Abstract The Homestake mine in the northern Black Hills, South Dakota is the largest iron-formation-hosted gold deposit known (Fig. 1), and has produced 1,113 tonnes (35.8 million oz.) of gold from 128 million tonnes of ore milled. In 1989 the mine produced 11.9 tonnes of gold. The deposit was discovered in 1876, and the mine has operated continuously to the present day (Fig. 2). Ore is currently mined from depths to 2,438 meters. Gold is the major commodity produced along with a minor silver by-product. The gold/silver ratio averages 5:1; base metal content is negligible. The Homestake gold deposit is hosted within quartz-veined, sulfide-rich segments of an early Proterozoic, carbonate-facies iron-formation in a sequence of originally calcareous, pelitic, and quartzose rocks (Fig. 3). Strata that contain the Homestake deposit were complexly deformed by a series of tight isoclinal and sheath fold events, and synchronous, extensive ductile and ductile-brittle shearing. Mine area rocks have been subjected to upper greenschist-lower amphibolite facies metamorphism. Early-stage ductile shearing appears to have controlled fluid movement in the system thereby controlling thermal energy and metamorphic processes. Observed prograde metamorphism in the district is therefore characterized as dynamothermal. Intrusion of a 1.72 Ga Harney Peak-type granite in an area northeast of the mine post -dated regional prograde metamorphism, and appears to be contemporaneous with later stages of semi-brittle deformation (Bachman et al., 1990).

Abstract Gold in the Open Cut area was first discovered by two Frenchmen, Moses and Frederick Manuel, on April 9, 1876 (Irving, 1904). A quartz outcrop located somewhere near the center of the current Open Cut (see frontispiece) was the site of the original Homestake lode discovery in 1876 (see Frontispiece for this volume). By the following year the numerous claims filed in the area had been consolidated into four large companies. Irving further states: "Not long after the mines opened it was found advisable to work them under a single management, and as time went on the Homestake Company came either into control or into actual possession of the other properties...” The gold ore was free-milling, and by the summer of 1878, 80 stamps were in operation in mills near large open cuts on the steep hillside. By 1880, a total of 740 stamps were crushing 2 to 3 tons of ore each per day. The old headframe of the B & M No. 2 underground mine, one of the early underground operations on the Homestake gold deposit, remains beneath the hill crest. Mining in the Open Cut was renewed as a supplemental surface operation in 1984, with 1989 gold production at 2678 kg (86,400 oz).