Genesis of the “soft” iron ore at S11D Deposit, in Carajás, Amazon Region, Brazil

The origin of the soft ore at the S11D iron mine in Carajás was investigated using 20 samples from a diamond drill hole. The methods of analyses were X-ray diffraction (XRD), optical microscopy, whole-rock chemistry, and scanning electron microscope coupled with energy-dispersive X-ray spectroscopy (SEM/EDS). The drill hole presents a profile through the substratum (protore, a banded iron formation - BIF) and three weathering horizons, defined from the base to the top, saprolite (coarse and fine), and crust. The soft iron ore occurs distributed along the saprolite horizon, and it is composed mainly of hematite and subordinate magnetite. The amount of quartz decreases upwards, whereas the amount of Fe-Al-(Ti-P)-minerals increases towards the top. The total iron is enriched in the fine saprolite when compared to the protore (42.55 to 97.62 wt.% Fe2O3, respectively). Trace elements such as Zr, Cr, Y, and rare earth elements (REE) show relative enrichment upward because they are generally located in residual minerals (as zircon and anatase). The REEs in iron ore samples exhibit enrichment of light rare earth elements (LREE) and depletion of heavy rare earth elements (HREE), with pronounced positive Eu anomaly, which reaffirms the connection between iron ore and BIF. Based on the mineralogy, chemistry, textures, and structures, a genetic laterite-supergene model is proposed for the origin of soft ore at the S11D deposit.