Abstract
The Paraná Ri ver Subprovince is part of the Goiás Tin Province. It includes a basement composed mainly by orthogneisses and migmatites, named Granite-Gneiss Complex, predominantly Paleoproterozoic in age, overlaid by a metasedimentary sequence of the Ticunzal Formation, both of which were intruded by pegmatites. These units are overlain by metasedimentary and minor metavolcanic rocks of the Arai Group. Alkalic granites intrude these rocks, previously subdivided into the gl (~1,770 Ma) and g 2 (~1,580 Ma) families, the later being responsible for the tin mineralization. Rhyolites of the Arai Group are contemporaneous to the alkaline granites. Primary tin mineralization occurs associated with the pegmatites and with late to post-magmatic alterations (greisenization, albitization) in the alkaline granites. Coarse-grained muscovite from Xupé and Riacho dos Cavalos pegmatites yielded K-Ar ages of 2,129±26 Ma and 2,006±24 Ma, respectively. A cassiterite sample from Riacho dos Cavalos Pegmatite was analyzed by U-Pb, confirming the age between 2,023 Ma and 2,277 Ma as the time of tin mineralization. Isochronic Pb-Pb cassiterite dating of the two pegmatites yield Archean ages, indicating that the lead incorporated by the cassiterite and its inclusions and aggregate minerais is old, probably derived from the basement. K-Ar dating of muscovite concentrates from the alkalic granites of the Mangabeira Massif yielded ages of 567±14 Ma and 553±11 Ma, indicating overprinting of the Brasiliano Cycle in the pluton. U-Pb cassiterite dating yield na age of 1,535±57 Ma, which is concordant with the time of crystallization of the granite and indicates that cassiterite hás not behaved as an open system in relation to the Brasiliano deformation. Isochronic Pb-Pb age of the same cassiterite sample (1,425±79 Ma) agrees with the U-Pb age within the analytical error. From these data it is possible to identify two events of tin mineralization in the Subprovince, the oldest of Transamazonian age, represented by the pegmatites within the Granite-Gneiss Complex and the Ticunzal Formation, and the youngest of ca. l ,500 Ma, related to the g 2 granites intrusion
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