Quantitative geochemical comparisons show that the source material and rate of deposition of the manganese ore and associated Cambrian rocks of the Harlech dome area of north Wales were the same as those for the enclosing shales. Manganese enrichment is considered due to intensive weathering conditions on land and contemporaneous strong evaporation of the surface waters of the depositional basin, which together resulted in precipitation of manganese carbonate plus silica in a temporarily isolated body of sea water.

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