ABSTRACT THE PETROLOGY AND STRUCTURAL GEOLOGY OF THE TOLVAR PEAK AREA, GUNNISON COUNTY, COLORADO The Tolvar Peak Area, located on the northern flank of the San Juan Mountains, is characterized by canyons and mesas with rounded slopes. The San Juan Mountains comprise a broad dissected uplift in the eastern half of the southwestern quadrant of Colorado. The area of study covers approximately 17 square miles, the geology of which is described in the thesis and illustrated on the accompanying maps. The greater part of the area has irregular relief with the mean elevation around 8,800 feet. Tolvar Peak, whose summit elevation is 9481 feet, is the highest point in the report area. The units present in the area are Precambrian metamorphic and intrusive rocks, a sedimentary formation of Jurassic age, extrusive rocks of Tertiary age, and Quaternary surficial deposits. The Precambrian rocks consist of a complex of highly metamorphosed schists and gneisses. These include quartz-mica schist, amphibole schist, amphibolite, chlorite schist, and hornblende gneiss. Granite and augite syenite have intruded these units. The schists are in part metamorphosed basic igneous rocks and in part metamorphosed sediments. The hornblende gneiss is believed to be a metamorphosed mass of quartz diorite. Morrison sandstone of Jurassic age crops out in small isolated erosional remnants which directly overlie the Precambrian rocks. The Alboroto rhyolite, accruing as erosional remnants, unconformably overlies the Precambrian rocks in certain portions of the area and the Jurassic Morrison sandstone elsewhere. The Quaternary deposits, which include talus and alluvium, are confined, in general, to narrow, thin deposits along the Cebolla Creek Valley and the Goose Creek drainage system. The general structure of the report area is moderately complex with faults, joints, cleavage, and intrusive masses generally trending in a northeast-southwest direction. The faults indicate small displacement and are of limited linear extent. The dominant structural feature is a normal fault in the southern portion of the area; this fault has displaced the Morrison sandstone roughly 1000 feet. This structural feature may be a portion of the extensive Cimarron fault, believed by other workers in the area to pass near the Powderhorn post office. Structures formed in the metamorphic schist and gneiss complex are, for the most part, joints with northeast trends. Most of the intrusive igneous masses have similar orientations. Quartz-chlorite, quartz-carbonate, and barite veins have been recognized; all of these veins appear to be of Precambrian age. The quartz-chlorite varieties are the most profuse and widely distributed. These veins are principally in fissures and generally trend northeast-southwest and dip steeply to the northwest. The quartz-carbonate veins are restricted to the metamorphic complex and are usually much larger than the other types. These veins range from stringers a fraction of an inch wide and a few feet in length to bodies six to eight feet wide and several hundred feet in length. Several prospect adits and tunnels were located on these veins in quest for gold. The strike of these veins appears to follow a general northeast-southwest trend, and the veins dip steeply to the southeast. Along the western border of the report area these veins have been replaced with red and brown jasper and iron oxides. A single, but significant, vein of nearly pure barite was mapped in one locality. It is about six feet in width and can be traced for several hundred feet on the surface. The body strikes in a northwest-southeast direction and has a nearly vertical dip. Small amounts of radioactive minerals are also present in the quartz-chlorite and the quartz-carbonate veins. The quartz-carbonate-iron oxide veins were, for the most part, more radioactive than, the other types. A radioactivity survey was conducted in the Tolvar Peak Area using a portable scintillation counter. Radioactive vein samples were tested in the laboratory for determination of equivalent radioactive mineral content. Olson and Wallace (1956, p. 702) state that "thorium is known to occur as thorite or thorogummite" in the Powderhorn District. The radioactive samples have not been studied sufficiently to identify the thorium-bearing mineral or minerals. The data collected suggest that the Tolvar Peak area may be a possible economic source area for thorium.