The source and evolution of ore fluids in the Heiniuwa gold deposit, Baoshan block, Sanjiang region: Constraints from sulfide trace element, fluid inclusion and stable isotope studies

Abstract

The Heiniuwa gold deposit is located in the northwestern segment of the Longyang Au–Fe–Pb–Zn polymetallic ore cluster in the northern Baoshan block, Sanjiang region, SW China. This ore cluster is characterized by skarn Fe–Pb–Zn mineralization (∼120 Ma) in the central part and Au mineralization in the distal part. The genesis of the Heiniuwa deposit and its relationship with the skarn deposits remain unclear. All orebodies in Heiniuwa are hosted in the Upper Cambrian calcareous slate and siltstone of the Hetaoping Formation, and are controlled by the NS-striking faults. Three main paragenetic stages of mineralization have been recognized based on petrographic observation including, from early to late, a pyrite – arsenopyrite – quartz stage (stage I), a pyrite – pyrrhotite – sphalerite – galena – chalcopyrite – quartz – calcite stage (stage II), and a quartz – calcite – chlorite stage (stage III). Three types of fluid inclusions were identified in the quartz and calcite from the stage I to III in Heiniuwa, including liquid-rich (L-type), vapor-rich (V-type) and solid-bearing (S-type) inclusions. The S-type inclusions only occur in stage I quartz. The fluid inclusions from stage I homogenized at 185–326 °C (concentrating at 220–250 °C), with eutectic temperatures of −48.5 °C to −36.7 °C, final ice-melting temperatures of −11.34 °C to −0.8 °C and salinities ranging from 1.4 to 17.2 wt% NaCl equiv. The inclusions from stage II homogenized at 155–281 °C (concentrating at 190–230 °C), with eutectic temperatures of −51.6 °C to −37.5 °C, final ice-melting temperatures of −10.6 °C to −0.5 °C and corresponding salinities ranging from 0.9 to 14.6 wt% NaCl equiv. The inclusions from stage III homogenized at 116–202 °C (concentrating at 150–170 °C), with eutectic temperature of −45.5 °C to −37.5 °C, final ice-melting temperatures of −4.3 °C to −0.6 °C and salinities ranging from 1.3 to 7.2 wt% NaCl equiv. The microthermometry reveals that the ore-forming fluid of Heiniuwa gold deposit is a typical H2O–NaCl–CaCl2 fluid system. The carbon and oxygen isotopes of calcite show that δ13CPDB and δ18OSMOW values respectively range from −13.4‰ to −6.8‰ and 9.1‰ to 9.6‰ in stage II, and from −8.1‰ to −1.5‰ and 6.4‰ to 8.2‰ in stage III. The bulk marble and calcareous slate samples have δ13CPDB and δ18OSMOW values from −3.0‰ to −0.7‰ and 7.8‰ to 11.9‰, respectively. This suggests that the carbon in the ore-forming fluid was probably from a magmatic source. The δ18Ofluid values of ore-forming fluid vary from 1.6‰ to 2.5‰ in stage I, from −6.2‰ to 0.6‰ in stage II and from −10.8‰ to −2.5‰ in stage III. This indicates that the fluid in stage I is dominantly magmatic, whereas the fluid in stage III is close to meteoric water. The decrease of Na/Ca ratios of ore-forming fluid from stage I to stage III suggests that the interaction with carbonate wallrock is significant during fluid evolution. The sulfide δ34S values range from −5.7‰ to +4.6‰ (mean = +0.3‰, n = 19), agreeing with the explanation based on C–O isotopes and fluid inclusions that the ore-forming fluids are mainly magmatic in origin. In addition, LA-ICP-MS analysis of sulfides also supports this opinion. The stage I pyrite has Cu/Ni ratios greater than 1, and a high concentration of gold; in contrast, the stage II pyrrhotite samples display a very low concentration of gold. This suggests that gold mineralization mainly occurred in stage I. The mixing between magmatic hydrothermal fluid and meteoric water is considered to be a key factor controlling the gold deposition.