The San Francisco manganese deposit, located in western central Mexico in a Tertiary, continental-volcanic province, is composed of Mn oxides and Fe oxides. The small geographic extent of the tuffs which enclose the oxides, and their sorting, rounding of constituents, and distinct stratification, point to deposition in a small lacustrine basin. The stratigraphic conformity of the oxides with the tuffs and their purity and zoning pattern suggest precipitation as a chemical sediment from volcanic solutions, at a time of decreased volcaniclastic contributions. As the mineralizing solutions entered the depositional basin, the two oxides precipitated separately, Fe oxides closer to the source and Mn oxides farther from it, in response to a gradual increase in pH and Eh of the solutions through mixing with alkaline, oxygenated waters in the depositional basin.Spectrographic analyses for minor and trace elements reveal that both the Mn oxides and the Fe oxides in the San Francisco deposit are strongly enriched in As, Ba, Cu, Mo, Pb, Sr, and V compared to crustal averages. Except for V, the concentrations of these elements are higher in the Mn oxides than in the Fe oxides by factors ranging from 3 to 140. Thus, the Mn oxides reflect the composition of the mineralizing solutions much more strongly than the Fe oxides. Mineralogy and solubility calculations suggest that the minor elements were removed from solution by adsorption rather than by precipitation in response to saturation. Since both oxides when freshly precipitated adsorb minor elements strongly, the distinct partition is interpreted as a result of diagenetic recrystallization processes. During recrystallization, Ba, Sr, and Pb adsorbed on precipitating hydrous Mn oxides form manganates of the cryptomelane-hollandite group, and Cu can substitute for Mn in the lattice of mixed valence Mn oxides. These elements would be desorbed from recrystallizing Fe oxides because of their inability to form minerals with trivalent Fe or to substitute for Fe in the lattice of its oxides. Depending on the geochemical characteristics of the depositional basin, they would form sulfates or carbonates in situ, be built into spatially associated Mn oxides, or reprecipitate in genetically related sulfidic sediments. The less distinctly partitioned minor elements As, Mo, and V can be accommodated in both Mn and Fe oxides, by formation of arsenates, molybdates, and vanadates or by substitution for either Mn or Fe in their oxides.High concentrations of the minor elements which characterize the San Francisco deposit have been reported from other Mn and Fe oxide deposits of volcanic association. The presence or unusual concentration of these elements in Mn and Fe oxides, or in genetically related sulfates, carbonates, or sulfides, is supporting evidence for volcanic contributions to their formation.

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