The role of fluids in the formation of agates

Agates (banded, coloured chalcedony) are most frequently formed in SiO2-rich (rhyolite, rhyodacite) and SiO2-poor (andesites, basalts) volcanic rocks. The silica necessary for agate formation was discussed mainly by the alteration of the host rocks and is predominantly transported by diffusion as Si(OH)4. The alteration processes are reflected in the occurrence of typical paragenetic minerals such as clay minerals, zeolites and/or iron oxides as well as high concentrations of Al, Fe, Ca, K and Na in the agates. However, the occurrence of calcite and fluorite and unexpected high concentrations of e.g. Ge, U and B especially in agates of acidic volcanics indicate that other fluids can play a role in the alteration of volcanic rocks and the mobilization and transport of SiO2 and other chemical compounds. Chemical transport reactions (CTR) of gases and liquids could be realized by stable fluorine (and chlorine) compounds such as SiF4, BF3, GeF4 and UO2F2. Such CTR could explain Si (Ge, B and U) transport better than exclusive transport by solution of SiO2 in water.The HF-release during the heat treatment and the enrichment of Ge, B and U in agates are positive indications of interaction of volatile fluids (HF).