Isotope (δ34S, δ13C, δ18O) Compositions of Disseminated Sulfide Mineralization in Igneous Rocks of the Dukat Ore Deposit (Northeastern Russia)

The paper discusses a study of variations in the δ34S, δ13C, δ18O compositions of disseminated sulfides and the carbonate phase, occurring in trace amounts in igneous rocks, which control the outlines of the unique Dukat Au-Ag deposit (northeastern Russia). The parameters obtained were compared with similar isotope parameters of ore assemblages of the same deposit. The δ34S values in sulfides and jarosite sampled in igneous rocks lie in a narrow interval (from −3.4 to + 3.6‰), which is comparable with the interval of δ34S variations in sulfides from orebodies (from −4.5 to + 2.0‰). Sulfur in pyrite of the early generation from K-Na leucogranites and pyrite from orebodies originated from the same source. Pyrite formed at late magmatic stages is characterized by a lighter sulfur isotope composition. Carbonate phases in igneous rocks of the Dukat ore deposit have low δ13C values (from −12.8 to −8.8‰). Based on oxygen isotope composition, carbonates are subdivided into two groups: those in equilibrium with the silicate matrix of rocks at high temperatures and those with abnormally low δ18O values (from −0.8 to +0.9‰). The data obtained can be described by a model that proposes that the formation of the sulfur isotope composition in sulfide and carbonate occurs in the process of thermochemical sulfate reduction (TSR) due to oxidation of organic carbon. Calculations show that the δ34S and δ13C values measured in rocks and ore assemblages of the Dukat ore deposit may have appeared due to abiogenic reduction of marine sulfate in a temperature range of 300–450°C. Comparison of the isotope parameters of carbonates from rocks and ore assemblages show that the source of carbonates in orebodies may have been country (underlying) rocks and the fluid released from cooling intrusive bodies of K-Na leucogranites, in which about 80% CO2 is lost.