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Wharf, South Dakota, US

The Wharf mine is located seven kilometres west of Lead, South Dakota, in the northern Black Hills region. This forested area is the easternmost uplift of the Laramide orogeny, having risen from the surrounding plains approximately 50 million years ago. The elongate dome is about 100 kilometres in width by 200 kilometres in length.  It consists of a core of Precambrian metamorphic and igneous rocks, flanked by exposures of Paleozoic through Mesozoic sedimentary rocks, and is intruded by a trend of Tertiary igneous bodies in the northern Black Hills. These intrusions have uplifted and exposed older rocks in the Lead window near the Wharf mine.

Wharf is located in the Bald Mountain mining district. The mined units are the Cambrian Deadwood Formation and Tertiary porphyritic trachyte sills. Manto-like deposits of disseminated gold in the lower sandstone of the Deadwood Formation are the highest-grade ore. Gold is also concentrated along near-vertical fractures in the remainder of the Deadwood. Much of the ore mined is porphyry, which is mineralized within pervasive fracture zones.

The Deadwood Formation, composed of near-shore sandstones through shales with varying carbonate content, is in total approximately 110 metres thick. The most heavily mineralized unit is the lower sandstone, with ore being silicified and often oxidized and red-brown. The lower Deadwood comprises thin basal conglomerate and silicified quartz arenite, locally glauconitic, overlain by interbedded calcareous sandstone, siltstone, and shale. The middle Deadwood is dominated by thick gray calcareous shale, and also contains interbedded sandstone, dolomite, and intraformational conglomerate. The upper Deadwood grades into glauconitic sandstone and arenite. Overlying rocks present in the mine area are the Ordovician Winnipeg and Whitewood, Devonian Englewood, and Mississippian Pahasapa Formations.

Tertiary igneous intrusions of varying composition are present in the area, but the mineralized and most voluminous rocks are Tertiary trachyte porphyry, which typically form sills 30 to 50 meters thick. Dikes and sills of late phonolite, often showing argillic alteration, are also common but are unmineralized.

The sedimentary units are nearly flat-lying and dip gently away from the Lead window. Minor faults present are rare, and mineralized fracture zones trend toward a large igneous intrusion in the area.