Summary:
The iron deposits of the Northern Iron System are hosted in the Precambrian rocks of the Itacaiunas Supergroup. The basement of the region consists of the Pium Complex ortho-granulites, and Xingu Complex gneiss and migmatites. The volcanics and sediments of the Itacaiunas Supergroup overlie the basement, and are in turn overlain by Aguas Claras clastic sediments. Granites, gabbros, and granitoids intrude the sedimentary sequence. The Carajas ores are hosted by the Grao Para Group of the Itacaiunas Supergroup, composed of meta-basalts, meta-sediments, ironstones, and meta-rhyolites. The ore deposits lie within an approximately 300 to 400 meters thick banded chert-hematite jaspilite unit that occurs between thick volcanic units.
The lower volcanic unit is the Parauapebas Formation (4,000 to 6,000 meters thick), and consists of bimodal volcanics (dominantly massive, vesicular and porphyritic flows and agglomerate breccias of metabasalt, meta-basaltic andesite and meta-trachyandesites), with subordinate (10 to 15 percent) meta- rhyolitic tuffs and flows.
The Carajas Formation hosts the deformed banded-iron formations (BIFs) with some interbedded mafic meta-volcanics. The Cigarra Formation (upper volcanic unit) is similar to that of the Parauapebas Formation with mixed meta-sediments (fine grained tuffs, tuffaceous siitstones, phyllites, cherts and greywacke). The volcanic sequence has generally been weathered to a depth of 100 to 150 meters. The oxidation is observed to a depth of 500 meters in the banded iron formation (BIF) of the ore zone.
Serra Sul Complex
The Serra Sul Complex corresponds to the normal limb domain of the Carajás Syncline, characterized by a lower degree of deformation when compared to the inverse limb, which is reflected in the greater continuity of the iron formations.
The SS11 deposit corresponds to the largest plateau and the main mineralized body of Serra Sul. This plateau extends for 28 km in the NW-SE direction, with elevations ranging from 650 to 850 m. Its segmented shape, with directions that vary sharply between N-S and EW, configures a kink-type pattern. The deposit includes bodies A, B, C, and D, the latter of which presents the most economic interest. The plateau is predominantly composed of rocks from the Carajás and Igarapé Cigarra formations of the Grão Pará Group, which is in contact with the Parauapebas Formation to the south and the Águas Claras Formation to the north. In general, the layers present dips and azimuths that vary between the north and east directions, configuring a normal stratigraphic stacking.
Mineralization
The mineralization at Serra Sul depends mainly on the weathering of jaspilites, the Carajás ore protolith. The high-grade ore consists of friable hematite, compact hematite, and manganese hematite, which occur in a sub-horizontal tabular layer that tends to follow the topographical surface. This is usually covered by a layer of canga that is also considered a mineralized lithotype.
Friable hematite (HF) accounts for about 85% of the mineralization. It is essentially composed of hematite, with irregular masses of magnetite, goethite, and limonite inherited from the jaspilite ore protolith, in addition to local points of kaolin and clay minerals originating from volcanic rock weathering. HF occurs from near the surface to depths greater than 450 m, with average Fe grades around 68.8% and relatively low levels of phosphorus, silica, alumina, and loss on ignition. Loss on ignition and phosphorus are generally found near canga contacts, where a transition zone measuring from centimeters to meters can be identified; alumina is most commonly associated with centimetric to metric intercalations of mafic rocks. At the jaspilite contact points, there is a sharp drop in iron content, and gradational contacts are rarely observed; when found, these do not exceed the metric scale.
Compact hematite (HC) corresponds to 1% of the mineralization and is restricted to some regions of bodies C and D, mostly as lenses below the canga and, more rarely, in deeper layers within the jaspilites, suggesting a hypogenic origin. Its structure is massive or foliated, with up to 30 m thickness, and an average Fe content of around 66%, with slightly higher levels of contaminants than HF.
Manganese hematite (HMN) is subordinate and has no representation in the deposit. It occurs under low continuity lenses, up to 50 m thick, usually close to contacts with jaspilites and mafic rocks, suggesting a hypogenic origin. In chemical and granulometric terms, it preserves characteristics similar to those of friable hematites, differentiating mainly by higher Mn contents, around 2.4% on average, and average Fe contents around 63%.
The cangas occur widely on the surface of Plateau SS11 as the product of weathering of different rocks in the region. They differ according to the substrate and can be classified as chemical canga (CQ), which covers mafic rocks, enclosing or intrusive in the iron formation, and structured canga (CE), developed directly over the iron formations and economically usable, therefore classified as ore. CE represents 14% of the mineralization and its thickness can range from a few meters to 60 m, averaging at around 15 m. It can be observed locally on hillsides, indicating a low transport rate. CE is predominantly compact and may preserve the banded texture. It is a very hydrated lithotype; its mineralogy is hard to define by the naked eye. Its average Fe content is 64.2% and the main contaminants are alumina and phosphorus, in addition to high amounts of loss on ignition.
Serra Norte Complex
The Serra Norte Complex corresponds to the inverted limb geological domain of the Carajás Syncline. The high deformation pattern of this domain is reflected in the geometry and the distribution of the iron formations, whose segmentation by faults and folds of different orientations and scales, individualizes plateaus from N1 to N9.
The plateaus of the Serra Norte Complex are limited to the north by the domain of volcanic rocks of the Parauapebas Formation, and to the south, by the domain of terrigenous sediments of the Águas Claras Formation.
The mineralization at The Serra Norte Complex is mainly formed from supergenic alteration of jaspilite, which constitutes the Carajás protolith. The high-grade ore is mainly represented by friable hematite, the structured canga is also considered as a mineralized lithotype.
Deposit N1 is in the extreme west of Serra Norte and corresponds to part of the homonymous plateau. This plateau has an elongated shape in the NW-SE direction. It is predominantly composed of rocks from the Carajás and Igarapé Cigarra formations. In general, the layers show medium to high angle dips to the northeast, configuring an inverted stratigraphic stack. The thickness of the layer varies between 250-600 m and is strongly controlled by folds and faults.
Deposits N2 and N3 are located on the homonymous plateaus, with an elongated shape in the E-W direction, in the central portion of the Serra Norte. The rocks of the Igarapé Cigarra Formation are predominant in these plateaus. In general, the layers show a medium to high angle dip to the north, configuring an inverted stratigraphic stacking.
Most of the iron formations of plateaus N2 and N3 were attributed to the Igarapé Cigarra Formation, they do not stand out in the relief due to the low thickness. Structural controls and their relative chronological ordering are like those described for N1, differing by the low continuity of iron formations, which reflects the importance of the faults for the local geological framework.