甘肃西成地区南北铅锌矿带矿床成矿特征及相互关系

Metallogenic characteristics and relationships of ore deposits in the north and south lead-zinc zones in the Xicheng area, Gansu

长期以来,甘肃西成地区的SEDEX型铅锌矿被分为南矿带沉积改造型与北矿带沉积变质型,二者容矿层位、容矿地层层序及成矿地质特征均存在较大差异。通过对二矿带成矿地质地球化学特征的对比,笔者发现,二矿带成矿的相似性远多于差异性,厂坝—李家沟矿床容矿地层层序倒转,灰岩系统在下,片岩系统在上,层序与南矿带相同。西成盆地存在多个近东西向的铅锌矿带,北矿带仅仅只是其中的一个。两个矿带成矿的差异性更多地表现在后期改造作用的方式上,厂坝—李家沟矿床的改造是在"原地"相对"干"而"封闭"的体系内完成,从而保留有完整的条纹条带状沉积特征,而南带众多的铅锌矿床的改造作用是原地的成矿物质经过迁移,"异地"、"湿"的(经历热液过程)和相对开放的条件下在构造虚脱部位沉淀富集成矿,并有部分外来物质参与。而引起这种改造作用差异的根本原因则是区内印支期与酸性侵入岩浆作用有关的变质作用程度不均一。

SEDEX lead-zinc deposits in the Xicheng （Xihe-Chenxian） area, Gansu, have long been divided into the reworked sedimentary type of the south ore belt and the metamorphic sedimentary type of the north ore belt. The two types show appreciable differences in host horizon, sequence of host strata and geological characteristics. Comparing the geological and geochemical characteristics of the two ore belts, the authors find that the two ore belts have much more similarities than differences. The sequence of the host strata in the Changba-Lijiagou deposit in the north ore belt is inverted, with limestone lying below and schist above, being similar to that in the south ore belt. In the Xicheng basin, there are several nearly E-W-trending lead-zinc ore belts, and the north ore belt is only one of them. The differences of the two ore belts are mostly manifested in the manner of late- stage reworking. The reworking of the Changba-Lijiagou deposit occurred in a relatively ＂dry and enclosed＂ system and thus the intact striped-banded sedimentary character may be found. By contrast, the reworking of many lead-zinc deposits in the south ore belt occurred after the in-situ ore-forming material was transported and precipitated and concentratied in a structural saddle in an ＂allochthonous＂, ＂wet＂ （through hydrothermal processes）, relatively open environment, with some exotic material introduced into it. The cause for the difference in the manner of reworking is the difference in the metamorphic grades related to acid intrusive magrnatism in the study area in the Indosinian period.