Summary:
The Araxá and Patrocínio phosphate deposits are part of a series of Late-Cretaceous, carbonatite-bearing alkaline ultramafic plutonic complexes belong to the Alto Paranaiba Igneous Province.
The tropical weather regime prevailing in the region and the inward drainage patterns developed from the weather-resistant quartzite margins of the dome structures resulted in the development of an extremely thick soil cover in most of the complexes. The extreme weathering was responsible for the residual concentration of apatite.
Phosphate rocks are derived principally from two primary sources, namely phosphorous rich sediments known as phosphorites, and from carbonatite and alkaline igneous intrusive rocks. The phosphate deposit at Araxá is the latter type. Carbonatite and alkaline igneous intrusive sources of phosphate rocks are typically considerably smaller deposits than the sedimentary sources. However, these igneous deposits are often of a significant local importance for fertiliser industries or, alternatively, produce phosphate as a by- product of other minerals mined within the carbonatite host. The host mineral for phosphate at Araxá is apatite.
The Araxá Project forms a small part of the carbonatite complex and is situated on the northern limit of the pipe. The local geology, within this part of the complex, is comprised of phlogopite and subordinate beforite rock types. The surface is covered by hard iron-rich laterite, locally called “Canga”. Colluvial material covers the laterite on a saddle between the slopes of two small hills. There are limited exposures of highly weathered carbonatite.
The Araxá Project has defined a mineralised target area of 650m x 250m, as a result of the location of the boreholes associated with the exploration programme. Mineralisation is known to extend to a depth of 80m based upon this drilling. It is important to note that mineralisation is open ended in all directions and has been limited along the permit boundaries, positioning of the boreholes and by the maximum drill depth.
Host Minerals
The three most common and economically exploitable phosphorous minerals are the oxy- salt phosphate minerals, xenotime YPO4, monazite (Ce,La,Nd,Th)PO4 and apatite Ca5(PO4)F,Cl,OH). Phosphorous does not occur in its elemental form and is readily oxidised at the earths surface into PO43- or, alternatively, HPO4- under acidic conditions.
Classification of Phosphate Deposits
Phosphate rocks are derived principally from two primary sources, namely phosphorous rich sediments known as phosphorites, or from carbonatite and alkaline igneous intrusive rocks, The predominant source of the massive worldwide fertiliser phosphorous industry is the sedimentary phosphorites deposits which are of variable ages and typically are deposited along continental shelves and in shallow marine settings. Most major phosphorite deposits are located close to current day coastlines within the tropical regions of the globe.
Phosphate Mineralisation in Carbonatites
The predominant phosphorous mineral of economic importance hosted in carbonatite is apatite with lesser amounts of monazite and xenotime. Indeed, some carbonatite complexes have separate apatite rich intrusive phases where apatite is the dominant mineral in that sequence of the intrusion, such as the Phalaborwa Complex, Limpopo Province, South Africa.
The Araxá is located within 1km of the Vale's phosphate mine and SSP fertiliser plant, which is also hosted within the Barreiro Carbonatite.
The Araxá is classed as a carbonatite deposit with REE (rare earth elements), niobium and phosphate mineralisation which has been enriched as a result of the effects of lateritisation in a tropical climate.
Exploration within the licence boundary has defined a mineralised area with the dimensions 650m x 250m x 80m deep. The areal extent is defined by the limit of the licence boundary and extent of drilling, whilst the depth is limited to the maximum depth of the drilling. Therefore, it must be noted that mineralisation is open ended in all directions.