Structural control and wall-rock alteration at the Wilbert Mine, Dome District, Butte County, Idaho

Abstract

The ore at the Wilbert mine in Butte County, Idaho, consists largely of fine-grained galena deposited mainly by replacement of a dolomitic member in a thick series of complexly folded and faulted Ordovician quartzites. Ore has been mined almost continuously from the surface to a depth of 800 feet and laterally for a distance of about 2,000 feet. In the upper levels the ore bodies have been localized by fracture and breccia zones along the crest of a sharply bent, overturned anticline, in the intermediate levels by a fissure in the plane of a curved thrust fault, which lies on the underside of the overturned anticline, and in the lower levels by a weak zone of fracturing and slipping along the contact of dolomite with overlying quartzite. The fracturing along the contact has apparently been localized by a minor bend in otherwise uniformly dipping beds; the ore body extends laterally downward at first and then assumes a horizontal position in the deep part of the mine.Mineralization was accompanied by marked alteration of the dolomite, which was feldspathized, carbonatized, silicified, and somewhat sericitized and pyritized prior to ore deposition. Feldspathization apparently was accomplished by high-temperature solutions emitted during a late stage of magmatic differentiation and was followed by intrusion of lamprophyre. Subsequent carbonatization and silicification (chiefly chalcedonization) apparently took place at relatively low temperatures.The mineralization is probably related genetically to early Tertiary magma, and ore deposition apparently occurred at moderate depth under essentially telethermal conditions.