Abstract
This work aims at the geochemical study of Pitinga cryolite mineralization through REE and Y analyses in disseminated and massive cryolite ore deposits, as well as in fluorite occurrences. REE signatures in fluorite and cryolite are similar to those in the Madeira albite granite. The highest ΣREE values are found in magmatic cryolite (677 to 1345 ppm); ΣREE is lower in massive cryolite. Average values for the different cryolite types are 10.3 ppm, 6.66 ppm and 8.38 ppm (for nucleated, caramel and white types, respectively). Disseminated fluorite displays higher ΣREE values (1708 and 1526ppm) than fluorite in late veins(34.81ppm). Yttrium concentration is higher in disseminated fluorite and in magmatic cryolite. The evolution of several parameters (REEtotal, LREE/HREE, Y) was followed throughout successive stages of evolution in albite granites and associated mineralization. At the end of the process, late cryolite was formed with low REEtotal content. REE data indicate that the MCD was formed by, and the disseminated ore enriched by (additional formation of hydrothermal disseminated cryolite), hydrothermal fluids, residual from albite granite. The presence of tetrads is poorly defined, although nucleated, caramel and white cryolite types show evidence for tetrad effect.
Highlights
Cryolite (Na3AlF6) is one of the fluorine minerals of major economic importance due to its utilization for aluminum metallurgy
The other three have rare-earth elements (REE) patterns do not differ too much from the core albite granite (CAG) and are similar to border albite granite (BAG) patterns presented by Costi (2000)
Existing genetic models for cryolite in granites include one metasomatic model (Horbe et al 1985, Teixeira et al 1992), which considers the albite granite an apogranite, and two different magmatic models, the first by Lenharo (1998) who suggested that the massive cryolite deposit (MCD) was formed from a F-rich residual liquid which has become immiscible in the silicate liquid, and the second by Costi (2000) who considered that increasing water content as the crystallization of the albite granite progressed led to the separation of aqueous fluids responsible for the formation of coarse-grained rocks in the CAG, whereas a F-rich residual phase formed the massive cryolite bodies of the MCD
Summary
Cryolite (Na3AlF6) is one of the fluorine minerals of major economic importance due to its utilization for aluminum metallurgy. This mineral is so rare that, until now, it had only been exploited economically in Ivigtut (Greenland), from the beginning of last century until reserve exhaustion in 1986. The Pitinga ore deposit is associated with the albite granite facies of the Madeira granite. It is a worldclass Sn deposit containing Nb, Ta and cryolite as coproducts, as well as Zr, rare-earth elements (REE), Y, Li and U, which may be exploited as sub-products. Cryolite mineralization occurs as disseminated ore (reserves around 110 Mtons, with 4.2% of Na3AlF6), as well as a massive cryolite deposit (MCD), with a reserve of 10 Mtons (32% of Na3AlF6)
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