Microbanded manganese formations; protoliths in the Franciscan Complex, California

Abstract

The Buckeye manganese deposit, 93 km southeast of San Francisco in the California Coast Ranges, preserves a geologic history that provides clues to the origin of numerous lenses of manganese carbonate, oxides, and silicates that occur with interbedded radiolarian chert and metashale of the Franciscan Complex. Compositionally and mineralogically laminated Mn-rich protoliths were deformed and dismembered, in a manner that mimics in smaller scale the deformation of the host complex, and then were incipiently metamorphosed at blueschistfacies conditions. Eight phases occur as almost monomineralic protoliths and mixtures: rhodochrosite, caryopilite, chlorite, gageite, taneyamalite, braunite, hausmannite, and laminated chert (quartz). Braunite, gageite, and some chlorite and caryopilite layers were deposited as gel-like materials; rhodochrosite, most caryopilite, and at least some hausmannite layers as lutites; and the chert as turbidites of radiolarian sand. Some gel-like materials are now preserved as transparent, sensibly isotropic relics of materials that fractured or shattered when deformed, creating curved surfaces. In contrast, the micrites flowed between the fragments of gel-like materials. The orebody and most of its constituent minerals have unusually Mn-rich compositions that are described by the system MnO-Si02-02-C02-H20. High values of Mn/Fe and U/Th, and low concentrations of Co, Cu, and Ni, distinguish the Buckeye deposit from many high-temperature hydrothermal deposits and hydrogenous or diagenetic manganese and ferromanganese nodules and pavements. This chemical signature suggests that ore deposition was related to fluids from the sediment column and seawater. Tungsten is associated exclusively with gageite, in concentrations as high as 80 parts per million. The source of the manganese is unknown; because basalts do not occur near the deposit, it was probably manganese leached from the sediment column by reducing solutions. Low concentrations of calcium (CaO approximately 0.6 weight percent) suggest that the host sediments formed beneath the carbonate-compensation depth. The most probable cause of the microbanding is changing proportions of chemical fluxes supplied to the sediment-seawater interface. The principal fluxes were biogenic silica from the water column, carbon dioxide from organic matter in the sediment column, 02 and other seawater constituents, and Mn+2-bearing fluid. The presence of A1203 and Ti02 (supplied by a detrital flux) in the metashale but not the ore lens suggests rapid ore deposition. Material supply-rate changes were probably due to a complex combination of episodic variations in the hydrothermal flux and periodic flows of radiolarian sand (silica and C02 fluxes) that may be related to climate variations.

Mineralogical and Oxygen Isotopic Constraints on the Origin of the Contact-Metamorphosed Bedded Manganese Deposit at Nagasawa, Japan

“…Minerals containing B or W have been reported from several bedded manganese deposits in Japan, such as hübnerite, tinzenite, and wiserite (e.g., Watanabe et al 1970), although no such minerals have been recognized in this study; the reasons for their absence remain unclear. Huebner & Flohr (1990) showed a positive correlation between tungsten contents and modal proportion of gageite in the cherthosted manganese ores of the Franciscan Complex. The absence of tungsten in the Nagasawa ores may imply the absence of specific precursor minerals such as gageite.…”

“…Momoi et al (1988) identified clay minerals enclosing spherulitic rhodochrosite from the Komaoka mine, Japan, as interlayered chloritesaponite. Huebner & Flohr (1990) The formation of pyroxmangite continued until rhodochrosite was completely consumed. The sequence of reactions is consistent with the petrographic observation that pyroxmangite veins occasionally fill the interstices of garnet-1 crystals (Fig.…”

“…For the chert-hosted bedded manganese deposits in other areas, no analyses have been made of chlorite or pyrite fractions in the rhodochrosite-rich ores except several bulk-rock chemical compositions (e.g., Matsumoto 1987, Hein et al 1987, Huebner & Flohr 1990, Sugisaki et al 1991. Sugisaki et al (1991) analyzed (X-ray fluorescence) bulk-rock samples of the manganese carbonate ores, the "manganese bands", composed of rhodochrosite spherules of up to several mm in size in a siliceous or argillaceous matrix, from the Mino Belt.…”

“…Similar manganese deposits are distributed in marine volcanosedimentary sequences, e.g., in the Franciscan Complex in California (e.g., Hein et al 1987, Hein & Koski 1987, Huebner & Flohr 1990, in the Gambatesa district in eastern Liguria (Marescotti & Frezzotti 2000), in the Proterozoic Vittinki Group in Finland (Mancini et al 2000), and at Val Ferrera in the eastern Swiss Alps (Brugger & Gieré 2000). Many of these deposits have been subjected to metamorphism under various conditions of temperature and pressure.…”

Mineralogy, paragenesis and genesis of the braunite deposits of the Mary Valley Manganese Belt, Queensland, Australia

“…The occurrence of native copper in the Skyring Creek material is interesting, as native copper occurs in the Blue Jay Mine, Franciscan Complex, California, in association with hausmannite, braunite and neotocite (Huebner and Flohr 1990). These investigators suggest that this deposit is a product of sea floor volcanism.…”