Brazilian Quartz Deposits with Special Emphasis on Gemstone Quartz and its Color Treatment
Ricardo ScholzMario Luiz de Sá Carneiro ChavesKlaus KrambrockM. V. B. PinheiroSandra de Brito BarretoMessias Gilmar de Menezes
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Quartz veins in the south of Santiago del Estero. A study of the mineralogy, textural features and geometry of large quartz deposits, in Ambargasta and Sumampa ranges, allow us to divide them into two different groups: small pegmatites and large hydrothermal veins. The former were related to wall-rock emplacement, whereas the latter were controlled by regional lineaments that affected the Proterozoic basement. Hydrothermal veins are mainly composed of quartz with minor amount of opal, chalcedony and iron oxides. The most important hydrothermal vein, called Cantamampa, is approximately 3 km long, up to 8 m wide and 70 m high. The limited alteration produced by the hydrothermal fluids suggests that their precipitation temperatures must have been low. Only epidote, sericite and chlorite occur as wall-rock alteration. Pegmatites bodies are smaller and, although quartz and alkaline feldspars are the main components, muscovite is also present in minor amounts. Quartz from the most important pegmatite (Piedra Blanca) contains about 99.9% of SiO2. However, in the hydrothermal veins, where Si is partially replaced by Al, the SiO2 amount in the quartz decreases to 93%. Key word: Quartz, pegmatite, hydrothermal veins, Sumampa, Ambargasta.
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Quartz vein-type gold deposit is the most important type of gold deposits in the world. However, the lack of minerals suitable for most conventional isotopic dating methods constrains the direct and precise dating. Recent development in mass spectrometry makes it possible to determine the age of auriferous quartz veins by U-Pb dating of zircons from quartz veins. Unfortunately, hydrothermal zircons that grow directly from mineralizing fluids and inherited magmatic or metamorphic zircons from wall rocks may coexist in the same vein. Such complexity poses significant problem while interpreting the U-Pb data. Thus, the key for zircon U-Pb dating of quartz vein-type gold deposit will be to distinguish hydrothermal zircon precipitated from ore-forming fluids from the inherited zircons. Combined studies of zircon morphology, internal texture, trace elemental geochemistry (including rare earth element), compositions of mineral and fluid inclusions will permit identification of hydrothermal zircons, which then can be precisely dated by SHRIMP or LA-ICP-MS methods to provide reliable age constraints of quartz vein-type gold deposits.
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A study of the mineralogy, textural features and geometry of large quartz deposits, in Ambargasta and Sumampa ranges, allow us to divide them into two different groups: small pegmatites and large hydrothermal veins. The former were related to wall-rock emplacement, whereas the latter were controlled by regional lineaments that affected the Proterozoic basement. Hydrothermal veins are mainly composed of quartz with minor amount of opal, chalcedony and iron oxides. The most important hydrothermal vein, called Cantamampa, is approximately 3 km long, up to 8 m wide and 70 m high. The limited alteration produced by the hydrothermal fluids suggests that their precipitation temperatures must have been low. Only epidote, sericite and chlorite occur as wall-rock alteration. Pegmatites bodies are smaller and, although quartz and alkaline feldspars are the main components, muscovite is also present in minor amounts. Quartz from the most important pegmatite (Piedra Blanca) contains about 99.9% of SiO2. However, in the hydrothermal veins, where Si is partially replaced by Al, the SiO2 amount in the quartz decreases to 93%. © 2005 Asociacion Geologica Argentina.
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