Noble-metal mineralization of the Adycha-Taryn metallogenic zone: Geochemistry of stable isotopes, fluid regime, and ore formation conditions

Abstract

The regional geologic setting of the Adycha-Taryn metallogenic zone, one of the areas most productive for noble-metal mineralization in northeastern Russia, is discussed. The intricate metallogenic history of the zone and the prolonged geodynamic activity of its ore-hosting structures are documented. Different types of mineralization, such as hydrothermal-metamorphogenic, gold-bismuth, gold-quartz, gold-antimony, and silver-antimony, are described. New data on the isotopic compositions of oxygen in quartz, sulfur in sulfides, and oxygen and carbon in carbonates from different mineralization types are presented. The early metamorphogenic quartz beyond the ore zones has δ18O = +20.1 ± 2.0‰. At the gold-bismuth deposits, the δ18O values of quartz are within the narrow range of +12.5 ± 0.4‰. Quartz from the gold-quartz mineralization shows much wider variation in δ18O values, from +14.2 to +19.5‰. A similar range (δ18O = +16.1 to +19.2‰) is observed for the gold-antimony mineralization. Cryptograined quartz from the silver-antimony mineralization is enriched in light oxygen isotopes (δ18O = -3.2 to +4.7‰). The following δ34S values (‰) have been established in sulfides of mineralization of different types: gold-bismuth -3.7 to -2.2 (Apy) and -6.7 to -6.8 (Py); gold-quartz -2.1 to +2.4 (Apy), -6.6 to +5.4 (Py), and -6.1 to +4.2 (St); gold-antimony -2.0 to +1.6 (Apy), -3.5 to +2.1 (Py), and -5.3 to +0.2 (St); and silver-antimony -2.0 to -1.9 (Apy), -2.2 ± 0.1 (Py), and -5.7 to -5.6 (St). The δ13C and δ18O values are contrasting in the studied types of mineralization, varying respectively from -6.9 to -5.9‰ and from +2.1 to +5.7‰ (gold-bismuth), from -9.1 to -6.1‰ and from +12.4 to 18.7‰ (gold-quartz), from -12.1 to -9.5‰ and from +15.0 to +16.3‰ (gold-antimony), from -11.6 to -11.1‰ and from +1.5 to +4.7‰ (silver-antimony). Metamorphogenic calcites are rich in both heavy C (-1.1 to -1.7‰) and heavy O (+20.3 to +20.5‰) isotopes. Microthermometric study and crush-leach analysis of fluid inclusions have revealed differences in the composition of ore-forming fluids and formation conditions for different types of mineralization. The isotopic compositions of O, C, and S of mineral-forming fluids suggest a significant input of magmatic fluids to the formation of gold-bismuth and gold-antimony deposits, the contribution of metamorphic fluids increases at gold-quartz deposits, and meteoric water is involved in the formation of silver-antimony deposits.