There are several hundred ophiolitic manganiferous chert deposits, primarily of late Jurassic to early Cretaceous age, known within the Franciscan assemblage of California. The sequences typically consist of one to three massive, manganiferous chert lenses containing 30 to 50 percent Mn, and averaging 1 m in thickness by 15 m subcircular diameter; these are separated by an average 2 to 10 m of thin-bedded radiolarian cherts and overlie basalts or greenstones. Both their geology and chemistry indicate that the ore lenses are hydrothermal and may have formed on the flanks of a mid-ocean ridge or within a back-arc basin. It is proposed that the sequences developed as a result of sea-floor spreading over a series of deep hydrothermal seawater convection cells paralleling a spreading center and spaced roughly 5 to 10 km apart. Chemical profiles of Mn, Fe, Si, Al, Cu, Ni, Zn, Co, Ba, Ti, the rare earth elements, and 87 Sr/ 86 Sr have been determined through two complete sections. These profiles indicate hydrothermal input of Mn, Si, Cu, Ni, Zn, and Ba and detrital or hydrogenous input of Al and Co; they illustrate the use of Ti as a measure of relative detrital sedimentation rates. Fe is strongly fractionated from Mn within the ores (Fe/Mn < 0.1), and Fe/Mn ratios decrease upward throughout each section suggesting preferential deposition of Fe within the sediment, and of Mn at the seawater interface. Rare earth element distributions reflect the interaction of sea-water and underlying basalts. Sr isotopic ratios of the ores and basalts demonstrate both strong and moderate seawater influences, respectively. Fluid inclusion analyses on veins of undetermined age show seawater salinity, temperatures of roughly 200 degrees C, and tentative entrapment pressures corresponding to 1,700-m water depth. Early and intermediate veins were injected into unconsolidated siliceous sediment producing a characteristic bleached and pseudobrecciated texture. An analogy is drawn with the present-day field of hydrothermal mounds near the Galapagos rift and with ophiolitic complexes of the northern Apennines and other localities.

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