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

Abstract

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.% Fe 2 O 3 , 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.

The role of fault reactivation in the development of tropical montane lakes

“…Due to reactivation of the E–W fault system in the Pliocene–Pleistocene or even older, when rocks were exposed by normal faulting, differential erosion started to build South Hill (Serra Sul) at the southern border of the Carajás Structure and the S11 plateau (Figure 9B). Weathering gradually altered the rocks, forming the Fe‐rich crust on the tops of hills (Silva and Costa, 2020). During uplift of the rocks, the erosion of the plateau border was controlled by the E–W faults and NE–SW and NW–SE fractures and faults (Pinheiro and Holdsworth, 1997a; Holdsworth and Pinheiro, 2000).…”

“…Since the formation of the lateritic rocks took place as a result of weathering, the differential dissolution of altered layers of the lateritic crust was increased by complex chemical reactions related to the circulation of incoming fluid (Abreu et al ., 2016; Cabral et al ., 2016). This process has been pointed out as an important mechanism for weathered rock dissolution, able to build pseudokarst structures such as sinkholes, caves and anomalous drainages (Silva and Costa, 2020). The role of this mechanism has to be taken into consideration as part of the definitive lake formation episodes that may have contributed to the regional landscape history, together with climate microscale oscillations, groundwater mobility, erosion by runoff drainage and deposition of sediments.…”

“…The four main stages of Violão Lake: (A) Proterozoic structural rock reorganization along the southern border of the Carajás Structure; (B) uplift and formation of South Hill, mostly by differential erosion; (C) Fe-rich crust formation by weathering and gravitational collapse along the E-W plateau border, starting lake formation; (D) episodic fault-fracture reactivation by gravitational collapse causing pulses of subsidence in the lake and defining its faulted borders. [Colour figure can be viewed at wileyonlinelibrary.com] 3741 FAULT REACTIVATION IN THE DEVELOPMENT OF LAKES crust on the tops of hills (Silva and Costa, 2020). During uplift of the rocks, the erosion of the plateau border was controlled by the E-W faults and NE-SW and NW-SE fractures and faults (Pinheiro and Holdsworth, 1997a;Holdsworth and Pinheiro, 2000).…”

“…They have been deeply weathered under humid equatorial climate conditions, forming an extensive supergenic cover of iron–aluminous laterite, hematitic breccias, and both ortho‐ and paraconglomerates (Maurity and Kotschoubey, 1995). The dissolution processes were recently described as an important mechanism also responsible for the partial collapse of the weathered upper crust present in the Violão Lake region (Abreu et al ., 2016; Cabral et al ., 2016; Auler et al ., 2019; Silva and Costa, 2020). K–Ar dating of supergene K‐bearing manganese oxides formed during lateritization indicates that weathering started at approximately 72 ± 6 Ma, with episodic precipitation of K–Mn oxides resulting from paleoclimate changes throughout the Cenozoic (Vasconcelos, 1999).…”

Mineralogy and geochemistry of immature lateritic crusts discriminating granitoids and Cr-Ni-bearings ultramafic rocks in southeastern Carajás

“…In the specific case, derived from granitoid and partly ultramafic rocks, according to textural and geochemical signatures. It is likely that the region studied has already been covered equally by mature laterites that have shaped the Carajás upper land since the Paleogene until the Miocene or Pleistocene (Costa, 1991;Costa & Araújo, 1997;Monteiro et al, 2018;Silva & Costa, 2020;Negrão & Costa, 2021), which were largely removed by erosive denudation events. The erosive debris is currently resting in deep valleys and forms thick debris cover inside the Carajás Range and plateaus in part similar, only to a lesser extent, to Pilbara in Australia (Ramanaidou & Morris, 2003;Morris & Ramanaidou, 2007).…”