DESCRIPTIVE MODEL OF FRANCISCAN-TYPE VOLCANOGENIC MANGANESE.

MODEL 24 c-1

By Dan L. Mosier and Norman J Page

DESCRIPTION Lenses and stratiform bodies of manganese oxide, carbonate, and silicate in chert associated with sedimentary and mafic volcanic rocks. Genesis related to volcanogenic processes.

GENERAL REFERENCES Trask and others (1943), Koski (1986).

GEOLOGIC ENVIRONMENT

Rock Types Chert, shale, sandstone, graywacke, jasper, tuff, basalt, and serpentine. Thin- and thick- bedded red or white chert and jasper are the predominant host rocks. Tholeiitic and alkaline volcanic rocks.

Textures White, red, brown, and green chert in thin-bedded or massive lenses, commonly with shale partings. Some of chert contains radiolarians.

Age Range Paleozoic to Jurassic.

Depositional Environment Sea-floor hot spring, deep water in a zone of oceanic upwelling at or near a continental margin.

Tectonic Setting(s) Oceanic ridges and rifted marginal basins (backarc setting) obducted onto a continental margin.

Associated Deposit Types Hot-Spring mercury, silica-carbonate mercury, podiform chromite.

DEPOSIT DESCRIPTION

Mineralogy Abundant psilomelane, pyrolusite, rhodochrosite, hausmannite, braunite, and neotocite; minor bementite, wad, rhodonite, inesite, pyrochroite, tephroite, ganophyllite, dannemorite, pyroxmangite, stilpnomelane, spessartine, pyrophanite, nsutite, manganite, cryptomelane, jacobsite, manjiroie, Mn-phlogopite, todorokite, piedmontite, hollandite, manganocalcite, birnessite, alleghanyite, galaxite, and alabandite.

Texture/Structure Fine-grained massive crystalline aggregates, botryoidal, colloform in bedded and lensoid masses; veinlets and disseminations.

Alteration Primary carbonates and silicates altered to oxides; some silicates altered to carbonates.

Ore Controls Sufficient structure and porosity to permit sub-sea-floor hydrothermal circulation and sea-floor venting; redox boundary at sea floor/seawater interface around hot springs; supergene enrichment to upgrade Mn content.

Weathering Strong development of secondary manganese oxides (psilomelane, pyrolusite, todorokite, birnessite, manganite) at the surface and along fractures.

Geochemical Signatures Mn, Fe, Cu, Hg, and Ba.

EXAMPLES

Ladd mines, USCA (Hein and others, 1987); Noda-Tamagawa, JAPN (Watanabe and others, 1970; Yamada and others, 1980).