UM VEIO CARBONÁTICO COM TERRAS RARAS E TÓRIO NO MACIÇO DE ITATIAIA, RIO DE JANEIRO

HENRY MAU, J. MOACYR V. COUTINHO

Resumo


In the Itatiaia alkaline massif, one of the largest in the world and Brazil's largest, a narrow vein was found, containing up to 1.9% rare earths, 7% strontium and 0.3% thorium. Mineral idelntification was rendered difficult by extreme fineness of grain, but the following ones were reeognized: grossularite, wollastonite, vesuvianite, calcite, fluorite, apatite, diopside, hornblende, magnetite, hematite, titanite, strontianite, in addition to three non-identified minerals. Semi-quantitative spectrographic analyses of fresh material taken as a section through the vein, contact and into the country rock, showed a more or less gradual decline in grade, from the vein into the syenite, for rare-earths, strontium, thorium, lead, scandium, yttrium, silver, manganese, phosphorus, calcium and magnesium, among others. It is believed 'that vein and syenite were at high and similar temperatures at the time of the intrusion of the vein material. Concentration clarkes were figured for the analyzed elements but no interpretation could be offered at this time. The same is true for the comparison between the fresh samples and one weathered vein sample that was also analyzed. The rare-earths relationship in the vein shows remarkable similarity with the apatite-type distribution. The relatively high grades of, among others, strontium, rare earths and thorium can only be accounted for by diadochic replacement of calcium, in addition, possibly, to the non-identified minerals. It is believed that the vein is carbonatitic on account of occurrence, mineral composition and clement distribution, as compared with examples drawn from litetature. Further studies in the vast, geologically little known massif are necessary to gather more evidence on carbonatites in that area.

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