Structure-fluid-metallogeny And Dynamics In The Muru Gold Belt,Jiaodong Peninsula,China

The Muru gold belt in the Jiaodong gold province is located in the Sulu terrane The precise gold mineralization timing and duration remain controversial,and processes involved in auriferous quartz vein deposition are not clear.In addition,the robust constraints on its preservation and exhumation are rare.On the basis of careful observation on mineralization-alteration features and internal texutres of the orebodies,and careful measurement of structural deformations,we conducted multiple geochronological methods including sericite 40Ar/39Ar,zircon(U-Th)/He and apatite fission track dating,combined with pyrite LA-ICP-MS trace element analyses,we tried to probe the processes of ore-fluid infiltrations and ore-forming fluids evolution,and frame the structural-thermal history of the gold belt after mineralization.Rushan gold deposit,which is a representative deposit in the Muru belt,formed between ca.122 Ma and 117 Ma in a single and protracted gold mineralization event lasting for about 5 My and including multiple fluid eventsInternal textures of the high-angle shear veins and low-angle extension veins in the Rushan gold deposit indicate that episodic fluctuations from supralithostatic to hydrostatic fluid pressure triggered the multiple ore-controlling fault ruptures.In this model,the controlling fault starts to slip with both reverse and sinistral kinematics,and dilatational zones are created.The ore-forming fluids then migrate upwards along the fault,due to the large pressure difference between the dilatational zones and the deep parts of the fault,causing hydraulic fracture in the host rocks and earlier quartz veins.Large volumes of quartz and pyrite are precipitated due to fluid immiscibility caused by pressure fluctuations,and new quartz veins form along both sides of the earlier quartz veins.The fault is now sealed by these new quartz veins and the fluid pressure starts to build until the next breach cycles.Orebodies with complex internal textures were thus deposited incrementally in these cycles,closely fitting the classic fault-valve model,in this case under a weak regional NW-SE transpressional stress.Three types of pyrite representing three consecutive mineralization stages in one single fluid event were identified in the Rushan gold deposit.Composition for trace elements in all types of pyrite is broadly the same.Cobalt,Ni and As show different distribution behaviours from the other elements,attributed to an oscillatory precipitation environment induced by episodic fluid press fluctuations.Episodic fluid pressure fluctuation and its resulting fluid immiscibility triggered the massive precipitation of coarse-grained milky-white quartz and pyrite firstly,which was succeeded by smoky-gray medium-fine quartz and pyrite occurring either as infillings in veinlets or disseminated particles.Lately,free gold and sulfide minerals were precipitated from a progressively Au-Ag-Te-Pb-Zn-Cu concentrated fluid.Large amount of reducing gas including H2S escaped from the fluids because of the episodic fluid immiscibility,the gold bisulfide in which gold was transported was thus destabilized and decomposed,and gold minerals were subsequently chemisorbed onto the hydraulic microfractures of pyrite.The Muru gold belt successively underwent NWW-SEE extension(110-100 Ma),NW-SE compression(100-85 Ma),NNW-SSE extension(80-65 Ma),NE-SW compression(65-50 Ma)and NW-SE compression(50 Ma-)after gold mineralization.Correspondingly,the gold belt experienced rapid cooling,slightly heating and thermal stagnation,moderate cooling and thermal stagnation successively,and was finally monotonically cooled downwards the current ambient temperature in a slow cooling rate.The gold belt might formed in a depth of slightly less than 7.1-14.2 km,and exhumation magnitude estimation suggests this belt has been denudated for 5.5-7.2 km,meaning only the shallowest part the oreboies were eroded away and large reserves probably exists at depth.Multiple subduction direction shifts of the Pacific Plate might be the dorminant geodynamic divers for all the structural magmatic and metallogenic events in this area since late Cretaceous.