FRACTURE CONTROL ON THE TERTIARY EPITHERMAL-MESOTHERMAL GOLD DEPOSITSNORTHERN BLACK-HILLS, SOUTH-DAKOTA

The schist-, sediment-, and intrusion-hosted Tertiary Au-Ag hydrotherm al deposits of the northern Black Hills, South Dakota are localized an d controlled by various types of fractures. This study is an attempt t o distinguish between and discuss the possible causes of these fractur es in three mines: Richmond Hill, Annie Creek - Foley Ridge (ACFR), an d Gilt Edge. Five models based on the structural and tectonic settings of the northern Black Hills are proposed for the fracture origin. In Richmond Hill mine, mineralized fractures within the schist-hosted dep osits are in the form of veinlets and stockworks. Larger veins filling open spaces along schistosity planes are also present. Fractures in R ichmond Hill mine are-related to breccia pipe emplacement and influenc ed by the Precambrian foliation. Fractures in sediment-hosted (ACFR) a nd intrusion-hosted (Gilt Edge) deposits are mode I systematic joints. These fractures, and similar ones examined in seven locations outside the mines, are related mostly to the formation of the Lead-Deadwood d ome. Fracture trends vary from one area to another depending on their location with respect to the dome. Other factors, such as the Black Hi lls uplift, pluton emplacement, and Precambrian foliation, may also af fect the fracture trends. The proximity of the mines to domes formed b y multiple intrusions, the associated fracture patterns and density, a nd existence of breccias, may be used in exploration for similar depos its in the northern Black Hills or other areas of similar geologic set tings.