Manganiferous cherts of the Franciscan assemblage: I. general geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers

David A. Crerar, Jay Namson, Michael So Chyi, Loretta Williams, Mark Feigenson

Research output: Contribution to journalArticle

127 Citations (Scopus)

Abstract

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 8'Sr/ 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 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°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 pseudobrec- ciated 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.

Original languageEnglish (US)
Pages (from-to)519-540
Number of pages22
JournalEconomic Geology
Volume77
Issue number3
DOIs
StatePublished - Jan 5 1982

Fingerprint

spreading center
Geology
geology
Seawater
basalt
convection
minerals
analogs
Ores
seawater
veins
sediments
rare earth elements
Rare earth elements
lenses
sea floor spreading
convection cells
Lenses
chert
Sediments

All Science Journal Classification (ASJC) codes

  • Engineering(all)
  • Economic Geology
  • Geochemistry and Petrology
  • Geology
  • Geophysics

Cite this

@article{e84d18902d964afda80af3c3ad810ad5,
title = "Manganiferous cherts of the Franciscan assemblage: I. general geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers",
abstract = "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 8'Sr/ 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 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°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 pseudobrec- ciated 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.",
author = "Crerar, {David A.} and Jay Namson and Chyi, {Michael So} and Loretta Williams and Mark Feigenson",
year = "1982",
month = "1",
day = "5",
doi = "https://doi.org/10.2113/gsecongeo.77.3.519",
language = "English (US)",
volume = "77",
pages = "519--540",
journal = "Economic Geology",
issn = "0361-0128",
publisher = "Society of Economic Geologists, Inc",
number = "3",

}

Manganiferous cherts of the Franciscan assemblage : I. general geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers. / Crerar, David A.; Namson, Jay; Chyi, Michael So; Williams, Loretta; Feigenson, Mark.

In: Economic Geology, Vol. 77, No. 3, 05.01.1982, p. 519-540.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Manganiferous cherts of the Franciscan assemblage

T2 - I. general geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers

AU - Crerar, David A.

AU - Namson, Jay

AU - Chyi, Michael So

AU - Williams, Loretta

AU - Feigenson, Mark

PY - 1982/1/5

Y1 - 1982/1/5

N2 - 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 8'Sr/ 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 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°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 pseudobrec- ciated 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.

AB - 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 8'Sr/ 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 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°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 pseudobrec- ciated 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.

UR - http://www.scopus.com/inward/record.url?scp=0020125565&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0020125565&partnerID=8YFLogxK

U2 - https://doi.org/10.2113/gsecongeo.77.3.519

DO - https://doi.org/10.2113/gsecongeo.77.3.519

M3 - Article

VL - 77

SP - 519

EP - 540

JO - Economic Geology

JF - Economic Geology

SN - 0361-0128

IS - 3

ER -