Genesis of two different types of gold mineralization in the Linglong gold field, China: Constrains from geology, fluid inclusions and stable isotope

Abstract

The Dongfeng and Linglong gold deposits are located in the northwest Jiaodong Peninsula, North China Craton. The deposits are mainly hosted in the Mesozoic granitoids and structurally controlled by the Zhaoyuan–Pingdu fault zone. Gold mineralization at Dongfeng occurs as disseminated ores and sulfide stockworks, typically enveloped by broad alteration selvages. In contrast, mineralization at Linglong is characterized by massive auriferous quartz veins with narrow alteration halos. Three stages of mineralization were identified in both deposits, with the early stage represented by quartz±pyrite, the middle stage by gold+quartz+pyrite or gold+quartz+base metal sulfides, and the late stage by quartz+carbonate±pyrite, respectively. Four types of fluid inclusions were distinguished based on petrography, microthermometry, and laser Raman spectroscopy, including (1) pure CO2 fluid inclusions (type I), (2) H2O–CO2–NaCl fluid inclusions (type II), (3) H2O–NaCl fluid inclusions (type III), and (4) daughter mineral-bearing or multiphase fluid inclusions (type IV).In the Dongfeng gold deposit, the early- and middle-stage quartz mainly contains primary type II fluid inclusions that completely homogenized at temperatures of 276–341°C with salinities of 2.8–11.7wt.% NaCl equivalent, and temperatures of 248–310°C with salinities of 3.3–10.8wt.% NaCl equivalent, respectively. A few primary type I fluid inclusions could be observed in the early-stage quartz. In contrast, the late-stage quartz contains only the type III fluid inclusions with homogenization temperatures of 117–219°C, and salinities of 0.5–8.5wt.% NaCl equivalent. The estimated pressures for the middle-stage fluids are 226–338MPa, suggesting that gold mineralization mainly occurred at paleodepths of deeper than 8.4–12.5km. The mineralization resulted from extensive water–rock interaction between the H2O–CO2–NaCl fluids and wallrocks in the first-order fault.In the Linglong gold deposit, the early-stage quartz mainly contains primary type II fluid inclusions and a few type I fluid inclusions, of which type II fluid inclusions have salinities of 3.3–7.5wt.% NaCl equivalent and homogenization temperatures of 271–374°C. The middle-stage quartz mainly contains all four types of fluid inclusions, among which the type II fluid inclusions yield homogenization temperatures of 251–287°C and salinities of 5.5–10.3wt.% NaCl equivalent, while the type III fluid inclusions have homogenization temperatures of 244–291°C and salinities of 4.1–13.3wt.% NaCl equivalent. Fluid inclusions in the late-stage quartz are type III fluid inclusions with low salinities of 0.3–8.2wt.% NaCl equivalent and low homogenization temperatures of 103–215°C. The trapping pressure estimated for the middle-stage fluids is 228–326MPa, suggesting that the gold mineralization mainly occurred at paleodepths of about 8.4–12.1km. Precipitation of gold is possibly a consequence of phase separation or boiling of the H2O–CO2–NaCl fluids in response to pressure and temperature fluctuations in the open space of the secondary faults.The δ34S values of pyrite are similar for the Dongfeng and Linglong deposits and show a range of 5.8 to 7.0‰ and 5.9 to 7.4‰, respectively. Oxygen and hydrogen stable isotopic analyses for quartz yielded the following results: δ18O=−3.8 to +6.4‰ and δD=−90.5 to −82.7‰ for the Dongfeng deposit, and δ18O=0.0 to +8.9‰ and δD=−77.4 to −63.7‰ for the Linglong deposit. Stable isotope data show that the ore-forming fluids of the two gold deposits are of magmatic origin, with gradual incorporation of shallower meteoric water during/after mineralization.