The chemical composition and strontium isotopes of brines from the Tunguska basin: Implications for their formation

The results of the examination of the chemical composition and Sr isotopic ratios in brines of Cl-Ca, Cl-Ca-Na, and Cl-Na types from the Tunguska basin with the total salinity within the range of 87-626 g/1 are discussed. The pH is 2.9 to 7.7, and the Sr isotope ratios are 0.70825 ± 5 to 0.72010 ± 11. It is shown that the examined brines strongly differ drastically in their composition from the marine brine formed at the stage of halite precipitation. Based on their composition in terms of a number of indicative indices, we differentiated the brines into infiltrational and sedimentational, with a wide range of mixing between sedimentational and infiltrational (fresh and saline) waters. The sedimentational brines are strongly altered, which is evident from their higher Ca, Sr, Br, and radiogenic ⁸⁷Sr contents. Sr isotope composition shows that all the brines interact at a various extent with aluminosilicate minerals of the terrigenous or basement rocks, which has led to an enrichment of brines in radiogenic Sr by 2-100%. Two groups of brines have been discriminated by type of strontium alteration: (1) brines altered in sulfate-carbonate rocks with a low ⁸⁷Sr content and (2) brines altered in aluminosilicate rocks with a high ⁸⁷Sr content.