Comportamento geoquímico de etr durante evolução magmática e alteração hidrotermal de granitos: exemplos da província estanífera de Goiás

Abstract

The A-type granitic massifs of the Parana (SPP) and Tocantins (SPT) tin sub-provinces have anomalous rare earth element (REE) contents, sometimes 10 3 greater than the chondrite. Two suites of granitic rocks are present in the SPP, g1 and g2. Rocks of g1 suite (g1a, g1b and g1c facies) display an alkalic affinity whereas g2 granites (g2b, g2c and g2d facies) are metaluminous to peraluminous. Granites of the Serra Dourada Massif, in the SPT, are distributed between Border, Main and Porphiritic facies. REE-bearing minerals described in these rocks are apatite, allanite, monazite, basnaesite, REE-oxyfluorides, fluocerite, zircon and xenotime. Those, only thorite wasn’t observed in the Serra Dourada Massif (SPT). The REE are incompatible at the early stages of g1 magma evolution and compatible at the late stages. In all g2 granites, LREE are compatible, whereas HREE concentrations remain unchanged during magma differentiation. In the Serra Dourada granites, the LREE contents are almost constant in all facies while the HREE are incompatible. In all situations, the REE behavior is in agreement with a magma evolution by fractional crystallization. The REE behavior during hydrothermal alteration is contrasting between different granites of both sub-provinces In the SPP massifs, where greisenization dominates, the amount of REE decreases in the altered rocks, whereas in the Serra Dourada Massif, where albitization and biotitization are the main alteration process, the amount of REE increases in the altered rocks. LREE patterns and LREE concentrations of the granites and altered rocks are controlled by allanite, monazite, apatite and the secondary REE minerals. Except for apatite, all these minerals are also important HREE carriers, controlling HREE patterns together with xenotime and thorite. The influence of zircon in these parameters is inexpressive, despite its importance as the main accessory mineral and its enrichment in Y and HREE in some granites.