The genesis of the early Neoproterozoic Shilu banded iron formations: Could it be applied to other iron ore deposits?

The early Neoproterozoic Shilu iron deposit is located in Hainan Island, South China. It is an unusual Fe deposit with high-grade (average FeO of 51 wt%) and giant scale (proven Fe-ore reserves > 500 Mt). It is one of a few examples of banded iron formation (BIF) type deposits formed in the interval between 1.8 and 0.8 Ga, thus providing an opportunity to understand the source of the iron and the metallogenic processes involved. Based on Fe content and modal composition, three types of ores are recognized from the Shilu iron deposit, comprising quartz itabirites, amphibole itabirites, and ferruginous sandstones. Except for the ferruginous sandstone, a detrital component is also present in quartz and amphibole itabirites as indicated by the positive correlation between TiO2 and Al2O3 and the negative correlations of TiO2 and Al2O3 with Y/Ho ratio. Ti and Al are immobile elements during weathering and hydrothermal alteration processes and so they retain a constant ratio in terrigenous materials, and they have lower Y/Ho ratios than sea water. The detrital component is consistent with a generally flat pattern of Post-Archean Australian shale normalized REE plus Y (REYPAAS). A large contribution of seawater is rejected because of the absence of Ce and Y anomalies and significant left-sloped REYPAAS. Due to the lack of a stable Eu positive anomaly or elevated Ho/Lu ratio, the contribution of high-temperature hydrothermal fluids is ruled out. A large amount of barite occurs in the Shilu deposit, which is a signal of lowtemperature hydrothermal processes. The subdued positive Eu anomalies displayed by some quartz and amphibole itabirites from the Shilu deposit arise from the presence of barite as indicated by a strong positive correlation between delta Eu and Ba. Fe isotope ratio data show a notable fractionation in the sequence of ironbearing rocks as shown by a systematic increase in delta 57Fe (-1.14 to 2.41 parts per thousand) from the base to the top. The fractionation trend is in agreement with a stratified ocean where the water is increasingly oxidized towards the surface. A metallogenic model is proposed where the Shilu BIFs are deposited from low-temperature Fe-bearing hydrothermal fluids vented into a redox-stratified shallow-water basin on a passive continental margin where extensional faults possibly acted as fluid paths. The depositional environment is similar to Superior-type BIFs, but the iron source here is related to a proximal hydrothermal vent system.