GEOLOGIA E CONTROLE ESTRUTURAL DOS DEPÓSITOS DE OURO DO GRUPO AGUAPEÍ - REGIÃO DA LAVRINHA, SUDOESTE DO CRÁTON AMAZÔNICO

Abstract

GEOLOGY AND STRUCTURAL CONTROL OF THE GOLD DEPOSITS IN THE AGUAPEI GROUP - LAVRINHA REGION, SOUTHWESTERN OF THE AMAZONIAN CRATON - BRAZIL The southwestern portion of the Amazonian Craton, in the Mato Grosso state, Central Brazil contains important gold concentrations that are associated with the tectonic and therm al evolution of the Aguapei folded belt. In these concentrations gold is associated with metasedimentary rocks of the Aguapei Grou p that cross the Brazilian border and has been called as Sunsas Group in Bolivia. The Lavrinha region at the central portion of the Ag uapei folded belt, is located in the fold-hinge of the Cagado anticline and represents a domain of low angle tangencial tectonism tha t demonstrate an allochthonism of the Aguapei Group sediments from SW to NE. In this site, the deformation on the Aguapei group generates a boundary of this group with the basement rocks which is represented by granitic-gnaissic rocks. The boundary forms a 2 to 3 metres large shear zone, called Morro Solteiro Shear Zone. Nine gold deposits are mapped at the Lavrinha region, distrib uted in three levels (inferior, intermediate and superior). The inferior level is represented by the Morro Solterio Shear Zone, and has been considered to be the most important for gold exploration. The intermediate level is represented by a series of stick-slip movem ents along the bedding plane developed in the metaconglomerate and quartzites with metassiltites and metamudstone intercalation. The superior level is associated with metassiltites and metasandstone. Gold mineralization is generated by the percolation of hydro thermal- metamorphic fluids of aqueous to aqueous-carbonic composition presenting a elevated H 2 O/CO 2 ratio, low salinite and high sulphur activity. These fluids were generated by the end of the Sunsas-Aguapei orogeny (1.1 - 0.95Ga). The mineralogical assemblage is represented by pyrite, magnetite, hematite, ilmenite and chalcopyrite. The presence of pyrite boxworks and hematite associated with magnetite and leucoxene represents the weathering of the sulphide and oxide minerals. Thus, native gold disseminated in the hydrothermal altered mylonites indicate a secondary enrichment of gold at the Lavrinha region.