Paleozoic and Mesozoic silica-rich seawater; evidence from hematitic chert (jasper) deposits

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doi: 10.1130/0091-7613(2003)031<0319:PAMSRS>2.0.CO;2
Authors:Grenne, Tor; Slack, John F.
Author Affiliations:Primary:
Geological Survey of Norway, Trondheim, Norway
U. S. Geological Survey, United States
Volume Title:Geology (Boulder)
Source:Geology (Boulder), 31(4), p.319-322. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0091-7613
Publication Date:2003
Note:In English. 33 refs.; illus.
Summary:Laterally extensive beds of highly siliceous, hematitic chert (jasper) are associated with many volcanogenic massive sulfide (VMS) deposits of Late Cambrian to Early Cretaceous age, yet are unknown in analogous younger (including modern) settings. Textural studies suggest that VMS-related jaspers in the Ordovician Lokken ophiolite of Norway were originally deposited as Si- and Fe-rich gels that precipitated from hydrothermal plumes as colloidal silica and iron-oxyhydroxide particles. Rare earth element patterns and Ge/Si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 103 to 104, similar to modern examples. We propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids-as suggested by previous workers-but instead was deposited from silica-rich seawater. Flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume. A model involving amorphous silica (opal-A) precursors to the jaspers suggests that silica contents of Cambrian-Early Cretaceous oceans were at least 110 mg/L SiO2, compared to values of 40-60 mg/L SiO2 estimated in other studies. The evolution of ancient silica-rich to modern Fe-rich precipitates in submarine-hydrothermal plumes reflects a changeover from silica-saturated to silica-depleted seawater through Phanerozoic time, due mainly to ocean-wide emergence of diatoms in the Cretaceous.
Subjects:Algae; Amorphous materials; Biogenic processes; Chemical ratios; Chemically precipitated rocks; Chert; Colloidal materials; Cretaceous; Diatoms; Framework silicates; Gels; Geochemistry; Germanium; Hematite; Hydrothermal conditions; Jasper; Marine environment; Massive deposits; Massive sulfide deposits; Mesozoic; Metal ores; Metallogeny; Metals; Microfossils; Mineral deposits, genesis; Opal; Opal-A; Ophiolite; Ordovician; Oxides; Paleo-oceanography; Paleoecology; Paleozoic; Plantae; Plumes; Precipitation; Rare earths; Sedimentary rocks; Silica; Silica minerals; Silicates; Siliceous composition; Silicon; Submarine environment; Europe; Norway; Scandinavia; Western Europe; Lokken Norway
Coordinates:N630600 N630600 E0094300 E0094300
Abstract Numbers:03M/4323
Record ID:2003032325
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
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