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.
The features that crisscross the surface of the sandstone pictured above are deformation bands. They are micro faults that form where the bedrock is under strain. They typically develop near larger faults and the orientation of the bands is determined by the orientation of the stresses acting on the rock.
The grains along deformation bands have been crushed, rotated, and reorganized. The resultant bands are harder and less permeable than the rock they formed in, which causes them to stand in relief when the rock weathers.
Geologists look for deformation bands as indicators that a fault may be nearby. This photo was taken in Van Buren County, Arkansas on the downthrown block of a normal fault.