THE PETROLOGY AND GEOCHEMISTRY OF THE WILDCAT GULCH SYENITE, GUNNISON COUNTY, COLORADO

In the Wildcat Gulch area of south-central Colorado, the Proterozic Dubois Greenstone succession has been intruded by the 1.7 Ga Tolvar Peak and Powderhorn Granites. In addition, the 1.4 Ga Berthoud Intrusive suite and the 570 Ma Powderhorn Carbonatite are found in intrusive association with the greenstone. Within Wildcat Gulch, syenite plutons intrude these units, and little has been done to describe their lithologic character, origin, or significance to the geology of Colorado. This study proposes four origin hypotheses for these syenite intrusions. Major, trace and REE geochemical analyses have been performed to help constrain the origin hypotheses. The syenites are fine-to-medium grained, with quartz (<10%) + Microcline (50 to 70%) + plagioclase (10 to 50%) + sodic amphiboles + biotite with minor clinopyroxene and apatite. Hand sample and thin section analysis, coupled with field relationships, indicate that there are three varieties of syenite. These are classified as alkali feldspar and quartz syenites, and quartz monzonites according to the IUGS classification scheme. Fieldwork also indicates that the syenites are nonfoliated and have black-wall mineralization to blocky assimilation contacts with the Dubois Greenstone. Because the Dubois Greenstone has been metamorphosed and deformed, the lack of foliation indicates the syenite intrusions are younger.