Compositional trends in magmatic and hydrothermal silicates of the Petráčkova hora intrusive complex, Bohemian Massif - link between the magmatic processes and intrusion-related gold mineralization

Abstract

Mineralized zones of the Variscan (348-344 Ma) Petráčkova hora gold deposit locally exhibit high-temperature (~600 to ~500 °C) hydrothermal alterations: K-alteration-1 (selectively pervasive) and K-alteration-2 (fracture-related) in the granodiorite and granodiorite porphyries, and silicification-1 and -2 in the country rocks (biotite hornfels and feldspathic quartzite). Alteration processes led to partial recrystallization of primary magmatic silicates, and to crystallization of new hydrothermal phases (K-feldspar, biotite and hornblende). Electron microprobe analyses were used to trace magmatic to hydrothermal evolution of the mineral chemistry. The K-feldspar phenocrysts in the granodiorite and granodiorite porphyry exhibit complex magmatic crystallization and resorption textures. Compositionally, K-feldspars differ especially in the barium admixture. It decreases from early (5-0.8 % celsian or 2.6-0.4 wt. % BaO; phenocrysts) to late magmatic (1.1-0 % celsian; groundmass) and hydrothermal (1.2-0 % celsian) K-feldspars. No significant differences were found between the chemistry of magmatic and early hydrothermal amphiboles. They all correspond to magnesiohornblende or actinolite. Tetrahedral silica (Si IV ) ranges from 6.7 to 7.8 atoms per formula unit, however; values greater than 7.2 are the most common. They exhibit positive Si IV -Mg/(Mg + Fe 2+ ) correlation. Late hydrothermal amphibole associated with silicification-2 is also magnesiohornblende; however, it exhibits negative Si IV -Mg/(Mg + Fe 2+ ) trend. Differences in the biotite composition are less pronounced in comparison to amphiboles and comprise especially a decrease in the Ti content and an increase in the Mg/(Mg + Fe 2+ ) ratio from the magmatic to the hydrothermal biotites. Some of the fine-grained biotite aggregates (recrystallized magmatic or hydrothermal biotite) are depleted in potassium, probably due to alteration demonstrated by the presence of submicroscopic intergrowths with clay/chlorite minerals (unrecognizable in back-scattered electron mode). Broad similarities in the chemistry of magmatic and early hydrothermal silicate minerals support the high-temperature nature of the early hydrothermal stage, and point to probable admixture of exsolved magmatic fluid phase in the early hydrothermal fluids.

Sedimentary facies and mineral provenance of Upper Triassic sandstones offshore Kvitøya, Svalbard: implications for palaeogeographic interpretations in the northern Barents Shelf area

“…Figure 8b shows Ba-rich lining at crystal boundaries inside the igneous rock fragment. Similar features have been associated with late igneous processes involving hydrothermal infiltration of the igneous protoliths (Mehnert & Büsh 1981;Stimac & Pearce 1992;Zachariáš 2008). In total, the K-feldspar textures and chemistry reveal erosion of granitic rocks of different igneous history, and with different impacts from hydrothermal alteration in the source.…”

“…8a, b). Ba-rich K-feldspar is known from crystallization of high-T granitoids (Smith 1974;Vernon 1986;Stimac & Pearce 1992;Zachariáš 2008). Figure 8b shows Ba-rich lining at crystal boundaries inside the igneous rock fragment.…”

“…Spessartine garnets appear in Mn-rich sediments ranging from very low-to high-grade metamorphic conditions (Nyame et al 1998;Makrygina & Suvorova 2011). Almandinespessartine garnets are also described from granites and pegmatites, associated with late igneous or metasomatic crystallization (e.g., Kontak & Corey 1988;Stone 1988;Zachariáš 2008;Suggate & Hall 2013).…”

Geochemical and Geochronological Discrimination of Biotite Types at the Detour Lake Gold Deposit, Canada

“…Primary magmatic biotite can be distinguished from re-equilibrated or neoblastic biotite by comparing their Ti content [23] and their X Mg values [47]. Based on previous studies, the relatively lower Ti content, and slightly higher X Mg of re-equilibrated and neoblastic biotite, are the major distinguishing factors from primary magmatic biotite [23,47].…”

“…Primary magmatic biotite can be distinguished from re-equilibrated or neoblastic biotite by comparing their Ti content [23] and their X Mg values [47]. Based on previous studies, the relatively lower Ti content, and slightly higher X Mg of re-equilibrated and neoblastic biotite, are the major distinguishing factors from primary magmatic biotite [23,47]. By examining the Ti content, the mineral chemistry data suggested that biotite from one barren samples (DLR-03) is primary magmatic in origin, whereas, other barren samples and all of the mineralized samples are either partially re-equilibrated, or neoblastites by a hydrothermal fluid.…”