Abstract
The approximately 1874 Ma old Negaunee Iron Formation (NIF) forms part of the metasedimentary Menominee Group of the Marquette Range Supergroup (MRS) in northern Michigan and Wisconsin, United States of America. The Menominee Group comprises quartzite, slate, siltstone and the NIF. The NIF is classified as a Superior-type iron formation (IF) and is found within a predominantly marine sedimentary sequence.
The objectives of this research study are to analyse samples obtained from drill core intersections of the Paleoproterozoic NIF in Minnesota, USA, in order to evaluate the paleoenvironmental conditions during its deposition and subsequent post-depositional evolution. For this purpose, a set of previously collected samples was prepared into polished thin sections and sample powders to assess the mineralogy, petrography, and geochemistry.
Analytical methods used includes Optical Microscopy, SEM-EDS, ICP-MS, XRF and stable isotope mass spectometry.
Based on the stratigraphy and lithofacies recognized during drill core logging, the NIF can be subdivided into five lithostratigraphic units. These units are, from the base upwards, the lower undifferentiated IF (~70 meters thick), the silicate zone (~120 meters thick), the carbonate zone (also known as the good ore zone; ~110 meters thick), the clastic zone (~200 meters thick), and the upper undifferentiated IF (~80 meters thick).
The NIF can be subdivided into three main textural lithotypes based on diagnostic mesoscopic and microscopic features. These lithotypes are Quartz magnetite-rich arenitic iron formations (QMAIF), banded iron formations (BIFs), and Fe-rich mudstones. QMAIF can be further categorized into two subtypes: massive QMAIF and banded QMAIF. Similarly, Fe-rich mudstones can be divided into two subtypes: massive clastic-textured Fe-rich chlorite mudstones and laminated Fe-rich chlorite mudstones. The BIFs are classified into three textural subtypes, namely lutites, mixed lutite-rhythmite, and rhythmites. All mentioned lithotypes are frequently interbedded with greywackes, tuffs and turbidites. Additionally, the presence of interbeds of tuffaceous units observed in the IFs further highlights the penecontemporaneous occurrence of volcanism associated with the deposition of IFs. The mineralogical composition observed in the NIF suggests that the presence of fine-grained carbonate minerals, such as siderite and ankerite, along with oxide minerals like hematite and magnetite, and silicate
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minerals including stilpnomelane and minnesotaite, can be attributed to early diagenetic processes. These processes likely resulted from reactions between ferric oxyhydroxides and either Fe2+-rich fluids, organic carbon, or silica-rich gels. Late diagenetic to early metamorphic re- and neocrystallization processes are indicated by the presence of mineral overgrowths, such as magnetite porphyroblasts, hematite rims on magnetite, as well as coarse-grained stilpnomelane and minnesotaite aggregates. The occurrence of cross-cutting quartz veinlets as well as pyrite porphyroblasts illustrate localized epigenetic alteration.
The major element geochemistry of the QMAIF and BIFs in the NIF is dominated by SiO2 and Fe2O3 with low Al2O3 and TiO2 contents, indicative of minimal detrital input. Shale-normalized rare earth element and yttrium (REY) patterns plot well below shale abundances and are slightly enriched in heavy rare earth elements (HREE) relative to light rare earth elements (LREE). The REY plots also show positive Eu, Y, and Pr anomalies. This suggests that Fe and Si contents were sourced and precipitated from seawater that had a significant high temperature (>250°C) hydrothermal input. The carbonates in the NIF show depleted δ13CVPDB values (-6,33‰ to -11,98‰) and δ18OVPDB values (-12,24‰ to -17,76‰) throughout. The highly 13C depleted carbonate values suggest that the abundant siderite was a product of redox reactions between Fe3+-precipitates and organic carbon during diagenesis.
The proposed depositional model for the NIF suggests that Fe2+ was sourced from hydrothermal plumes mixed into seawater moving towards the coastline from a distal source. The mode of Fe deposition could have been the oxidation of Fe2+ by either free oxygen from cyanobacteria, photoferrotrophs, or chemolithoautotrophs, as suggested by the depleted carbonate δ13CVPDB values indicative of high organic carbon input into the system. The observations and data interpretations show that the NIF was deposited in a marine environment that was transitional between deep (i.e., below storm wave base on the outer shelf to shelf slope) and shallow (i.e., above storm wave base on the inner shelf) water. The former is supported by micritic lithologies (felutite and ferhythmite) lacking sedimentary structures interbedded with greywacke and the latter by coarse lithologies (QMAIF) indicative of wave reworking.
The NIF as well as its depositional setting suggest that iron and silica are both sourced from proximal volcanism in a narrow marine basin. This is clearly different to Superior-type BIFs deposited during or soon after the GOE. It appears, therefore, likely that depositional setting and processes of sedimentation are different.