滇西北北衙斑岩型金多金属矿田构造体系及其控矿作用

Ore-field structural system and its ore-controlling processes of the Beiyaporphyry gold polymetallic deposit in northwestern Yunnan,China

北衙斑岩型金多金属矿床是西南"三江"多金属成矿带新生代斑岩型矿床的典型代表,是滇西北新生代构造-岩浆-成矿事件的产物。北衙金多金属矿田构造发育,构成一套褶皱-断裂-节理控岩控矿构造系统,控制和形成了"五位一体"矿床类型组成两大成矿系统:Ⅰ斑岩成矿系统,(1)受斑岩体内断裂、节理裂隙及爆破角砾岩筒控制的斑岩型Cu-Au-(Fe)矿床,(2)受斑岩与围岩接触带构造控制的夕卡岩型Au-Fe-(Cu)矿床,(3)受围岩层间断裂破碎带、断裂、节理裂隙控制的热液脉型Au-Fe矿床,(4)沿北衙组灰岩与青天堡组砂泥岩界面及北衙组层间断裂破碎带产出的中低温热液型Pb多金属矿床;Ⅱ氧化淋滤成矿系统,(5)受岩溶洞穴和不整合面控制的氧化淋滤富集型Au矿床。通过构造形迹的几何学、运动学分析和力学性质鉴定,笔者认为该矿田自燕山晚期—喜马拉雅早期印度-欧亚板块碰撞以来主要经历了四期构造活动,其主压应力分别经历了近EW向→近SN向→NW向→近EW向的转变过程,依次形成了南北构造带、东西构造带和北东构造带,分别归属于三江—滇中经向构造体系金沙江—程海南北构造带、云南纬向构造体系汤丹—鹤庆—兰坪东西构造带和丽江北东构造带。笔者认为,早南北构造带构成矿田控岩控矿构造格架,东西构造带是控制与喜马拉雅中期富碱斑岩有关的斑岩成矿系统的主导性构造体系,晚南北构造带既对先前形成的金多金属矿体产生破坏或(和)改造富集,又是控制喜马拉雅晚期氧化淋滤成矿系统的主导性构造体系。

The Beiya porphyry gold polymetallic deposit, located in northwestern Yunnan, China and formed during the Cenozoic structure-magma-mineralization events, is one of the typical Cenozoic porphyry deposits in the Sanjiang polymetallic metallogenic belt. Well developed structures in the Beiya gold polymetallic ore-field composed a typical folds-faults-joints rock- and ore-controlling structural system and controlled the formation of the so called ＂five in one＂ deposits which constituted two metallogenic systems. I Porphyry metallogenic system： （1） the porphyry type Cu Au-（Fe） deposit that controlled by faults, joints and explosion breccia pipes in porphyry; （2） the skarn type Au-Fe-（Cu） deposit that controlled by the contact zone structure between country rock and porphyry; （3） the hydrothermal vein type Au-Fe deposit that controlled by interlayer fracture zone, faults and joints in country rock; （4） the low-medium temperature hydrothermal type Pb polymetallic deposit that controlled by the boundary between Qingtianbao formation sand-mudstone and Beiya formation limestone and interlayer fracture zone in Beiya formation. Ⅱ-Oxidation leaching metallogenic system： （5） the oxidized-leached-enriched type Au deposit that controlled by karst caves and unconformities. Through the analysis of geometry, kinematics and mechanical property of the typical rock- and ore-controlling structures, this paper dissects ore-field structural system and discusses its ore-controlling processes. Since the Late Yanshanian- Early Himalaya, affected by the continuous collision of the Indian and Eurasian plates, the main constructional compressive stress has experienced the transition sequentially from the nearly EW- to nearly SN- to NW- and to nearly EW-trending in the Beiya ore-field, and formed the SN, EW and NE structure belts which belong to the Jinshajiang - Chenghai SN structure belt of the Sanjiang - Central Yunnan meridional structural system, the Tangdan- Heqing- Lanping EW structure belt of the Yunnan latitudinal structural system and the Lijiang NE structure belt, respectively. It is believed that the EW structure belt was the dominant structural system which controlled the large-scale porphyry metallogenic system in the middle Himalaya. The early SN structure belt, formed in the early Himalaya, provided a favorable structural setting for porphyry emplacement and gold polymetallic mineralization. In contrast, the later SN structure belt was the dominant structural system that controlled the oxidation leaching metallogenic system in the Late Himalaya, which largely destroyed the deposits and/or led to transformation and enrichment.