Tectono-geochemical Comparisons Between Baguamiao And Ding-ma Gold-bearing Shear Belt,Western Qingling Mountains

The Baguamiao and Dingjialin in Shananxi,and Taiyangping and Dongjiayuan in Sichuan,are typical gold deposits which occurred in fine clastic sedimentary rocks of the western Qinling orogenic belt.The Baguamiao superlarge gold deposit is controlled by E-W ductile-brittle shear zone developed in Devonian strata,and there occur both granodiorite bodies,and veins of granite porphyry,diorite-porphyrite and quartz albit-ophyre of the Yanshanian.The Dingjialin-Taiyangping-Dongjiayuan gold ore belt is controlled by NE-trending Dingma ductile-brittle shear zone developed in Silurian strata,and there lacks magmatic activities.This study integrates structural-petrological analysis,petrogeochemical,microstructures-chemistry and SHRIMP U-Pb dating to draw the following main conclusions:1.These two types of gold-bearing shear zone both develop brittle-ductile shearing,ductile rheological structures and tectonites.They appear to be of obvious structural-petrological zoning and progressively deformed from country rocks to the center of shear zone.Gold orebodies are found to mainly occurred at alternation rocks by tectonic fluids for the Baguamiao gold,and at mylonite and cataclasite rocks for the Dingjialin–Taiyangping–Dongjiayuan gold ore belt.2.Their structural deformation sequence consists of ductile?brittle-ductile?brittle,but result from various mechanisms.The Baguamiao superlarge gold deposit,which belongs to the Carlin-like type gold deposit,suffered from bedding-parallel ductile shearing from Carboniferous to early Triassic?344.1±4.5 Ma³01.8±4.6 Ma,and 249.9 Ma?,cutting-bed brittle-ductile shearing of the Indochina period,and NE-trending brittle joints and magmatic fluids of the Yanshanian?141.9±2.5 Ma¹40±2.2 Ma?.The Taiyangping-dongjiayuan gold deposits suffered from brittle-ductile shearing of the Indochina period superimposed by brittle deformation of the middle Cenozoic.3.They are characterized by various petrogenic and ore-forming fluids and superimposed mineralization types.The ore-forming fluids of the Baguamiao gold deposit are rich in Fe-Mg-Ca-CO₂?iron dolomitization?and SiO₂?silicification?,and were superimposed by deep-seated F-Cl-B acid fluid driven by magma?spotted tourmaline-biotite?.The Dingjialin–Taiyangping–Dongjiayuan gold ore belt,which belongs to orogenic gold deposit,is characterized by the ore-forming fluids rich in Fe-Mg-CO₃^2-?sideritization,iron dolomitization?and SiO₂?silicification?.4.These two types of gold-bearing shear zone developed at various structural levels of the middle crust.The gold-bearing shear zone of the Baguamiao superlarge ore deposit shows ore-forming temperature of 350⁴50??T?and pressure of 0.12⁰.31GPa?P?,and belongs to albite-two mica type forming at relatively deep structural level.The gold-bearing shear zone of the Dingjialin-Taiyangping-Dongjiayuan gold ore belt shows ore-forming temperature of350³86??T?and pressure of 0.18⁰.25 GPa?P?,and belongs to chlorite-sericite type which develops at shallower structural level than that for the Baguamiao ore deposit.However,these two types of shear zone display great differences from the Carlin-type gold deposits occurring in brittle zone.5.These two types of shear zone suffered from various geochemical and petrographic conditions.The biotite-chlorite thermometer indicates the Baguamiao ore deposit underwent medium Temperature?T??High T?High T critical phase,and the ore-forming fluids shows strong oxidation-reduction,which is favorable for the concentration of gold deposit at a large scale.