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Abstract

Dolomites are the favored host rocks for low-temperature lead /zinc ore deposits, probably because many dolomites have formed in evaporitic environments. Association with a local sulfur source probably enhanced the chances of ore formation. Reduction of sulfate in the presence of organic matter such as gas or oil is the most likely intermediate step. If so, the presence of sulfide ores in apparently sulfate-free dolomites may indicate a former sulfate abundance. Ore bodies are commonly associated with solution-collapse breccias, which are often attributed to evaporite solution. The nearly complete subsequent removal of the evaporites makes this difficult to prove, but in the large, breccia-hosted ore deposits of southeastern Missouri, the evidence is highly suggestive of former evaporitic conditions.

A majority of randomly selected host dolomites that we have examined contain minute solid inclusions of gypsum or anhydrite. These can be released by solution of the enclosing carbonate, and greatly enlarged recognizable crystal aggregates can be cultured from the insoluble residues. In the Lockport Dolomke of southern Ontario, gypsiferous vugs are common, and chloride ions are so abundant that serious corrosion problems arise when structural steel is encased in concrete produced from crushed-stone aggregate derived from this source. Fluid inclusions within sphalerite in Mississippi Valley-type ore deposits typically indicate ore precipitation from strongly hypersaline brines. Geological evidence supports the growth of white sparry dolomite gangue contemporaneously with such sphalerite. We conclude that most ancient dolomites were formed in association with evaporitic facies and/or hypersaline brines.

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