Petrografia e Geocronologia U–Pb em zircão das rochas hospedeiras do depósito IOCG GT-34, Província Carajás

Abstract

The Carajás Province, located in the Amazon Craton, holds a large concentration of world- class mineral deposits. Amid this varied set of mineral systems, there are deposits of iron oxide-copper-gold (IOCG). These deposits are structurally controlled and occur along shear zones in the WNW-ESE direction, which juxtapose neoarchean supracrustal rocksand meso to neoarchean basement rocks of the province. The formation of mineralization in the IOCG occurred in three main pulses during the Neoarchean (ca. 2.72 –2.68 Ga andca. 2.54–2.45 Ga) and the Paleoproterozoic (ca. 1.90–1.88 Ga). The interpretations obtained for the literature data raised a great doubt about what would have promoted the large-scale circulation of hydrothermal fluids to form the high-tonnage IOCG deposits of the Neoarchean, since the mineralization ages do not overlap with the main magmatic event of the Neoarchean. province in this era (ca. 2.76–2.73 Ga). In an attempt to understand this scenario, this research proposal focused on petrographic descriptions and geochronological dating (U-Pb LA-MC-ICPMS) of the host rocks of the IOCG GT-34 deposit, in order to understand the evolutionary context of each rock and possibly a genetic relationship between the mineralization. In view of this, it appears that the rocks of the deposit are predominantly composed of tonalite-trondhjemite granodiorite (TTG) associations of Mesoarchean ages (ca. 3.05, 2.92 and 2.88 Ga) inserted in the Xingu Complex basement and intrusions mafic magmatism still unknown due to the absence of zirconium grains Two petrographic natures are recognized for the mesoarchean rocks, being Biotite-Hornblende-Actinolite Tonalitic Gneisses and Hornblende-Biotite Granodiorite Gneisses, sometimes encompassing amphibolitic enclaves. The occurrences are marked by similar structural and textural features originating from ultra-high temperature metamorphism accompanied by partial melting processes and crystallization of anathetic liquids. Orthogneisses exhibit sodic alterations, gray to pink colors, medium granulation, porphyroclastic texture marked by plagioclase megacrystals. They are accompanied by the development of incipient to well marked foliations and compositional banding, in addition to intensely deformed crystals. The analyzes made for zircon grains from orthogneisses generally show features associated with partial melting of rocks subjected to high temperature metamorphism. For ages between 3054 ±3Ma (MSWD: 7,9), 2924 ±8Ma (MSWD: 8,1) and 2950 ±7Ma (MSWD: 3,2) an association is proposed with the formation of the continental crust of the Carajás Domain under the influence of juvenile lower crust anatexia, while ages ranging between 2880 ±5Ma (MSWD: 5,7) e 2883 ±3Ma (MSWD: 6,3), would be related to the last episode of migmatization that affected the rocks of the Xingu complex. The intrusive gabbro hornblende has preserved igneous texture and is affected by an alteration with albite and hastingsite, which is characteristic of the GT-34 deposit. Correlate the Neoarchean ages of hydrothermal alteration and the ages of mineralization at ca. 2.75 – 2.72 Ga, the hornblende gabbro is younger than the orthogneisses and occurs before (or during) the mineralization event, which could be associated with Neoraquean magmatism. Furthermore, the absence of pyroxenes can be attributed to hydration associated with percolation of syn- or post-magmatic hydrothermal fluids. Mesoarchean rocks indirectlyparticipate in mineralization. Paragenetic associations of younger minerals in TTG felsic crust rocks could be Extensive sodium alterations based on basins with evaporites are consistent. Thus, the probably Neoarchean magmatism that generated the gabbro hornblende, correlated to the extensional environment at ca. 2.74 registered in the Carajás Domain, possibly presents a cogenetic connection with an important role in the movement of fluids for the formation of the deposit. However, a better positioning of these rocks inthe domain depends on new dating attempts.