Abstract:

Banded iron formations (BIFs) are a small but important component of the Precambrian rock record. Consisting of alternating bands of iron-rich minerals and chert, these sedimentary rocks can be used to learn a great deal about Earth processes at their<br/>time of deposition. The bulk of research on BIFs to date has focused on the enigmatic iron component, despite the great insight to be derived about their depositional environment from detailed study of the chert bands. Although cherts are commonly<br/>formed through direct precipitation of amorphous silica from seawater or through fluid movement through unlithified sediments, their trace element geochemistry can prove very sensitive to small contributions of other materials, allowing for the relatively easy detection of different precursor rock types in cherts.<br/>In Archean greenstone belts, such as the Abitibi greenstone belt, BIFs represent a stratigraphic departure from the volcanic rock-dominated stratigraphy of these belts. BIFs up to 50 m thick within the Deloro assemblage, Bartlett Dome region, of the Abitibi greenstone belt, are bounded above and below by volcanic rocks which display U-Pb ages with differences of up to 12 My, with the absence of any observable erosional surfaces. This suggests near-continuous deposition, albeit at very slow rates. Here I use a combination of a detailed mapping of sedimentary structures and textures, as well as high-precision trace element geochemistry to identify unique features in cherts of the BIFs of the Abitibi greenstone belt. Many of these features are indicative of slow to null depositional rates.<br/>Some of the primary structures observed include chert hardgrounds and prelithification nodules, loading structures, stratabound chert breccias, and coarse-grained, heterolithic debris flows, suggesting a combination of rapid, episodic deposition and<br/>periods of null deposition and violent fluid escape. The trace element geochemistry has revealed that chert formation was dominated by one of several processes; precipitation of amorphous silica from seawater, sub-seafloor hydrothermal circulation, and the input of subsequently silicified detrital materials. These groups were identified using Ni/Cr ratios to determine the degree of detrital vs. orthochemical (seawater or hydrothermal) contribution, and further refined within these chemical groups based on the MUQ-normalized REE+Y patterns, which have aided in identifying more specific fluid and detrital sources in these cherts.<br/>Based on the sedimentology and geochemistry of the cherts, a new depositional model for the BIFs of the Bartlett Dome is here suggested. Controlled by 4 main depositional and diagenetic processes, this model consists of: periods of null-deposition<br/>resulting in the formation of chert hardgrounds; the rapid deposition of amorphous silica and ultra-fine detrital volcanic material from seawater, forming load structures; the instantaneous deposition of coarse grained heterolithic debris flows; and sub-seafloor hydrothermal circulation, resulting in the replacement of other materials by silica and the brecciation of buried hardgrounds through fluid escape. The interplay of these 4 processes within the stratigraphic space of 50m strongly suggests that the principal control on BIF formation in the Abitibi greenstone belt was the prolonged period of deposition with highly limited material input, allowing for numerous processes to exert influence on deposition. The model presented here may be applicable in other greenstone belts worldwide, pending studies of the BIFs in those belts through this lens.