Journal of


  (Formerly Journal of the Czech Geological Society)

Original Paper

Jiří Zachariáš

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

Journal of Geosciences, volume 53 (2008), issue 1, 105 - 117


  Abstract References

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 (SiIV) ranges from 6.7 to 7.8 atoms per formula unit, however; values greater than 7.2 are the most common. They exhibit positive SiIV-Mg/(Mg + Fe2+) correlation. Late hydrothermal amphibole associated with silicification-2 is also magnesiohornblende; however, it exhibits negative SiIV-Mg/(Mg + Fe2+) 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 + Fe2+) 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.

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