Contexto geológico e litogeoquímica das unidades plutônicas-vulcânicas da região de União do Norte, setor leste da Província Aurífera de Alta Floresta (MT)

Abstract

The Alta Floresta Gold Province, eastern portion of the Amazon Craton, represents a tectonic unit composed primarily of plutonic-volcanic sequences generated in a continental arc during the Paleoproterozoic. Particularly in the União do Norte region, easternmost segment of this province and within the municipal limits of Peixoto de Azevedo (MT), the plutonic and volcanic rocks are the host of gold deposits with base metal sulphides (sphalerite+galena±chalcopyrite±digenite), some currently under exploitation by local prospectors (garimpeiros). The geological setting of this region consists essentially of an epiclastic volcaniclastic unit and granitic suites. The former unit contains feldspathic-arenite and feldspathic-wake volcaniclastics interpreted to have been deposited in a retroarc foreland basin in an active continental margin. The main source area for these sediments would have been intermediate volcanic rocks. The granitoids show composition ranging from granodiorite to alkali-feldspar porphyritic granite (União do Norte Porphyry). Mafic to felsic volcanic dikes crosscut both the volcaniclastic unit as the granitoids, which are still overlain by arenaceous sediments of the Dardanelos Formation. The plutonic and volcanic units represent essentially a high-K calc-alkaline, magnesium to ferrous, metaluminous to subordinate alkaline and peraluminous magmatism. Additionally, fi eld and geochemical data also indicate that the granitic suites represent a magmatic series that probably evolved from granodiorites (I-type granite) generated in the onset of a volcanic arc to the emplacement of highly-evolved granitic rocks, such as the União do Norte Porphyry (A-type granite), in a post-collisional setting.