Reactivation Of Ore-controlling Faults And Gold Mineralization Process In Sizhuang Gold Deposit,Jiaodong Peninsula

The formation of vein gold deposits is the result of coupling between ore-bearing fluid and ore-controlling fault zone.The ore-controlling fault often has multi-stage tectonic activity and has inheritance.Sizhuang gold deposit is located in the south of Jiaojia gold field in Jiaodong gold concentration area,strictly controlled by NNE-to ENE-striking Jiaojia fault zone.Multiple mineralization types coexist and multi-stage structures are superimposed in Sizhuang gold deposit,which is an ideal selection for the study of ore-controlling fault reactivation and gold mineralization process.Focusing on the mineralization diversity of the Sizhuang gold deposit,this paper analyzed the spatial and temporal structure of the Jiaojia fault zone and the geometry and kinematics of the orebody(vein)through detailed underground tunnel mapping and structural observation at hand specimen scale and thin section scale,and established the gold mineralization dynamics model.Obtain the following understanding:1.The spatial and temporal structure of Jiaojia ore-controlling fault zone in Sizhuang gold deposit was analyzed through detailed underground structure-alteration mapping,microscopic structure observation under microscope and quantitative analysis of structural strength.The upper and footwall fracture zones of the Jiaojia fault zone are asymmetrical,with the upper and footwall fracture bandwidth of 10m?20m and the footwall fracture width of 420m?630m.According to the frequency distribution of brittle structure,the footwall fracture zones are divided into fracture core(10m?200m),inner fracture zone(10m?30m)and outer fracture zone(400m).During the metallogenic period,the Jiaojia fault belt underwent three stages of tectonic activity,including compression(?₃ is near vertical,?₁ is NNW trending),strike slip(?₃ is NEE trending near horizontal,?₁ is NWW trending near horizontal)and extension deformation(?₃ is NE trending near horizontal,?₁ is NW trending near vertical).2.Geometry and kinematics analysis of orebody(vein)show that the orebody lateral deposit and ore-forming fluid flow direction in Sizhuang gold deposit are mainly controlled by the fault shear direction in the process of ore fault reactivation.The orebody shape parameter j(0.002?3.202)indicates that the orebody evolves from flat shape to flat shape.Among them,the j value of quartz sulfide vein type orebody and pyrite sericite type orebody have different changing trends,indicating that the fault shear direction has changed.The tension vector of quartz-sericite-pyrite vein,quartz-polymetallic sulfide vein and quartz-calcite sulfide vein are 215°?13°,289°?12°and 136°?9°respectively,reflecting the change of effective stress and related to the change of stress field.3.The study of fault zone alteration geochemistry shows that the water-rock reaction is the main metallogenic mechanism of Sizhuang gold deposit.The changes of Sb,Cu,Zn and Sr elements in the fault zone indicate that the gold enrichment is related to sericite,and the Cu and Zn elements in the fault core and fracture zone have different trends,indicating the existence of multi-stage mineralization events.At the proximal end of the main fault surface,the oxides of K and Al migrated in,while the oxides of Na,Mg,Fe and Si migrated out,indicating that the early sericite formed layered silicate with weak capability,and the late silicification led to strain hardening.The interaction between strain weakening and hardening widened the fault zone.4.The control of magmatic activity in the Early Cretaceous caused the local stress field to be reversed,leading to the reactivation of the ore-controlling faults in the Sizhuang gold deposit.Fluid plays an important role in the process of fault reactivation.The fluid reacts with the rock and the permeability of the fault zone increases.The overpressure fluid drives the pre-existing faults to occur and then move,and the temperature and pressure of the fluid decrease suddenly at the expansion and expansion sites such as fracture bending and overlapping conversion,and the gold element precipitates and enriches ore-forming.