Abstract

Copper sulfides and copper-iron sulfides have partly replaced or are closely associated with carbonaceous fossil plant material, mainly logs, in the Agua Zarca Sandstone Member of the Chinle Formation of Triassic age. Small amounts of native silver occur with the sulfides. Malachite, chrysocolla, and azurite fill the interstices and fractures in the clastic host rocks, forming irregular halos around the mineralized fossil plant material.Primary sulfide minerals, chalcocite, covellite, bornite, chalcopyrite, and pyrite, were probably deposited by ground-water solutions moving through paleochannel complexes in Triassic time shortly after deposition of the host rock; mineralization occurred where there was a favorable reducing environment provided principally by the carbonaceous material. The ultimate source of the copper may have been older deposits in the Precambrian rocks of the Uncompahgre highland in northern New Mexico and Colorado. The sulfide minerals were partly oxidized later, and copper in the form of malachite, chrysocolla, and azurite was disseminated in the adjacent host rock. Major faults post-date deposition of the sulfide minerals and can be expected to offset some of the mineralized paleochannels.Large-scale deposition of the sulfide minerals occurred at fossil logjams within paleochannels. Orientations of cross-bedding and fossil logs can be used to determine the trends of the paleochannels. These trends should be followed in subsurface exploration.

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