Isotope geochemistry of the Sarekuobu metavolcanic-hosted gold deposit in the Chinese Altay (NW China): Implications for the fluid and metal sources

Abstract

The Late Triassic Sarekuobu lode gold (Au) deposit (reserves: 8.00 t Au at 3.68 g/t) is situated in the Kelan volcanic basin of the Chinese Altay Orogen, NW China. Gold mineralization is hosted in the deformed metavolcanic sequences of the Devonian Kangbutiebao Formation. Multi-isotope analyses were conducted to constrain the ore-forming fluid and metal sources of the Sarekuobu deposit. The measured δDV-SMOW values of the fluid inclusions from the mineralized quartz (stages II and III) are of −132.9‰ to −78.5‰. The measured δ18OV-SMOW values of the quartz range from 11‰ to 13.6‰ (the corresponding calculated δ18OH2O values for the ore fluids: 4.2–6.9‰), which are consistent with those of typical orogenic Au deposits worldwide and fall between the metamorphic water and meteoric water fields in the δ18OH2O vs. δDV-SMOW diagram. This implies that the ore-forming fluids were mainly derived from metamorphic devolatilization with some meteoric water input. Calcite in the late mineralization (Stage III) veinlets contains δ13CV-PDB of −5.2‰ to −1.2‰ and δ18OOV-PDB of −20.4‰ to −19.9‰, similar to the carbonates (limestone and marble) of the Kangbutiebao Formation, and plots near the marine/continental carbonate field(s) in the δ13CV-PDB vs. δ18OOV-PDB diagram. This suggests that the carbon in the late-stage hydrothermal fluids was originated from marine/terrestrial carbonates with organic sediment decarboxylation input. The δ34S values for the Au-hosting sulfides (1.8–8.2 ‰) overlap with the volcanic wall rocks of the Kangbutiebao Formation (−4.7–18.7‰), which suggests that the sulfur was most likely wall rock-derived. The Sarekuobu sulfides contain 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.071–18.874, 15.520–15.669, and 37.860–38.961, respectively. The sulfide Sr and Nd isotopes are featured by their narrow compositional ranges and crustal affinities (i.e., (87Sr/86Sr)i = 0.70959–0.72216 and calculated εNd(t) = −3.20000 to −1.86000). All the Pb, Sr and Nd isotope compositions of the Sarekuobu ore sulfides are similar to those of the Kangbutiebao Formation wall rocks, indicating that the metals were mainly wall rock-derived. The structurally-controlled nature, the wall-rock metamorphism and alteration styles (e.g., silicic, chlorite, epidote and carbonate), and the abundance of CO2-rich and low-salinity fluid inclusions of the Sarekuobu deposit are similar to typical orogenic Au deposits. We therefore propose that the Sarekuobu Au deposit is a typical metavolcanic-hosted orogenic Au deposit, and was likely formed in a Triassic collisional-related setting.