Abstract: Coarse White Sparry Dolomite and Calcite Cements--Indicators of Former Sustained Porosity-Permeability Trends in Mississippi Valley Type Ore-Host Dolostones

Frank W. Beales, Cynthia R. Coron, Jenna L. Hardy, Francis T. Manns

Coarse white sparry dolomite and, to a lesser extent, calcite, commonly occlude regionally important, carbonate rock porosity systems. Collapse breccias with or without fine rock matrix and basal trash zones; fluxobreccias, i.e., those involving lateral translation of breccia clasts; and pseudobreccias which retain the original rock-fabric orientation are all characterized by an earlier generation of neomorphic or paramorphic dolostone clasts, or remnants, in a sea of coarse white sparry dolomite. Centripetal-layered crystal growth of carbonate and metallic sulfide minerals attests to the filling of open space. In addition, interstitial bitumen and less commonly quartz, bitumen, gypsum, sulfur, and color zonations and coatings of euhedral crystals indicate episodic cement tion.

Former porosity, superbly delineated by white sparry dolomite, was present within the host rocks of many Mississippi Valley type ore bodies and was probably paleokarstic in origin, possibly initiated by the solution of interbedded evaporites. Breccia-moldic and breccia-interstitial porosity, commonly involving keystone-supported cavities and cavern roof arches, certainly provided highly permeable, late diagenetic, fluid escape routes prior to the constriction of open throats by carbonate and sulfide cementation. Final occlusion of open space by coarse mosaic calcite is common. Rebrecciated blocks of breccia may be present.

Major lead-zinc ore bodies indicate long-sustained permeability, vast volumes of fluid migrating, and probably complex fluid-mixing phenomena. Although lead-zinc ores commonly are associated with some traces of bitumen, why has oil-field development drilling yet to encounter a major ore body?

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC