Granitos pórfiros da região de vila Riozinho, província aurífera do Tapajós: petrografia e geocronologia

Abstract

Palavras-chave: craton Amazonico, Provincia Aurifera do Tapajos, granitos porfiros, imagens retro-espalhadas. Abstract Porphyry granites of vila Riozinho region, Tapajos gold province: petrography and geo- chronology. The Tapajos Gold Province (TGP) is located in the center of the Amazonian craton. It is domi- nated by Paleoproterozoic calc-alkaline and A-type granitoids and intermediate to felsic volcanic sequences. The Vila Riozinho region, situated in the central-eastern part of the TGP and constituted by the Vila Riozinho, Moraes Almeida and Jardim do Ouro villages, has the more important geologic units that characterize this province. The aim of this paper is to present new petrographic and geochronological data on the porphyry granites associated with the dominant monzogranitic rocks of the Older Sao Jorge granite and with ignimbrites and rhyolites of the Moraes Almeida Formation and leucogranitic rocks of the Maloquinha suite. The geochronological data revealed at least two distinct periods of generation of porphyry granites in the region. The first, associated with 1.98 Ga calc-alkaline magmatismo, and the second with 1.88 Ga alkaline rocks. Backscattered images and analyses of EDS (energy dispersive spectrometry) obtained with a SEM (scanning electron microscope) showed that is frequent the occurrences of gold particles in the porphyry granites filling cavities in quartz and plagioclase, or associated with epidote and alkali feldspar. These data suggest that the porphyry granites can have contributed to gold mineralization of the Vila Riozinho region, and shows that detailed studies are necessary to elucidate metal- logenetic importance of these rocks in the gold mineralization of the region.