UC Santa Barbara

The Rochford District of the Black Hills, South Dakota was heavily prospected for gold in the late 19th Century, and more recent prospecting has identified many areas with anomalous gold values (Great Lakes Exploration, unpublished data; Bayley, 1972), including some samples exceeding 20 ppm Au. The Rochford District is only 30 kilometers south of the Homestake Mine, one of the largest gold producers in North American history. Gold mineralization at the Homestake Mine is intimately related to a complex Proterozoic deformational and metamorphic history that has affected the entire Black Hills Region, and shares many similarities with the Rochford District. Despite the similarities to Homestake, and compelling evidence for gold mineralization, the Rochford District has remained largely unexplored. This study integrates geologic mapping, structural measurements, and petrographic analyses with an extensive new gold assay database in order to identify the major controls on gold mineralization in the Rochford District.

Both the Rochford District and Homestake Mine contain early F1 folds and an S1 fabric that has been transposed by a later S2 fabric. This pervasive, NNW-trending, S2 foliation is associated with tight to isoclinal F2 folds. F2 folds display dramatic thickening in the hinge regions, which increases the volume of iron formation available to host mineralization in discrete, predictable locations. A late, S3 crenulation cleavage only observed in the Rochford District is associated with strong pressure solution and is believed to represent a major fluid migration event.

The gold is concentrated in the Homestake and Rochford iron formations, which act as chemical traps for gold and sulfides. The greenschist to amphibolite transition seems to represent the most favorable combination of host rock mineralogy and permeability for gold mineralization at the Homestake Mine and within the Rochford District. Gold mineralization in the Rochford District is texturally and chemically similar to that at Homestake, but the relationship between ore-type and gold grade is different, suggesting a similar source for the gold, but different mineralization mechanisms. Textural relationships between sulfides and metamorphic minerals suggest two distinct pulses of mineralization. The first is potentially synchronous with the gold mineralization event at Homestake. The second pulse is potentially associated with the late, S3 crenulation and pressure solution that remobilized and enriched the mineralization in the Rochford Formation. The close similarities between the Homestake Mine and the Rochford District in host rock composition, structural style, metamorphic grade, and gold mineralization suggest that the Rochford District has the potential to contain a world-class gold deposit. Exploration efforts should focus on areas of thickened F2 fold hinges of Rochford Formation within the greenschist to amphibolite transition zone.