Formation of the intrusion-hosted orogenic-type gold lodes: Exemplified by the Axile gold deposit in the Chinese Altai

Abstract

Unlike the gold deposits typically hosted in metamorphosed volcanic-sedimentary sequences, the recently discovered Axile gold deposit (over 13 t Au at 5.35 g/t) is a unique example in the Chinese Altai for evaluating the genesis of intrusion-hosted lode gold deposits in metamorphic terranes. The orebodies in this deposit occur as quartz veins and altered tectonites hosted in the sinistrally sheared quartz diorite and biotite granite zones in the Habahe intrusion. The quartz vein-type ores are composed of quartz-pyrite veins and quartz-polymetallic sulfide veins, in which gold-tellurides also occur. The altered tectonites, spatially associated with the quartz veins, are composed of deformed and altered rocks of quartz diorite and biotite granite. Main alterations include quartz, sericite, calcite, chlorite, albite, pyrite, chalcopyrite, and gold-tellurides. Two types of fluid inclusions are identified in the ores, i.e., (1) carbonic-aqueous (C-type) and (2) aqueous (W-type), with the C-type being dominated. These fluid inclusions yield salinity ranging from 3.6 to 9.0 wt% NaCl equivalent and homogenization temperatures in the range of 249 − 387 ℃, suggesting a typical mesothermal NaCl-CO₂-H₂O system. The Axile deposit thus corresponds to an orogenic-type gold deposit.

The hydrothermal titanites in the auriferous quartz-pyrite veins yield in situ U-Pb ages around 278.4 ± 2.4 Ma (MSWD = 3.1, 2σ), which constrain the gold mineralization timing as Permian. The mineralized quartz diorite and biotite granite yield zircon U-Pb ages of 377.0 ± 1.0 Ma (MSWD = 0.1; 1σ) and 394.0 ± 1.2 Ma (MSWD = 0.4; 1σ), respectively, which show that the ore-hosting intrusions were emplaced in the Devonian, obviously earlier than the gold mineralization. The light δ¹³C_V-PDB values (−15.5 ‰ to −6.7 ‰, average −10.7 ‰) of the CO₂ extracted from the fluid inclusions display a significant contribution from the organic carbon in sediments. Given the sedimentary rocks underwent Permian greenschist to amphibolite facies metamorphism in the Chinese Altai, accompanied by the synchronous structural deformation and shearing activities, it could be concluded that the Axile gold deposit is formed by a shear zone-controlled mesothermal system that originated from metamorphic-devolatilization. The Chinese Altai Orogen is an orogenic-type gold metallogenic belt, containing both the intrusion- and metamorphic rock-hosted gold deposits, illustrated by a consistent genetic model.