Evolution of the composition of hydrothermal mineral-forming fluid of ore deposits in early Precambrian of the Ukrainian Shield

Abstract

The aim of the study, the results of which are presented in this article, is to study changes in the composition of hydrothermal fluid ore systems of the early Precambrian on the example of gold and uranium ore deposits of the Ukrainian shield in connection with the evolution of exo- and endogenous processes in the outer shells of the Earth. The method used is a complex isotope-geochemical study of the composition of gas-liquid inclusions of hydrothermal fluid of ore systems. Based on experimental isotope-geochemical studies of the fluid of gas-liquid inclusions in the minerals of Precambrian deposits of uranium and gold of the Serednoprydniprovsky and Ingul megablocks of the Ukrainian Shield, a consistent change in the quantitative and isotopic composition of hydrothermal fluids of mineral formation from Mesoarchean to Paleoproterozoic was established. The content of H2O and CO2 was investigated and the isotopic composition of carbon CO2 fluids in quartz, pyrite, and feldspar were studied. The age range of the studied deposits is 3200—1750 million years. A decrease in the carbon dioxide content in the hydrothermal mineral-forming fluid of the Precambrian occurred in the Neoarchean, that is, before the Great Oxygen Event (Great Oxidation Event) — an increase in the oxygen content in the atmosphere in the Paleoproterozoic and is probably associated with the processes of exogenous and endogenous hydration of ultramafic rocks during the formation of the protocontinental crust. Molar fraction of CO2 in the mineral-forming fluid, in the Precambrian correlates to some extent with atmospheric pressure. The increase in the content of the carbon-12 isotope in the Paleoproterozoic in the mineral-forming fluid occurred due to the oxidation of organic matter with an increase in the oxygen content in the atmosphere.