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. The local stratigraphic sequence of the Itacaiunas Supergroup in the area of the Northern System, as shown in Figure 3-2, is as follows:

¦ Upper Group: Igarape Bahia Aquiri Group - meta sedimentary and metavolcanic rocks (including manganese beds in Aguas Claras Formation)

¦ Middle Group: Grao Para Group - meta-sedimentary and meta-volcanic rocks
- Upper Formation: Cigarra Formation - meta-volcanics.
- Middle Formation: Carajas Formation - predominantly banded iron formation with lesser mafic meta-volcanic units.
- Lower Formation: Parauapebas Formation - bimodal metavolcanic rocks and metasedimentary rocks with intercalated discontinuous banded iron formations.

The Serra dos Carajas basin is cut by major E-W and N70°W trending regional lineaments. The area is affected by numerous minor regional faults (sigmoid form). The most outstanding discontinuity is the WNW-trending Carajas Fault that divides the basin into two domains, North and South, with the N5 deposit located in the more structurally complex northern domain.

The structurally most complex northern domain contains folded, faulted, and rotated iron ore bodies (N1 to N9 and Serra Leste). Several N-S oriented minor sympathetic fractures control the orebody configuration.

The southern domain includes orebodies that dip to the north (SI to S4). These orebodies are part of the south flank of the major structure, and show no apparent block movement or rotation.

Jaspilite represents the proto-ore of the Carajas region deposits, typically with 15 to 45 percent Fe (but can range up to 57 percent) and 35 to 65 percent Si02. The jaspilite is characterized by alternate light and dark colored micro-bands. Light colored layers are generally white to pale red, and consist of crypto- to micro crystalline quartz with inclusions of cryptocrystalline hematite and lesser martitized magnetite plus occasional sericite. Dark colored layers consist of fine-grained hematite and martitized magnetite.

Deep leaching of the jaspilite has resulted in the progressive migration of silica, forming hard hematite at depth. With proximity to the surface, the weathering has resulted in the formation of soft hematite. Both hard and soft hematites represent enriched iron mineralization with iron contents typically ranging from 60 to 68 percent Fe. Near-surface weathering has created an iron laterite layer at the surface.

The main mineralized lithological units of the N5 deposit are shown in the photographs in Figure 3-3.

* Hard Hematite: Compact, blue-gray, massive hematite, with a metallic luster, high density, and low porosity. Iron grades range from 65 to 69 percent. It is primarily used in the production of the export lump ore. Hard hematite is an increasingly rare iron oretype in Vale's Carajas operations.

* Soft Hematite: Massive hematite occasionally pulverized, highly porous, very weak, and slightly magnetic, with average iron grades of around 65 percent. It is the primary ore mineral, and is generally sufficiently friable to be excavated without blasting. Comprises the main source of sinter feed and pellet feed products.

* Canga: Canga is the uppermost unit and consists of a lateritic-saprolitic material that is the product of surface weathering of the underlying iron mineralization (Structural Canga) or barren mafic rocks (Chemical Canga). Mineral Canga consists of blocks of hematite cemented by hydrated iron oxides (goethite and limonite). It is generally 15 to 20 meters thick.

The Northern System mines, located in the Carajas mineral province of the Brazilian state of Para, contain some of the largest iron ore deposits in the world. The reserves are divided into Serra Norte, Serra Sul and Serra Leste (Northern, Southern and Eastern ranges) situated 35 kilometers apart.

Serra Norte (Northern range) is divided into three main mining areas (N4W, N4E and N5) and two major beneficiation plants.

In 2014 Vale started a new mine (SL1) and beneficiation plant in Serra Leste.

Serra Sul integrated operation includes S11C and S11D deposits. In the fourth quarter of 2016, Serra Sul S11D started up the mining and processing operations.

The ore reserves in the Northern System are comprised of hematite. Because of the high grade (66.7% on average) of the Northern System deposits, Vale does not need to operate a concentration plant at Carajas. The beneficiation process consists simply of sizing operations, including screening, hydrocycloning, crushing and filtration.

In Serra Norte, one of the major plants applies the natural moisture beneficiation process, consisting of crushing and screening, and the other applies both the natural moisture and the wet beneficiation process in distinct lines. The wet beneficiation process consists simply of sizing operations, including screening, hydrocycloning, crushing and filtration. Output from this site consists of sinter feed, pellet feed and lump ore. Serra Leste and Serra Sul natural moisture beneficiation process consists of crushing and screening. Serra Sul produces only sinter feed and Serra Leste produces lump and sinter feed.

The processes used at Carajas to produce the three different sized products simply consist of well known and established methods of size reduction and sizing. The entire processing system consists of the following six steps:
Secondary crushing and wet and dry screening.
Tertiary crushing and wet screening.
Hydrocyclone desliming.
Ball mill grinding.
Dewatering and filtration.
Product stockpiling, blending and shipping.

The first is the beneficiation flowsheet for the production of lump ore, sinter feed and natural pellet feed. The second is for the production of processed pellet feed by additional wet grinding and wet processing of sinter feed material. The third is for the dry screening process.

Crushing and Wet and Dry Screening
After the ROM ore has been crushed at one of the stationary or semi-mobile primary crushing stations it is transferred by belt conveyor to storage silos ahead of secondary crushing where the ore processing begins. In secondary crushing the ore is dry screened at 75 mm with the oversize going to cone crushers operating in closed circuit with the screens. The screen undersize is conveyed to silo storage ahead of secondary screening Most of the time the ore requires very little crushing so the screen oversize and the crushing feed tonnage is very low.

Secondary screening is the main part of the ore processing system at Carajas. It is the beginning of where the first finished product is produced and provides an intermediate product which passes on downstream for further wet processing or dry screening. This is considered the heart of the beneficiation operations at Carajas. There are 17 parallel secondary screening lines for wet or dry screening of the minus 75 mm ore feed. Ten of the 17 lines can be operated as dry screens and eight of those lines have recently been in continuous operation. In addition, two 10 mtpy remotely located dry screening plants will operate to produce only sinter feed. The screen oversize (granular material) from the two remote dry screening plants will be stockpiled and used as feed to the existing beneficiation plant. It was reported that 10 percent of the ROM feed to dry screening becomes screen oversize granular product. The remaining 90 percent becomes sinter feed with no additional processing losses.

The screens in the existing plant are double deck with 20 mm openings on the top deck and 13 mm openings on the bottom desk. The top deck oversize (-75 mm +20 mm) is crushed in the tertiary crushers and the bottom deck oversize (-20 mm +13 mm) is stored as lump ore product or sent to feed the tertiary crusher double deck screens. The screen undersize feeds another group of single deck screens that make a +2 mm sinter feed product. The sinter feed can be produced either wet or dry. Since the dry screening is proprietary no additional details are available. The single deck screen undersize is processed in spiral classifiers to classify the -2 mm product and provide a -2 mm +0.15 mm product to feed dewatering screens with 0.5 mm openings. The dewatering screen oversize forms the second part of the sinter feed product and the -0.5 mm screen undersize is closed circuit with the spiral classifier. The spiral classifier overflow provides the feed to the downstream desliming hydrocyclone process.

Tertiary Crushing and Wet Screening
The tertiary crushing and screening simply crush, screen and dewater the ore. Cone crushers and double deck screens in closed circuit are used to produce -50 mm +20 mm lump ore and a +13 mm bottom deck oversize product that can be directed to either the lump ore or the sinter feed. The -13 mm screen undersize is dewatered at 2.0 mm. The oversize joins the sinter feed and the -2.0 mm undersize passes through a spiral classifier and a 0.5 mm dewatering screen to provide additional sinter feed. The -0.5mm dewatering screen undersize is recycled to the spiral classifier and the classifier overflow goes to the tailings pond.

Hydrocyclone Desliming
The -0.5 mm slurry from the secondary screening plant spiral classifier overflow is deslimed in three stages of desliming using 10 inch- and 4 inch-diameter hydrocyclones. The final overflow is the slime tailing that goes to tailings thickeners and then to the tailings pond. The desliming hydrocyclone underflow produces a pellet feed product called natural pellet feed. The pellet feed is dewatered in thickeners and vacuum disc filters. This pellet feed product, which up to now has been exported, will slowly be eliminated as more secondary screening is converted from wet to dry screening. The Vale pellet plant at Sao Luis will also be supplied with pellet feed from Carajas that is derived from sinter feed that has been ground in ball mills and is known as processed pellet feed.

Ball Mill Grinding
In the event that additional pellet feed is needed it can be produced to different specifications by grinding some of the -13 mm +0.15 sinter feed in ball mills to produce an artificial or processed pellet feed. Ball mills are operated in closed circuit with classifying hydrocyclones. The classifying hydrocyclone overflow is fed to a thickening hydrocyclone which produces an underflow product of pellet feed and an overflow slime tailing product. The thickening hydrocyclone underflow goes to a concentrate thickener to prepare it for feeding the filters and the overflow goes to a tailings thickener before it is pumped to the tailings storage basin. The tailings thickener overflow is recycled back to the plant as process water.

Dewatering and Filtration
The dewatering of various processing products is done to enable recycling of process water to the plant and to remove sufficient water from pellet feed for ease in storing, shipping and handling. The slime tailings from the secondary screening plant are treated in two tailings thickeners. The underflow is pumped to the Gelado tailings basin and the overflow is pumped back to the plant for reuse as process water.

Pellet feed products must be thickened and filtered for handling and shipping. The natural pellet feed is thickened in dewatering hydrocyclones and filtered in vacuum disc filters resulting in filtercake moistures of 10.5 to 12 percent. The processed pellet feed is thickened in a conventional thickener and filtered in pressure filters to a moisture level of about 9.5 percent.