Abstract

Flat-lying, elongate bedding-replacement fluorspar deposits in southern Illinois commonly exhibit structural features best explained as the results of net loss of rock volume during the ore forming process.Synclines found over many deposits are regarded as the results of a thinning of limestone strata localized in the ore-bearing areas. A statistical study of test borings shows that such localized thinning is a characteristic feature of the district. Also many evidences of subsidence are apparent within the deposits, such as measurable thinning and removal of part or all of certain limestone beds, spalling and collapse of slabs of roof-rock and other layers of rock into the body of the deposits, presence of V-structures, and cavities and tabular openings at the sides and tops of deposits.The mechanism responsible for the indicated volume shrinkage is considered to be in part the stoichimetric replacement of calcite in limestone by denser fluorite, and in part the actual removal of limestone by the action of solvent mineralizing fluids.

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