This year, employees of the Karelian Research Center RAS noted the manifestations of agate mineralization in shungite rocks. Agates are rhythmically banded and patterned concretions of aggregates of chalcedony and other types of silica, as well as carbonates, zeolites, mica, iron oxides and hydroxides. Agate deposits are numerous and occur all over the world. Most often, agates fill in cavities in volcanic rocks. Widely known are agates from Brazil, Uruguay, India, Germany, and Mongolia. In Russia, the main agate deposits are located in the Urals, Timan, Transbaikalia, Chukotka, Magadan region and Kamchatka.
Agates in smaller amounts have also been found in sedimentary rocks, usually quite young on the geological scale (<500 Ma). The 2.1 Ga old Karelian shungites, are likely the most ancient among known agate-bearing sedimentary rocks. Whether this is true was to be determined.
Specimen from the holdings of the Precambrian Geology Museum IG KarRC RAS. Photo by KarRC RAS Science Communication Service.
Scientists from the Institute of Geology KarRC RAS undertook to reconstruct the environment in which the agate mineralization had formed. They shared the results in an article published in the international Minerals Journal.
— Analysis of the literature shows that finding agates in highly carbonaceous sedimentary rocks, in our case in shungites, is a true rarity! No such ancient agate-bearing sedimentary rocks aged 2.1 Ga have been mentioned in the literature before. We were interested in the geological history of their formation, mineralogy of the agate formation process and, of course, its timeframe, — explained Evgeniia Svetova, Senior Researcher at Mineral Raw Materials Department of the Institute of Geology KarRC RAS.
It is noteworthy that the history of scientific study of agates in the world is 250 years long and is reflected in numerous publications. However, the questions of the genesis of these mineral formations have not yet been fully solved.
- The process of agate formation in volcanic rocks is associated with the activity of postmagmatic hot hydrothermal solutions containing, in addition to silica, a wide range of chemical elements. After volcanic activity is over, these solutions circulate for a long time through pores and cracks in solidified lavas, gradually filling the voids with siliceous material. The mechanism of agate formation in sedimentary shungite rocks is more complicated, as our study has shown. Agate formation was preceded by dissolution of primary carbonate nodules in shungites. And it was only in the second stage that the leached-out cavities were filled with agate material through circulation of hydrothermal solutions, — the scientist added.
An interesting mineralogical finding for the agates under study is the detection of the mineral moganite in their composition. Moganite is chemically identical to quartz (SiO2), but it is an unstable phase of silica, which is transformed into a stable phase of quartz under high temperatures. We know that the study area was influenced by regional metamorphic processes in the period of 2.1-1.5 Ga BP. The well-preserved moganite in the agates indicates that the geological process that led to agate formation in the shungites occurred much later than the formation of the host shungite rocks. Thus, the scientists have indirectly confirmed the hypothesis suggested by some authors that there existed magmatic processes in Central Karelia and Zaonezhye that caused significant hydrothermal activation of the territory in the Neoproterozoic or Phanerozoic time (within 1100-200 Ma BP).
— Agates are of interest for geologists not only as a decorative ornamental stone, but also as a hydrothermal formation genetically related to polymetallic ores (copper, noble metals, uranium, vanadium). Owing to the study of agates, we have previously dated the hydrothermal redistribution of carbon in Karelian shungites to 1980-1950 Ma BP, which is important to know when studying hydrocarbon migrations in natural systems. Thus, agates can reveal many secrets of Karelia's ore treasures, and their study is of great applied and theoretical importance, — Evgeniia Svetova summed up.
