The phosphate e laterite of Piracaua (state of Maranhão) was studied with basis on several lines of evidence, namely, structure of the deposit, mineral distribuition, variations in chemical composition and chemistry of ground waters in the region. The distribuition of elements during the formation of the deposit is interpreted and the conditioning factors analysed. The Pirocaua hill occurs on the coast in the northwestern part of Maranhão state, near the mouth of the river Maracaçume. The hill is 105 m high and the phosphate occurs near the top, and it was possible-to study across section of about 50 m. Five horizons were defined, in the order towards the topgparent rockm which was not' reached, but is probably phyllite of the Gurupi group; transition horizon, with partially.a1tered phyllite; kaolinite horizon; phosphate horizon; and iron riohcrust. The phosphate horizon has an average thickness of 5 to 6 m and contains mostly augelite and smaller amounts of crandallite-goyazite; these are also small amounts of variscite, wavellite and senegalite. Our results show that fluctuations of the hydrostatic level were important during the Formation of the phosphate horizon. When the deposit was formed there was also a decrease of the activity of the silicic acid and a parallel increase of acidity towards the top of the cross-section studied. In these conditions, Fe2+ migrated towards the top of the deposit and was precipitated as Fe3+ in the oxidizing zone. Migration of phosphate was in part due to its affinits to play minerals, in which mechanism ground water played asmajor role. The distribuition of several elements (Fe, P, A1, Si, Ti, Ca, Sr, Mg, Mn, Na, K, B, Mo, Ga, Cu, Ni, V e Cr] shows that: The increase in iron contents from the lowermost horizon to the iron-rich top is related to the increase in the contents of hematite and goethite, similarly, high contents of A1 and P are related to the occurence of Al-and Al-Ca-phosphate. Si contents show little variation in the laterite zone and phosphate horizon; as can be expected, its values are much higher in the kaolioite horizon. Ti shows only a smell variation in the erase section and is found mostly in anatose. Ca and Sr are found mostly in the phosphates, particularly ovandallite-goyazite. Mg, Na and K ware ale strongly bleached during the process. B is obviously associated with tourmaline (dravite). While Mn and Mo show only little variation, the highest concentrations of Ga,V and Cr are fund in the phosphate horizon and iron - rich crust. Cu end Ni show a similar distribuition and their behaviour is probably influenced by their chemical similarity to Mg. Mass balance Calculations indicate that of the parent-rock is a phyllite, it is neeessary an extreme enrichement in P and Sr to give the composition of the phosphate horizon. With basis on these observetions we conclude that: 1) the parent-rock must have contents of P and Sr higher than the average for phyllites; or 2) the phosphate has some other source; We support therefore that drill-holes be made in order to collect samples of the metassediments beneath the deposit to determine the concentrations of the two elements in these rocks.
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