Abstract

Gold and iron-ore production from the South Pass granite-greenstone terrain is estimated at more than 325,000 ounces of gold and more than 90 million tons of ore. The mineralized terrain lies in the southwest part of a fragmented Archean (>2.5 b.y.) cratonic block known as the Wyoming Province. Rock types in this part of the Wyoming Province can be generalized into three basic lithologic groups: 1) Early quartzo-feldspathic gneiss, 2) supracrustal terrains and 3) late Archean granite.

The South Pass area is a supracrustal terrain intruded along its flanks by late Archean granite. Supracrustal rocks at South Pass have been folded into a tight synclinorium. The basal succession consists of quartzite, schist and banded iron formation known as the Goldman Meadows Formation. The Goldman Meadows Formation is overlain by mafic metavolcanics and greenschist mapped as the Roundtop Mountain Greenstone Formation. Unconformably overlying the Roundtop Mountain Greenstone Formation are metagreywackes of the Miners Delight Formation.

Two separate mining districts, the South Pass-Atlantic City and the Lewiston districts, are located in the supracrustal rocks of the mineralized belt. The districts have similar geology but are located on the opposing limbs of the South Pass synclinorium. Both were developed in metasediments of the Miners Delight Formation and gold mineralization occurs principally as shear-zone deposits.

Untested gold mineralization in the South Pass greenstone belt includes discrete veins in metavolcanics of the Roundtop Mountain Greenstone Formation and sulfide and silicified zones in the iron formation of the Goldman Meadows Formation.