Agates from Mesoproterozoic Volcanics (Pasha–Ladoga Basin, NW Russia): Characteristics and Proposed Origin

Agate gemstones occurring in the Mesoproterozoic volcanic rocks of Priozersk Formation (PrF) within Pasha–Ladoga Basin (Fennoscandian Shield, NW Russia) were investigated to characterize mineral and geochemical composition agates provide new information concerning their origin. Optical scanning electron microscopy, EDS microanalysis, X-ray powder diffraction, fluorescence spectrometry, Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), C-O isotope analysis used for study. mineralization appears mostly as an infill fissures, cavities, gas vesicles massive vesicular basalts, lava-breccias. The is dominated by alpha-quartz (fibrous chalcedony, microcrystalline macrocrystalline quartz), but it also displays abundances calcite. characteristic red-brownish agate’s coloration caused multiple hematite inclusions distributed agate matrix. study revealed two phases formation PrF volcanics, which are most likely controlled distinctly different fluids and/or mixture. At first, appeared due post-magmatic iron-rich fluids. late hydrothermal activity was probably triggered intrusion gabbro-dolerite sill resulted second phase formation. We suggest that remobilized iron compounds from crust weathering underlying led additional vein filling with hematite-rich calcite/silica matter.