DESCRIPTIVE MODEL OF OLYMPIC PENINSULA-TYPE VOLCANOGENIC MANGANESE.
MODEL 24 c-3
By Dan L. Mosier and Norman J Page
DESCRIPTION Tabular, comfortable lenses of manganese oxides and silicates in or between argillaceous red limestone and pillow basalt. Genesis is related to volcanogenic processes.
GENERAL REFERENCES Green (1945), Magill (1960), Cady (1975), Koski (1986).
GEOLOGIC ENVIRONMENT
Rock Types Pillow basalt and red argillaceous limestone. Associated tuff, agglomerate, argillite, and graywacke. Alkaline or tholeiitic volcanic rocks.
Textures Basalt flows have massive and pillow structures; limestone is very fine grained, argillaceous, and siliceous; argillite is calcareous and siliceous; argillite is calcareous and ferruginous.
Age Range Eocene or older.
Depositional Environment Deep to shallow marine on or near basaltic seamount or islands; sea-floor hot spring.
Tectonic Setting(s) Ocean basin with seamounts and volcanic islands (midplate setting) obducted onto a continental margin.
DEPOSIT DESCRIPTION
Mineralogy Bementite, hausmannite, neotocite are common; inesite, manganite, rhodochrosite, rhodonite, pyrolusite, manganocalcite are less abundant; braunite, wad, tephroite, manganophyllite, piedmontite, jacobsite, spessartine are minor.
Texture/Structure Finely crystalline massive lenses and layers (banding), spongy or colloform layering of hausmannite, brecciation, nearly round microstructures to oolitic structures, algal structure or fecal pellets.
Alteration Silicification, greenschist-facies metamorphism.
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 Secondary manganese oxides (pyrolusite and manganite) are formed as oxidation products of bementite or other manganese minerals; restricted to surface and along joints.
Geochemical Signatures Mn, Fe, Cu, Hg, and Ba.
EXAMPLES
Crescent mine, USWA (Magill, 1960).