Abstract
The Ipitinga Group (2.27 Ga) at the Serra do Ipitinga consists of a meta-volcano-sedimentaryunit of mafic and ultramafic metavo1canics, cordierite-antophyllite-and quartzo-chlorite-bearing rocks under1ying chemical (oxide and silicatetype BIFs) and clastic metasedimentary rocks. Part of the mafic and ultramafic lavas was hydrothermally altered, which gave rise to quartz-chlorite-bearing rocks and hydrothermal syndepositional exhalative vo1canogenic sulfide mineralization, formed by pyrrhotite, pyrite, chalcopyrite and sphalerite with Au and Ag and traces of galena , molibdenite and silver telluride. Ali the meta-volcano-sedimentary unit was deformed by ductile-brittle shearing and transformed in present-day NW-SE trending shear belts with a subvertical - NE dip. During this defomational shearing episode, hydrothermal fluids yielded gold-sheared Iodes. Twelve types of fluid inclusions identified in the Ipitinga Group distinguish two fluid systems related to three peculiar hydrothermal events. The quartz of the quartz-chlorite-bearing roeks (hydrothermally - altered basalts), the main host of the volcanogenic sulfide mineralization, eontains eleven types of fluid inclusions. Eight of these oceur only in these rocks, which can be considered to have resulted from fluid-rock reactions related to the first hydrothermal event (1). This event involved an aqueous earbonic fluid consisting mainly of CH 4 and Hp, with traces of N 2 and H 2 S. CO 2 -bearing inclusions are subordinate and contain traces of CH 4 , H 2 S and N 2. The predominance of CH 4 over CO 2 in the aqueous earbonic fluid system is coherent to proposed models for the generation of exhalative volcanogenic sulfide mineralization in mid - oceanic ridge hydrothermal systems. The presence of CH 4 in the first hydrothermal event in the Ipitinga Group is evidence of an ocean-floor metarnorphic-hydrotherrnal process, in an environment similar to present-day ocean spreading eenters. Pressure and temperature values for the sulfide mineralization were considered to be in the range of 0.7 - 2.3 kbar and 250 - 450°C, respectively. Aqueous saline fluid related to the second hidrothermal event (II) oceurs only in the cordierite-anthophyllite rocks. Quartz veins are the products of a third hydrothermal event related to duetile shearing (III) and contain inclusions of an aqueous saline fluid which can also be found in quartz of the quartz-chlorite-bearing roeks. The fluid inelusion study also pointed out the relative chronology for the hydrothermal-metarnorphic events in the Ipitinga Group.
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