NORTHWESTERN OHIO : CALCITE , DOLOMITE AND FLUORITE 1
Calcite is by far the most abundant mineral in the Devonian rocks at Pugh Quarry. The large crystals, from 5 mm to 10-15 cm long, are predominantly brown with wide variations in hue and intensity. The small crystals, smaller than 5 mm, range from colorless to gray-yellow to pale yellow. Both large and small crystals consist almost entirely of scalenohedral forms (dog-tooth spar habit). The most common and most easily recognized type of twinning in the large crystals is on the (0001) plane. Both dolomite and fluorite are minor minerals at Pugh Quarry, although fiuorite is found in all parts of the Quarry. Dolomite crystals occur as secondary encrusting material and are most common in vugs. OHIO J. SCI. 79(1): 24, 1979 In this third paper on the mineralogy of Pugh Quarry, calcite, dolomite and fluorite are described. The quarry is located in the SW3^, SW^, Sec. 6, T.4N, R.9E in Milton Township, Wood Co., Ohio. Its sulfide mineralization, including marcasite, pyrite, and sphalerite has been previously described (Parr and Change 1977, 1978).
BLUE JOHN FLUORSPAR
Summary The paper presents an account of the mineralogy, geological occurrence and history of the blue and white banded fluorspar found near Castleton in Derbyshire. Blue John is shown to be a fluorspar, coloured by bituminous inclusions, deposited as cavity-linings and replacements in Carboniferous Limestone of reef and massif facies. Alternative hypotheses of the cause of the colouring in Blue John are discussed. Lying more or less parallel to the bedding of the limestones, the deposits are in pipe-veins rising up the flanks of a pseudo-anticlinal structure formerly covered by impervious Namurian shales, which now outcrop only round the flanks of Treak Cliff. The relationships between Blue John and the associated minerals (calcite, barytes, galena, pyrites and quartz) are described, and an account is given of the history of the use of Blue John and the methods of manufacturing ornaments. Reasons are given for the conclusion that the story of the use of Blue John by the Romans is without foundation.
An aspect of sedimentary basin evolution; the concentration of Mississippi Valley-type ores during late stages of diagenesis
ABSTRACT During the normal evolution of a sedimentary basin the chemistry of escaping compaction-generated fluid will evolve through time. Gas and oil pools are associated with early diagenetic processes and metallic mineralization with later diagenetic and metamorphic processes. The principles which govern the localization of many ore bodies are sedimentary principles and the petroleum geologist should include the search for ore deposits in his normal exploration practise. Specifically, lead-zinc deposits of Mississippi Valley type are concentrated in sedimentary carbonate rocks because hydrogen sulphide tends to form and accumulate in carbonate reservoirs, and it subsequently acts as a precipitant of metallic sulphides. Ore bodies are found where regional permeable porous trends, acting as a "plumbing system" for escaping basinal saline connate waters that carry the ore metals, traverse local H2S-bearing carbonate reservoirs. Geochemical data are consistent with derivation of the ore metals from the sedimentary basins. Base-metal ions may be sparsely disseminated in argillaceous sediments, or deposited as sulphides in concentrations approaching ore tenor in black shale. Carbonate rocks in general have low concentrations of base metals. The metals can be derived from the sediments by desorption probably mainly from clays, or organic material and other sediments. Burial to critical depths will control the reorganization of clay minerals, the concentration of brines, the release of metals, the breakdown of organic complexes, and the expulsion of connate fluids. These expelled fluids will be migrating from deeper source areas towards the surface and will in consequence be a variety of "hydrothermal" solution to some geologists. However, no essential addition of juvenile waters is considered necessary to the system. As most authors appear to regard a magmatic association as implicit in the term "hydrothermal", the fluids referred to here should perhaps be designated as "stratafugic" to emphasize their distinction and to avoid further compromising the former term. In contrast to shale-sandstone associations, porosity in carbonate rocks commonly harbours hydrogen sulphide which, on compaction, also must escape with connate fluids. Given a suitable "plumbing system", metal-bearing fluids expelled from the basin will meet these sulphide-rich waters with resulting precipitation of metallic sulphide. The Pine Point ore field, Northwest Territories, is used as an example to demonstrate that such a hypothesis is consistent with field data, which are in part difficult to explain in any other way. This hypothesis, subject to local variations, appears to explain the basin association and the localization of many ore occurrences of Mississippi Valley type. End_Page 383------------------------