The San Francisco manganese deposit is a well-stratified mass of manganese oxides and iron oxides enclosed in a continental sequence of Tertiary volcanic rocks. The two oxides form irregular layers and lenses within the deposit and are clearly separated from each other: iron oxide layers predominate to the northwest, and manganese oxide layers and lenses to the southeast.The deposit's conformity with the surrounding tuffs and its textural and compositional stratification point to a syngenetic sedimentary origin. Ripple marks and cross-bedding in underlying sandstones and shales and an abundance of iron oxides in the adjacent tuffs indicate deposition in shallow water under oxidizing conditions. Empirical data from present-day environments and the mineralogic composition of the deposit give a probable range of Eh-pH for its formation and point to the mechanisms involved in the separation of iron and manganese in the depositional basin.The purity of the oxides, the large enrichment of manganese relative to iron as compared to crustal averages, and the trace element composition of the oxides suggest volcanic solutions as a source of the two metals.

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