Precambrian geology along parts of the Gunnison uplift of southwestern Colorado
— D. C. Hedlund and J. C. Olson

The Gunnison Uplift of southwestern Colorado consists of extensive Precambrian rocks along the south side of the Gunnison River that extend for as much as 120 km from the Black Canyon area through the Powderhorn district and eastward to Cochetopa Creek (fig. 1). The part of the Gunnison Uplift discussed in this report occupies an area of about 60 by 30 km and is a part of the Uncompahgre-San Luis Highlands. The Cimarron fault bounds the Gunnison Uplift on the south and is well exposed in the valley of the Lake Fork of the Gunnison River, where the Upper Cretaceous Mancos Shale is faulted downward on the south against Precambrian rocks. The fault block in this region is tilted 5 to 10 degrees to the north-northeast. Along the Gunnison River, the Precambrian rocks dip under the cover of Upper Jurassic and Cretaceous sedimentary rocks, and the contact slopes northward forming a part of the south flank of the Piceance Creek Basin.

 

The Precambrian rocks have been separated into two major formations, the younger dominantly metasedimentary Black Canyon Schist and the older dominantly metavolcanic Dubois Greenstone. The Black Canyon Schist is at least 10,000 m thick, and in the Black Canyon W. R. Hansen (1971) recognized five different map units. In the Powderhorn region, the Dubois Greenstone is at least 8,000 m thick and the base of the formation is not exposed. Olson and Hedlund (1973) separated the Dubois Greenstone into three main rock types: (1) metabasalt and metaandesite flows, (2) felsite porphyries and metatuffaceous rocks, and (3) epiclastic rocks derived from the older volcanic strata. The purpose of this paper is to give a review of the Precambrian stratigraphy, structure, and intrusive activity.

 

Uplift and erosion in the late Paleozoic and (or) Triassic resulted in the removal of all Paleozoic strata and peneplanation of the Precambrian basement prior to the deposition of the Upper Jurassic Junction Creek Sandstone. Laramide movement on the Cimarron fault resulted in at least 610 m of displacement. Subsequent erosion removed Upper Jurassic and Cretaceous sedimentary rocks from most of the area prior to the eruption of extensive Oligocene lavas, laharic breccias, ash-flow tuffs, and Miocene or Pliocene flows onto the stripped surface of Precambrian rock.

 

Numerous small gold and silver mines within the Dubois Greenstone gave rise to the term Gunnison Gold Belt, and such small mining towns as Dubois, Spencer, Vulcan, and Midway were formed in the late 1880's. In the middle and late 1970's, with the recognition of volcanogenic massive sulfide deposits in the region, there was a renewed interest and exploration of the gold- and silver-bearing stratabound base-metal sulfides. D. M. Sheridan, W. H. Raymond, and L. J. Cox (this guidebook) discuss this type of mineralization in an accompanying paper (see also Drobeck, this guidebook).