The manganese deposits occur in two lower Cretaceous formations. The lower formation is marine, with andesite flows and contains one manganiferous zone; the upper formation is continental, composed largely of clastic material, and contains two manganese zones. Braunite is the predominant ore mineral, with pyrolusite, manganite, psilomelane, and hausmannite less abundant. The manganese is considered a product of submarine volcanism, concentrated and deposited in lagoons.

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