Summary:
The Tapira phosphate deposit is part of a series of Late-Cretaceous, carbonatite-bearing alkaline ultramafic plutonic complexes belong to the Alto Paranaiba Igneous Province. The Tapira igneous rocks intrude the phyllites, schists, and quartzites of the Late-Proterozoic Brasilia mobile belt.
The Tapira igneous complex is roughly elliptical, 35 square kilometers (km2) in area and consists predominantly of alkaline pyroxenite rocks with subordinate carbonatite, serpentinite (dunite), glimmerite, syenite, and ultramafic potassic dikes. Locally, the pyroxenites are divided into:
1. Bebedourites: a local name for a variety of biotite pyroxenite composed essentially of aegirine-augite, and biotite with perovskite and opaques.
2. Phoscorites: plutonic ultramafic rocks, containing magnetite, apatite, and one of the silicates, forsterite, diopside, or phlogopite. Phoscorites almost always occur in close association with carbonatites.
The Tapira phosphate deposit was formed by the supergenic alteration of bebedourites, phoscorites, and carbonatites rich in apatite. The extreme weathering process was responsible for the residual concentration of apatite. The weathering processes are typically related to the partial hydrolysis of silicate rocks and the dissolution of carbonates, with a general loss of calcium (Ca), magnesium (Mg), potassium (K), and silica (Si), and the accumulation of aluminum (Al), iron (Fe), and titanium (Ti), from the base to the top.
The main geological types identified in the deposit are a combination of the igneous protoliths (bebedourites, phoscorites, and carbonatites) and the products of the weathering process.
According to the level of weathering, the products are:
- Alloterite: The top layer consisting of intense reddish autochthonous soils.
- Top isalterite (saprolite): Profile with an average depth of 25 m with clayish-sandy material that can be yellow to reddish. Homogenization of mineral phases does not allow the rock structures to be identified.
- Bottom isalterite (saprolite): Profile with an average depth of 25 m, resulting from the advanced weathering of the altered rock horizon. Some primary rock structures can still be observed, but the overall appearance is that of homogenous altered soil. This is the mainly phosphate mineralized horizon.
- Semi-weathered rock: Weathered horizon in which the rock structure is mostly preserved.
- Fresh rock: Mainly bebedourites and phoscorites intruded by carbonatite veins.