Agate Mineralization in Paleoproterozoic Organic Carbon-Rich Sedimentary Rocks of the Onega Basin (NW Russia): Insights into Genesis

Abstract

In this contribution, we present the results of mineralogical investigation of the agates in Paleoproterozoic organic carbon-rich sedimentary rocks within the Onega Basin (Fennoscandian shield, Russia) aimed at reconstructing the agate-forming processes. Optical and scanning electron microscopy, EDS microanalysis, thermal analysis, X-ray powder diffraction, Raman spectroscopy, and carbon isotope analysis were used for the study. Three main varieties of agates differing in morphology and texture were identified, including concentrically zoned nodules, fine-banded, and carbon-rich moss agates. Mineralogical evidence indicates the participation of hydrothermal fluids in agate formation. Concentrically zoned nodules could be formed due to the dissolution of carbonate concretions in the organic carbon-rich siltstones and their silicification as a result of late hydrothermal processes. Fine-banded vein agates occur in stockworks crosscutting organic carbon-rich rocks and are widely accompanied by sulfides, selenides, carbonates, sulfates, and iron oxides. Carbonaceous matter in moss agates is present as poorly ordered carbon and is characterized by a low δ13Corg value (−25.64‰), suggesting a biogenic origin. Raman spectroscopy data showed an elevated amount of moganite besides alpha quartz in the concentrically zoned nodules compared to other agate varieties, indicating different ages of the mineralization processes. We suggest that the revealed varieties of agates were formed at different stages of long-term hydrothermal processes occurring in the Onega Basin.