Morphological-textural and chemical study of quartz from the Mocambo and Velho Guilherme Granites – Velho Guilherme intrusive suite (Southeast of the Amazonian Craton): Petrological and metallogenetic implications

Abstract

Quartz crystals from the Mocambo and Velho Guilherme granites, greisenized rocks, and associated veins within the Velho Guilherme Intrusive Suite (Southeast of the Amazonian Craton) are key to understanding magmatic-hydrothermal evolution and Sn-W mineralization in this region. The study aims to characterize the textural and chemical evolution of quartz across magmatic to hydrothermal conditions in these granites, establish a genetic sequence for quartz types, and evaluate their relationship to mineralization using scanning electron microscopy-cathodoluminescence (SEM-CL) imaging and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis. Cathodoluminescence imaging established an evolutionary sequence for quartz in these granites, progressing from magmatic Qz1 (found in less evolved, minimally altered rocks) progressing through Qz2 and Qz3 (common in moderately altered rocks) to Qz4 and Qz5 (dominant in highly evolved and hydrothermally altered rocks, including greisenized rocks and associated quartz veins). These later quartz types serve as markers for cassiterite, wolframite, and sulfide mineralization within the suite. Chemical analyses revealed that quartz types from the Mocambo Granite, source of a significant cassiterite deposit, are more enriched and exhibit greater variability in trace element concentrations. Magmatic quartz is enriched in Ti and displays variable levels of Al, K, P, Na, Fe, and other elements. Hydrothermal quartz is distinguished by higher Al, K, and Li contents. Qz4 in the Mocambo Granite shows frequent and notable concentrations of Al, K, Li, Na, Fe, Sn, W, and Zn, whereas Qz4 in the Velho Guilherme Granite lacks W and contains rare Sn. Ti, Al, and Li concentrations can reach 172, 1841, and 72 ppm, respectively, in the Mocambo Granite, while in the Velho Guilherme Granite, these values can reach 101 ppm for Ti, 1928 ppm for Al, and 117 ppm for Li. Magmatic quartz contains relatively high Al contents in both granites (above 250 ppm), while Al concentrations exceeding 400 ppm occur in quartz veins and greisenized mineralized rocks. The defined quartz sequence and chemistry robustly track magmatic-hydrothermal evolution. Distinct geochemical signatures, particularly the enrichment and presence of Sn and W in Mocambo Granite quartz (Qz4), correlate with its significant cassiterite mineralization potential compared to the Velho Guilherme Granite. Al, Li and K concentration is a key indicator of hydrothermal quartz in veins and greisens.