Oxygen Isotope System of Leucogranites of the Omsukchan Trough (Northeastern Russia): Conditions and Mechanisms of Water–Rock Interaction

The oxygen isotopic composition of minerals (Qtz, Zrn, Kfs) and bulk samples of Late Cretaceous porphyritic leucogranites of the Omsukchan trough and the adjacent territories, hosting Au–Ag-, Sn-, and Sn-polymetallic mineralization, is studied. The δ 18 O values of bulk samples of leucogranites increase from ‒10.6‰ (within the Dukat Au–Ag-ore field) to +9.7‰ (in the other ore-bearing regions). The isotopically light characteristics of rocks are an attribute of feldspars and correspond to isotopic equilibrium with meteoric fluid (δ 18 О = –15‰) in a wide range of T at limited water–rock ratios (W/R from 0.9–1.6 to 0.3–0.1). Unlike feldspars, the δ 18 O values of quartz and zircon are partially changed under fluid action: in the first case, probably, by the diffusion mechanism, and in the second, by the dissolution-crystallization mechanism.