Fracture Control on the Tertiary Epithermal-Mesothermal Gold Deposits Northern Black Hills, South Dakot

Exploration & Mining Geology, Vol. 4, No. 3, 1995

MUSTAFA M. HARIRI Earth Sciences Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia, ALVIS L. LISENBEE and COLIN J. PATERSON, Department of Geology and Geological Engineering, South Dakota School, of Mines and Technology, Rapid City, South Dakota 57701, U. S. A

The schist-, sediment-, and intrusion-hosted Tertiary Au-Ag hydrothermal deposits of the northern Black Hills, South Dakota are localized and controlled by various types of fractures. This study is an attempt to distinguish between and discuss the possible causes of these fractures in three mines: Richmond Hill, Annie Creek – Foley Ridge (ACFR), and 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 deposits are in the form of veinlets and stockworks. Larger veins ?lling open spaces along schistosity planes are also present. Fractures in Richmond Hill mine are related to breccia pipe emplacement and in?uenced by the Precambrian foliation. Fractures in sediment-hosted (ACFR) and 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 dome. Fracture trends vary from one area to another depending on their location with respect to the dome. Other factors, such as the Black Hills uplift, pluton emplacement, and Precambrian foliation, may also affect the fracture trends. The proximity of the mines to domes formed by multiple intrusions, the associated fracture patterns and density, and existence of breccias, may be used in exploration for similar deposits in the northern Black Hills or other areas of similar geologic settings
Keywords: Fracture control, Hydrothermal deposits.