SYNGENETIC AND DIAGENETIC MINERALIZATION IN YORKSHIRE

Abstract

Summary The bedded iron ores, sulphide-bearing black shales and evaporites of Yorkshire are described. The presence of abundant decaying vegetable or animal matter is held to be fundamental to the formation of both the clayironstones of the Coal Measures and the sideritic chamosite-oolites of the Jurassic, to provide the required conditions of low pH, low pH and available CO 2 . Oxide, silicate-carbonate, carbonate and sulphide facies are recognized in the Cleveland ores and associated rocks, and the primary chamosite ooliths are held to have been formed in the region between the carbonate and oxide zones rather than near the sulphide facies. It is noted that Al and Si in solution are necessary, in addition to Fe++, for the formation of the silicate ores. Hypotheses, including that of Borchert, for the transportation and precipitation of the iron are considered, and an attempt is made to assess the role of diagenesis in this type of mineral concentration. A consideration of the zonal distribution of minerals within the Permian evaporites leads to a suggestion that a straits or submerged bar lay to the north-west between the Zechstein Sea and the open ocean. The zones tended to migrate towards the margin of the basin as deposition proceeded. Boundaries between the various members of the evaporite sequences are considered to be markedly diachronous. Emphasis is laid upon the massive and complex effects of later, probably diagenetic, changes under the influence of migrant concentrated brines.