The 290-km-long Black Hills uplift extends from the South Dakota-Nebraska border to southeast Montana. A structurally higher east block of northward trend and a northwest-trending west block are separated by the west-facing Fanny Peak monocline. Paleozoic, Mesozoic, and Paleocene sedimentary rocks 2,220 to 3,000 m thick overlaid the anisotropic Precambrian igneous, metasedimentary, and metaigneous basement during Laramide uplift. Draping of the sedimentary section over basement fault blocks produced monoclines with, opposed to, and parallel to regional dip and ramps, terraces, and anticlines as subsidiary structures. Individual monoclines as much as 170 km long consist of linear segments commonly joined in gentle arcs. The dominant fold trend is northward, although one major segment of the Black Hills monocline trends northwest. The folds terminate at intersections with other monoclines, by decreasing stratigraphic offset along strike, or by splaying into several folds of lesser structural relief.Gravity data suggest single basement faults beneath steep, narrow folds and multiple faults beneath wider structures. In a single locality the basement is exposed in fault contact with Paleozoic rocks that show a narrow zone of rotation in both fault blocks. Normal and reverse faults locally extend as high as the massive Mississippian and Pennsylvanian carbonate rocks, but elsewhere these units are folded into five linear segments. Mesozoic siltstone and shale are believed to be more rounded in section view due to flexural flow. In the inclined limb conjugate fault sets and horizontal tension cracks affect the brittle rocks, and ductile layers are thinned by flexural flow.The uplift is separated from the Powder River Basin on the west by the Black Hills and the Fanny Peak monoclines which have structural relief as great as 1,670 m and dips as great as 90°. Gently inclined planar sedimentary units, inferred to parallel the surface of the basement, show changes of a few degrees dip across the folds. Such differences may result from rotation of rigid blocks of the anisotropic 165 on May 31, 2015 memoirs.gsapubs.org Downloaded from 166 ALVIS L. LISENBEE basement along curved faults in the style outlined by Stearns (1975).The arcuate eastern boundary with the Interior Lowlands province forms a partial dome. Strain was distributed across at least 32 km, possibly by shear along the Precambrian schistosity. North-and east-trending folds, believed to reflect longitudinal and transverse faults in the arched basement, locally parallel the basement fabric. At this easternmost margin of the Wyoming province, the uplift and adjoining province were both elevated, although unequally, whereas the basin to the west of the uplift was an area of subsidence. This difference in absolute movement patterns is believed responsible for the asymmetry and composite structural style of the Black Hills uplift.

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Since the discovery of gold in 1874, the Black Hills has been well-known as a gold metallogenic province. In fact it is one of the richest areas in the world, having produced approximately 354 kg/km2 (31,750 oz/mile2). The premier mine in this province, theHomestake mine, is one of the oldest and longest -operating in the world, having been in production since 1876. Furthermore, the variety of gold deposit types in such a small area is unique. These include Au-U quartz pebble conglomerate deposits of early Proterozoic age, iron-formation-hosted and quartz vein gold deposits of middle Proterozoic age, paleoplacer Au in basal conglomerates of Cambrian age, epithermal igneous-hosted and sediment-hosted Au-Ag deposits of early Tertiary age, and recent gold placer deposits (see summary of gold deposits in Paterson et al., 1988; reprinted in this volume). Although the history of mining here is a long one, the origins of the Homestake and other gold deposits in the Black Hills are yet to be fully explained. This is not a result of lack of interest or investigation. Significant studies regarding these deposits were conducted as long ago as 1904 by Irving, and subsequently by Connolly (1927) and Connolly and O'Harra (1929), and by Noble (1950) and Noble and Harder (1948). Then as now, there were opposing schools of thought regarding the origins of the various deposit types. For example, for the Tertiary sediment-hosted replacement deposits, Irving (1904) favored ore deposition from meteoric waters heated by the Tertiary igneous intrusions, whereas Connolly (1927) was a proponent of the magmatic-hydrothermal origin for the sediment-hosted replacement gold-silver deposits. Simultaneously, it was recognized that there were important structural and stratigraphic controls on ore localization, and that the mechanics of the sill and laccolith emplacement influenced the continuity and distribution of ores. There remain many important questions to be answered regarding the origin and distribution of the gold deposits in the Black Hills. We summarize here some of the more important ones for your consideration during this field conference. Is the Homestake deposit epigenetic (Noble, 1950; Slaughter, 1968; Bachman and Caddey*; Kath and Redden) or syngenetic but later remobilized (Rye and Rye, 1974; Rogers)? There is no consensus here, even among geologists working directly or indirectly with the Homestake Mining Company. There is general agreement however, that the mineralization is Proterozoic in age (Bachman and Caddey), and not Tertiary as reported inadvertently in the introduction to the

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