Origin and geochemistry of agates in Permian volcanic rocks of the Sub-Erzgebirge basin, Saxony (Germany)

- agates in ignimbrites form during alteration processes at elevated temperatures (> 150 °C).
- specific formation processes result in high concentrations of Ge (> 90 ppm), B (46 ppm) and U (> 18 ppm) in the agates.
- normalized REE distribution patterns of the agates show strong negative Eu-anomalies, enriched HREE as well as tetrad effects.
- the microstructure of agates points to crystallization via an amorphous silica precursor under non-equilibrium conditions

Mineralogical and geochemical investigations of agates from Permian volcanic rocks of the Sub-Erzgebirge basin (Saxony, Germany) were made to constrain the genesis and characteristics of these spectacular forms of silica in acidic volcanic host rocks. Samples from the main agate occurrences of Chemnitz, Hohenstein-Ernstthal, St. Egidien and Zwickau were selected for detailed analyses.The results of the study show that agate formation can be related to volcanic activity (Rochlitz ignimbrite) and the subsequent alteration of the volcanic rocks. Most agates originate from the infill of silica into cavities of lithophysae (high-temperature crystallization domains), which formed during cooling of welded ignimbrite. Agate formation temperatures of probably > 150 °C were calculated from fluid inclusion and oxygen isotope studies, which indicate that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activity.The trace-element composition of chalcedony and macrocrystalline quartz in agates is different from that of quartz from magmatic or metamorphic rocks and pegmatites. Elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) were released during the alteration processes and accumulated in the SiO2 matrix of the agates. Extraordinary high contents of Ge (> 90 ppm), B (46 ppm) and U (> 18 ppm) were also detected, which can exceed the Clark concentration and sometimes the element concentration in the surrounding host rocks. In addition, chondrite-normalized REE distribution patterns of the agates show strong negative Eu-anomalies, enriched HREE as well as tetrad effects. These patterns are different from the REE patterns of the volcanic host rocks and point to fractionation processes during agate formation. The specific geochemical features indicate interactions of the host rocks with magmatic volatile fluids and heated meteoric water, and transport of SiO2 and other elements both in aqueous solution and via stable fluorine (and chlorine) compounds such as SiF4, BF3, GeF4, and UO2F2.The results of cathodoluminescence (CL) microscopy and spectroscopy revealed a microstructure of the agates that is similar for all occurrences. Characteristic features are irregular internal textures and sector zoning in quartz as well as luminescence colours and spectra, respectively, which are caused by a typical high defect density (oxygen vacancies, silanol groups). According to these results, the formation of agates can be explained by crystallization via an amorphous silica precursor under non-equilibrium conditions.

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