The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile and Bakers Bridge Granites invaded the metavolcanic sequence between 1,680 and 1,700 Ma. The timing of deposition of the conglomerates, clean quartzites, and pelites of the Vallecito Conglomerate and the Uncompahgre Formation with respect to events in the metavolcanic sequence is poorly constrained, largely because shear zones lie along crucial contacts. Recent work suggests that much if not all of this clastic sedimentary sequence may be parautochthonous and was probably deposited sometime after 1,680 to 1,700 Ma. Despite uncertainties in timing of deposition, it is clear that a second event of deformation and metamorphism affected the region, producing polyphase features in both the metavolcanic sequence and the clastic sedimentary sequence, as well as a weak foliation in the Tenmile Granite. The Eolus Granite and related intrusions, ranging in age from 1,320 to 1,450 Ma, crosscut all structural features in the region and provide a minimum age limit for the latest deformation in the region. This younger event is recorded nowhere else in Colorado but may correlate with events in northern New Mexico.

Abstract 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/km 2 (31,750 oz/mile 2 ). 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

Abstract Tertiary gold-silver deposits in the Lead-Deadwood area have been exploited almost as long as the Precambrian Homestake gold deposit. The deposits were first described by Irving (1904) who was able to visit and map many of the mines. Recent papers by Shapiro and Gries (1970), Norton (1989), Paterson et al. (1988), and Redden and French (1989) summarize and update data on the deposits. The major mines of the late 1800's-early 1900's were developed along relatively small, high-grade replacement deposits of dolomitic sandstone beds at one or more levels in the Cambrian Deadwood Formation. Ore extended laterally from obvious vertical feeder fractures or joints which were commonly referred to as "verticals". Some deposits were also influenced by the emplacement of igneous sills in the Deadwood Formation that acted as barriers or traps for hydrothermal solutions. Because mineralization was closely related to the vertical joints and the favorable host beds were of limited thickness, the Tertiary deposits were of limited size. Nevertheless, approximately 90,000 kg (3 million oz) of gold were recovered from such deposits by the time production essentially ceased about three decades ago. With the major increase in gold price in the late 1970's and early 1980's, and the development of low cost open pit mining and heap leach processing, exploration was undertaken by several companies to determine whether there were low-grade, large, disseminated-type deposits which were amenable to low cost mining and leaching operations. At present four companies are operating open pit mines in this type of mineralization.

Abstract The northwest-southeast oriented Hampshire DieppeBasin, approximately 280 km long (Hamblin etal., 1992), is a structural basin formed by Miocene inversion of the underlying Mesozoic Hampshire-DieppeHigh and the adjacent Jurassic-Lower Cretaceous extensional sedimentary basins (Chadwick, 1993; Hibschet al., 1993). The onshore portion in southern England, the Hampshire Basin (Figure 1)/ covers much of the county of Hampshire and extends into the adjacent counties of the Isle of Wight, Dorset, and West Sussex. Its southern boundary is formed by east-west, north youngingmonoclinal folds involving Upper Cretaceousand Paleogene strata, draped over reversed normal faults in Jurassic-Lower Cretaceous strata(Chadwick, 1993). The other boundaries correspond to the Paleogene-Upper Cretaceous (Chalk) junction andwere exposed as a result of post-Miocene erosion. The Paleogene succession (Daley, 1999) was depositedunconformably on weakly folded and eroded Upper Cretaceous (Campanian) Chalk (Melville and Freshney,1982) and subsequently uplifted and deformed duringmid-Paleocene (Laramide) crustal compression (Hamblinet al., 1992). The Paleogene strata dip steeply in the vicinity of the monocline, whereas to the north, dips are shallow in low-amplitude folds. At the beginning of Paleogene time, the British area was some 12 south of its present position (Irving,1967) and considerably warmer than at present (Reidand Chandler, 1933; Daley, 1972). Britain lay on the western margins of what has been called the Northwest European Tertiary Basin (Vinken et al., 1988). At the western end of this basin, the Tertiary North. Sea Basin extended northward between the British and Scandinavian land masses. The present Hampshire(tectonic) Basin was at the southeastern end of this369sedimentary basin. The Paleogene succession here is up to 652m thick (Edwards and Freshney, 1987a) (Figure 2).It begins with upper Paleocene coastal plain sediments and continues with lower to mid-Eocene (Ypresian-Bartonian,NP10-NP17), cyclically alternating marine, nearshore, and coastal plain sediments. Extreme shallowing and possibly subaerial exposure(Edwards and Freshney, 1987b) at the mid-upper Eocene (NP17-NP18) boundary preceded the onset ofpredominantly argillaceous, strongly river-influenced, shallow-marine, laguno-Iacustrine and lacustrine sedimentation(represented by the Solent Group), which continued at least into the early Oligocene (Rupelian,NP23). This major change in sedimentary style signifies the establishment of a better-defined sedimentary basin of unknown extent. The change coincided with gradual uplift of the Weald-Artois axis (coincidentwith the major Midi basement fault), linking extreme southeastern England to northeastern France (Hamblinet al., 1992). As a result of post-Miocene erosion, the middle and upper formations of the three comprising the Solent Group are now confined to the Isle of Wight.