Rb-Sr and Re-Os Ages of Fault Gouges and Copper Mineralization from the Paradox Basin, Utah

Clay-rich fault gouge and sulfides associated with copper mineralization in the eastern Paradox Basin were dated using Rb-Sr and Re-Os, respectively. Multiple analyses of leached residues of the clay size fraction with the greatest percent of likely authigenic 1Md illite (Reitveld modeling of randomly oriented XRD data) yielded an imprecise age for the Moab fault of 60 ± 17 Ma (initial 87Sr/86Sr of 0.7166 ± 0.0052, MSWD = 0.21) , and the data combined from three faults of the Lisbon Valley fault system (Lisbon Valley, GTO, and Keystone faults) yielded a more precise age of 50.9 ± 3.5 Ma (initial 87Sr/86Sr of 0.71144 ± 0.00033, MSWD = 0.16). The leachates of all four faults are identical within uncertainty and average 0.7095, indicating that the leachable minerals (e.g. calcite) in the fault gouge had a likely origin from mixing of evaporite derived fluids with more evolved country rock derived fluids. The illite-rich clay residues from the four faults also had overlapping initial 87Sr/86Sr values consistent with derivation of authigenic illite solely from the evolved country rock derived fluids.