Crerar et al.(1982)による〔『Manganiferous cherts of the Franciscan assemblage: I. General geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers』(519p)から〕


There are several hundred ophiolitic manganiferous chert deposits, primarily of late Jurassic to early Cretaceous age, known within the Franciscan assemblages 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 87Sr/86Sr 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 interation of seawater 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゜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.』

Regional geology

Origin of the Franciscan assemblage
Geology of the manganese deposits
The Blue Jay mine
The South Thomas mine
Petrography of the cherts
Chemical stratigraphy

Tests for a hydrothermal sea-floor origin
Chemical trends and processes
Rare earth element distributions
Strontium isotopes
Basalt chemistry
Fluid inclusion analyses
Discussion: ancient and modern analogues and submarine hydrothermal activity
Genetic model for the Franciscan deposits

Paleotectonic setting