Summary:
The main Carajás iron ore deposits are associated with flat-topped elevated plateaus, in general, elevated areas, between 650-800 meters, defined along two main morphological alignments
corresponding to Serra Norte and Serra Sul. These alignments materialize the limbs of the Carajás Syncline.
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.
S11CD Plateau
Deposit dimensions
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%.
Although jaspilite (JP) is not a mineralized type in Carajás, it will be described here, as it is genetically related to mineralization. These are iron formations characterized by the alternation of hematite bands and jasper/silica, subordinately, chlorite and carbonates bands. They can be grouped, according to mineralogy and texture, into carbonate, siliceous, chlorite, and breccia types. The SS11 jaspilites are greyish and may resemble itabirites, but present geomechanical characteristics similar to those of Serra Norte jaspilites, constituting an extremely compact, hard to sample lithotype. Fe average content is 45.6% and contaminant levels are lower than in HF, with alumina as the main contaminant at around 0.6% or higher in the vicinity of mafic rock contacts. They occur at the base of the iron formations package, with unknown thickness in contact with mafic rocks, but also as centimeter-thick lenses and up to 200 m, immersed in the large mass of friable hematites.
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.