Abstract
Zircon from granites of the Jamon (JS), Serra dos Carajas (SCS) and Velho Guilherme (VGS) suites were studied by SEM trough backscattered and cathodoluminescence images and EDS analyses. Granites and greisens of the VGS have zircons dominantly anhedral, altered and intensely corroded, enriched in Hf and with the lowest Zr/Hf ratios. In the granites, these ratios decrease towards the more evolved varieties. Zircons of the JS are euhedral to subhedral, zoned and little altered, comparatively depleted in Hf and displaying the highest Zr/Hf ratios, indicating limited potential for tin associated mineralization. Zircons from granites of the SCS are subhedral to anhedral, altered and corroded showing Hf contents and Zr/Hf ratios intermediate to those of the JS and VGS. The granites of the VGS with associated mineralization of Sn, Ta, and W contain zircons with Zr/Hf ratios varying from 7 to 22. It is concluded that ratios of that order can be used as a prospecting guide of specialized granites. On the other hand, zircons from the greisens associated with the Cigano Granite of the SCS have average Zr/Hf ratios around 23, but no cassiterite was found in these rocks. This indicates that those zircons preserved their magmatic geochemical signature. This study distinguished the three granitic suites in terms of zircon composition and demonstrated the importance of their geochemical signature, especially in terms of Zr/Hf ratio, in the identification of specialized granites. EDS-SEM analysis can thus be used in a preliminary assessment of the metallogenic potential of tin granites.
Highlights
Zircon is a mineral with a high chemical resistance that is often used in dating rocks, characterizing magmatic sources, determining the degree of magmatic fractionation and studying provenance
The greisens mineralized in Sn, W and Ta of this suite contain zircons with the Zr/Hf ratios between 5 and 22, indicating that zircons from reduced granites with the Zr/Hf ratios of this order may be used in preliminary prospective studies of specialized granite bodies
Zircons from the Serra dos Carajás suite (SCS) showed medium contents of Hf + Y + Th + U and Zr/Hf ratios intermediate between those found in the zircons of the Jamon suite (JS) and Velho Guilherme suite (VGS) (Tab. 2)
Summary
Zircon is a mineral with a high chemical resistance that is often used in dating rocks, characterizing magmatic sources, determining the degree of magmatic fractionation and studying provenance. CL is used as a petrological tool as certain minerals, including zircon, exhibit CL when bombarded with electrons The use of these two techniques has allowed the identification of (1) distinct patterns of zoning, a feature typical of magmatic zircon and which indicate variations in the concentrations of Zr, Si, Hf, Y, P, Th, U, REE, Ca and other elements present in the crystal structure of the zircon (Hanchar & Miller 1993; Hanchar & Hudnick 1995; Fowler et al 2002); (2) inherited cores in magmatic zircons, which are surrounded by overgrowths (Paterson et al 1992b; Vavra et al 1996); (3) modifications caused during the later and/or post-magmatic stages such as an interruption of zoning, which can cause an appearance of patches and irregular edges enriched in certain components (e.g., Hf and Ca) and (4) the presence of xenocryst inclusions (i.e., apatite or garnet) or mineral phases introduced after the formation of the zircon crystal (i.e., thorite, xenotime, cassiterite, monazite or fluorite). It has been demonstrated that zircons of specialized granites have a significant enrichment of Hf, Y, Th, U and Ca, and have low Zr/Hf ratios, indicating that the composition of zircons can be a useful guide for the preliminary assessment of the metallogenic potential of tin-mineralized granites and associated metals (Wang et al 2000; Kempe et al 2004; Lamarão et al 2007, 2010, 2012)
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