Geochemical nd Geochronological Constraints on the Genesis of Au-Te Deposits at Cripple Creek, Colorado

The Cripple Creek district (653 metric tons (t) of Au) consists ofAu-Te veins and disseminated gold deposits hat are spatially related to alkaline igneous rocks in an Oligocene intrusive complex. Vein paragenesis includes quartz-biotite-K feldspar-fiuorite-pyrite followed by base metal sulfides and telluride minerals. Dis-seminated deposits consist of microcrystalline native gold with pyrite that are associated with zones of pervasive adularia. New 4øAr/a9Ar dates indicate that there was a complex magmatic and hydrothermal history. Relatively felsic rocks (tephriphonolite, trachyandesite, and phonolite) were emplaced into the complex over about i m.y., from 32.5 _+ 0.1 (la) to 31.5 _+ 0.1 Ma. A younger episode of phonolite mplacement outside of the complex is indicated by an age of 30.9 _+ 0.1 Ma. Field relationships suggest that at least one episode of mafic and ultramafic dike emplacement occurred after relatively more felsic rocks and prior to the main gold mineralizing event. Only a single whole-rock date for mafic phonolite (which indicated a maximum age of 28.7 Ma) was obtained. However, constraints on the timing of mineralization are provided by paragenetically early vein minerals and K feldspar f om the disseminated gold pyrite deposits. Early vein minerals (31.3 + _ 0.1-29.6 _+ 0.1 Ma) and K feldspar (29.8 _+ 0.1 Ma) from the Cresson disseminated deposit, ogether with potassicall