Abstract

Agates of Paleoproterozoic volcanics (2100−1920 Ma) within the Onega Basin (Karelian Craton, Southeast Fennoscandia) were studied using optical and scanning electron microscopy, X-ray powder diffraction, X-ray fluorescence spectrometry XRF, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and C-O isotope analysis. Agate mineralization is widespread in the lavas gas vesicles, inter-pillow space of basalts, picrobasalts, basaltic andesites, as well as agglomerate tuffs. Agates are characterized by fine and coarse banding concentric zoning; moss, spotted, veinlet, and poor-fancy texture types were identified. Agate mineralization is represented by silicates, oxides, and hydroxides, carbonates, phosphates, sulfides, and sulfates. Among the silica minerals in agates only chalcedony, quartz and quartzine were found. The parameters of the quartz structure according to the X-ray diffraction data (well-develops reflections (212), (203), (301), large crystallite sizes (Cs 710–1050 Å) and crystallinity index (CI 7.8–10.3) give evidence of multi-stage silica minerals recrystallization due to a metamorphic (thermal) effect. The decreasing trend of trace element concentration in the banded agates from the outer zone to the core suggests a chemical purification process during crystallization. C-O isotope characteristic of agate-associated calcite reflects primary magmatic origin with the influence of hydrothermal activity and/or low-thermal meteoric fluids. Agates were formed under low PT-parameters and related to hydrothermal activity on the first stage of Svecofenian orogeny within 1780−1730 Ma. Thus, it can be believed that the temporal gap between continental flood basalts outflow and agate formation is about 190 Ma.

Highlights

  • Agates are fascinating minerals that have long attracted people’s attention with the beauty and uniqueness of the pattern

  • The present paper examines the mineralogical and geochemical properties of agates from the famous agate-bearing Paleoproterozoic volcanic complex on the Southeast Fennoscandia

  • The LSH section is represented by an association of terrigenous, siliceous, carbonate rocks with high organic carbon content, as well as by mafic and ultramafic volcanic complexes related to continental flood basalts

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Summary

Introduction

Agates are fascinating minerals that have long attracted people’s attention with the beauty and uniqueness of the pattern. The question about the temporal gap between the formation of gas vesicles in volcanic rocks and their mineral infilling is under discussion [6,13]. The present paper examines the mineralogical and geochemical properties of agates from the famous agate-bearing Paleoproterozoic volcanic complex on the Southeast Fennoscandia. This complex belongs to the Ludicovian Superhorizon Timofeev in 1912 [27] His thin section micrographs of the agates from Ludicovian basalts of the Suisari Island in Lake Onega were the earliest agate thin sections to be found in the scientific literature. The present investigation aims to highlight both the mineralogical and geochemical features of Onega agates with a reconstruction of the possible conditions of their formation. The Onega Basin experienced the Svekofennian Orogeny (~1860 Ma) [30] and the agates could provide novel information on the evolution of Paleoproterozoic magmatic complexes in the Karelian Craton

Geological Setting
Agate Occurrences
Methods
Surface
Results and Discussion
Results anddata
Macrophotographs
Optical Tulguba
X-Ray Powder Diffraction
SEM and Microprobe Investigation
Geochemistry Investigation
C-ORidge
Conclusions
Full Text
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