Quartzo e cassiterita como marcadores da evolução magmático-hidrotermal dos Granitos Mocambo e Velho Guilherme, Província Carajás.

Abstract

Quartz crystals from the Mocambo and Velho Guilherme Granite and cassiterite from the Mocambo Granite present in greisenized rocks and hydrothermal veins belonging to the Velho Guilherme Intrusive Suite, Xingu region, Amazonian Craton, were analyzed by scanning electron microscopy - cathodoluminescence (SEM-LC) and by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Morphological and textural analyses obtained through CL images allowed the identification of similar types and the definition of an evolution pattern for the quartz present in the rocks of both plutons. The magmatic type (Qz1), present in the least evolved and little altered rocks, passing through Qz2 and Qz3, common in moderately altered rocks, up to types Qz4 and Q5, dominant in the most evolved and hydrothermalized rocks, including the greisenized ones and the associated quartz veins, both markers of the mineralization of cassiterite, wolframite and sulfides present in this suite. Chemical analyses show that all quartz from the Mocambo Granite, which contains an important secondary cassiterite deposit, are more enriched and present greater variability in the elements analyzed. Magmatic quartz is more enriched in Ti and present varying levels of Al, K, P, Na and Fe. The higher contents of Al, K and Li identified in hydrothermal quartz differentiate them from magmatic quartz. In turn, cassiterite crystals show varied textures in cathodoluminescence images, such as concentric/oscillatory and sectoral zoning, as well as banding formed by more and less luminescent bands. Homogeneous dark zones are common and relict crystals, usually present in quartz veins associated with wolframite, are less luminescent. The main trace elements found in cassiterites are represented by Ti, Fe, Nb, Ta, W, Zr, Al and Hf, similar to those of tin mineralizations present in quartz and greisens veins of deposits from different geological environments. However, the pattern formed by the normalized “spidergram” is unique, presenting positive peaks of Sc, Ti, Nb, Sb, Ta and W and negative peaks of V, Mn and Fe.