A TEXTURAL AND MINERALOGICAL INVESTIGATION OF EARLY DIAGENETIC REACTIONS IN THE ~1.88 GA BIWABIK IRON FORMATION, MN

We present textural and mineralogical evidence for early silica cementation and diagenetic reactions preserved in samples from the near un-metamorphosed ~1.88 Ga Biwabik Iron Formation (IF). The IF is divided into four members representing repeated transgressive-regressive depositional cycles. The lowermost IF units consist predominantly of granular layers associated with high-energy sedimentary structures, interpreted as a shallow marine shelf depositional sequence. Up-section is marked by an abrupt gross-scale shift to banded units interpreted as a transition to quiescent deeper water facies, followed by a return to granular units. We performed textural and mineralogical analysis of thin sections of both granular and banded IF samples from drill core selected to document mineral replacement reactions interpreted to occur during early diagenesis and cementation. In samples of granular IF, sand-sized granules consist of microcrystalline quartz (chert), and/or radiating aggregates of Fe-silicate minerals, encased in mega-crystalline quartz cement. Iron-oxides are present in banded sections, or as inclusions in chert granules. ‘Patches’ - flat, pebble-size objects interpreted as local rip-up clasts surrounded by reaction rims - are a common feature in granular IF. One example consists of micro-crystalline quartz surrounded by a rim of carbonate, quartz, magnetite, and rare hematite. Surrounding the clast and rim are sand-sized, mega-quartz cemented chert and Fe-silicate granules. We interpret minimal mechanical compaction of granules, and the presence of large areas of mega-quartz cement, to be textural evidence for early silica cementation in granular IF. We interpret the minerals surrounding the chert clast (‘patch’) to represent a diagenetic reaction rim, providing evidence for replacement of primary micro-crystalline quartz and/or Fe-silicates by carbonate ± magnetite and/or hematite. This work is a part of a larger project aimed at using petrography and geochemistry to identify primary mineral phases in Precambrian IF. As chemical sedimentary rocks, the primary minerals in IF are thought to precipitate from seawater, and, if identified, may provide insight into geochemical conditions of Earth’s early oceans where nascent life is hypothesized to have first evolved.