POCOS DE CALDAS

Thanks to Peter G.Seroka for the Woolley book, which gave me the geological information, and - as always - to László Horváth for his mineral list !!

Pocos de Caldas is one of the largest intrusions of the world (some 800 km²) and extends into the two brazilian states of Sao Paulo and Minas Gerais.

Emplaced in precambrian basement rocks and jurassic sandstones the intrusion consists of 78% tinguaite/phonolite, 13.5% nepheline syentie, phonolitic lavas 5%, volcanoclastics 3% and lujavrite/khibinite 0.5%. Between nepheline syenite and tinguaite there are intrusive boundaries. In the W and S there volcanics (tuffs, breccia etc) and sediments. In the N there are eudialyte-bearing bodies of lujavrite/khibinite. In the W there are also sheeted rocks with rich mineralisation.

Hydrothermal aletration and ore deposition have occured in all rock types, with emphasis on the inner tinguaite and nepheline syenite.

The age of the intriusion is between 52.9 and 89.8 million years.

Pocos de Caldas is one of the world´s largest baddleyite deposits (veins of baddeleyite and zircon) with just small-scale production left nowadays. Bauxite is still mined extensively. Urianium mining has also been done.

Mineral list (still incomplete):

Aegirine

Albite

Analcime

Arfvedsonite

Astrophyllite

Baddeleyite

Bastnaesite-(Ce)

Biotite

Burbankite

Cancrinite

Catapleiite

Cerianite

Eudialyte

Fluorite

Gaidonnayite

Hainite

Lamprophyllite

Lavenite

Magnetite

Microcline

Molybdenite

Mosandrite

Pyrite

Rinkite

Rosenbuschite

Titanite

Zircon

&Suggested reading:

Valeton,I et al: Supergene alteration since the upper Cretaceous on alkaline igneous and metasomatic rocks of the Pocos de Caldas ring complex, Minas Gerais, Brazil. APPLIED GEOCHEMISTRY, 12(1997), 133-154

Woolley, A.R.: Alkaline Rocks and Carbonatites of the World. Part 1: North and South America. British Museum (Natural History), London, 1987

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