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https://hdl.handle.net/2142/68063
Description
Title
Geology and Geochemistry of Sphalerite in Coal
Author(s)
Cobb, James Collins
Issue Date
1981
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Language
eng
Abstract
Middle Pennsylvania (Desmoinesian) coals of certain areas of the Illinois and Forest City Basins contain epigenetic sphalerite and associated minerals in veins, clastic dikes, collapse structures, and other small-scale structural features. The mineral assemblage in the sphalerite veins consists of silicates, sulfides, and carbonates. The paragenesis is kaolinite-(quartz?)-pyrite-sphalerite-pyrite-calcite.
The morphology of sphalerite vein walls indicates that banding in the coal had achieved nearly its present stage of development at the time of sphalerite deposition. Sphalerite veins postdate cleats and are often developed in cleats. Some sphalerite deposition was contemporaneous with subsidence of coal into sinkholes. Compactional features in the coal show that from 8 to 14 percent of the compaction in the coal occurred after sphalerite deposition commenced. A relationship between porosity, inherent moisture, and compaction of the host coals suggests that the sub-bituminous (B) rank had been achieved at the time of sphalerite deposition.
Homogenization temperatures for the sphalerite fluid inclusions are nearly identical for the sphalerite from the Illinois and Forest City Basins and range from 75(DEGREES) to 113(DEGREES)C. There is a moderately high correlation coefficient (r = .75) between the vitrinite reflectance of the host coals and homogenization temperatures. This data may eliminate the possibility of fluid flow initiated by thermally-produced convection and raises the possibility of convection by compaction and loading of basin sediments. This data and a paleogeothermal gradient of 2.35(DEGREES)C per 100m suggest a maximum depth of sphalerite deposition of 3.9 kilometers and a minimum depth of 2.2 kilometers.
The chemical and isotopic composition of the fluid inclusions are nearly the same for the six coal beds and two coal basins represented suggesting a similar fluid depositing the sphalerites. They are also similar to fluids observed in inclusions from nearby Mississippi Valley-type ore deposits, and bare close resemblance to basinal brines.
The occurrence of sphalerite in coal may record basinal processes that contribute to the genesis of Mississippi Valley-type ore deposits.
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