Contrasting Depositional Environment of Iron Formation at Endengue Area, NW Congo Craton, Southern Cameroon: New Insights from Trace and Rare Earth Elements Geochemistry

摘要

The Endengue Banded Iron Formation (BIF) is located in the northwestern edge of the Congo craton in Cameroon. Here , we report geochemical data of trace and rare earth elements (REE) of the Endengue BIF samples from the Archean Ntem complex and investigate their environmental setting. Two types of BIF occur at Endengue area, both containing minimal contamination from terrestrial material. Total REE ( Σ REE) contents in the Type 1 BIF are extremely low, ranging from 0.34 to 1.83 ppm, similarly to pure chemical sediments while Type 2 BIF displays Σ REE contents ranging from 2.98 to 24.26 ppm. The PAAS-normalized REE + Y patterns of the two BIF types display LREE enrichment relative to HREE and weak positive Eu anomaly, most likely suggesting that the source of iron and siliceous of the Endengue BIFs is mainly from the contribution of low-temperature hydrothermal alteration of the crust. Type 1 BIF shows very low Nd content (<1 ppm) with positive correlation between Nd and Ce/Ce * and positive Ce anomalies which suggests suboxic or anoxic seawater similar to the depositional environment of Elom BIF in Archean Ntem complex. In contrast, Type 2 BIF displays low to moderate Nd contents (1 and 100 ppm with the exception of sample LBR65) with negative correlation between Nd and Ce/Ce * and negative Ce anomalies. These features indicate precipatation of Type 2 BIF from oxic iron-rich solution that changed to oxidized surface by rapid precipitation of the hydrothermal Fe. The Endengue BIFs were deposited in the continental margin ocean in presence of low-temperature hydrothermal fluids mixed with seawater, similar to Paleoproterozoic Kpwa - Atog Boga BIFs within the Nyong group and other Paleoproterozoic Superior - type BIFs worldwide.