Mineralogy and geochemistry of the lower and middle magbesian limestone of county Durham

Abstract

The Magnesian Limestone is of Permian age. In Durham, shelf and basin fades can be recognized within the Lower division while lagoon, reef and basin facies occur within the Middle division. Calcite, dolomite, gypsum and anhydrite form the main mineralogical components, although minor amounts of limonite, quartz, pyrite and an illitic clay mineral are common. Gypsum and anhydrite are characteristic of the two basin facies but are also associated with the shelf and lagoon facies in South-East Durham. In North-East Durham, the three shallow-water facies are predominantly dolomitic; calcite occurs most commonly as void filling, but also replaces dolomite and occurs in a "felted" texture which is believed to have been formed by replacement of a sulphate mineral. Extremes of carbonate composition are found within the two basin facies. Sulphates only occur in abundance where dolomite is the predominant carbonate. Calcite is believed to have formed at depth by the reaction of dolomite with locally derived, sulphate- rich solutions. The process might still be occurring. Differences in pore-size distribution make the shallow-water facies more porous and permeable than the basin facies. The relationship between porosity and permeability is discussed. Strontium occurs as a solid-solution impurity in dolomite and calcite and, to a much greater extent in gypsum and anhydrite. Dolomite also contains considerable manganese. During dedolomitization by the "sulphate-agent" mechanism, strontium is incorporated into, and manganese largely excluded from, the lattice of the replacement calcite. Rubidium and at least some barium, lead, boron and nickel are considered to occur as detrital constituents within the illitic clay material. Copper, in part at least, is associated with organic carbon. Zinc, lead and barium are enriched in rocks of the three shallow-water facies. The elements are considered to have been introduced into porous and permeable rocks by metal-bearing solutions. The mode of occurrence of these elements and the source of the "mineralizing" solutions is discussed.