Summary:
The Paragominas bauxite deposits are developed on a 10,000km2 plateau, situated 50 m to 150 m above the drainage-incised valley floors. The average altitude does not exceed 150 m in the plateaus.
The Miltônia 3 (M3) and Miltônia 5 (M5) gibbsitic bauxite deposits have formed by deep tropical weathering of the Ipixuna Formation. The bauxite layer forms a nearly continuous tabular body, less than 5 m thick, but extending 20km north–south, and as much as 8km east–west, beneath the plateau surface.
Prolonged weathering and oxidation of aluminum-rich sediments allowed the formation of gibbsite, an aluminum hydroxide, which can occur in a microcrystalline, porcelain-like form (amorphous bauxite), or as fine-grained euhedral crystals that occasionally reach 1 mm in size. Minerals within the deposit display either ‘detrital’ or ‘secondary’ character. Minerals that are ‘detrital’ were originally deposited as part of the host water-borne sediment (e.g., quartz and anastase); those that are ‘secondary’ are weathering products of kaolinite and feldspar.
The geological profile of the Miltônia deposit has eight horizons, with lateral variation in thickness to characterize the following lenticular horizons: Clay Overburden (CAP), Nodular Bauxite (BN), Crystallized Nodular Bauxite (BNC), Ferruginous Laterite (LF), Crystallized Bauxite (BC), Crystallized/Amorphous Bauxite (BCBA), Amorphous Bauxite (BA), and Variegated Clay rocks of the District pertain to the Itapecuru Group (south and central portions) and the Ipixuna Formation (north portions) belonging to the Upper Cretaceous, according to Santos Jr. and Rosseti (2002).
The Nodular Bauxite is characterized by nodules which increase in size towards the base. This material forms quite discontinuous bodies. At the top, there are millimetres to centimetre sized bauxitic nodules, coloured from yellow to lilac, associated with ferruginous pseudo-pisolites. At the base the nodule become concretions, with a diminishing quantity of pseudo-pisolites and increased degree of crystallization of the gibbsite.
With increasing depth, the nodular bauxite grades into Crystallized Nodular Bauxite, constituted by nodules and/or concretions of reddish colour, with a relative absence of ferruginous pseudopisolites. This material sits discordantly on the Ferruginous Laterite level. Together, the thicknesses of Nodular and Crystallized Nodular can be up to 2 meters.
The Ferruginous Laterite is predominantly in the form of pseudopisolites or nodules, at the top and in concretions at the base. Sometimes, this horizon contains medium to coarse grained crystalline bauxite, associated with ferruginous concretions. Thickness of this layer varies from a few centimetres to 2 meters.
The Crystallized Bauxite horizon is that which has the best lateral continuity. It consists of reddish coloured bauxite in the form of blocks or concretions. Frequently, the top is enriched in iron, due to contact with the Ferruginous Laterite, but iron grades decrease towards the base. The contact with the underlying horizon, Crystallized/Amorphous Bauxite is represented by a very irregular transition. The average thickness of the horizon is about 1.5 meters.
The Crystallized/Amorphous Bauxite occurs as nodules and concretions, having yellow to lilac coloured micro to macrocrystalline gibbsite, which is associated (or not) with light grey to light brown porcelaneous (micro or crypto-crystalline) gibbsite. The contact with the Amorphous Bauxite is characterized by an increase in the clay matrix material found between nodules and strong indications of kaolinization, appearing as variegated clay. The average thickness of this horizon is about 1 metre.
The Amorphous Bauxite is characteristically formed of elongated nodules and blocks of micro-crystalline gibbsite, of yellow colour set in a clay matrix, which is typically variegated. This horizon is the part of the bauxitic zone with the highest silica content.
The Variegated Clay, or kaolinized sapprolite, characterized by multicoloured kaolinitic clay, is the basal limit of the Laterite profile.
There are two zones of alumina enrichment in the bauxite/Laterite profile found at Miltônia. These are denominated Upper and Lower Enrichment Zones by the authors of the paper. They correspond to the BNC and BC horizons, respectively.
Despite the gradational contact between BN and BNC, the BNC exhibits strong alumina enrichment, with consequently strong suppression of iron content. Silica is slighthy depressed coming to grades of less than 5.5%. The LF acts like as inversion zone for iron grades, while silica is slightly enriched here.
The top of the BC is marked by the presence of ferruginous blocks and nodules, usually forming concretions less than 20 centimetres thick. This explains why iron oxide grades are about 15% in this portion, dropping off below. Visually, the gradational contact between BC and BCBA is marked by a change in texture of the reddish concretions with clearly crystalline bauxite. The nodules become yellow and lilac; the crystals are less well defined and frequently show evidence of degradation. Chemically, this contact is marked opposite trends in the silica and iron curves.
The lower horizons are marked by successive enrichment of reactive silica concomitant with suppression of iron and available alumina.
The variation in mineralogy is relatively small over Miltônia. The principal mineral constituents are gibbsite, kaolinite, quartz, hematite, anatase, goethite and, in lesser quantities, micas, zircon, tourmaline, ilmenite, pyrophyllite, and locally a little boehmite.
Characteristics of the Ore
The Crystallized Bauxite and the upper portion of the Crystallized/ Amorphous Bauxite are the principal components of the ore, because of the high grades of available alumina and lower quantities of reactive silica found here. This combination also has an almost constant presence over the entire plateau and affords the best thickness for mining. The Upper Enrichment Zone offers a subordinate option in places where grades and thickness are adequate and the intervening Ferruginous Laterite is not too thick to be included without upsetting overall grades, or it is thick enough to be separated with efficient use of the mining equipment.