『Abstract
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.』
Introduction
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
Acknowledgments
References