Régis Munhoz Krás Borges

Tin-Bearing Sodic Episyenites Associated with the Proterozoic, A-Type Água Boa Granite, Pitinga Mine, Amazonian Craton, Brazil

This paper reports the first occurrence of tin-mineralized episyenites in the Amazonian craton. The Agua Boa and Madeira plutons in the Pitinga region are stanniferous Proterozoic A-type granites of the rapakivi series featuring metasomatic episyenitization. The biotite granite facies of the Agua Boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite. The sodic episyenites formed by: (a) albitization of K-feldspar, (b) vug formation by dissolution of magmatic quartz, (c) vug filling by albite, chlorite, lithian muscovite, cassiterite ± fluorite ± K-feldspar and (d) deposition of late quartz±cassiterite in remaining cavities. In the potassic episyenites, vugs produced by quartz dissolution are filled by secondary K-feldspar, which also replaces magmatic phases. Micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites. In contrast to the sodic episyenites, the potassic and micaceous episyenites contain only traces of cassiterite. Relative to unaltered biotite granite, the sodic episyenites are strongly depleted in SiO2, K2O, LREE and Sr, moderately depleted in HREE, and enriched in Na2O, Al2O3, Rb and Sn. The potassic episyenites resemble the sodic episyenite in SiO2 and Al2O3but exhibit higher values of K2O, Ba, Y and Rb; their Na2O, REE, and Sn concentrations match those of the biotite granite host. Metasomatic episyenitization is attributed to: (1) a high temperature gradient and fluid-rock ratio during subsolidus cooling of the Agua Boa pluton, (2) quartz dissolution in the biotite granite facies by reaction with an high-temperature, silica-undersaturated fluid of probable magmatic origin, (3) hydrothermal cavity filling by a low-temperature, silica- saturated fluid of meteoric, or mixed magmatic/meteoric character. Significant tin values in metasomatic sodic episyenites near the Pitinga mine suggest that exploration of similar rocks in other A-type granites may be worthwhile.

Quartzo e zircão como marcadores da evolução magmático-hidrotermal do Granito Antônio Vicente, Suíte Intrusiva Velho Guilherme, Província Carajás

Four morphological and textural types of quartz, informally labeled Qz1, Qz2, Qz3 and Qz4, were identified in the different facies of the Antonio Vicente Granite, Carajas Province, by scanning electron microscope-cathodoluminescence (SEM-CL) images. In the less evolved rocks, containing amphibole and biotite, well developed anhedral to subhedral, luminescent and intensely fractured crystals dominate, named Qz1. Hydrothermal fluids that percolated the granite modified the magmatic quartz (Qz1) into Qz2 and Qz3 through processes of alteration, dissolution and recrystallization, with these changes much more evident in the intensely altered syenogranite rocks. Qz4 constitute medium-to-coarse grained crystals, usually luminescent and comparatively little fractured. Its occurrence is restricted to strongly hydrotermalized syenogranite rocks and bodies of greisens, suggesting the beginning of the greisenization process. In the greisens, medium-to-coarse grained euhedral, concentrically zoned quartz crystals dominate, with typical features of hydrothermal origin (Qz5). Fine crystals of zoned cassiterite (≤ 100 µm) are common and fill cavities in the types Qz4 and Qz5. Zircon crystals dominantly anhedral, corroded, with the highest contents of Hf and the lower Zr/Hf ratios belong to more evolved and hydrothermally altered rocks and to associated greisens, both carriers of Sn mineralization. This fact suggests that the geochemical signature of zircon, especially Zr/Hf ratio, can be used for the preliminary assessment of metallogenic potential of tin granites.

Variação morfológica e composicional de zircão e suas implicações metalogenéticas: o exemplo das suítes Jamon, serra dos Carajás e Velho Guilherme, Cráton Amazônico

Zircon from granites of the Jamon suite (JS), Serra dos Carajas suite (SCS) and Velho Guilherme suite (VGS) were studied by scanning electron microscope (SEM) through backscattered electron and cathodoluminescence images and energy dispersive spectroscopy (EDS) analyses. Granites and greisens of the VGS have predominantly anhedral zircons that are altered and intensely corroded, are enriched in Hf and have the lowest Zr/Hf ratios of the studied suites. In the granites, these ratios decrease towards the more evolved varieties. Zircons of the JS are euhedral to subhedral, zoned and slightly altered; they are also comparatively depleted in Hf and display 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, and show Hf contents and Zr/Hf ratios intermediate to those of the JS and VGS. The granites of the VGS with Sn-, Ta- and W-associated mineralization contain zircons with Zr/Hf ratios varying from 7 to 22. It is concluded that ratios of similar magnitude can be used as a prospecting guide for specialized granites. Zircons from the greisens associated with the Cigano granite of the SCS have average Zr/Hf ratios of approximately 23, but no cassiterite was found in these rocks, indicating that the 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 their 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.