The Ruby Range igenous complex, west-central Colorado, is characterized by mid-Tertiary granodiorite plutons which intrude Paleozoic, Mesozoic, and Tertiary strata. Conventional contact metamorphism is limited to a mile-wide zone adjacent to the Ruby Range axis. Geochemical and related studies, including hydrocarbon levels and types, kerogen composition, visual kerogen, and vitrinite observations, on the organic matter in intruded sediments demonstrate that low-grade thermal metamorphic effects extend about 6 mi (9.6 km) from the Ruby Range intrusive axis. This is an unusually wide thermal aureole and indicates important heat sources in addition to the intrusions along the Ruby Range axis. Peripheral sills and laccoliths caused little thermal effect on the sediments. It i concluded that the thermal aureole adjacent to the Ruby Range is enlarged because of subjacent intrusions. Specifically, it is postulated that the White Rock pluton projects into the subsurface beneath the sedimentary cover. Recognition of organic matter metamorphism in the overlying sediments permits tracing the approximate margin of the pluton from its outcrop area in the northeast for a distance of about 10 mi (16 km) to the southwest along its subjacent trend. Relations between the trend of metamorphic isotherms and topography indicate that the pluton has a steep (stocklike) southeast margin. Significantly, the recently discovered molybdenum deposit beneath Mt. Emmons, near Crested Butte, Colorado, is located approximately along the projected trend of the subjacent extension of the hite Rock pluton. Recognition of very low grades of thermal metamorphism of organic matter may also have application to the location and evaluation of geothermal reservoirs.

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