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Mid‐Proterozoic Ferruginous Conditions Reflect Postdepositional Processes
Author(s) -
Slotznick Sarah P.,
Webb Samuel M.,
Kirschvink Joseph L.,
Fischer Woodward W.
Publication year - 2019
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2018gl081496
Subject(s) - geology , diagenesis , pyrrhotite , pyrite , siderite , geochemistry , proterozoic , authigenic , sedimentary rock , metamorphic rock , facies , paleontology , structural basin , tectonics
To evaluate the mechanics of mid‐Proterozoic environmental iron transport and deposition, we coupled microscale textural and bulk rock magnetic techniques to study the ~1.4‐Ga lower Belt group, Belt Supergroup, Montana and Idaho. We identified a pyrrhotite‐siderite isograd that marks metamorphic iron‐bearing mineral reactions beginning in subgreenschist facies samples. Even in the best‐preserved parts of the basin, secondary overprints were common including recrystallization of iron‐bearing sulfides, base metal sulfides, and nanophase pyrrhotite. Despite these overprints, a record of redox chemistry was preserved in the early diagenetic framboidal pyrite and detrital iron oxides including trace nanoscale magnetite that remained after sulfidization in anoxic and sulfidic sedimentary pore fluids. Based on these results, we interpret the Belt Basin as having oxic waters, at least in shallow‐water environments, with no indication of abundant ferrous iron in the water column; this is consistent with the cooccurrence of early eukaryotic fossils within the same strata.