REE, Y, Nb, U, and Th contents and tetrad effect in zircon from a magmatic-hydrothermal F-rich system of Sn-rare metal-cryolite mineralized granites from the Pitinga Mine, Amazonia, Brazil

The geochemistry of zircon from the granites that host the Sn-Rare-Metal-cryolite deposit of Pitinga Mine in northern Brazil, Amazonia, is discussed based on data obtained by LA-ICP-MS. The ore deposit is one of the largest in the world and is related with F-rich A-type granite intrusions of 1822 +/- 2 Ma. REE, Y, U, Th, Nb, Ta, Pb, and Hf contents were determined in zircon grains from the albite-bearing facies that contains the ore deposit and from less evolved facies composed of amphibole-biotite and biotite granites. The trace-element contents of zircon were compared to those of their host rocks and the calculated zircon/rock ratios are like the values of zircon/melt partition coefficients for natural granitic compositions. The concentrations found for all analysed elements are highly variable, even for determinations made in the same grain. However, the average contents and patterns are like those of typical magmatic zircon and can indicate the composition of the melts from which they were crystallized. The interpretation of trace element contents in the zircon grains suggests that: (i) in the albite-bearing fades, zircon crystallized after the volatile phase exsolution and shows typical geochemical features such as: Th/U ratios from 1 to 10, Y/Ho is lower than 20, Sm/Nd ratios are generally higher than 0.5, Nb/Y is higher than 0.08, and Hf is over 2 wt%; (ii) M-type tetrad effects were produced in the REE patterns of most differentiated melts by F-complex stabilization, and were preserved in some zircon grains; (iii) ore deposition in the Pitinga mine initiated in the late stages of magmatic crystallization mainly following resurgent boiling. The trace element contents of zircon are particularly relevant for provenance studies if mineral/melt partition coefficients are taken into account, so that the approximate trace element pattern of their igneous source can be estimated. The geochemistry of trace elements in zircon, in spite of the wide range of contents, can produce a very significant set of information useful for metallogenetic, petrogenetic, exploration, geochronological, and provenance studies. (C) 2011 Elsevier Ltd. All rights reserved.