THE SUBSURFACE PROJECTION OF THE WHITE ROCK STOCK AND IMPLICATIONS TO THE MID-TERTIARY HISTORY OF THE COLORADO MINERAL BELT IN GUNNISON COUNT, COLORADO

Mid-Tertiary plutons and associated mineral deposits which occur along the crest of the southwest-northeast trending Ruby Range define the central axis of the Colorado Mineral Belt in northern Gunnison county. The CMB is 12 miles wide and includes dikes, sills, large laccoliths, and the Snowmass and White Rock stocks. These granitic plutons intrude Paleozoic, Mesozoic and early-Tertiary strata; age dates indicate mid-Oligocene, ca 30 my, as the principal time of intrusion. The spatial, temporal, and compositional associations of the igneous rocks of this segment of the CMB indicate a common magma source and history.

Conventional contact metamorphism is limited to a mile-wide zone adjacent to the Ruby Range axis; however, organic geochemical studies of the Cretaceous Mancos shale reveal that low-grade thermal metamorphic effects extend about 6 miles southeast of the Ruby Range. This is an unusually wide thermal aureole and indicates an important heat source in addition to the Ruby Range intrusions. The peripheral dikes, sills, and laccoliths were found to cause little thermal effect on intruded strata. It is concluded that the thermal aureole adjacent to the Ruby Range is enlarged because of the presence of a subjacent intrusion.

Evidence indicates that the White Rock stock projects into the subsurface beneath the sedimentary cover. Recognition of organic matter transformations within the Mancos shale permits tracing the approximate southeast margin of the pluton from its outcrop for a distance of about 10 miles southwest along its subjacent trend. Similarly, the wide zone of thermal metamorphism on the northwest side of the Ruby Range, most apparent across the Treasure Mtn dome, is controlled by a subjacent heat source due to the projection of the Snowmass stock into the subsurface.

Speculative interpretations: the Snowmass and White Rock stocks are contiguous, both project southwest into the subsurface; rather than classic stocks these features are the surface truncation of a thick slab-shaped igneous layer intruded into the upper levels of the crust; the molten slab was the source of the granitic magma emplaced as laccoliths and related intrusions.