318 2. GEOLOGICAL CONTEXT In the Serra Norte mines (N4 and N5), basalts, diabase dikes, and gabbros—collectively classified as mafic rocks—record multiple magmatic events associated with the geological evolution of the Carajás Mineral Province (CMP). These igneous units occur either as volcanic flows interlayered at the base (Parauapebas Formation) and top (Igarapé Cigarra Formation) of the banded iron formation, or as younger intrusive bodies represented by diabase and gabbro dikes and sills that cut the supracrustal sequence. Mineralogical analyses reveal that the volcanic rocks underwent extensive post-emplacement modification, resulting in the partial to complete replacement of their primary mineral assemblages. The abundant biotite, chlorite, and potassium feldspar indicate pervasive post-magmatic hydrothermal alteration, which reequilibrated the original mafic mineralogy under variable physicochemical conditions. This alteration is consistent with regional hydrothermal systems widely documented in Carajás and linked to the circulation of heated fluids during successive tectono-magmatic episodes. Fluid–rock interaction—facilitated by fractures, faults, and shear zones—promoted recrystallization and mineral replacement, generating alteration patterns of variable intensity dominated by potassic associations. The N4 Mine is subdivided into N4WN, N4W, N4WC, N4WS, N4EN, and N4E, distributed along the central eastern portion of the N4 plateau (Figure 1). This extensive plateau comprises rocks of the Parauapebas, Carajás, and Igarapé Cigarra Formations, arranged in tabular bodies that predominantly dip westward, forming a normal stratigraphic stacking. Prior to mining, the plateau was naturally divided into eastern and western sectors by a drainage channel separating the two main banded iron formation bodies. The eastern block is elongated N–S (8.5 × 1 km), whereas the western block is more regular (4 × 2 km), both ranging from 650 to 750 m in elevation. Geologically, N4 encompasses Archean and Proterozoic units dominated by iron-formation sequences affected by widespread hydrothermal alteration. Terrigenous sediments of the Águas Claras Formation overlie the BIFs, while younger arkosic deposits of the Gorotire Formation occur along normal faults that define a graben structure in the southern portion of the plateau (Nogueira, 1995; Azevedo et al., 2018). Across the plateau, the Parauapebas, Carajás, and Igarapé Cigarra Formations collectively define the main lithostratigraphic and structural domains. The Carajás Formation forms two distinct belts (N4W and N4E), separated by an east-verging thrust fault. N4W is characterized by a relatively simple homoclinal structure segmented by NW–SE dextral strike-slip faults, whereas N4E displays more complex patterns, including rotated banding and a J-shaped fold produced by drag along major shear zones. The Parauapebas Formation consists predominantly of Upper Proterozoic volcanic basalts originally composed of olivine, pyroxene and feldspars. These rocks underwent intense hydrothermal alteration and weathering, generating secondary mineral phases such as chlorite, zeolites, hematite and magnetite. The Carajás Formation—representing about 40% of the plateau—is the thickest and most continuous iron-formation unit, composed of hematite- and magnetite-rich BIFs locally referred to as jaspilites and strongly controlled by strike-slip deformation. The Igarapé Cigarra Formation crops out west of the Carajás Formation and includes
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