陕西八卦庙金矿脆-韧性剪切带控矿特征及成矿构造动力学机制

Baguamiao superlarge gold deposit in the Qinling orogen:the characteristics of its control by the brittle-ductile shear zone and dynamic mechanism for ore-forming structure

八卦庙金矿是秦岭造山带唯一的超大型金矿床,曾被认为是微细浸染型(卡林型)金矿。但最新研究表明该矿床金为粗粒可见金,矿床空间定位和矿体展布受二里河—长沟—八卦庙—空棺沟脆-韧性剪切带控制,矿区内岩石剪切变形特征明显。在脆韧性剪切过程中形成了NW向顺层无根揉皱状含金石英脉,石英脉Ar-Ar坪年龄为(232.58±1.59)Ma,等时线年龄为(222.14±3.45)Ma,是印支期脆-韧性剪切-动力变质分异的产物。这种NW向石英脉含金1×10-6～4×10-6。在燕山早期,剪切带抬升进入浅层叠加了脆性变形,在热液作用下形成了NE向石英脉。NE向石英脉是重要的富矿石,含金4×10-6以上。石英强烈的脆-韧性变形特点记录了两期变形,从早到晚石英位错和差应力具从大到小的演化趋势。韧性剪切和后期脆性变形每一期构造-热液脉动式活动都是从早到晚△σ值降低,在两次脉动式活动的交替时期出现的构造亚稳定期成矿,应力周期形成的“应力泵”或“地震泵”作用是金矿成矿的力学机制。

The Baguamiao gold deposit is the only superlarge gold deposit in the Qinling orogen.It was once considered to be a sediment-hosted finely disseminated gold deposit(Carlin type).But the latest study shows that it is characterized by gold-bearing quartz veins with vis ible and coarse-grained gold.Its spatial lo-cation and distribution of orebodies are controlled by the Erlihe-Changgou-Baguamiao-Kongguangou brittle-ductile shear zone and the rocks in the deposit show distinct features of shear deformation.During brittle-ductile shearing,NW-trending,rootless,corrugated,bedding-parallel quartz veins were formed.The Au-bearing quartz veins ,which have an Ar-Ar plateau age of 232.58±1.59Ma and an Ar-Ar isochron age of 222.14±3.45Ma ,are the product of Indosinian brittle-ductile shearing-dynamic metamorphism and differenti-ation.In the Early Yanshanian,the ductile shear zone was uplifted to the shallow level and reworked by brittle deformation,thus enabling the NE-trending Au-bearing quartz veins to be formed under the action of hydrothermal fluids.Such Au-bearing quartz veins are rich ores and contain1×10 -6 ～4×10 -6 Au.The strong brittle-ductile shear deformation records that the quartz dislocation and differential stress(△σ)had an evolutionary trend of becoming small from the early to late phases.The△σvalues tended to decrease toward the end for the shearing event and brittle deformation event.Mineralization took place during the metastable stages of tectonic activity in the periods of alternate occurrence of two pulsations of tectonic events.The“stress pump ”or “shock pump ”formed in the stress periods was the dynamic mechanism for the formation of the Baguamiao superlarge gold deposit.