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The McKinley Mountain area, a 22-square mile tract of Precambrian rocks that contain thorium-bearing veins, lies on the west flank of the Wet Mountains, Ouster and Fremont Counties, Colo. The bedrock in this area is a complexly interlayered sequence of gneisses of metasedimentary origin, migmatite, and granitic gneisses that has been transected by an albite syenite stock and numerous northwest-trending dikes, veins, and fractures. Hornblende-plagioclase and biotite-quartz-plagioclase gneisses are the principal metasedimentary rocks; garnet, sillimanite, quartzite, and pyroxene-scapolite zones are locally present. A poorly foliated alaskitic granite gneiss is common as layers ranging from more than 500 feet in thickness to migmatitic lit-par-lit type layers less than an inch thick. Of less wide distribution, but of similar occurrence, are quartz monzonite gneiss and light-colored granodiorite gneiss. The albite syenite, one of the youngest rocks in the area, is not foliated and is about 595 million years old (late Precambrian), by the Larsen zircon method of age determination. Many of the dikes are similar in composition to the albite syenite stock. The foliation of the gneisses generally is steep and trends northeast in most of the area. Several northeasterly trending folds have been mapped in the northern half of the area; a vertically plunging fold occurs in the southwest quarter of the area. Faults, shear zones, and joints, some filled with veins and dikes, trend northwest across the area and cut the foliation at large angles. More than 800 radioactivity anomalies were found along northwesterly trending veins within the area, and 29 localities were examined outside the area. Most anomalies do not exceed 5 times the background count, but relatively rich concentrations of radioactive elements giving strong anomalies are scattered along the veins in pockets and lenses. The radioactivity, as shown by representative analyses, is due almost entirely to thorium. A hydrated thoritelike mineral is visible at some localities; the same mineral doubtless is finely disseminated among red, yellow, and brown iron oxides and (or) hydroxides at radioactive localities where no thorium-bearing mineral is visible. Besides iron oxides and, locally, thorium minerals, many veins contain abundant carbonate minerals, barite, quartz, and minor quantities of sulfide minerals. 491 492 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Geology and thorium deposits of the Wet Mountains, Colorado; a progress report