GEOLOGIC CONTROL ON ALTERATION AND MINERALIZATION OF THE WHARF MINE, SOUTH DAKOTA

The Wharf mine is located in the northern Black Hills, South Dakota, in the Bald Mountain mining district. Wharf is a structurally controlled disseminated gold deposit, hosted by Paleozoic sedimentary rocks and Tertiary alkalic intrusive rocks. Mining has occurred at Wharf for over 30 years as an open pit heap leach operation. Despite the long history of production, the alteration styles and ore types have not been characterized prior to this study.

Host rocks are sandstones of the upper and lower members of the Deadwood Fm, intrusive breccias and alkalic intrusive sills, including alkali feldspar trachyte and quartz alkali feldspar trachyte.

Alteration styles in the deposit demonstrate both lithological and structural control. Silicic alteration is strong and spatially pervasive in the lower member of the Deadwood Fm, particularly in gold mineralized areas. Silicification is also closely related to faults, fault breccias and fractures forming halos that extend from 10 cm to 2 m into the host rocks. Alteration in the intrusive rocks is sodic-potassic, characterized by bladed adularia, albite, microcline and overgrowth of orthoclase. Sodic-potassic alteration is locally overprinted by phyllic alteration, precipitation of white micas, quartz and dissolution of feldspars. Argilic alteration characterized by illite, smectite, chlorite and kaolinite is spatially related with main fault zones. Calcite occurs as veinlets in the intermediate member of the Deadwood Fm; whereas in the intrusive rocks and lower and upper sandstones traces of calcite occur disseminated. Arsenosiderite veinlets and disseminated arsenopyrite occur in the oxidized upper sandstones. Late barite, fluorite, (Ca,Ce,La,Th) phosphates, sphalerite, galena, calcite and anhydrite fill vugs in the intrusive rocks and are disseminated in the basal arenites. Banded chalcedony, tridymite after opaline silica, and bismuth chlorides locally fill vugs in breccias and quartz veinlets.

In the oxide zone, gold occurs natively and as electrum filling pores and fractures. In the sulfide zone, gold occurs natively, as electrum and gold tellurides. Gold phases are 2 to 10 µm in size and disseminated between grains in the sedimentary rocks. Arsenopyrite-rimmed pyrite grains also host inclusions of gold tellurides and electrum.