Petrologia e química mineral dos greisens associados ao Granito Água Boa - Mina Pitinga (AM): um estudo dos processos de formação de greisens

Abstract

This Master's Dissertation has as its main objective the petrographic and mineral chemistry study of the greisens associated with the Água Boa Granite, in the Pitinga mine (AM), based on core samples from a drilling network carried out on the west edge of the body by the Grupo Paranapanema S/A. The Pitinga mine is one of the world's largest producers of tin, in addition to containing considerable mineralization of cryolite and rare metals, such as Zr, Nb, Ta, Y, REE, etc. The tin-bearing granites of the region present Paleoproterozoic ages, and are inserted in the geological-geotectonic context of the Central Amazon Province. The data accumulated until today show that the Pitinga Tin Province has three primary sources for mineralizations, the albite-granite associated with the Madeira Granite, the greisens and epi-syenites associated with the Água Boa Granite. The host granites of the greisens are predominantly alkali-feldspar-granites, with local variations for syenogranites, exhibit a medium to coarse serial texture, gray to grayish-pink colors, are isotropic, and locally have rapakivitic features. Hornblende and biotite are the varietal mafics; Alanite, opaque minerals, zircon, apatite and fluorite are accessories and pistacite, chlorite and carbonates are secondary minerals. The greisens studied are endogreisens, being located essentially in the apical portions of the granite, being controlled by joints. In the petrographic study carried out, two main typologies were distinguished based on their mineralogical and textural characteristics: Greisen Gs1: this type of greisen is the one with the greatest areal expression, forming continuous zones of up to 5 meters, interdigitated with greisenized granites. It is a black rock, with a medium granular texture and essentially composed of quartz, reddish brown siderophyllite and topaz, accompanied by variable amounts of sphalerite, pyrite, chalcopyrite, cassiterite, zircon, fluorite, siderite and anatase. In facies where topaz is more abundant, there is a noticeable decrease in the content of siderophyllite, sphalerite and pyrite, as well as textural evidence of replacement of the latter. Cassiterite preferentially associates with partially chloritized siderophyllites, or forms thin crowns around pyrite and sphalerite. Sphalerite is an important phase in this greisen, associating with chalcopyrite. Greisen Gs2: this greisen normally occurs as bands or veins of up to 3.5 meters thick, interdigitated with greisenized granites. Topaz, sphalerite, zircon, fluorite, anatase, pyrite, chalcopyrite, galena, cassiterite and, locally, light green siderophyllite, siderite and beryl, complement the mineralogy. Fluorite predominates markedly over topaz. The association of phengitic muscovite with green chlorite is characteristic of this greisen. Both are interdigitated and, sometimes, there is evidence of substitution of chlorite by muscovite. The petrographic data indicate the presence of greater volumes of cassiterite in this greisen. The mineral chemistry studies were carried out from quantitative analyzes (WDS) in electronic microprobe of the minerals that form the main paragenesis of greisens, such as in the siderophyllites of greisens Gs1 and Gs2, and in the phrengites and chlorites of greisen Gs2, in addition to some analyzes in biotites, amphiboles and primary plagioclase from the host granite. Greisens Gs1 and Gs2 occur in different domains in the Guinho-Baixão grid, establishing a well-defined geographical-mineralogical zonation. Data relating to the chemistry of the main minerals forming these greisens indicate that the paragenesis of Gs1 was formed at relatively higher temperatures than those forming Gs2. This is corroborated by the greater amount of Cu and Pb sulfides existing in the latter, typical of lower temperature associations in the evolution of hydrothermal processes. Sphalerite may have been superimposed at lower temperatures in Gs1, not having formed in equilibrium with the higher temperature fluids. Furthermore, F apparently played an important role in the evolution of greisens, since its contents are lower in the Gs2-forming minerals, which may have been decisive for the formation of different paragenesis. The hydrothermal alteration halo associated with greisens, Gs2 is greater than that associated with Gs1, given that in their areas of occurrence there is a greater volume of intensely transformed granitic rocks, with obliterated primary textures and radically reduced quartz contents, in addition to the formation of dissolution cavities, generated by hydrothermal leaching. These features may be associated with episyenitization processes, detected east of the Guindo-Baixão grid. The greisens associated with the Água Boa Granite are different from the majority of the greisens studied in the literature, since their paragenesis are called siderophyllites and chlorites, with muscovites in smaller quantities, contrary to world examples. The petrographic study confirmed the close association of greisens with tin mineralizations. The analysis of petrographic and mineral chemistry data opens up some possibilities to explain the contrasting nature of greisens Gs1 and Gs2, and the evolution of their generating fluids: a) a hydrothermal fluid of the same global composition reacting with granitic rocks of different composition at the time of formation of greisens. This hypothesis is not supported by the variations of the greisens host observed on a microscopic scale; b) a hydrothermal fluid of identical initial composition, but which differed at some point in its evolution, which would have generated the geographic-mineralogical zonation observed in greisens. In this case, it is considered that the host rock has the same global composition, and that local physicochemical conditions have caused its differentiation. c) in addition to possible variations in the nature of the fluids over time, it is likely that the more intense hydraulic fracturing observed in the areas where Gs2 occurs has contributed to the differences between greisens Gs1 and Gs2. It would lead to a dispersion of fluids over a larger volume of host rock, facilitating hydrothermal alteration at first, but making it difficult later on, as it requires an exceptionally high volume of fluids, not available in the hydrothermal system in question.