DESCRIPTIVE MODEL OF VOLCANOGENIC Mn
MODEL 24c (Superceeded by descriptive models 24c1-4)
By Randolph A. Koski
APPROXIMATE SYNONYM Volcanogenic-sedimentary (Roy, 1981)
DESCRIPTION Lenses and stratiform bodies of manganese oxide, carbonate, and silicate in volcanic sedimentary sequences. Genesis related to volcanic (volcanogenic) processes.
GENERAL REFERENCE Roy (1981).
GEOLOGICAL ENVIRONMENT
Rock Types Chert, shale, graywacke, tuff, basalt; chert, jasper, basalt (ophiolite); basalt, andesite, rhyolite (island-arc); basalt, limestone; conglomerate, sandstone, tuff, gypsum.
Age Range Cambrian to Pliocene.
Depositional Environment Sea-floor hot spring, generally deep water; some shallow water marine; some may be enclosed basin.
Tectonic Setting(s) Oceanic ridge, marginal basin, island arc, young rifted basin; all can be considered eugeosynclinal.
Associated Deposit Types Kuroko massive sulfide deposits.
DEPOSIT DESCRIPTION
Mineralogy Rhodochrosite, Mn-calcite, braunite, hausmannite, bementite, neotocite, alleghenyite, spessartine, rhodonite, Mn-opal, manganite, pyrolusite, coronadite, cryptomelane, hollandite, todorokite, amorphous Mn02.
Texture/Structure Fine-grained massive crystalline aggregates, botryoidal, colloform in bedded and lensoid masses.
Alteration Spilitic or greenschist-facies alteration of associated mafic lavas, silicification, hematitization.
Ore Controls Sufficient structure and porosity to permit subsea-floor hydrothermal circulation and sea-floor venting; redox boundary at seafloor-seawater interface around hot spring; supergene enrichment to upgrade Mn content.
Weathering Strong development of secondary Mn oxides (todorokite, birnessite, pyrolusite, amorphous Mn02) at the surface and along fractures.
Geochemical Signature Although Mn is only moderately mobile and relatively abundant in most rocks, Mn minerals may incorporate many other trace elements such as Zn, Pb, Cu, and Ba.
Examples
Olympic Peninsula, USWA (Park, 1942, 1946; Sorem and Gunn, 1967)
Franciscan type, USCA, USOR (Taliaferro and Hudson, 1943;