Detailed Geochemistry and Nd isotopes of a 2.85 Ga old BIF from the Slave Craton: Unraveling the Seawater Sources and Precipitation of Iron and Silica in BIF Formation

English Abstract:

Banded iron formations (BIFs) are metamorphosed chemical sediments that initially precipitated from Precambrian ocean water. The alternating bands of silica and iron contain important information about the composition of seawater at the time of BIF formation, as well as the relative contributions of hydrothermal discharge and continental weathering. In this work we use the combination of geochemistry and Sm-Nd isotopes on a layer-by-layer basis of the ~2.85 Ga old BIF from the Central Slave Cover Group (CSCG). The bulk of the CSCG is composed of oxide BIF underlain by fuchsitic quartzite that unconformably rests on basement gneisses. Seven silica bands and seven iron bands were separated from the BIF. Geochemical analyses show that in all the bands Al₂O₃ is < 1 wt.%, with less in the silica bands (average 0.12 wt.%) relative to the iron bands (average 0.41 wt.%). Highly insoluble elements, such as Ti, Zr, Nb and Th, are elevated in the iron bands relative to the silica bands. Both the total REE concentrations and the shale-normalized REE patterns reveal a systematic difference, with higher REE content and a more elevated LREE patterns for the iron bands. This, together with the higher abundances of insoluble elements, points towards a larger terrigenous input during precipitation of the iron. However, a well-developed Eu anomaly occurs in all the bands suggesting some mixing of high-T hydrothermal fluids with the upper seawater. The iron bands have consistently depleted e_Nd(t) values between +1.0 to +2.8 (average of +1.7) reflecting that the dissolved REEs in the source water during ferric iron precipitation may have contained a mixture of submarine hydrothermal fluids and bulk upper seawater dominated by more evolved continental sources. The silica bands, on the other hand, show a higher spread in their e_Nd(t) values. Four of the bands have enriched e_Nd(t) values of -2.2, -1.2, -0.6 and -0.4 (average of -1.1) and three of the bands have depleted e_Nd(t) values of +1.7, +2.4 and +3.4 (average of +2.5). This trend suggests that both evolved and more depleted continental landmasses contributed to the dissolved REE budget of the ambient surface seawaters during silica precipitation.