HYDROTHERMAL PLATINUM-GROUP ELEMENT MINERALIZATION AND ALTERATION, MEDICINE BOW MOUNTAINS, SOUTHEASTERN WYOMING

Platinum-group element (PGE) mineralization is commonly magmatic in origin, and hydrothermal PGE mineralization remains poorly understood. The Medicine Bow Mountains in southeastern Wyoming are host to several historically mined, shear-hosted Au ± PGE deposits, such as the historic New Rambler mine. The greenschist to amphibolite facies host rocks in this region are transected by the 1.76 Ga Proterozoic Cheyenne belt, which separates the Archean Wyoming province to the north from accreted Proterozoic island arcs associated with the Colorado Province to the south. Some authors have suggested that the nearby 1.77 Ga Mullen Creek complex, a layered mafic intrusion, was the original source of the PGEs in the shear-hosted deposits, with the Cheyenne belt providing a conduit for the transport of PGEs in hydrothermal fluids. Other authors have proposed that younger, felsic intrusions provided fluids necessary for Au ± PGE mineralization in the region. This study utilizes 1:12,000 scale mapping of study areas which cover portions of the Centennial, Keystone, and Overlook Hill 7.5’ quadrangles, in combination with petrography, to document alteration associated with the historic Au ± PGE deposits. The main rock units in the study area include amphibole schist, amphibolite, lime-silicate marble, and small-volume felsic intrusions. Initial mapping indicates that alteration is spatially associated with mylonites and locally intense shearing in the southern Centennial Ridge area. The alteration consists of common chlorite alteration of amphibole-rich host rocks, as well as rare, local epidote ± feldspar veins and multiple episodes of epidote ± Fe-oxide veins and fracture coatings. Local alkali metasomatism accompanies some samples of sheared material, with significant addition of K-feldspar in graphite-bearing samples. Locally in the study area, gabbroic rocks of the Mullen Creek Complex are altered pervasively to epidote-rich, chlorite-bearing assemblages. Preliminary mapping results indicate that much of the alteration is broadly spatially associated with contacts between the gabbroic rocks and 1.73 Ga felsic intrusions. Continued mapping and petrographic investigation will be used to document alteration and evaluate timing relations between alteration, mineralization, felsic igneous bodies, layered mafic intrusions, and the Cheyenne belt to better determine the source and timing of ore deposition in the area.