Abstract

Saline basinal fluids, at temperatures from 60 to 250 °C, have affected almost every sedimentary basin in the world including rocks from Palaeoproterozoic to Cenozoic age. These fluids commonly precipitate base-metal sulphides (pyrite, sphalerite, galena, etc.) and associated minerals (barite, fluorite, calcite, dolomite, etc.) ranging in volume from trace amounts to large economic ore deposits. Such deposits are commonly referred to as Mississippi Valley-type (MVT) after the large Palaeozoic deposits of this kind found in the Mississippi Valley of North America. They are primarily hosted by platform carbonates, typically dolomite, and are usually associated with hydrocarbons.

Dolomites not affected by mineralizing fluids commonly display micron- to decimicron-size planar textures, and have well-developed micro- and mesoporosity networks dominated by intercrystal and vug porosity. However, these and other carbonate rocks affected by basinal fluids may undergo massive geochemical and textural alteration. This occurs even when the affected rocks are distal from the main loci of sulphide mineralization. Alteration includes: dolomitization of limestone; neomorphic recrystallization of existing dolomite; and precipitation at intervals of large volumes of open-space-filling dolomite, calcite and quartz cements alternating with dissolution. Dolomitization of limestone and/or neomorphic recrystallization of dolomite, at elevated temperatures, commonly results in centimicron and larger size crystals, and development of nonplanar textures that increase pore-throat tortuosity. Open-space-filling dolomite, calcite and quartz cementation causes a dramatic reduction of porosity and blockage of pore throats. Periods of carbonate dissolution, proximal to intense sulphide mineralization, result in the development of large-scale macroporosity such as breccias that are commonly superimposed on karst and tectonic fractures.

Exposure to mineralizing basinal fluids substantially alters porosity and permeability distribution, and thus the potential reservoir properties of the dolomite. The resulting reservoir may have little resemblance to its precursor. Understanding the epigenetic history of a dolomite is critical, therefore, as this will ultimately affect its development strategy and production history.