OS DIAMANTES DE SÃO JOÃO DO ARAGUAIA NO ESTADO DO PARÁ (AMAZÔNIA): HISTÓRICO, EXTRAÇÃO, MINERALOGIA E VALORIZAÇÃO

Abstract

The stretch of Baixo Rio Tocantins between the cities of São João do Araguaia and Tucuruí, geologically is placed at the borders of the Amazon Craton. Historically, it was one of the richest Brazilian diamond producing areas. The produced stones had good acceptance on the market, being characterized by their abundance, their relatively small size (rare above l ct), the predominance of yellow colors, external morphology many times abraded, ratio gems/industrial stones said until 90%. The largest stone found is said to have been of 51 ct (gem quality). At present times the garimpagem activity is practically extinct. During the present work, alluvial diamonds collected during the years of 1996/97 at São João do Araguaia/PA were characterized mineralogically. Some inclusions, hosted by these diamonds, were submitted to analysis by scanning electron microscope with energy dispersive spectroscopy (MEV/EDS). The predominant weight of the diamonds was found to be among 0.5 to 0.75 ct. The most frequent colors were yellow with different hues, the most frequent crystalline forms dodecahedrons. The medium value of the collected diamonds was evaluated at US$ 100. – per carat. Inclusions hosted by diamonds sampled at São João do Araguaia, can be related to the ultra-mafic suite. The samples collected at São João do Araguaia apparently represent a mix of two different diamond populations at the Baixo Rio Tocantins area. One subjected to strong corrosion and dissolution conditions acting during her emplacement summed to a relatively long geological history. The other population apparently was only weakly subjected to those conditions allied to a relatively shorter geological history. The first population may correspond to most of the diamonds mined during the production boom. Their origin apparently could be related to the Poti Formation. The other is apparently related to a nearer situated, may be primary, origin. Keywords: Amazon region; mineral inclusion; carat; dodecahedron; gold.