Meso- and Neoarchaean Banded Iron Formations of the Slave Craton, NW Canada: Seawater Chemistry, Source Characteristics, Depositional Setting and Age Constraints

  • Author / Creator
    Nielsen, Rasmus Haugaard
  • Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks that have been widely used as a proxy for the chemical and redox composition of Achaean seawater. Within the Slave Craton (Northwest Territories, Canada), two different BIF deposits exist: (1) an ~ 2.85 Ga unit deposited in a continental rift basin, associated with fuchsitic quartzite, conglomerate and basement gneisses, and (2) an ~ 2.62 Ga unit deposited in an arc-type basin and found interbedded with extensive greywackemudstone turbidite sequences. The time frame within which these BIFs were deposited records major changes in global Archaean environmental conditions, such as mantle plume activity, continental assembly and changes in oceanic and atmospheric chemistry. Although these palaeoenvironmental factors can potentially be traced in BIFs, this study shows that these deposits tend to reflect phenomena that are strongly dependent on localto- regional palaeoenvironmental conditions. This thesis presents new petrologic and geochemical insights into the deposition of these poorly studied Slave Craton BIF deposits. It interprets their geochemical seawater signature from a local-to-regional perspective as it relates to the ambient environment, and it provides a new understanding of the interplay of the dominant solute sources controlling seawater composition at the time. Both BIF deposits display seawater derived shale-normalized rare earth elements + yttrium (REY) patterns enriched in heavy-REE relative to light-REE (HREE>LREE) and positive lanthanum (La) and yttrium (Y) anomalies. High-resolution, layer-by-layer geochemical and samarium-neodymium (Sm-Nd) isotope analyses further reveal that both BIF deposits were precipitated in a basin where seawater was a mixture of deep, iii hydrothermal waters and surface waters with solutes sourced from land. The high silica contents (up to 20 wt.% SiO2) in the iron bands and low iron contents (down to 4 wt. Fe2O3) in the silica bands show that background silica sedimentation within the water column was frequently interrupted by the convective upwelling of iron-rich water masses. In the older 2.85 Ga BIF, the iron-rich bands all have positive εNd(t) values which demonstrate that the dissolved REY in the source water during ferric iron precipitation was provided by deeper-marine hydrothermal fluids with relatively uniform 143Nd/144Nd. The surface water, on the other hand, was heterogeneous and controlled by a wide range of REY sources all related to both radiogenic (positive εNd(t) values) and unradiogenic (negative εNd(t) values) continental landmasses. In the younger 2.62 Ga BIF, high germanium/silicon (Ge/Si) ratios in the iron bands, and low Ge/Si ratios in the silica bands are further evidence for the interplay between upper silica rich and deeper iron rich seawater during BIF formation. However, a difference between the ambient seawater behind deposition of the younger and older BIF deposits is the size of the hydrothermal contribution and the presence of a strong oxidant (e.g., O2) in the 2.62 Ga BIF. During active rifting of the basement, a large excess of reduced europium (Eu2+) from hightemperature hydrothermal fluids impacted the basin water and was incorporated in the precursor precipitate of the ca. 2.85 Ga BIF. The positive Eu anomaly is larger than in similarly-aged BIF deposits, as well as to the younger 2.62 Ga BIF in this study. Whereas the older BIF do not show any anomalies of cerium (Ce), parts of the younger BIF displays negative Ce anomalies, which indicate the presence of an oxidizing agent that was able to fractionate Ce3+ to Ce4+ in the 2.62 Ga seawater. These findings clearly indicate the importance of addressing each BIF within its own depositional context, as iv they can reveal detectable changes in the evolution of relative water mass contributions in a particular depositional basin, over time. The 2.62 Ga BIF is found interstratified with large volume of greywackemudstone turbidite deposits. As a modern analogue turbidites are deposited by density driven currents containing suspended debris. The extensive turbidite sedimentation associated with the BIF has noticeably impacted its elemental budget. As such, the slow chemical rainout of the precursor precipitates in this BIF likely took place on the basin floor adjacent to a steep submarine ramp. BIF deposition was episodically interrupted by turbidite currents and slump-generated debris flows sourced from the adjacent, tectonically active arc-terrain. Until now, the depositional timing of the 2.62 Ga BIF deposit has been based entirely on maximum depositional ages in detrital zircons from the associated turbidites. In this study we found a felsic-to-intermediate, ~3-cm-thick, tuff ash bed, interlayered within the turbidite-BIF sequence at Slemon Lake in the southwest part of the Slave Craton. The tuff yielded a single zircon age population with a U-Pb zircon crystallization age of 2620 ± 6 Ma. This crystallization age defines the age of these turbidite-BIF. The date defined by the tuff ash bed, together with maximum detrital zircon ages carried out on greywackes across the craton, separates the turbidite-BIF deposits into its own stratigraphic group, which we propose to be named the Slemon Group. The trace element signature of the tuff ash bed is reminiscent of arc-type volcanics, likely associated with the Defeat suite granitoids, thereby linking the younger turbidite-BIF basin to its overall tectonic environment.

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    Doctor of Philosophy
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