Genesis of the Cambrian Manganese Carbonate Rocks of North Wales

Abstract

ABSTRACT Quantitative geochemical comparisons show that the manganese ore and associated rocks of the Cambrian Harlech Series were formed from the same materials as supplied the enclosing shales, except that these materials were more intensely broken down and were diluted with large quantities of manganese carbonate. The rate of deposition of the ore sediment was of the same order as that of the enclosing shales, despite the cessation of detrital-hydrolysate sedimentation. The sudden influx of such vast quantities of manganese was not accompanied by even partially comparable enrichment of any other element whatsoever. Enrichment of manganese relative to iron must have occurred at, if not within, the source rocks and was not due to separation during transport. The coincidence of manganese conce tration with cessation of detrital influx suggests an origin owing to intensive weathering conditions on land, these same conditions causing strong evaporation from the surface waters of the depositional basin and thus leading to precipitation of manganese carbonate and silica in a temporarily isolated body of sea-water.