Agates of Onega structure Paleoproterozoic volcanic rocks (Central Karelia)

Agate mineralization in Central Karelia (the North-West Russia) is associated with the Paleoproterozoic volcanic rocks of the Ludicovian system (2.05—1.95 Ga) in the Onega structure. Agates and parent volcanic rocks were studied by optical and electron microscopy, electron microprobe and X-ray diffraction analyses, Raman spectroscopy, ICP-MS. It is shown that the main role in the agates structure has silica minerals: fine-grained quartz, fibrous and fine-flaked chalcedony. Agates are characterized by inclusions of coarse-crystalline calcite, microinclusions of chlorite, iron oxides and hydroxides, hydroxylapatite, epidote, mica (phengite), apatite, pyrite, chalcopyrite, titanite, leucoxene. Presence of contrasting rhythms in agates are marked by a change in composition of impurity mineral phases and different microtexture of silica layers represented by different-grained aggregates of quartz, fine flaked and fibrous chalcedony, quartzin. This indicates a stage-by-stage crystallization process with different temperature and pressure conditions of agate formation, which may also reflect the heterogeneity of the initial hydrothermal fluid. High concentrations of Ti, Cr, Mn, Ni, Cu (10—120 ppm) and low concentrations of Li, Co, Ga, Zn, Sr, Zr, Mo, Sn (0.5—10 ppm) have been found characteristic for quartz-chalcedony agates. Calcite in agates is characterized by high concentrations of Mn (1253—6675 ppm), Sс, Ti, Ni, Sr, Y, La, Ce, Nd (5—56 ppm). The chondrite-normalized REE distribution in agates shows a decay profile from La to Lu and a negative Eu anomaly in some samples. Low contents of rare metals and REE in agates, in comparison with parent volcanic rock, indicates a gradual chemical depletion of circulating fluids at the agate formation stage.