Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga Gunflint Iron Formation; implications for the depositional setting, and the effects of diagenesis and metamorphism

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doi: 10.1016/0301-9268(92)90061-R
Authors:Winter, Bryce L.; Knauth, L. Paul
Author Affiliations:Primary:
Arizona State University, Department of Geology, Tempe, AZ, United States
Volume Title:Precambrian Research
Source:Precambrian Research, 59(3-4), p.283-313. Publisher: Elsevier, Amsterdam, International. ISSN: 0301-9268
Publication Date:1992
Note:In English. 76 refs.; illus. incl. strat. cols., 5 tables, sketch map
Summary:Isotopic studies of 97 samples of chert from a approx 220 km transect through the Gunflint iron formation, which represents a variety of depositional environments, display a narrow delta18O range from 21.3-24.7 per mille ; there is no delta18O variation related to location or chert type. It seems that the isotopic compositions of these cherts have not been affected by metamorphism, nor does this appear to be the cause either of the narrow delta18O range or the relatively low delta18O values of the Gunflint cherts as compared to Phanerozoic cherts. It appears that silicification and lithification occurred very close to the sediment-water interface. Although some silica may have initially precipitated as hydrous silica, the small delta18O range suggests that any conversion to quartz chert must have occurred during early diagnesis, so chert delta18O values can be used to put constraints on the origin of the Gunflint iron formation. It can be concluded that all cherts precipitated over a relatively common T interval (approx 20 degrees C) from a common parent water with relatively uniform delta18O. Siderite in the deep-water, banded facies of the Gunflint displays a lower range of delta13C than siderite in the more shoreward, organic-rich, shale facies. This can be explained by primary precipitation in an ocean system that was layered with respect to C isotopes, the banded facies siderite precipitating from a water mass upwelling from submarine regions onto the continental shelf, and the shale-facies siderite precipitating from a nearer shore, shallow sea-water mass. Ankerite in the Gunflint occurs as a replacement of primary siderite in the banded facies, and as a replacement of greenalite and chert in the arenite facies. It seems that shallow sea-water was a major component of the diagenetic fluid. Later neomorphism of the replacement ankerite at higher burial T resulted in a wide range of delta18O values (13.8-22.5 per mille). The unique mineralogy of the uppermost Gunflint (calcite, serpentine and magnetite) compared to the majority of the formation, the very low delta values, and overlying diabase sills suggest that the non-ferroan nature of the carbonate in the upper Gunflint is the result of contact metamorphism. [R.E.S.]
Subjects:Ankerite; C-13/C-12; Carbon; Carbonates; Chemically precipitated rocks; Chert; Deposition; Diagenesis; Gunflint Iron Formation; Isotopes; Metamorphism; O-18/O-16; Oxygen; Petrography; Precambrian; Precipitation; Sedimentary rocks; Siderite; Stable isotopes; Canada; Eastern Canada; Ontario; Animikie Basin
Abstract Numbers:93M/3160
Record ID:1993002256
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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