Caracterização das rochas hospedeiras e da mineralização sulfetada do Alvo Estrela (Cu-Au), Serra dos Carajás, Pará

Abstract

The host rocks for the Estrela Cu-Au deposit in the Serra dos Carajás region are calc alkaline and cogenetic andesites and gabbros of the Grão Pará Group, of the Itacaiunas Supergroup, formed by 2.7 Ga. The deposit is in a 400-600 m thick sequence of altered andesites and gabbros, composed by hastingsite, Fe-pargasite, Fe-hornblende, oligoclaseandesine, albite, quartz, magnetite and biotite, with minor chamosite, dravite and schorlite. Relict ophitic to subophitic igneous textures are still preserved in these rocks. The gabbros and andesites are from magmatic arc origin, as suggested by Sc/Ti ratios of 0.02-2.28 x 10 -3 for the gabbros and 3.25 x 10 -3 - 1.67 x 10 -3 for the andesites. Crustal contamination is also indicated by the Nd (T) negative values of –3.2. The andesites present higher REE content (ΣREE = 347 a 1786.12 ppm) than the gabbros (ΣREE = 227.38 a 1028.28 ppm), which may reflect the original igneous content or an advanced alteration stage. The second possibility is favored by the similarity of the MORB normalized spidergrams of the Estrela mafic rocks and the Archean and Paleoproterozoic Canadian Basalts from Birch Uchi and La Ronge Domain. The host rocks of the Estrela Cu-Au Deposit have been affected by an early calcicsodic alteration followed by a potassic alteration, accompanied by ferrification and sulfidation, which transformed the igneous protoliths into biotite-rich rocks. The early calcicsodic alteration is represented by hastingsite, Fe-pargasite, Fe-hornblende, oligoclaseandesine, albite, quartz, magnetite and minor Fe-biotite, Fe-epidote and chlorite. The potassic alteration overprinted the calcic-sodic mineral assemblage and is caracterized by siderophyllite, biotite, Fe-epidote, fluorite, radioactive minerals, quartz, chamosite, dravite, schorlite, magnetite, chalcopyrite, pyrite, pyrrhotite, molybdenite and minor bornite. The late alteration stage is represented by a greisenization at localized sites, mainly in the andesites. The greisen mineralogy is quartz, zinnwaldite, Li-muscovite, dravite-schorlite, fluorite, topaz, titanite, F-apatite and chlorite. The last alteration stage post dates the mineralization and greisenization. It is marked by calcite, fluorite, chamosite, topaz, quartz and tourmaline. The ore is epigenetic, occurring in vein breccias, stockworks, and also disseminated in the host rocks, mostly in the andesites. The vein and breccia ore formed at about 1.8 Ga and consist of chalcopyrite, pyrite, minor bornite, molybdenite and magnetite along with quartz, fluorite, albite, siderophyillite, tourmaline, epidote, chamosite, topaz and occasionally calcite. Except for the calcic-sodic alteration, the same mineralogy is observed in the vein fillings and host rocks. The older veins, pre dating the ore, are composed by quartz, albite and magnetite, characterizing a silicification process, which is followed by a potassic alteration, accompanied xi by ferrification and sulfidation. The next stage of veins contains siderophyllite, biotite, Feepidote, fluorite, radioactive minerals, quartz, chamosite, dravite, shorlite, magnetite, chalcopyrite, pyrite, pyrrhotite, molybdenite and minor bornite. Chalcopyrite replaces pyrite and is replaced by pyrrhotite. Gold (0.116-0.759%) was found manly in chalcopyrite. The late alteration veins present quartz, zinnwaldite, Li-muscovite, dravite-shorlite, fluorite, topaz, titanite, F-apatite and chlorite. Calcite, fluorite, chamosite, topaz, quartz and tourmaline veins post date the mineralization and greisenization. Chorite geothermometry temperature indicate an average of 235°C for late stage veins. This alteration sequence suggests that hotter fluids, responsible by the potassic alteration and albitization were oxidizing, alkaline and held high K and Cl activities in addition to high Na:Ca ratios. During the cooling path a decreasing in the Na:Ca ratio probably occurred accompanied by a sharp increasing of F activity, as evidenced by the massive presence of fluorite. Rare epidote and calcite attest to the slightly growing Ca activity towards the latest hydrothermal phase. During the greisenization stage the fluids became reducing and acidic permitting the stabilization of the Li-muscovites and the other greisen mineral assemblages. The vein filling pattern also suggests that the fO2 of the early fluids was buffered by Quartz-Magnetite. The fluids dominating the potassic veins were still oxidizing and probably slightly alkalic, turning into reducing and acidic during the greisenization stage. The pH decrease would increase the chalcopyrite solubility, which may explain its scarcity associated to the greisen. 18OSMOW on vein quartz ( 18O = 9.6-10.2‰), chlorite ( 18O = 1.2‰ e D-47‰) and biotite ( 18O = 3.7‰ e D-7.8‰) indicate that the mineralizing fluids were metamorphic in origin and that the mixture of meteoric water played an important role on the cooling hydrothermal system. This mixture may have reduced the chloride concentration in the fluid, decreasing the chalcopyrite solubility.