The genesis of gold deposits in the Hetai goldfield, South China: New constraints from geochronology, fluid inclusion, and multiple isotopic studies

Abstract

The Hetai goldfield is one of the largest gold mining districts in the South China Block. A complex paragenesis consists of three mineralization stages: syntectonic metamorphic stage, hydrothermal stage, and supergene stage. The syntectonic metamorphic stage is characterized by banded quartz (Q1) and invisible Au in mylonites. The hydrothermal stage, which is the main gold mineralization stage, can be divided into three substages: an early substage characterized by coarse‐grained quartz (Q2) + pyrite + arsenopyrite + pyrrhotite, an intermediate substage characterized by gray fine‐grained quartz (Q3) + electrum + poly‐sulphides + sericite + chlorite and a late substage with predominate white quartz (Q4) + calcite as well as lesser sulphides. During the supergene stage, malachite, covellite, and limonite were formed. Four types of fluid inclusions (FIs) in these types of quartz were identified: aqueous FIs (type 1), CO2‐bearing aqueous FIs (type 2), CO2‐rich FIs (type 3), and solid‐bearing FIs (type 4). Q1 only contains type 1 FIs; Q2 contains types 2 and 4 FIs; Q3 contains types 2 and 3 FIs; and Q4 only contains type 1 FIs. Microthermometric results show that the homogenization temperatures of fluid inclusions range from 350°C to 400°C for Q1, 250°C to 320°C for Q2, 138°C to 245°C for Q3, and 110°C to 207°C for Q4. Salinities of fluid inclusions range from 3.4 to 5.7 wt.%, 2.6 to 12.9 wt.%, 0.5 to 11.5 wt.%, and 0.2 to 7.3 wt.% for Q1, Q2, Q3, and Q4, respectively. Alteration minerals (sericite and chlorite) from the main mineralization stage have δDH2O‐VSMOW values of −62‰ to −98‰ and calculated δ18OH2O‐VSMOW values of 6.3‰ to 4.4‰. The in situ sulphur isotope analysis of pyrite yields a narrow δ34SCDT range of −1.5‰ to −5.2‰ (average −3.4‰), combined with the Pb isotopic compositions of the sulphides, wall rocks, and Wucun pluton, revealing that the ore‐forming material was mainly derived from magmatic source. The mineralization age (sericite 40Ar/39Ar: 159.3 ± 0.8 Ma) is close to the emplacement age of Wucun biotite monzonitic granite pluton (LA‐ICP‐MS zircon U–Pb age: 158.1 ± 1.9 Ma). These geochemical and geochronological data suggest that the main phase of gold mineralization in the Hetai goldfield is genetically related to the granitic activities in the Late Jurassic rather than the mylonitization event in the Late Triassic to Early Jurassic.

Ore Genesis of the Changkeng–Fuwan Au-Ag Deposit in Central Guangdong, South China: Evidence from Fluid Inclusions and C-H-O-S-Pb-He-Ar Isotopes

“…In the Ag orebody, the 3 He/ 4 He values ranged from 1.41 to 5.70 Ra (average 3.88 Ra). The 40 Ar/ 36 Ar values varied from 295.6 to 314.7 with a mean value of 300.9, higher than that of air (295.5). The 4 He concentrations ranged from 0.49 to 19.70 (10 −8 cm 3 STP/g) with an average value of 6.20 (10 −8 cm 3 STP/g), and those of 40 Ar varied from 2.35 to 30.00 (10 −8 cm 3 STP/g) with a mean value of 9.29 (10 −8 cm 3 STP/g).…”

“…The calculated results ranged from 17.85 to 72.59% (average 44.84%), reflecting that the ore-forming fluids of the deposit may mainly derive from the MDF with the addition of the CDF. In addition, the values of 40 Ar/ 36 Ar varied from 295.6 to 314.7, with a mean value of 300.9, which was also slightly higher than that of air (295.5). The 40 Ar/ 36 Ar values of the Au and Ag orebodies also showed a similar distribution in the R/Ra-40 Ar/ 36 Ar diagram (Figure 13b).…”

“…In addition, the values of 40 Ar/ 36 Ar varied from 295.6 to 314.7, with a mean value of 300.9, which was also slightly higher than that of air (295.5). The 40 Ar/ 36 Ar values of the Au and Ag orebodies also showed a similar distribution in the R/Ra-40 Ar/ 36 Ar diagram (Figure 13b). All data fell between the MDF and ASW values, which revealed that the fluids were formed by mixing meteoric water and mantle-derived fluids.…”

“…It has undergone multiple tectonic events associated with mineralization. As a result, the region is an important location for hydrothermal polymetallic mineralization [35][36][37][38][39]. Zhou et al [4] divided the QHMB into three sections (Figure 1a).…”

“…In the Au orebody, the 3 He/ 4 He values varied from 2.46 to 2.50 Ra, where Ra (=1.4 × 10 −6 ) represents the atmospheric 3 He/ 4 He values [66]. The 40 Ar/ 36 Ar values of the two analyzed samples were 304.6 and 300.2, slightly higher than the atmospheric standard values (295.5) [67]. The 4 He concentrations in the pyrite showed a variation between 1.30 and 3.65 (10 −8 cm 3 STP/g).…”