Emplacement and diagenesis of gypsum and anhydrite in the late Permian Raisby Formation, north-east England

Abstract

Cavities, pores and crystal pseudomorphs which are lined or filled by calcite are ubiquitous in Raisby Formation dolostones at outcrop in north-east England. Their distribution in most exposures is random on a metre scale. On a smaller scale, cavities, pores and crystal pseudomorphs are most numerous in bioclastic and resedimented beds. Deep borehole cores of the Raisby Formation contain nodules and euhedral crystals of gypsum and anhydrite of very similar size and shape to the cavities and pseudomorphs. This indicates that calcium sulphates were once abundant in the Raisby Formation at outcrop, but almost all have been removed subsequently.Calcium sulphate minerals were formed early in the diagenetic history of the Raisby Formation, predominantly by selective replacement of carbonate rocks. Gypsum was precipitated first, probably as a by-product of dolomitization, by brines refluxing basinwards through the Raisby Formation from the hypersaline Zechstein Sea during deposition of the Edlington Formation and the Hartlepool Anhydrite Formation. Later, replacive anhydrite formed following both pressure-solution and dehydration of the gypsum. During further burial, some anhydrite was replaced by barite, dolomite and sphalerite. Upon uplift of the Raisby Formation during the Tertiary, anhydrite rehydrated to porphyroblastic gypsum and this was then dissolved by meteoric groundwaters. The resultant pores were lined or filled by two generations of calcite and also, locally, by kaolinite. The earlier calcite generation is luminescent and was precipitated partly as a passive cement and partly by replacement of surviving gypsum and anhydrite, a reaction thought to have been mediated by sulphate-reducing bacteria. The later calcite generation is predominantly non-luminescent and was precipitated from near-surface meteoric groundwaters.

Mineralogy and Petrography of Fatha gypsum rocks in Zurbatiyah area, eastern Iraq

“…The crystal shapes within spherulitic pseudomorphs indicate formation as radiating clusters of platy crystals which were probably originally anhydrite. The tabular euhedral pseudomorphs initially crystallized as gypsum, whereas the acicular single crystal pseudomorphs have a shape which is most closely comparable with that of anhydrite [27]. The primary gypsum directly pricipitate from brines and preserve the original texture and have not any anhydrite relics [28] ( Dronkert,1985) whereas the secondary gypsum formed due to the replacement of a precursor anhydrite or has been occurred during diagenesis [18].Two basic properties are considered by the present study in order to classify the evaporitic rocks, the structural terms are based on variations in external features of the evaporitic masses, while the external terms are based on the optical mineralogy and details of crystal fabric within the evaporitic masses.…”