胶东大庄子金矿成矿流体及稳定同位素研究

Ore-forming fluid and stable isotope studies of Dazhuangzi gold deposit in Jiaodong Peninsula

胶东大庄子金矿床位于胶北地体西南缘,中生代胶莱盆地北缘,西侧靠近郯庐(沂沭)断裂带。矿体产出在古元古界荆山群变质岩中,发育碎裂(糜棱)蚀变岩型和石英脉型2种矿石。流体包裹体显微测温表明,主成矿期温度为240~280℃,成矿流体盐度w(NaCleq)为7%~8%,属于H2O-CO2-NaCl体系。包裹体显微测温及岩相学观察发现,成矿期流体发生了相分离作用,这可能是导致矿化的主要原因之一。氢、氧同位素测试显示石英δ18O石英值变化于10.9‰~11.8‰之间,石英包裹体δ18OH2O变化于4.3‰~5.2‰,δDH2O变化于-65.8‰^-74.5‰,表明大庄子金矿成矿期流体以岩浆水为主,成矿后期混入了一定量的大气降水,岩浆水可能来自于与基性脉岩同源的基性岩浆的去气作用。硫同位素分析获得硫化物δ34S值为7.9‰~11.3‰,与围岩荆山群变质岩硫同位素组成非常接近,而与胶东群变质岩和中生代花岗岩的硫同位素组成差别较大,因此认为大庄子金矿床成矿流体中的硫主要来自于荆山群变质岩。

The Dazhuangzi gold deposit lies in Pingdu City,Shandong Province.Tectonically,it is located in the southwestern corner of Jiaobei terrain and on the northern margin of the Mesozoic Jiaolai basin;to the west,it is adjacent to the Yishu fault zone.The ore bodies,which are hosted in metamorphic rocks of Paleoproterozoic Jingshan Group,can be divided into two types,namely the ＂altered cataclasite or mylonite type＂ and the ＂quartz vein type＂.Microthermometric investigation reveals that gold precipitation occurs at 240～280℃ from CO2-rich,low salinity（w（NaCleq）7%～8%） hydrothermal fluids in which there are no other volatiles except H2O and CO2.More importantly,phase separation is observed and is firmly believed to be responsible for the main stage mineralization.Results of hydrogen and oxygen isotope studies suggest that ore-forming fluids were a mixture of magmatic water which might have originated from degassing of mafic magmas which produced the widespread mafic dikes in this region and meteoric water.Sulfur isotope study demonstrates that sulfides have a δ34S value range of 7.9‰～11.3‰,which is quite similar to that of metamorphic rocks of Jingshan Group,thus indicating that sulfur in the ore-forming fluids was dominantly derived from the metamorphic rocks of Jingshan Group.