Estudo da alteração hidrotermal,com ênfase no metamorfismo sódico, de rochas granitóides e máficas da região de Canaã de Carajás, Província Mineral de Carajás

Abstract

The Sossego and Serra Dourada areas are located in the southern part of Carajas ridge, at the contact zone between the basement rocks (Xingu Complex; ~2.8 Ga) and the Grão Pará Group (2.76 Ga). Serra Dourada lies 15 km east of the Sossego deposit, at the transition strip that bounds the Itacaiúnas block and the Rio Maria granite-greenstone terrane. In the Sossego deposit granitoids, metavolcanic rocks, biotite-rich and mafic rocks were investigated, all of them were hydrothermalized and/or deformed to a greater or lesser extent. In Serra Dourada granitoids, their enclaves and mafic rocks were studied, the former being represented by unaltered, altered and milonitized varieties. The likely protoliths of the apogranitoids and metavolcanic rocks that occur in Sossego were syenogranites/granodiorites and porphyritic quartzodiorite/diorite, respectively, while the mafic altered rocks were derived from microgabros. The porphyroclastic texture of the biotite-rich rocks may be an evidence for their protoliths being, at least in part, the same as those of the metavolcanic rocks. Apogranitoids, metavolcanic and biotite-rich rocks have similar REE patterns, suggesting that they might be comagmatic, although some samples of the biotite-rich rocks show higher La/Yb ratios. Concerning the contents of less mobile elements (Nb and Zr), the metavolcanic and biotite-rich rocks are much more similar. Albitization, scapolitization, amphibolitization, chloritization, biotitization, epidotization and potassic feldspatization were the most important hydrothermal processes that affected those protoliths. Albitization is more common in the apogranitoids and acid metavolcanic rocks, whereas scapolitization is more typical of the biotite-rich and mafic rocks. In the apogranitoids two albite generations (I and II) were identified. The albite I represents crystals with chessboard texture produced by the replacement of potassium feldspar. Albite II is present in monomineralic veinlets which correspond to one of the latest stage of hydrothermal activity. Only albite I occurs in all granitoid varieties. In the mafic rocks two scapolite generations were recognized. One refers to crystals aggregates that replaced the primary plagioclase and the other to fine sinuous veinlets of scapolite+Mg-hornblende that, in these rocks, mark the latest stage of the hydrothermal alteration. In the biotite-rich rocks only scapolite I was identified. The scapolite that occurs in both the mafic and biotite-rich rocks have similar meionite (Me=27-28%) and Cl (3-4%) contents, and were most likely formed by a NaCl-rich fluid, with little influence of the rock composition. This fluid might have also been responsible for the production of albite in all analyzed samples. This albite is purer in the apogranitoids (Ab=98.5-99.3%) and the metavolcanic rocks (Ab=99-99.3%) than in the other rocks. In Serra Dourada area, the granitoids include syenogranites, granodiorite and tonalites as well as enclaves of tonalitic composition. Their mineralogical characteristics and calc-alkaline affinity allow them to be classified as I-type granitoids related to subduction with some influence of extensional regime. Most geochemical data indicate that they are cogenetic and have been produced by fractional crystallization. Albitization, scapolitization and minor biotitization were the most important types of alteration that affected those rocks, the first being restricted to the syenogranites, while the other two types are recorded only in the tonalites. The syenogranites present different degrees of alteration ranging from aposyenogranite without hydrothermal albite to albitite with ~75% of hydrothermal albite. In all granitoids, the hydrothermal albite is purer than the magmatic albite. Mass-balance calculations shows that, in relation to the average composition of syenogranites and for volume factors of 0.8-1.1, the albitites gained Na2O and Cu, and lost K2O, Ba, Rb, Sr and W. The scapolitized rocks resulted from the alteration of tonalites and are mainly characterized by scapolite-rich (up to ~70%) veins/veinlets that cross-cut the rocks. In the moderately altered varieties, the scapolite composition is richer in Na (Me=24%) and Cl (4%) in comparison to that present in veins and in the mylonitized granitoids. Massbalance calculations indicate that the formation of the scapolitized rocks was accompanied by losses of Al2O3, MgO, CaO, Sr and Zr and gains of Fe2O3(t), Na2O, K2O, volatiles, Rb, Ba and Cu. The fluids that caused albitization and scapolitization of the rocks were Na and Cl-rich and, apparently, the albitization of the syenogranites preceded the scapolitization of the tonalites. The scapolite composition seems to have been strongly controlled by the composition of primary plagioclase, as evidenced by its more calcic character in the mafic rocks than in the apotonalites. Estimates of Cl content (1-2%) of rocks from both areas were based on its concentrations in modal scapolite, biotite and amphiboles. The stability of scapolite requires high salinity fluids as those that have been reported in the Sossego deposit. No fluid inclusion data concerning the Na-rich fluids responsible for albitization and scapolitization in the Serra Dourada area are yet available, although the evidences, notably the abundance of scapolite, suggest an evaporitic or similar source.