Ore Genesis of the Kalatongke Cu–Ni Sulfide Deposits, Western China: Constraints from Volatile Chemical and Carbon Isotopic Compositions

摘要

The Kalatongke Cu–Ni sulfide deposits located in the East Junggar terrane, northern Xinjiang, western China are the largest magmatic sulfide deposits in the Central Asian Orogenic Belt (CAOB). The chemical and carbon isotopic compositions of the volatiles trapped in olivine, pyroxene and sulfide mineral separates were analyzed by vacuum stepwise-heating mass spectrometry. The results show that the released volatiles are concentrated at three temperature intervals of 200–400°C, 400–900°C and 900–1200°C. The released volatiles from silicate mineral separates at 400–900°C and 900–1200°C have similar chemical and carbon isotopic compositions, which are mainly composed of H₂O (av. ∼92 mol%) with minor H₂, CO₂, H₂S and SO₂, and they are likely associated with the ore-forming magmatic volatiles. Light δ13C_CO2 values (from –20.86‰ to –12.85‰) of pyroxene indicate crustal contamination occurred prior to or synchronous with pyroxene crystallization of mantle-derived ore-forming magma. The elevated contents of H₂ and H₂O in the olivine and pyroxene suggest a deep mantle-originated ore-forming volatile mixed with aqueous volatiles from recycled subducted slab. High contents of CO₂ in the ore-forming magma volatiles led to an increase in oxygen fugacity, and thereby reduced the solubility of sulfur in the magma, then triggered sulfur saturation followed by sulfide melt segregation; CO₂ contents correlated with Cu contents in the whole rocks suggest that a supercritical state of CO₂ in the ore-forming magma system under high temperature and pressure conditions might play a key role in the assemblage of huge Cu and Ni elements. The volatiles released from constituent minerals of intrusion 1* have more CO₂ and SO₂ oxidized gases, higher CO₂/CH₄ and SO₂/H₂S ratios and lighter δ13C_CO2 than those of intrusions 2# and 3#. This combination suggests that the higher oxidation state of the volatiles in intrusion 1# than intrusions 2# and 3#, which could be one of key ore-forming factors for large amounts of ores and high contents of Cu and Ni in intrusion 1#. The volatiles released at 200–400°C are dominated by H₂O with minor CO₂, N₂+CO and SO₂, with δ13C_CO2, values (–25.66‰ to –22.98‰) within the crustal ranges, and are considered to be related to secondary tectonic– hydrothermal activities.