Criteria for Recognition of Diverse Dolostone Types from Host Rocks for Mississippi Valley-Type Ore Deposits: ABSTRACT

Abstract

Dolostones are the favored host rocks for low-temperature lead and zinc ore deposits, probably because these dolostones formed in evaporitic environments. Sulfate concentration, providing a local sulfur source, probably enhanced the chances of ore formation. Reduction of sulfate by organic reducing agents, commonly gas or oil, is the most likely intermediate step. If this is so, the presence of the sulfide ores in apparently sulfate-free dolostones may indicate a former sulfate abundance. Orebodies are commonly associated with solution-collapse breccias, many of which are attributed to evaporite solution. The nearly complete, subsequent removal of the evaporites renders proof of the supposition difficult. In the big southeast Missouri breccia-hosted ore deposits, evidence in favor of former evaporitic conditions is conclusive. Most randomly selected dolostones have minute solid inclusions of gypsum or anhydrite that can be released by solution of the enclosing carbonate. Greatly enlarged and readily recognizable crystal aggregates can be cultured from the insoluble residues. In the Lockport Dolostone of southern Ontario, gypsiferous vugs are common. Chloride ions are so abundant in the Lockport that serious corrosion problems arise when structural steel is encased in concrete using crushed-stone aggregate derived from this source. Fluid inclusions within sphalerite in Mississippi Valley-type ore deposits typically indicate ore precipitation from strongly hypersaline and even saturated brines. Geologic evidence supports the growth of white sparry dolomite gangue contemporaneously with such sphalerite. We conclude that most ancient dolostones are formed in association with evaporitic facies and/or hypersaline brines. End_of_Article - Last_Page 416------------