挤出构造对凤县八卦庙金矿床的控制

八卦庙金矿床的控矿构造为脆-韧性剪切带,其变形从早至晚经历了变形分解阶段、构造挤出阶段与倾竖褶皱阶段。在变形分解阶段,剪切带内形成线性强变形带与透镜状弱变形域,并在强变形带中发育一系列"顺层"石英脉,且递进剪切为一系列A型褶曲;在构造挤出阶段,剪切带产状由缓变陡,构造体制逐渐由非共轴剪切转变为近共轴挤压,形成挤出构造控制的构造蚀变岩型成矿构造;在倾竖褶皱阶段,剪切带叠加了顺层非共轴右行剪切,形成八卦庙倾竖褶皱,并在挤出构造体中形成一组NE向剪裂控制的构造脉型成矿构造。挤出构造主要包括:剪切带边部呈叠瓦状分布的内倾弧形逆冲断层系;剪切带内条带状构造大理岩与构造千枚岩间的滑动构造;构造千枚岩带中发育一系列呈倒水滴状、平面上呈不对称"鱼状"的挤出构造体;挤出构造体中含炭泥砾-角砾岩脉。蚀变岩型金矿体发育于挤出构造体中,其空间排布规律及挤出构造结构完全受控于构造挤出时的动力学机制。单个蚀变岩型金矿体向西侧伏,侧伏角约65°;而南、中、北矿带则整体向东侧伏,侧伏角约25°。

Control of the Extrusion Tectonics on Baguamiao Gold Deposit, Feng County,Shaanxi Province

The ore-controlling structure of Baguamiao gold deposit is brittle-ductile shear zone,which experienced the deformation partitioning stage,the tectonic extrusion stage and the plunging vertical fold stage from early to late.In the deformation partitioning stage,the shear zone developed lens-like weak-deformational domain and the linear strong-deformational zone where a series of bedded quartz veins was formed,then deformed progressively into a series of A-type folds;in the tectonic extrusion stage,the occurrence of shear zone changed from low-angle to high-angle,the tectonic regimes transformed gradually from non-coaxial shearing to nearly coaxial extrusion,resulting in the formation of altered-rock type metallogenic structures controlled by the extrusion tectonics;in the plunging-vertical fold stage,the bedded right-lateral shearing with non-coaxiality is superposed upon the shear zone to form the Baguamiao plunging vertical fold,and the vein-type metallogenic structures controlled by a group of NE-trending shear fractures in the extrusion tectonic bodies.The extrusion tectonics mainly includes:① The arc-shaped thrust fault systems with imbricate distribution in the edge of shear zone,their dips are inclined to the shear zone;② The sliding structures between the banded structural marbles and the tectonic phyllites in the shear belt;③ A series extrusion tectonic bodies with shape-inverted water drops and asymmetric fish-like shape in planes in the tectonic phyllite belts;④ The C-bearing clays and gravels,and breccia veins in the extrusion tectonic bodies.The altered rock type gold ore bodies were developed in the extrusion tectonic bodies,whose distribution patterns and textures of the extrusion tectonics completely controlled by the dynamics in the process of tectonic extrusion.Single altered rock type Au ore body shows West pitch direction with pitch angle at ~ 65°while the south,middle and north ore belts show East pitch direction with the pitch angle at ~ 25°.