Mountain and the Aspen area.