四川木里菜园子金矿床成矿机制——来自构造解析、流体包裹体与H-O-S同位素制约OA

The Caiyuanzi gold deposit in Muli,Sichuan Province,is located in the southern section of the Ganzi-Litang ophiolitic mélange belt and is a large-scale gold deposit(Au resources>20 t)discovered in recent years.The orebody positioning is controlled by the brittle-ductile shear structural system above the ductile shear zone.However,previous studies lack a systematic understanding of the intrinsic relationship between the ductile shear zone and the genesis of the deposit,which has constrained the understanding of ore-controlling regularities and the determination of further exploration directions.To clarify the genetic type of this deposit,this work systematically investigated the characteristics of the ductile shear zone,ore-controlling structures,ore-forming fluid features,and gold precipitation mechanisms through methods including field geological profiles,drill core logging,microstructural observation,fluid inclusion petrography and microthermometry,laser Raman spectroscopy,and H-O and S isotope analyses.Results show that:(1)The shear zone experienced superimposed modification from early deep sinistral ductile shearing to late shallow brittle-ductile shearing.The ore bodies are strictly controlled by the brittle-ductile shear structural system,displaying a left-stepping en echelon arrangement in plan view,constituting a Riedel shear ore-controlling structure.(2)The ore-forming fluid is characterized by medium-low temperature(homogenization temperature:104.5～405.7℃,peak at 260～280℃),low salinity(avg.8.29%NaCleqv),and medium-low density(avg.0.87 g/cm³),belonging to the H2O-NaCl-CH4-CO2 system.H-O isotopes(δD=-116.2‰ to-76.0‰,δ18OH2O=3.6‰ to 6.5‰)and S isotopes(δ34S=10.0‰ to 18.8‰)indicate that the ore-forming fluids and materials were mainly derived from deep metamorphic fluids,with meteoric water added during the late stage of mineralization.(3)Decompression boiling induced by spatial dilation is the dominant mechanism for gold precipitation.(4)The genetic type of the deposit can be classified as a ductile shear zone-type gold deposit,with the mineralization type being structurally altered rock type.Based on the regional geological evolution background,it is proposed that the formation of the Caiyuanzi gold deposit is related to the post-orogenic crustal extension and thinning of the Songpan-Garzi orogenic belt during the Yanshanian period.Accordingly,a four-stage metallogenic model is established:initial gold emplacement,gold source formation,gold migration and enrichment,and gold precipitation.This study aims to provide a reference for the exploration of similar deposits within the Ganzi-Litang metallogenic belt.