Gold Mineralization And Post-ore Denudation In The Muping-Rushan Gold Belt,Jiaodong Peninsula,eastern China

The Muping-Rushan Gold Belt in the Jiaodong Peninsula,East China,is located in the transition belt of the North China Craton and Sulu UHP metamorphic belt.It is mainly quartz vein type gold deposits and is one of the important gold deposit types in the Jiaodong area.Due to its special geotectonic location,the host rock(Kunyushan granite complex)can provide a record of the thermal and chemical evolution of lithosphere in a continental collisional zone.Field relationship and dating results revealed five intrusive phases of the Kunyushan granitic complex were modified into gneissic fine-grained garnet-bearing monzogranite(G1),gneissic medium-to fine-grained monzogranite(G2),medium-to coarse-grained monzogranite(G3),medium-grained monzogranite(G4)and porphyritic coarse-to medium-grained monzogranite(G5).All these phases have similar rock assemblages of peraluminous,calc-alkaline,I-type monzogranites.The monzogranites had primitive mantle-normalized patterns enriched in light rare earth elements and lithophile elements(Sr,Ba and Rb),but depleted in heavy rare earth elements and high-field-strength elements(Nb,P and Ti).LA-ICP-MS zircon U-Pb dating shows that the crystallization age of the Kunyushan granitic complex extends at least from 157.7-142.0 Ma.The weighted average ages of G1-G5 were 145.6±1.9 Ma,145.8±0.5 Ma,147.2±0.7 Ma,154.2±0.6 Ma,and 155.0±0.7 Ma,respectively.G5 to G1,from the middle to the edge shows a trend of gradually becoming younger.Hf-Sr-Nd-Pb isotopes and inherited zircons suggest that the parental magma of the Kunyushan granitic complex was derived from melting of basement rocks of the North China Craton and the Yangtze Craton.The magmatism presumably took place under the background of subduction and back-arc extension of the Paleo-Pacific plate.We infer that the Kunyushan granitic complex was produced by partial melting of the thickened lower crust,through delamination of the thickened lower crust during the late Jurassic.This magmatic event may mark the beginning of the extensive Mesozoic thinning of the lithosphere in East China.EPMA mapping for pyrite and pyrrhotite of typical gold deposits in the Muping-Rushan gold belt shows that Ag and Au are mainly thin veins filled in the cracks of stage ? pyrite(Py1)or around stage ? pyrite(Py2).The As content is higher in the crystal edge and cracks of stage ?pyrite(Py1)and the around of stage ? pyrite(Py2),which is positively correlated with the content of gold.As in some pyrite particles are in the form of rings and bands.The results of helium and argon isotopes indicate that the ore-forming fluid of gold deposits in the Muping-Rushan gold belt has the characteristics of crust-mantle mixing.While hydrogen and oxygen isotope shows that the ore-forming fluid is a mixture of magmatic water and meteoric water.In situ sulfur isotope results of sulfide from gold sulfides in the study area shows that ?34S range from 5.8‰ to 10.1‰,suggesting the ?34S of metallogenic stages(? and ?)derive from Jiaodong Group,Jingshan Group and Kunyushan complex.The lead isotopic composition shows the characteristics of the lower crust and orogenic belt.LA-ICP-MS hydrothermal monazite U-Pb obtained a metallogenic age of 118.9±2.1 Ma for the Tangjiagou gold deposit.It is preliminarily proposed that the main metallogenic age of the gold deposits in the Muping-Rushan gold belt are between 114.6 Ma and 104.8 Ma,with a weighted average of 108.9±0.7 Ma.Based on the above conclusions,the metallogenic model of deposits in Muping-Rushan gold belt is preliminarily proposed.Post-ore change and preservation studies shows that the uplifting exhumation depth of the Kunyushan granitic complex from late Jurassic to early Cretaceous is about 6.9 km.Since the Cretaceous,the total uplifting exhumation depth is about 6.6 km.At this denudation depth,the gold deposits in the Muping-Rushan gold belt have not yet been fully uplifted to the surface,which is favorable for the preservation of the deposits after mineralization.