Cebolla Mining District (Goose Creek Mining District), Gunnison County, Colorado, USAi

Regional Level Types
Cebolla Mining District (Goose Creek Mining District)	Mining District
Gunnison County	County
Colorado	State
USA	Country

General Geology.
The Cebolla district lies on the north flank of the San Juan Mountains south of the Gunnison River. It is characterized by dissected mesas sloping toward the Gunnison River at an altitude of about 8,500 to 9,000 feet. The pre-Cambrian basement rocks are capped by erosion remnants of the Alboroto rhyolite and latitic breccias of the San Juan Mountain volcanic rocks. Beneath the volcanic capping there are a few thin remnants of Jurassic sedimentary rocks bordering the Gunnison River. The early Tertiary erosion surface on which the volcanic rocks rest and the late Paleozoic or early Mesozoic surface on which the Jurassic rocks rest are closely coincident. This is of possible significance with regard to cycles of erosion and leaching to which the vein deposits have been subjected.

The pre-Cambrian rocks are exposed over large areas between the dissected mesas and in the canyons of the principal streams, the Lake Fork Gunnison River, and Cebolla Creek. They consist of chlorite, mica, and hornblende schists and greenstones, which are intruded by granitic and syenitic rocks and cut by pegmatite dikes and younger volcanic rocks. The granitic and pegmatitic rocks are of pre-Cambrian age, but some dikes and other intrusive rocks may be younger.

Vein Deposits.
Two types of vein deposits occur in the Cebolla area besides the pegmatite dikes. They are capped unconformably by Jurassic sedimentary rocks and Tertiary volcanic rocks. The oldest veins are quartz-chlorite and contain copper and gold like those of the Cochetopa area. These are generally nonradioactive although a few minor exceptions were noted. The principal radioactive deposits are quartz-carbonate veins containing minor amounts of pyrite, sphalerite, and galena. Locally the deposits were prospected for silver. The quartz in many veins is typically smoky and crackled and is associated with a gray carbonate gangue of dolomitic or ankeritic aspect. The sulfides and radioactive materials are intergrown with these gangues in exposures of the veins along the valley and canyon walls. On the mesa surfaces the veins are locally much altered to a vericolored jaspery material, which has chiefly replaced the carbonate gangue. The jaspery material retains some of the radioactivity of the original vein matter. The veins are locally jasperized to depths of 50 to 100 feet beneath the volcanic capping of the mesas. Jasperization was also prevolcanic and presumably related to older Mesozoic or late Paleozoic erosion surfaces. It may be partly related to hot-spring activity.

Exposures of the veins in the canyons and dissected mesas through a vertical range of 800 to 1,000 feet show that oxidation extended locally to depths of several hundred feet, depending upon local relief and ground-water conditions. Leaching and dispersion of the radioactive material could have taken place during several cycles of erosion since Paleozoic time.

The greater part of the present radioactivity is due to the thorium content of the veins, but conceivably some uranium could have been leached from the veins near the different erosion surfaces. Deposition of postmine radioactive carbonate stalactites indicates that some leaching of uranium disintegration products is still in progress. Further chemical research is desirable to form a basis for judging the significance of these occurrences.

The veins are not of economic value at present, but,should appreciable leaching of uranium become established, further testing by deep drilling would be desirable.