Agate genesis including alteration processes of the rhyolitic volcanics
Investigations were carried out about the marginal facies of Lower Rotliegend-rhyolites to clarify the specific genetic conditions of quartz- respectively agate-bearing rhyolitic spherulites in result of various post-magmatic alteration processes. These volcanics of the Thuringian Forest represent endogenous dome-like structures and some coherent lava flows. The outer part of their marginal facies, which forms the spherulite zone as an important facial part with dimensions in the deca-metre range up to 40 metres, reflects processes of degassing and spherulitic crystallization at higher temperatures as well as alteration processes later on. The spherulite zones generated within marginal parts of the rhyolitic volcanics which were relatively rich in water. Their petrochemical data differ significantly from those of the potassium-rich rhyolites in regular facies.
The internal cavities of larger rhyolitic spherulites hosted within the spherulite zones contain epigenetic fillings of quartz, illite/chlorite, calcite, fluorite and other paragenetic minerals sometimes (eg Figures 11-15). The rhyolitic wall rock, which surrounds the spherulites (cf. Figures 5, 6) has a distinct perlitic structure (altered residual volcanic glass). Formation of spherulites has been completed within the marginal facies (cf. Fig. 2) before solidification of the rhyolitic volcanics.
SEM-investigations revealed that the first wall-layered chalcedony (agate I) often has not a globular but a fine-grained crystalline microstructure. Following this first wall-layered agate, predominantly globular particles of silica of up to 0,7 µm form the later long-fibrous wall-layered agate. The globular silica particles of horizontally-layered bands crystallized as granular micro-quartz attaining sizes of up to around 2,5 µm deposited by sedimentary processes. Proportioning and distribution of globular silica particles, with respect to size, may indicate the structural major types of agates (wall-layered or wall-banded type and horizontally-layered type; cf. Figures 5, 11, 12) generated in the rhyolitic spherulites of the Thuringian Forest. In accordance with increase of microquartz grain-sizes of horizontally-layered bands, phyllosilicates, essentially illite and chlorite, can be observed inside of the microquartz immediately below the border to macroquartz. The succession of wall-layered type-/horizontally-layered type agate/ macroquartz which repeat sometimes reflects the change from colloidal to dilute silica solution.
The specific distribution of alkalis and alkaline-earth elements within the spheroidal shell of the rhyolitic spherulites was caused by crystallization processes of the so-called primary spherulites. After these post-magmatic distribution changes at higher temperatures, various alteration processes caused a further petrochemical and mineralogical discrimination, which was due to potassic alteration (K metasomatism), sodic albitization (Na metasomatism) and mainly low temperature hydration processes (hydrogen metasomatism) in sense of argillic alteration (Table 2). The petrochemical features (K/Na ratio of spherulitic/perlitic host rocks and agates respectively Na-Ca-K distribution of agates) point to the source of silica in those parts of the marginal facies which enclose the spherulites. The course and intensity of metasomatic host rock alteration define the features of each spherulite zone respectively agate locality and determine the lokal mode of origin of the epigenetic mineralizations.
Silica enrichment within the rhyolites of the spherulite zone up to more than 80 wt.% SiO2 was probably associated with K metasomatism. Regarding coherent lava flows silica assimilation may come further from the underlying sedimentary rocks. The silified spherulite zone served as the source for silica mobilization due to hydrogen metasomatism which caused deposition of chalcedony, micro- and macroquartz (cf. Table 2). The hydrogen metasomatism as one of the most important alteration processes within the spherulite zones was caused by interactions of these rhyolites with meteoric and primary igneous waters as the source of the δ18O-enriched solutions. Alteration processes affecting mafic minerals mobilized iron which was bond by formation of secondary haematite microlites. Illite accumulations reaching 3 up to 18 wt.% within the altered residual volcanic glass are characteristic for alkali mobilization. The mobilization of alkaline-earth elements reduced their contents within the spherulite zone, whereby calcium was bound at calcite in the internal cavity of larger rhyolitic spherulites. The scalenohedral calcite Ia (cf. Figure 13) is considered to be cogenetic with the beginning of redeposition of silica as first wall-layered type agate. Later on habit of calcite sequences changed to platy (blades of calcite IIa: Figure 14), rhombohedral and prismatic. Whereas calcite of the sequences I and II has been replaced by quartz, it was preserved regarding the sequences III (exclusively rhombohedral) and IV (prismatic). Isotope compositions of agates and calcite III indicate low temperatures of formation (Table 3).
Relating to the system thermal water/K-feldspar, illite-chlorite, calcite, chalcedony, chemistry of fluid inclusions is in accordance with low-temperature formation of agates (eg localities 7 and 16). Alteration minerals (1M-illite) of the altered residual volcanic glass as well as illite/chlorite, secondary quartz and calcite in the internal cavity of rhyolitic spherulites, which were generated by low-temperature hydration, are characteristic for very low-grade metamorphism below 200°C to demonstrate the low anchizone.