The world-class Natalka gold deposit, northeast Russia: REE patterns, fluid inclusions, stable oxygen isotopes, and formation conditions of ore

REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H2O-CO2-NaCl-MgCl2 with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO2-CH4, and (3) H2O-NaCl-MgCl2 with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H2S and destabilization of the Au(HS)2− complex. The fluid with the third composition arose due to the boiling of the H2O-CO2-CH4-NaCl-MgCl2 liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ18O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \delta ^{18} O_{H_2 O} $$\end{document} = +6.3 to +8.8‰ at 350°C and +3.6 to +5.9‰ at 280°C, respectively. The narrow interval of oxygen isotopic compositions shows that this source was homogeneous. The data obtained allow us to suggest that the deposit formation was related to magmatic activity, including the direct supply of ore components from a magma chamber and mobilization of these components in the processes of dehydration and decarbonation during contact and regional metamorphism.