SETTING THE STAGE FOR SEDIMENT-HOSTED COPPER: FAULT RELATED HYDROCARBON(?) BLEACHING AND COPPER ORE AT THE CASHIN MINE, MONTROSE COUNTY, COLORADO

Field mapping in the area of the Cashin Cu Mine in western Montrose County, Colorado reveals sub-conformable zones of bleaching in gently dipping Jurassic sandstones (Wingate through Entrada), centered upon steeply dipping, northeast-trending normal faults associated with the Paradox Valley salt anticline. Bleaching of red sandstones has converted hematite to pyrite along these structures for at least 250 meters vertically and from meters to kilometers laterally. Around the Paradox Valley several of the same faults that acted as conduits for bleaching fluids also host copper mineralization such as at the Cashin and Cliffdweller mines. Copper mineralization locally extends for tens of meters away from the faults in high permeability zones, predominately the Wingate Sandstone. Copper distribution in the Wingate Sandstone at the Cashin Mine indicates that copper was deposited only in bleached zones where the host rock was chemically prepared by reducing fluids. The presence of pyrobitumen associated with copper sulfide mineralization suggests that the reducing fluid may have carried hydrocarbons. Detailed petrography of samples from unbleached, bleached, and mineralized Wingate Sandstone has defined the distribution and paragenetic sequence of the important minerals (iron oxides, pyrite, kaolinite, illite-smectite, chlorite, mafic minerals, and copper sulfides) associated with both events.