Manganese deposits and the associated cherts and argillites of the Auckland area

Abstract

Manganese deposits, cherts and argillites have arisen as volcanogenic neoformations in the Permian to Jurassic New Zealand geosyncline. Released when seawater reacted with hot basaltic lava, Si, Fe, Mn, Al, Mg and Ca precipitated in the slightly alkaline environment provided by normal seawater as silica, clay, calcite, hematite, chlorite, manganese oxides and silicates. Segregation took place in these sediments during diagenesis. In silica-rich horizons the silica separated from clay giving rise to rhythmic chert-shale bedding. Manganese-rich horizons (primary mineralization) segregated into dense, lenticular beds, laminae and nodules. These formed parallel to the original stratification and are composed of concretionary oolites and microlenticules, ranging in diameter from tenths of a millimetre to 3mm. Intergrowth of oolites or the formation of a cement composed of the same primary minerals--bementite, braunite, manganite, and crytpomelane—gives crystal aggregate textures. Post-depositional movement of the spilite-chert-argillite sequences has produced folding and breakup of strata into blocks. Now horizons have little lateral continuity. Mode of deposition, rather than composition of volcanic emanations, determines chemical composition of primary bodies. The virtual absence of iron and manganese minerals in quartzveining indicates an immobility of these elements during deep burial and low-grade metamorphism. This is due to the mildly alkaline nature of the interstitial fluids from dissolved silica. Oxidized and remobilized during surface and near-surface weathering, iron and manganese precipitate as psilomelane, cryptomelane, lithiophorite, manganite, pyrolusite, nsutite and goethite (redeposited mineralization). Although more difficult to extract, stratabound primary mineralization bodies have more potential economic value than redeposited accumulations. Electromagnetic geophysical prospecting should locate manganese mineralization.