Plúton Desemborque: exemplo de corpo a metais raros (Nb, Zr, Hf) e estanho no estado de São Paulo

Abstract

The Desemborque pluton shows a biotite sienogranite composition and crops out in the southernmost part of the Sao Paulo State, in an area of ca. 50 km 2 . It has a circular shape and is probably correlated to the Neoproterozoic III orogenic event. High SiO 2 , K 2 O, Na 2 O and trace elements (Nb and Y) values, low CaO contents and high Fe/Mg ratios point to the alkalic afinity of this singular A-type body. High Zr and Y concentrations are associated with the occurrence of important accessory minerals like zircon, allanite, columbite and possibly xenotime. The Desemborque pluton accessory phases (columbite, cassiterite and hafnian zircon) were analyzed by SEM-EDS. The columbite Nb 2 O 5 and Ta 2 O 5 contents range from 75.4 to 58.9 wt% and 2.1 to 13.7 wt%, respectively. This mineral presents mica (biotite?) as solid inclusion phase. The hafnian zircon HfO 2 values vary from 12.6% to 15.6 wt% (~ 15 mol. %) HfSiO 4 . Thorite, uraninite, magnetite, quartz, mica, tremolite, monazite, xenotime and bastnasite (?) were found as exsolutions and inclusions into the hafnian zircon. Zircon inclusions hosted in cassiterite crystals present the highest HfO 2 values of 21.7 wt%. The analyzed cassiterite grain shows 98.32% to 99.46 wt% SnO 2 , 0.40% to 0.60 wt% Ta 2 O 5 and 0.0% - 0.30 wt% Nb 2 O 5 and columbite, magnetite, ilmenite, wolframite, ilmenorutile, hafnian zircon and ytrio-columbite were the solid inclusions detected in this mineral. The Desemborque pluton resembles rare metals peralkaline granites mineralized in Nb and REE. Meanwhile, Y and REE enriched minerals, as xenotime and monazite, were not ostensively found in the focused granite.