Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas. The brown mineral is sphalerite: an ore of zinc. The pink mineral is dolomite – it’s pretty, but not economically valuable. They were both deposited on the gray dolostone; you can just make it out on the right, in back.
Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena. They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together. That area was mined for lead and zinc in the late 1800s and early 1900s.
It’s typical to find rich ore deposits in rock that’s been fractured by faulting. The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures. It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.
Dolomite (pink) and Sphalerite (brown) In Dolostone
Dolomite and sphalerite are two minerals present in limestone and dolostone in the lead and zinc districts of north Arkansas. Dolomite commonly occurs with the sphalerite, however it is not an ore mineral and is considered worthless. Sphalerite is the primary ore of zinc. Zinc was mined in the lower end of the Buffalo National River in the late 1800s and early 1900s. One of the largest mining communities was located at Rush, Arkansas. Zinc is used as a coating of iron or steel to protect it from corrosion. It is also used in batteries, small non-structural castings, and alloys, such as brass. This mineralization is present in the Everton Formation. It is thought that migration of warm mineral-rich fluids expelled by the pressure of the mountain building event that caused the Ouachita Mountains is responsible for the mineralization in northern Arkansas. Note the brecciated texture (angular fragments) of the rock. Open spaces, called cavities, in the rock caused the overlying rock to collapse, and break into angular pieces. Mineralized water then flowed around the broken pieces and the dolomite and sphalerite precipitated in the open spaces.