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Thomas J. Campbell


            The historic Homestake Gold Mine is located in the Lead Window in the northern Black Hills of South Dakota and is hosted in Precambrian rocks that are approximately two billion years old. Gold was the major commodity produced by the mine with silver recovered as a byproduct.  Base metal content was negligible. The deposit was mined over a time span of 126 years with a large infrastructure that is comprised of two surface shafts (Ross and Yates), three underground winzes (numbers 4, 6, and 7), hundreds of kilometers of drifts (mine workings excavated parallel to foliation/fabric) and cross-cuts (mine workings that transect foliation/fabric), thousands of stopes (mined ore bodies), and numerous surface structures. Most stopes have been filled with rough waste (from development work) and/or sand fill (tailings) from processed ore. Workings are developed in a block of the Early Proterozoic Earth’s crust that is approximately 2.7 x 3 x 5 km with 6.5 km of plunge length.

Homestake rocks comprise a variety of metamorphic rock types subdivided into three distinct units: Poorman, Homestake, and Ellison formations, listed from oldest to youngest. The base of the Poorman Formation consists of metamorphosed tholeiitic basalt with possible back-arc basin affinities whereas the remaining Poorman lithologies are metasediments that include a complex succession of rock types. These include metamorphosed equivalents of dirty dolomite, banded carbonate-rich claystone and siltstone, marl, iron formation, carbonaceous pyrrhotite-bearing siliceous exhalite, and interbedded tuffs. Poorman metasediments are interpreted as chemical precipitates dominated by Ca and Mg carbonates admixed with fine-grained terrigenous detrital material that constitute hemipelagic basin fill followed by input from seafloor volcanic exhalative activity and minor volcanic ash. This metasedimentary sequence was followed by a transition to Fe and Mg carbonate chemical precipitation and iron formation near the top of the Poorman Formation and into the Homestake Formation.  Multiple horizons of carbonate facies iron formation interlayered with marl are abundant throughout the Homestake Formation. Finally, tectonism was rejuvenated in the area and deep marine fans encroached into the basin that formed the superjacent the Ellison Formation which is interpreted as a metaclastic sequence dominated by feldspathic litharenite with abundant shale, siltstone, and tuffaceous units.

Metamorphic grade of these rocks ranges from middle greenschist in the western part of the mine to middle amphibolite facies in the deep, eastern section of the mine. The entire rock package was subjected to several periods of deformation during the Precambrian resulting in complex fold patterns, and significant, localized shear zones. Regional prograde metamorphism was overprinted by a metamorphic event that is related to the emplacement of the Crook Mountain Granite (very similar to the Harney Peak Granite found in the central and southern Black Hills). This granite is located in the subsurface northeast of the mine and is interpreted to be contemporaneous with some of the younger, Precambrian age deformation and fluid events found in the mine area. During the Tertiary period, rocks in the Lead Window were subjected to a brittle phase of deformation accompanied by the emplacement of alkalic igneous intrusions. This late, brittle phase of deformation is responsible for the joint sets found in the Lead Window along with local faults exhibiting minimal displacement. Aside from man made accesses into the mine, it is the network of these later stage brittle features that influence the influx of meteoric water into the mine workings. Drifting, cross-cutting, and diamond drilling have locally intersected these fracture systems or “watercourses”, as they are called by the miners, and encountered medium to high temperature (45 to 85ºC) water under low to high pressure.

Ore mineralization consists of pyrrhotite, arsenopyrite, +/- pyrite, and native gold associated with chlorite group minerals, quartz, siderite, ankerite, and biotite. This Precambrian mineralization is intimately associated with a specific stage of quartz veining and ductile shear zones. Individual ore bodies are relatively undeformed, pipe-like to tabular bodies that vary from sparse to densely clustered, depending on a variety factors that include shear intensity, quartz vein development, rheologic contrasts, and host rock mineralogy. Ninety-five percent of the gold mineralization is hosted in the Homestake Formation with the remainder in the Poorman Formation and to a much lesser extent, the Ellison Formation.

In terms of mine geography as it relates to the geology and fold structures, the large-scale synforms and antiforms have smaller scale fold structures associated with them termed “ledges”. If these ledges are synclines, they are odd numbered from east to west (3, 5, 7, 9, 11, and so on) and if they are anticlines, the structures are appropriately even numbered (6 and 10 ledges are the most significant). The exception to this is found in the historic, easternmost ledges that are termed Caledonia and Main ledges; Main Ledge was the most prolific ore system in the mine that technically starts out as an anticline and down-plunge, the ore progressively rakes across the structure and favors the synclinal part of the fold. Over 40 million ounces of gold were produced from all of the ledges that were mined during the 126 year history of this world class gold deposit that is the prototype for Precambrian iron formation-hosted gold deposits. 


General location map of the Black Hills and Homestake


Geologic map of the Lead Window


Plan view of the ledges in Homestake


Generalized cross-section of a portion of the Homestake Mine


Characteristics of the Yates Unit Amphibolite (PDF)


General view of underground


Black Hills Stratigraphy