Geological Overview of the Black Hills

Thomas Loomis

Geologic Overview

The geology of South Dakota varies considerably from across the State (Figure 1). For the most part the State lies within the drainage of the Missouri River, which divides the State geographically and geologically. To the east of the river, South Dakota is mostly buried by glacial rock. A broad arch, called the Sioux arch, extends from the southeastern corner of Minnesota to central South Dakota. This arch is composed of the ancient red Sioux quartzite, which was deposited as a blanket sand about 1.7 billion years ago. The quartzite is exposed in varies places along the arch. West of the Missouri River the plains begin to rise with the Cretaceous sediments, which form the bedrock deposited by shallow seas. The Williston Basin occupies the northwestern quadrant of the State and was formed during continuous downwarping during the Paleozoic time. Approximately 16,000 feet of sediment accumulated in this basin and is one of the largest structural and sedimentary basins in North America (Denson, N. M, 1964). The famous Badlands of South Dakota are the product of erosion and deposition of Eocene and Oligocene clays and silts of the White River group southeast of the Black Hills.

Figure 1. Major Structural Features of South Dakota (Gries, 1996)

Little did General George Custer know that when he headed south along the western border of the Black Hills, that he was skirting the geologic fence of what would be perhaps one of the greatest mining regions the world has ever known. From Custer’s vista high on the Inya Kara Mountain in Wyoming west of the Black Hills he would never know that huge quantities of gold in excess of 55 million ounces would eventually be produced in the Black Hills to the east. Hundreds of millions of tons of coal would be produced in the Powder River basin to the west. World-class deposits of uranium would be discovered in the sandstone bluffs to the south. Mica, lithium, beryllium, tantalum, and feldspar deposits would be found in the interior of the Hills and would supply the requirements for both World Wars.  

Custer actually traveled along the Tertiary red-beds adjacent to what is now called the “hogback” of the Black Hills (Figure 2). This term was used to describe the layer of Cretaceous age rocks, which form a ridge and valley around the Hills. These rings are a mere reflection of the overall geologic structure of the Black Hills – a kidney shaped anticline. The core of the Black Hills is represented by the Harney Peak Batholith, a 1.7 billion-year-old granitic intrusion. This intrusion subsequently formed the present day Black Hills as the eastern-most extent of the Laramide orogeny, and in the process eroded away thousands of feet of Paleozoic and Mesozoic sediments. This event probably began about 62 million years ago (Gries, 1996). The result of which provides a magnificent window into to the past. Virtually every period of the Paleozoic stratigraphic column is represented in the Black Hills except the Silurian. Some of the oldest rocks on earth are found in the Black Hills dating back 2.5 billion years ago with the emplacement of the Little Elk Creek granite west of Tilford, South Dakota. The original “country” rock laid down before the emplacement therefore dates back even further in time.  

 Figure 2. 1880 Arial Depiction of the Black Hills by Henry Newton showing the "Hogback" rings around the Hills.
Dept. of Interior Geological Survey   
Courtesy Ed Gerken, Black Hills Trails of Time

Gold deposition in the Black Hills has been the subject of thorough debating throughout the years. The Homestake gold mine in Lead (Figure 3) has captured worldwide speculation as how the gold was deposited. Was the gold syngenetic and deposited with host Precambrian iron formation or was it hydrothermally emplaced? Likewise, geologists have often debated the origin of the quartz veins containing gold in the southern Hills. Even the Tertiary intrusives emplaced in the Cambrian sediments of the northern Hills carry gold and raise eyebrows. Were the Tertiary intrusives responsible for carrying gold and subsequent deposition into the overlying Cambrian sediments?  It’s a good question since it is now fact that the mines around the Homestake, which are all located in the Cambrian Deadwood formation have now outlasted the great Homestake mine.

Figure 3. Homestake Mine, Lead, South Dakota
Photo by Tom Loomis 

For the mineral collector however, Black Hills gold has had only limited appeal. Beside the Homestake mine, nice gold specimens have been produced from vein deposits such as the Rattlesnake Jack and the Holy Terror mine (Figure 4). Silver mines in the Galena and Carbonate areas have produced silver sulfosalts, phosphates and arsenates. These districts are considered epithermal and are spatially associated with an east-west trending zone of Tertiary alkalic intrusions. The Silver City district host polymetallic deposits found in graywacke and are associated with quartz veins in fault zones (Paterson et al, 1990). Connolly in 1927 surmised that a zonal distribution of metal exists outward from the Homestake mine in Lead. With the town of Lead at the center, gold and tungsten occur. Outward are gold and silver deposits, lead-zinc and finally around the perimeter are lead-silver deposits. Each zone substantiated by proven districts, which have produced the said metal.  

Figure 4. Gold - 1cm in Quartz matrix, Holy Terror Mine, Keystone, SD
Photo by Tom Loomis 

If the northern Hills are famous for its gold mines, the southern Hills are equally famous for it pegmatites. More than 20,000 pegmatites exist in the Black Hills. These pegmatites probably crystallized about 8 miles below the paleosurface shortly after the emplacement of the Harney Peak batholith (Gries, 1996).  Figure 5 and 6 are photos of the highest point in the Black Hills - Harney Peak at an elevation of 7,242 feet the center of the Harney Peak batholith. Most of the pegmatites occur near Hill City, Keystone and Custer. However, the Tinton area in the northern Hills also contain pegmatite deposits. Most of the pegmatites are tabular and of small dimension ranging from inches to more than 500 feet wide. The greater numbers of mined pegmatites were dome-like bodies (Connolly, 1927) on the scale of 200 to 1,000 feet along the longest axis (Page et al 1953).  The composition is essentially quartz and feldspar with smaller quantities of other minerals, but approach the composition of granite (Page et al, 1953). However, nearly sixty primary minerals have been recorded in the pegmatites (Connolly, 1927). More than 175 different mineral species have been identified in the pegmatites. Some of the more famous pegmatites in the Hills in terms of economic production include the Etta, Tin Mountain, Bob Ingersoll, Beecher No. 2, Mateen, Hugo, and Peerless mines.

Figure 5. Harney Peak
Photo by Tom Loomis

Figure 6. Harney Peak Look Out Station
Photo by Tom Loomis

Outward from the Black Hills several economic mineral deposits have been mined which adds to the mineral wealth of the Hills. In 1951 the first commercial uranium deposits were found and occur as lens shaped bodies in the sandstones of the Cretaceous Inya Kara group south of the Hills. To the north of the Hills, economic quantities of bentonite have been mined since 1923 and continue today. The bentonite occurs in Cretaceous shales of the Mowry Shale.  

References  

Connolly, J. P., O’Harra, C. C., 1929, The Mineral Wealth of the Black Hills, South Dakota School of Mines, Bull. No. 16.

Denson, N. M. 1964, Mineral and Water Resources of South Dakota. Report prepared by the  USGS, USBR in cooperation with SDGS and SDSM&T.    

Gerken, Ed. Trails of Time Publishing POB 747, Hill City, SD 57745 USA

Gries, J.P., 1996, Roadside Geology of South Dakota, Mountain Press Publishing Company, Missoula, Montana  

Paterson, C. J., Lisenbee, A. L., Redden, J. A., Gold Deposits in the Black Hills, South Dakota: Guidebook Series Vol. 7, “Metallogeny of Gold in the Black Hills, Society of Economic Geologist, Field Conference, 1990.