Evolution Of Ore-forming Fluids Of Sanshandao Gold Deposit, Jiaodong Peninsula, Eastern China

Sanshandao gold deposit is a typical super-large “Jiaojia” type gold deposit inSanshandao-Cangshang ore controlling fault. We identified four main mineralizationstages, which include pyrite-beresite(Ⅰ), quartz-pyrite(Ⅱ), quartz-metal sulfides(Ⅲ),and quartz-calcite(Ⅳ). In this paper we did much research on the fluid inclusionpetrography, thermometry, component analysis and isotopic geochemistry fromdifferent ore-forming stages. Meanwhile, we discussed the evolution of ore-formingfluids and its metallogenic mechanism. At last we established the followingunderstandings.(1) Most of fluid inclusions during the mineralized stage are composed ofCO2-rich two-phase inclusions and CO2-bearing three-phase inclusions. Thehomogenization temperature mostly ranges from160°C to380°C, with the salinity,density, and pressure of ore-forming fluids concentrating on the range of1.22%~14.32%NaCl.eqv.,0.51~1.06g/cm3,0.35~29.37MPa, respectively. This type ofore-forming fluids belongs to the medium-low temperature, medium-low salinity, lowdensity category. With the evoluting of gold mineralization the fluids show a featureof sustaining declining in the temperature and salinity.(2)The gas composition in the fluid inclusions is mainly H2O and CO2, and theremaining is CH4, N2, C2H6and Ar with a limited quantity. There was a negativecorrelation between CO2and H2O content significantly. The positive ion in the liquidcomposition mostly is Na+and K+, which the amount of has a trend of declining fromthe earlier mineralization stage to the later stage, while the amount of Mg2+and Ca2+is so limited. The quantity of the negative ion of Cl-has the highest value in quantity,and then it’s the SO42-. The Cl-begins to climb after a short decreasing trending in theearlier mineralization stage. The SO42-increase rapidly in the main mineralizationstage. The ore-forming fluids belongs to the Na+(K+)-Cl-(SO42-)-CO2-H2O fluidssystem as a whole.(3) The HH2O-OH2Oisotopic composition from the fluids inclusions indicate thatthe ore-forming fluids were mainly derived from magmatic fluids, with a part fromthe atmospheric water. The13CCO2-18OCO2isotopic compositon from later themineraliation stage indicate that the carbon originate from mantle material and marinecarbonate. After comprehensive analysis of the origin of ore-forming fluids, it showsthe characteristics of mixed origin of the ore-forming fluids. The ore-forming fluids ofhe early mineralization stage most likely originated from the magmatic hydrothermal,caused by dehydrolysis of from mafic to intermediate mantle magma. As theevoluting of gold mineralization more meteoric water began to mix into theore-forming fluids, which include a little seawater in later mineralization stage.(4) The S isotopes composition of sulphides have a rich34S characteristic, with arange of10.4~12.6‰, which also significantly indicate the ore-forming materialsmainly originated from crust. We think that the ore-forming materials possibly camefrom the deep Guojialing granite and the Linglong granite. However, the ore-formingmaterials possibly stem from the Jiaodong metamorphic rocks or othergranite-ophiolites mass indirectly. (5) All the evidences from geological characteristics and fluid geochemistry ofthe Sanshandao gold deposit indicate that the migration of gold is mainly in the formof sulfur complexes in early mineralization stage, and is mainly in the form ofchlorine complexes in main mineralization stage. The boiling phenomenon caused bydecompression, which initially caused by the conversion of the tectonic system duringMesozoic, mainly lead to the deposition of Au element dissolved in the ore-formingfluid. After combining all the characteristics of region tectonic evolution andmetallogenic epoch, we think Sanshandao gold deposit, the origin of which had closerelation with the subduction mechanism of the Pacific plate to North China plate, isone of the atypical orogenetic gold deposits.