秦岭造山带中段马蹄沟金矿床伸展剪切构造系统及其控矿作用

马蹄沟金矿床位于陕西省宝鸡市南部秦岭造山带中段的商丹断裂带(华北板块与扬子板块碰撞缝合带)西段南缘,在矿集区划分上属于凤太矿集区北部。金矿区发育一套由面理构造、a-型线理、a-型褶皱和配套张扭性裂隙组成的伸展剪切构造系统。面理构造产状平缓,走向NWW,倾向NNE,倾角以20°~30°为主;a-型线理产状总体倾伏向为NEE,部分向E倾伏,倾伏角比较平缓,以5°~20°为主,个别达30°~50°;a-型褶皱构造往往轴面产状比较平缓或近水平,枢纽与a-型线理一致;张扭性裂隙构造单体呈约350°走向,倾向E,倾角50°~70°,组合形式平面上为右列式、剖面上为前列式。金矿床宏观上受发育于早古生代罗汉寺岩组浅变质碎屑岩中的韧脆性剪切伸展构造系统控制,金矿体沿该韧脆性变形带中的各个方向微裂隙发育,形成宽度0.5~2 mm的微细含硫化物含金石英网脉,这些微裂隙包括平行面理、小角度(10°~15°)斜交面理、中角度(25°~30°)斜交面理,对应于剪切带的P、D、R型含矿裂隙;最特色的是发育一组产状350°/NE50°~70°、大角度(50°~60°)斜交面理的较粗大含硫化物富金石英脉,其对应于剪切带的T型含矿裂隙。商丹断裂带西段地区金成矿作用与区域构造演化密切相关,秦岭造山带勉县?略阳地区陆陆碰撞在早?中三叠世已经完成,晚三叠世早期存在伸展构造形成的构造背景。区域上的进一步找矿方向是发育伸展剪切构造系统的地区。

Extensive-shear Structure and its Controls on Mineralization in the Matigou Gold Deposit,Middle Qinling Orogenic Belt,NW China

The Matigou gold deposit is located to the south of Baoji city,Shaanxi province,or rather the northern part of Fengxian-Taibai ore concentration area.It occurs in the southern margin of the west part of the Shangdan fault,a suture zone between the North China and Yangtze block,in the Qinling orogenic belt.An extensive-shear structural system(ESSS)consisting of well-developed foliation,a-type lineation,a-type fold and tenso-shear fracture was recognized in the gold ore field.The foliation is dipping to NNE at 20°–30°,a-type lineation is pitching mainly to NEE and partly to east at angle of mainly 5°–20°and partly 20°–30°.The a-type fold has flat or horizontal axial plane,with hinge parallels to a-type lineation.The monomer tenso-shear fracture has strike of 350°and dipping to east at 50°–70°,while combined tenso-shear fractures exhibit as sinistral echelon in plane and forward structure in profile.The gold mineralization is controlled by the extensive-shear structural system which developed in the epimetamorphic detrital rocks of the early Paleozoic Luohansi Group.The gold ore-bodies occur in these fractures in the ductile-brittle shear zone,such as parallel to foliation,cut foliation at low(10°–15°)and middle angles(25°–30°),corresponding to the P,D,and R-type ore-bearing fractures in ductile shear zones,and formed fine(only 0.5–2 mm wide)sulphide quartz stringer lodes.The gold mineralization is also characterized by well-developed large Au-rich sulphide quartz veins that cut through foliation at high angle(50°–60°)which correspond to the T-type ore-bearing fracture in ductile shear zones.Gold mineralization in the western part of the Shangdan fault zone is closely related to the tectonic evolution.The continent-continent collision in the Mianxian-Lueyang area of the Qinling orogenic belt completed in the Early-Middle Triassic,and the extensive tectonic deformation should be began in the early stage of Late Triassic epoch.So,the direction of the future prospecting for gold deposits should concentrate on places with well-developed extensive-shear structural system.