Ore-controlling Model Of Structures In Fault Zone,jiaojia Gold Orefield, Shangdong Province, China

As one of the most important gold-producing ore field in Eastern China and the geologic site where the Jiao Jia-type gold deposit was named after, the Jiaojia gold orefield is famous around the world for its special ore-forming mechanism. Particularly for the key scientific problem that modeling the structural ore-controlling pattern and ore-forming processes in Jiaojia faulting field has not been fully understood. Revealing the forming rules and processes of gold mineralization objectively and comprehensively have yet been solved. Therefore on the paper, guided by the regional metallogenic theory and employing interdisciplinary methods we selected specific representative deposits and focus on its structural faulting net and gold mineralization effects to specifically interpret the Jiaojia fault net, zonation and deformation environment, to reveal its ore-forming, and-controlling mechanism, to build up the exploring model in the Jiaojia gold field, and then further to forecast and locate the concealed ore-bodies.From Late Jurassic to Early Cretaceous, the gold mineralization in the Jiaojia fault field developed during the transition period of regional kinetic field switching from subduction-extrusion to strike slip-tension. The prior regional principal stress is compression with NW-SE trending, while it switches to NNE-NEE trending during the mineralization stage. Accordingly, the Jiaojia faulting field transferred from predominated dextral ductile shear stress to sinistral extension- shearing stress. Based on evidences from fieldwork, microscopy and EBSD analysis, all associated deposits are hosted in the Jiaojia faulting zone, and mostly distributed in the footwall. Distinct structural zonation is recognized vertically with the fault plane, the fault gouge zone, extruded schistosity zone, tectoniclens zone, dense joints zone and sparse joints zone. The extruded schistosity zone, controlled by the Jiaojia main fault, develops the protomylonite and/or mylonite, while they are overprinted by the cataclasite related with gold mineralization. The combination of the mineral deformation in earlier stage indicates that they were formed in the high greenschist facies with the temperature of about 450-500°C, and dextral ductile shear stress. The tectoniclens zone, which is controlled by the NNE- and NE-trending secondary faults, develops protocataclasite and/or cataclasite in the condition between high greenschist facies and low greenschist facies with sinistral shearing stress. The joint zone, also controlled by the NNE- and NE-trending secondary faults, develops the cataclastic granite. The combination of mineral deformation shows the proofs that it was formed in the low greenschist facies with the temperature of 300-400°C and extension- shearing stress kinetic field. Horizontal zonation of mineralization-alteration is the main signature for the ore-controlling mechanism in Jiaojia faulting ore field. The types of hydrothermal alteration in the extruded schistosity zone mainly are sericitization and sulfuration, and the deposit can be also considered as the pyrite-sericite-quartz type gold deposit. The hydrothermal alterations in the tectoniclens zone are signified by the(pyrite)-sericite-quartz and silicification veins, and being as the quartz-sulfide vein type ore bodies. The alterations in the joint zone are the sericite-quartz and potassium veins, and changed into the potassium fine ore veins ore bodies along the joint surface.Based on detailed fieldwork and mass balance calculation, the ore-forming elements, chemically related to Au, migrated and precipitated in a large scale in the hydrothermal alteration and tectonic deformation zone, and form the pyrite-sericite-quartz type gold ore bodies. The δ34S values of sulfides indicate that the ore sulfur may come from the Mesozoic granites and originally derived from Jiaodong group, the metamorphosed basement. The ore-forming fluids have a low-moderate salinity, CO2-rich and CH4-slight hydrotherm with P-T conditions ranging from 280 to 2900 bar and 225 to 320°C, respectively. The gold is transported as Au(HS)2-complex in the ore-foming fluids.Studying results show that the ore-forming fluids develop the sulfofication and fluid immiscibility when they were migrating to the extruded schistosity zone and joint zone, through changing the metallogenetic physic-chemical conditions(such as Pressure, Temperature and Oxygen Fugacity). The Au-bearing complex is then subjected to damage and the ore-forming elements are deposited from the fluids. Therefore, The gold mineralization formed in the pyrite-sericite-quartz type ore-bodies in the extruded schistosity zone, and become the potassium fine vein type ore bodies along the joint surfaces. Hereby, ore-controlling structural model in Jiaojia fault field is proposed. Predicting mining area in the Xincheng and Sizhuang gold ore field were targeted.