Fluid evolution and ore genesis of the giant Sanshandao gold deposit, Jiaodong gold province, China: Constrains from geology, fluid inclusions and H–O–S–He–Ar isotopic compositions

Abstract

The Sanshandao gold deposit is located at the northwestern edge of the Jiaodong peninsula, North China Craton. It is mainly hosted in the Mesozoic granitoids. Gold mineralization occurs as disseminated- and stockwork-style ores, typically enveloped by broad alteration selvages. Mineralization and alterations are structurally controlled by the Sanshandao–Cangshang fault. Three paragenetic stages were identified 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, 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). The early- and middle-stage quartz mainly contain primary type II fluid inclusions that completely homogenized at temperatures of 241–390°C with salinities of 2.96–18.39wt.% NaCl equivalent, and temperatures of 207–336°C with salinities of 2.06–17.57wt.% NaCl equivalent, respectively. A few primary type I fluid inclusions can be observed in the early-stage quartz, while the middle-stage quartz contains a few primary type I, type III and type IV fluid inclusions. In contrast, the late-stage quartz only contains type III fluid inclusions showing homogenization temperatures of 101–268°C, and salinities of 0.17–15.47wt.% NaCl equivalent. CH4, derived from extensive water–rock interaction between the ore-forming fluids and wall rocks in the main fault, is the key factor controlling the gold mineralization. CH4 changed the physico-chemical conditions of the ore fluids, and led to gold precipitation.Oxygen and hydrogen stable isotopic analyses for quartz from different ore-forming stages show that the ore-forming fluids are of magmatic origin, with gradual incorporation of meteoric water during/after mineralization. δ34S values of pyrite from different ore-forming stages vary between 1.9‰ and 11.9‰. 3He/4He ratios of fluid inclusions in the middle-stage pyrite mainly range from 0.14 to 2.94Ra, while it is from 675.10 to 5926.44 for 40Ar/36Ar. All these isotopic signatures suggest that the ore fluids could be derived from a combination of crustal and mantle components.