Journal of

GEOsciences

  (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

DOI: http://doi.org/10.3190/jgeosci.021

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 + Fe²⁺) correlation. Late hydrothermal amphibole associated with silicification-2 is also magnesiohornblende; however, it exhibits negative Si^IV-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.