CONTRASTING MINERALOGICAL-GEOCHEMICAL SIGNATURES OF MANGANESE OXIDES - GUIDES TO METALLOGENESIS

Manganese oxides have a strong sorption capacity for cationic species in the depositional fluid. This sorption capacity is reflected in the chemistry of the oxides, which is different from that for the oxides precipitated from fresh water, seawater, and hydrothermal fluids. Characteristic mineralogical-geochemical enrichment and geochemical association (defined statistically) signatures, in conjunction with several diagnostic plots, can be employed to distinguish between the following different genetic types of manganese deposits: supergene hydrothermal; supergene marine terrestrial; supergene dubhite; and supergene marine hydrothermal sedimentary exhalative (sedex). "Dubhites" are manganese oxides derived from the weathering of a mineralized sequence (i.e., base and precious ore minerals and gangue). Hydrothermal deposits characteristically show geochemical enrichments in the assemblage As-Ba-Cu-Li-Mo-Pb-Sb-Sr-V-Zn and an Mn-As geochemical association. Supergene marine deposits show Na-K-Ca-Mg-Sr and Co-Cu-Ni geochemical enrichments, whereas supergene terrestrial deposits tend to have very high Ba contents and an Mn-Ba association. Dubhites contain high levels of Pb and Zn, show an Mn-Pb-base metal association, and are composed of significant quantities of base metal-bearing manganese oxides. Bixbyite, braunite, hausmannite, hubnerite, jacobsite, and pyrochroite occur predominantly only in hydrothermal deposits, whereas chalcophanite, coronadite, crednerite, delta-MnO2, groutite, hollandite, lithiophorite, manganite, nsutite, quenselite, ramsdellite, romanechite, todorokite, and woodruffite are generally of supergene origin. In addition to established plots presently used to identify marine sedimentary exhalative deposits, the following diagnostic graphs were devised: Na vs. Mg distinguishes deposits of marine and fresh water origin, Co + Ni vs. As + Cu + Mo + Pb + V + Zn discriminates between hydrothermal and supergene oxides, and Pb vs. Zn can be used to recognize dubhites and therefore prospective sequences.