The Cu-Au Visconde deposit lies at the contact zone between the basement (>3.0Ga) and the Grão Pará Group (2.76 Ga) within the so-called Transition Domain of the Mineral Carajás Province. It is located at about 15 km east of the Cu-Au Sossego mine in the county of Canaã de Carajás, Pará state. Felsic metavolcanic rocks, probably belonging to that group, as well as granitic and mafic intrusions, dominate in the deposit and neighboring area. Subordinate ultramafic bodies occur within the mafic units. All these rocks are moderately to strongly hydrothermally altered and show varying degrees of deformation. Mafic dikes and a granitoid isotropic body, the latter probably related to the granitogenesis of 1.88 Ga, represent the last igneous activity in the area and cut the pre-existing rock package. Despite the mineralogical and textural changes, it was possible to infer a monzogranitic to granodioritic composition to the original granitoids largely based on the amounts of chess-board albite. Moreover, the scarcity of primary mafic minerals makes these granitoids similar to the Planalto Granite, which crops out approximately 7 km east of the deposit area. The mineral content and the partially preserved subophitic texture are suggestive that gabros and/or quartz diorites were potential protoliths of the mafic intrusions. On the other hand, the recognition of plagioclase, quartz and K-feldspar primary phenocrysts and the use of trace elements with limited mobility in the hydrothermal environment allowed discriminating a riodacitic composition for the protolith of the felsic volcanic rocks. Grading from ductile to brittle regimes, the hydrothermal alteration changed from early sodic-calcic assemblages, characterized by ubiquitous albitization, scapolitization or amphibolitization, to late potassic assemblages, in which the K-feldspar and Cl-biotite are the diagnostic minerals. Then the alteration restored its sodic-calcic character as indicated by albite, epidote, apatite, tourmaline and fluorite that replaced pre-existing minerals or filled open spaces. At last, it is recorded a calcic-magnesian stage during which clinochlore/Fe-clinochlore, actinolite, carbonate and subordinate talc were equilibrated. In the granitoids, albitization, epidotization and tourmalinization were the most prominent alteration processes, whereas scapolitization, biotitization, amphibolitization and magnetization were more remarkable in gabros/quartz diorites and K-feldspatization in the felsic metavolcanic rocks. The ore, whose formation began at the final stages of the potassic alteration, was essentially controlled by brittle structures. Initially represented by weak chalcopyrite, molybdenite and pyrite disseminations in the zones altered to tremoliteactinolite, scapolite, albite and magnetite, the mineralization evolved to sulfide concentrations in veins and breccias. Among the sulfides chalcopyrite, bornite, molybdenite are dominant, but pyrite and pentlandite also occur together with apatite, scapolite, actinolite, epidote, magnetite, martite, hematite, calcite, and gypsum or fluorite as the main gangue minerals. The typical metallic suite of the sulfide breccia is Fe–Cu–Ni–ETR±Au±Zn±Y±Co±Se, with ƩETR as high as 1030 ppm. Fluid inclusions trapped in quartz, scapolite, apatite and calcite crystals unraveled at least three aqueous fluids. Fluid 1, simplified by the system H2O-NaCl-CaCl2±MgCl2 and present in all host minerals, was hot (450–500ºC) and very saline (up to 58 wt% equivalent NaCl). The alteration and mineralization haloes should have resulted from the interaction of the host rocks with this fluid, which might have experienced cooling and dilution probably due to mixing with surficial waters. After the mineralization event, the deposit recorded the successive inflow of fluid 2 (H2O-NaCl-FeCl2±MgCl2, up to 30 wt % equiv. NaCl) and fluid 3 (H2O-NaCl±KCl, up to 18 wt % equiv. NaCl), both cooler than fluid 1. With a restrict circulation and preserved only in quartz and apatite crystals, fluid 2 might have been related to the intrusion of the late mafic dikes, whereas fluid 3 migration would have taken place in response to the emplacement of the alkali granite (1.88 Ga?), being trapped, similarly to fluid 1, in all host minerals, but as secondary IF. The high salinity and no evidence of boiling, coupled with the presence of Cl-rich minerals, suggest that a purely magmatic source is unlike for fluid 1. As an alternative, it is assumed a mixed source, involving the migration of magmatic or metamorphic fluids throughout carbonatic-evaporitic sequences from which Na, Ca and Cl have been largely leached. The mineralogical, chemical and microthermometric data allowed to characterize the mineralizing fluid as an aqueous solution consisting of NaCl, CaCl2, KCl, FeCl2 and MgCl2(?) that also carried P, B, F, Y, Ba, Sr, Rb and ETR, Cu, Ni and Co, besides S species. The Sossego and the Visconde deposits present similarities in terms of (1) the nature of the host rocks (felsic metavolcanics, granitoids, and mafic intrusions), (2) the types of alteration, highlighting the intense and widespread sodic-calcic metassomatism, (3) the occurrence of the major ore bodies in brecciated zones and (4) the Fe-Cu-Ni-ETR±Au±Co as the metallic signature of the ore. Regarding the main differences, the sub-economic sulfide accumulations and the smaller amounts of massive magnetitites of the Visconde deposit could be listed.
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