New insights into the genesis of the Sipingshan gold deposit in eastern Heilongjiang, north-eastern China: Evidence from geology, U-Pb and Rb-Sr geochronology, geochemistry, and H[sbnd]O[sbnd]S-Pb isotopes

Abstract

The Sipingshan gold deposit is intermediately sized and located in the eastern Wandashan Terrane. It has attracted much attention for its uncommon genetic type of hot spring. Gold orebodies mainly occur in the upper and lower siliceous rocks, siliceous cemented breccia is found between the upper and lower siliceous rocks, and silicified rhyolite porphyry is observed below the lower siliceous rocks. Two main mineralisation periods (i.e. exhalative sedimentation and hydrothermal superimposition) have been identified at Sipingshan based on mineral paragenesis and crosscutting vein relationships. In this study, analyses of whole-rock geochemical compositions, pyrite Rb–Sr age, zircon U–Pb–Hf contents, and S–PbOH isotopes were conducted to determine the Au mineralisation time and ore genesis of this deposit. The ore-bearing siliceous rock revealed weighted mean 206Pb/238U ages of 121.8 ± 1.1 Ma and 117.6 ± 1.0 Ma, representing the mineralisation age of the exhalative sedimentation period. The Rb–Sr isochron age of pyrites (106 ± 3 Ma) was consistent with the Au–Cu mineralisation age in the Wandashan area and represented magmatic–hydrothermal mineralisation. The siliceous rocks were characterised by high SiO2 (97.92–99.18 wt%), low TiO2 (0.01–0.03 wt%), low Al2O3 (0.22–0.55 wt%), negative δCe (0.78–1.00), positive δEu (1.12–4.04), and low Sb, As, Cs, Cd and Li contents, indicating the genesis of hydrothermal sediments. The OH isotope results suggested that the ore-forming fluids were mainly derived from magmatic water and mixed with meteoric water in the later stage. The δ34S values of pyrites from the siliceous rocks (–25.8 to –8‰) indicated a biogenic sulfur-dominated source, corresponding to the exhalative sedimentation period. The δ34S values of pyrites (9.8 to 29.7 ‰ and –1.9 to –4.7 ‰) formed in the magmatic–hydrothermal period indicated that sulfur was most likely derived from sedimentary rock and magmatic sulfur sources. The Pb isotope data also implied a mixture of upper crust and mantle sources. Therefore, we propose that the Sipingshan gold deposit is a continental exhalative sedimentation-type gold deposit superimposed by later magmatic–hydrothermal fluids, and formed during the subduction of the Paleo-Pacific Plate during the Early Cretaceous.