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Author: Admin | 2025-04-28
As the placers became depleted, the Homestake Co., through acquisition of other properties and intelligent development, became the leading operator in the Black Hills and the largest gold producer in the United States (fig. 24). Several smaller companies also developed gold lodes in the Deadwood-Two Bit, Garden, Bald Mountain, Squaw Creek, Hill City, and Keystone districts.The Black Hills consist of a core of Precambrian rocks surrounded by concentric bands of Paleozoic and Mesozoic rocks that dip gently away from the core. They are intruded by sills, stocks, and dikes of igneous rocks that range in composition from rhyolite and phonolite to quartz monzonite (Darton and Paige, 1925).The Precambrian rocks are exposed in a northwest-trending oval area about 60 miles long and 25 miles wide, and they form the highest part of the Black Hills. These rocks consist of (1) intensely folded metasedimentary rocks, including schist, slate, and quartzite, (2) amphibolites that cut the metasedimentary rocks, and (3) granite that intrudes the other two (Noble and Harder, 1948). The famous Homestake ore body occurs in a cummingtonite schist unit, known as the Homestake Formation, which ranges from 200 to 300 feet in thickness and lies stratigraphically about 2,000 feet above the base of the Precambrian rocks exposed in the Lead area.Encircling the outcrop area of Precambrian rocks is a thick section of Paleozoic sedimentary rocks which are, in ascending order, the Cambrian Deadwood Formation, the Ordovician Whitewood Formation, the Mississippian Englewood and Pahasapa Formations, the Pennsylvanian Minnelusa Formation, and the Permian Opeche Formation and Minnekahta Limestone. The outer rim of the Black Hills is made up of a series of rims and hogbacks of Mesozoic rocks. The main period of deformation that produced the Black Hills dome occurred in post-Cretaceous time, during the Laramide orogeny. The structures seem to be a
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