|
Araxa |
|
|
Minas Gerais, Brazil |
| Main commodities:
P Nb REE
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Araxá apatite-niobium deposit, that is hosted by the Barreiro carbonatite-alkaline igneous complex, is situated ~6 km south of the city of Araxá, ~300 km west of Belo Horizonte in Minas Gerais, 410 km SSE of Brasilia, in Brazil, #Location: 19° 38'S, 46°56'W.
The Barreiro complex is located ~30 km NNW of the similar Tapira complex within the 1500 km long NNW-SSE trending Meso- to Neoproterozoic Brasilia fold belt, to the SW of the Archaean to Palaeoproteozoic São Francisco craton, and immediately adjacent to the northeastern margin of the Palaeozoic-Mesozoic Paraná Basin. It has been dated at between 90 to 80 Ma (number of determinations) and 89 Ma (K/Ar biotite), i.e., Late Cretaceous and comprises a circular, 4.5 km diameter intrusion covering ~16 sq. km, the central part of which is mainly occupied by a carbonatite that is predominantly beforsitic in composition.
The igneous complex is intruded into metasediments of the Brasilia fold belt to the west of the São Francisco craton. The intruded basement comprises a sequence of quartzites and schists of the Mesoproterozoic Araxá Group that rest on basic and ultrabasic magmatic rocks, and is a member of the Espinhaço Supergroup that was deposited within an aulacogen (rift basin) to the west of, and laps onto, the São Francisco craton. These sediments were most likely metomorphosed when they were thrust eastward onto the São Francisco craton during both the ~1100 Ma Uruaçu and 600 Ma Brasiliano orogenies. The Paraná Basin to the SW includes the thick, extensive Cretacous Paraná Flood Basalts that temporally overlap the extensive NW-SE-trending, 300 x 100 km, Alto Paranaíba Igneous Province that includes the Barreiro Carbonatite Complex, and is related to deep seated faults within the Brasilia Fold Belt.
The intrusion of the Barreiro complex into quartzites and schists of the Araxá Group caused arching which gave rise to a very conspicuous dome structure with concentric and radial fractures in the country rocks, as well as intense shattering of the quartzites. The complex is composed of carbonatite, glimmerite and phoscorite. The carbonatite occurs as a network of concentric and radial dykes, quite variable in dimension, and as small veins ranging from a few mm to several cm in thickness intruding either alkaline or country-rocks. Additional lithologies include mica-rich rocks and lamprophyres.
Alkali metasomatism of these rocks (especially the quartzites) is evident as an up to up to 2.5 km wide contact aureole exhibiting clear textural, mineralogical and chemical evidence of fenitisation. The fenitisation takes the form of veins and veinlets mainly composed of Na-amphibole cutting the host quartzites. In addition to the metasomatic minerals, which include arfvedsonite and sodic pyroxene, hematite, goethite, anatase, rare-earth phosphate, titanite and pyrochlore also occur in the wallrocks
The Barreiro complex comprises a ~2 km diameter central core of medium- to coarse-grained dolomitic carbonatite, in which dolomite is the major mineral, accompanied by subordinate calcite and ankerite. Barite, apatite, magnetite, perovskite, quartz, pyrite, phlogopite, Na-amphibole, isokite and strontianite may also be present, with bariopyrochlore as a common accessory mineral. Below the thick lateritic cap, the fresh plutonic carbonatite cumulates in the core of the structure contain magnetite with exsolved ilmenite, phlogopite, pyrochlore and ilmenite, with associated apatite, and are cut by thin veins of different widths and grain sizes composed of various mineral assemblages, including the major components dolomite+magnetite+phlogopite, dolomite+magnetite, and dolomite+Fe-Cu sulphide minerals+barium carbonate (norsethite). These mineralised rocks have been encountered to depths in excess of 850 m below the current surface.
Thick masses of phoscorite are associated with the dolomitic carbonatite in the central part of the complex. The fine- to coarse-grained phoscorite is mainly composed of magnetite, apatite, phlogopite and carbonates and contains the highest concentration of pyrochlore within the complex, occurring as individual grains of bariopyrochlore or associated with magnetite in veins. Glimmerite is found as an annulus between the dolomitic carbonatite core and the fenitised quartzite ring. The glimmerite is fine- to coarse-grained and consists mainly of phlogopite, with subordinate dolomite and accessory magnetite and apatite, and contains rare relict pyroxene and olivine. It was produced by metasomatism of ultramafic igneous rocks, probably peridotite and pyroxenite. The metasomatism may have occurred during the magmatic, alkaline-carbonatite stage which involved the necessary alkali-rich fluids.
A circular lateritic cover sheet ~1800 m in diameter and up to 230 m thick occurs in the central part of the domal structure of the complex. Residual bariopyrochlore has become concentrated within the lateritic profile, giving rise to the largest known niobium reserves in the world. The laterite mineralogy is composed of limonite, goethite, barite and magnetite as major phases, with subordinate bariopyrochlore, gorceixite, monazite, ilmenite and quartz.
Two types of niobium ores are found at Araxá, the primary ore as described above, with a mean grade of 1.5% Nb2O5, and a maximum grade of 8% Nb2O5, and the friable residual ore with a mean concentration of 2.5% Nb2O5. Only the latter is exploited by open-pit mining. Weathering enriched the niobium ore minerals, which were physically preserved but chemically modified during leaching, and concentrated through through volume reduction during the lateritisation process. Pyrochlore was progressively transformed into bariopyrochlore along fractures and at grain boundaries by the substitution of calcium and sodium by barium, probably due to the action of meteoric waters.
The deposit has a resource of residual ore up to 250 m thick, totalling ~450 Mt @ 2.5 % Nb2O5, although drilling in 2001 revealed additional zones with 4.6 to 7 % Nb2O5
Primary apatite and late-formed strontium apatite are restricted to the primary phoscorite ore. The main phosphate-bearing mineral in the residual ore is very fine-grained neoformed crandallite-group mineral gorceixite (BaAl3(PO4)(PO3OH)(OH)6), believed to have been formed at the expense of apatite, which supplies P, as well as part of the Ca and REE; tetraferriphlogopite (supplying Al and some Ba) and carbonate minerals (Ca, Sr, REE and some Ba).
Reserves in 2006 (DNPM 2006 Mineral Annuary) were 88.7 Mt @ 11.12% P2O5, concentrated to grades of 33 to 35% P2O5.
This summary was drawn from Traversa et al., 2001, Issa et al., 2003, Nasraoui and Waerenborgh 2001 and Northolt et al., 1989.
The most recent source geological information used to prepare this decription was dated: 2005.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
|
|
|
|
Carvalho, L.M.M., Pires, A.C.B. and Vidotti, R.M., 2015 - Phosphate prospection in the alkaline province of Alto Do Paranaiba, Brazil, based on airborne geophysical data: in Revista Brasileira de Geofisica (Brazilian Journal of Geophysics), v.33, pp. 237-249.
|
Nasraoui M and Waerenborgh J C, 2001 - Fe speciation in weathered pyrochlore-group minerals from the Lueshe and Araxa (Barreiro) carbonatites by 57Fe Mossbauer spectroscopy: in Canadian Mineralogist v.39 pp. 1073-1080
|
Smith, M.P., Moore, K., Kavecsanszki, D., Finch, A.A., Kynicky, J. and Wall, F., 2016 - From mantle to critical zone: A review of large and giant sized deposits of the rare earth elements: in Geoscience Frontiers v.7, pp. 315-334.
|
Traversa G, Gomes C B, Brotzu P, Buraglini N, Morbidelli L, Principato M S, Ronca S and Ruberti E, 2001 - Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxa complex (Alto Paranaiba Province, Brazil): in Anais da Academia Brasileira de Ciencias v.73 pp. 71-98
|
Wang, Z.-Y., Fan, H.-R., Zhou, L., Yang, K.-F. and She, H.-D., 2020 - Carbonatite-Related REE Deposits: An Overview: in Minerals (MDPI) v.10, 26p. doi:10.3390/min10110965.
|
|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
