In the complex pre-Cambrian terrane of the Lead area of the northern Black Hills of western South Dakota is a group of amphibolites parallel to the structure of the enclosing sedimentary rocks and found at many stratigraphic horizons. These bodies have long been assumed by most workers in this area to be altered igneous intrusions.
The present study was undertaken to determine with certainty the origin of these amphibolites and to correlate their history with that of the district as a whole. Structural and petrographic evidence supports the previous conclusion, indicating that the amphibolites were originally gabbroic sills intruded into the sedimentary rocks toward the close of the dynamic metamorphism and then altered in various degrees by the movements. A zonal distribution of minerals in the sedimentary series is ascribed to the temperature effect of depth of burial and igneous intrusion, and it has been found that the amphibolites also were considerably affected by this thermal phase of the metamorphism. Hydrothermal alteration was the final chapter in the metamorphic history of the amphibolites.