The geochemistry of banded iron formations in the sukumaland greenstone belt of Geita, northern Tanzania: evidence for mixing of hydrothermal and clastic sources of the chemical elements

Abstract

Major and trace element compositions of samples of Banded Iron Formations (BIF) from the Neoarchaean Sukumaland Greenstone Belt of Geita in northern Tanzania reveal that the BIF precipitated from hydrothermal solutions. Fe-Ti-Al-Mn systematics suggest that the hydrothermal deposits have been contaminated, by up to 20% by weight, with detrital material having a composition similar to modern deep-sea pelagic clays. SiO2 and Fe2O3 contents are 48.2 to 88.5% and 8.9 to 49.1% respectively. Al2O3 contents lie between 0.33 and 2.1% and show no correlation with either Fe2O3 or SiO2. Al2O3 is, however, positively correlated with Ti, Ga, Hf, Rb,Th, Zr and Sr but not with CaO, the alkalies and the total Rare Earth Elements (REE). The other major element oxides are generally present in negligible amounts. The samples are characterised by mean Zr/Hf and 144Sm/143Nd ratios of 48± 5 (2 SE) and 0.10±0.01 (2 SE) respectively, similar to mean upper continental crustal values. Shale-normalised REE patterns are nearly flat, except for small positive Eu and very slight negative Ce anomalies and reveal that, compared to average upper crust, the abundances of the REE in the BIF are up to an order of magnitude lower. Chondrite-normalised patterns are characterised by light (L) REE enrichment, flat to slightly depleted heavy (H) REE, slightly positive Eu anomalies and very small negative Ce anomalies. The HREE-depleted patterns are similar to patterns derived from granite-dominated upper continental crust and indicate that the bulk of the REE in the Geita BIF can not have been derived from a mixture of Neoarchaean sea water and bottom hydrothermal solutions. The trace element data, and the REE in particular, indicate that, despite their relatively low proportions, granitic detritus probably derived from contemporaneous felsic flows and pyroclastics are the cause of the dominant trace element geochemical signature of the BIF.