History- The Homestake mine is located in Lead South Dakota in the northern Black Hills. In July of 1874, gold was first discovered in the Black Hills by General Custer’s expedition. This discovery signaled the gold rush to the Black Hills area in 1876. Moses and Fred Manuel first discovered gold in the northern Black Hills on April 9, 1876 on what is called the Homestake mine. The brothers discovered an outcrop of ore which was termed as a lead, hence the town name. They sank their discovery shaft in the side of a draw, then in the spring of 1876 took out $5,000 worth of gold. The following year the brothers sold their claim to a trio of mining entrepreneurs for $70,000. The Homestake mine stayed open until 2001 and was the second largest gold producer in the United States and was once the largest and deepest gold mine in North America. The mine produced 39.8 million ounces of gold for more than a billion dollars. The mine then closed in 2002. Currently the mine is now home to the Sanford Underground Laboratory which is conducting studies in Neutrinos.
Geologic History- Sometime between 30-60 billion years ago, rocks beneath the northern Black Hills were subjected to increasing temperatures sufficient enough to melt them deep below the subsurface. This heat generated molten igneous masses that intruded upward, forcibly shoving apart the rock and making numerous dikes as well as forcibly injecting beds to form igneous masses like nearby Terry Peak.
The oldest dated granites in the Lead area have been dated back 1.7 billion years old. It is reasoned to believe that the rocks in the bottom of the Open Cut may be 2 billion years old. These older rocks which the granites intruded were shales, silts and sands deposited in vast seas in which 30 thousand feet of sedimentary rock accumulated. These very ancient rocks are termed Precambrian in age, meaning they formed prior to the time when fossils first began to appear in the geologic record of rock formations. Closer inspection of these rocks reveal they have undergone extensive physic-chemical reconstitution, meaning there original mineralogical, chemical and physical makeup have changed. New complex minerals developed from older simpler minerals through a process called metamorphism, caused by great pressure and temperature changes. Temperatures were in upwards in the range of 400 degrees centigrade. Pressures of such magnitude were maintained over great periods of time to change hundreds of cubic miles of solid rock. The history can be readily seen in the Open Cut for it is revealed in the open strata of the rocks.
Quartz veins are the most prominent features of mineralization. They are of all sizes and shapes and there are often large numbers of these veins in the ore bodies. Usually near the quartz veins will contain gold.
Open Cut-The coordinates will take you to an observation deck overlooking the Homestake Open Cut. Here you will view the spectacular presence of the mine. The observation deck is free of charge. There are tours available, for a nominal fee.
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1. What are the 3 main prospective minerals taken from the Homestake Open Cut?
2. Please describe what you see from the observation deck.