小秦岭金矿田共轭剪切矿脉的发现及意义

Discovery of conjugated shear veins in Xiaoqinling gold orefield and their significance

小秦岭金矿田近EW走向的倾向南与倾向北金矿脉群金储量占全部储量的90%以上。以往大量矿床勘探工作未揭露到两组矿脉的交汇部位,有关断裂切割关系及组合形式始终存在疑问。通过采矿坑道的调查发现两组控矿断裂为共轭剪切关系。在平面和剖面上南倾与北倾矿脉均呈舒缓波状延伸,交叉部位充填无明显互相错开的膨大石英脉。两者在空间分布上还具有等间距性,北倾脉分布间距为200~400 m,南倾脉间距为400~500 m。共轭剪切断裂具有早期韧性、晚期脆性两个发展阶段:早期韧性阶段最大主应力(σ1)为近直立状态,σ1方向两组韧性断层共轭夹角约107°;晚期脆性阶段的破裂面继承早期韧性断层作伸展活动,充填含金石英脉,保持了原韧性阶段共轭剪切角。研究认为:早白垩世大规模花岗岩岩浆底辟侵位导致形成了热穹窿构造,以共轭韧-脆性断层为代表的伸展断裂系统发育于整个金矿田;含金石英脉可能并非形成于多级热液导矿渠道,而是以超临界流体(气态)充满热穹窿,降温后方成为热液。有关共轭剪切矿脉的认识不仅具有局部追索找矿意义,而且其分布的普遍性拓宽了找矿思路和范围。

The gold veins that occur in the near EW-trending fractures which dip to south and north in Xiaoqinling gold ore-field,have the gold reserves for more than 90%of the total reserves.Lots of previous deposit exploration work did not reveal the intersection of the two vein groups,had the questions left over the fracture-cutting relationship and their combination form.The investigations of the mining tunnel suggested that the two groups of ore-control fractures come in conjugate shear relations.In terms of the plane and section,both the veins of diping to south and to north extend in a gently wavy trend,filling with enlarged quartz veins at the intersecting parts with no obviously staggered.The veins in the two groups also have the equidistance of spatial distribution,the equidistance between the veins dipping to north is about 200-400 m,between the veins dipping to south is about 400-500 m.The conjugate shear fractures have had two development stages—early ductile deformation stage and late brittle deformation stage.In the early ductile deformation stage,the maximum principal stress(σ1)is to stand upright and the conjugate angle of the two sets of ductile faults along theσ1 direction is about 107°.Inherited from the early ductile faults,the fractures stretch in the brittle deformation stage,filling with gold-bearing quartz veins and maintaining the conjugate shear angle of the original ductile deformation stage.It is believed that the Early Cretaceous large-scale granite magma diapir emplacement led to the formation of the thermal dome structure,the gold-bearing quartz veins might not be formed by a multi-level hydrothermal channel,but the supercritical fluid(gaseous)fills in the thermal dome,forming hydrothermal solution after cooling.The understanding about the conjugate shearing veins not only has the significance for local recourse and prospecting,but also for more prospecting in wide area because of the universality of conjugate shearing fractures.