Ambiente geológico e mineralizações associadas ao granito Serra Dourada (extremidade meridional) Goiás

Abstract

The Serra Dourada granite belongs to a set of domic structures, generally mineralized in cassiterite, located in the center-east of Goiás, where rocks of the Uruaçu and Brasília folding belts and the Goiás median massif dominate. In order to contribute to the petrological, metallogenetic and stratigraphic knowledge of these granitic rocks, the southern tip of Serra Dourada was selected for this study. The methodology adopted was mapping at a 1:45,000 scale, petrographic, minerographic and geochronological studies, in addition to the determination of the contents of major elements in rocks and some minerals, and of trace elements in rocks. The granitic rocks of Serra Dourada were classified as syenogranites, presenting three varieties: amphibole-biotite granite, muscovite-biotite granite and biotite granite, the latter being dominant. The K-Rb graph indicates an advanced degree of fractionation for these rocks and suggests a trend that starts from granite to amphibole and ends in muscovite. In turn, the systematic variation of the contents and ratios of some trace elements reveals an intimate relationship between these varieties, meaning multiple intrusions that correspond to different degrees of partial fusion of the original material. In attempts at dating by the Rb-SR method, it was observed that the phenomena subsequent to the initial lodging in the crust introduced possible isotopic rejuvenations. However, these granitic rocks provided maximum conventional ages close to 2 b.a. The last magmatic phases of the Serra Dourada granite were the pegmatites which, in the core of the batholith, are zoned and contain aquamarine, while at the edge they bear tantalite-columbite, emerald, muscovite and monazite. Then, large amounts of hydrothermal solutions enriched in Sn and F reached both the granite and its host, changing them to greisens. Upon contacting the enclaves, the solutions precipitated cassiterite, magnetite, fluorite and sulfides. Veins with wolframite and rutile lodged in the nearest enclaves. At lower temperatures, these solutions generated kaolin when reaching the pegmatites of the contact range. Several types of enclaves have been identified in the granite: biotitite, soda-gneiss, xenoliths of schists and quartzites, and amphibolites. The soda-gneiss enclaves are trondhjemitic in nature and also have amphibole and biotite, biotite and biotite and muscovite varieties. The similarity of the assemblage and chemistry of some mineralogical phases suggests a consanguinity between soda-gneiss and granite, with the possibility that they are partially intact fragments of the rocks that gave rise, by anatexia, to the granitic material. On the other hand, the contents and anomalous ratios of some elements of soda-gneiss indicate reaction with magma, which is emphasized by the position of these rocks in the K-Rb graph. This reaction certainly affected the isotopic ratios, allowing only to suggest an Archean age. In turn, the biotites are possibly restricted. The sequence where the Serra Dourada granite was lodged is composed of intercalations of schists and quartzites from the Serra da Mesa Group. The typical mineralizations of acid magmatism, greissens, pegmatites and granitic sills in the metasediments, in addition to xenoliths from the enclosing rocks and pronounced foliation at the edges of the body, testify to the intrusive character of the granite in these metamorphites, whose late-syntectonic event is associated with the formation of the brachyanticlinium, which agrees with the regional structural pattern. Through the mineralogical assemblage of these rocks, conditions of low amphibolite facies were attested for its formation, where pressures above 4.5 Kb and temperatures around 550°C.