内蒙古碾子沟钼矿床成矿流体来源、演化及成矿机理

Origin,Evolution of Ore-Forming Fluids and Metallogenic Mechanism of Nianzigou Molybdenum Deposit,Inner Mongolia

内蒙古自治区碾子沟钼矿床地处华北地台北缘西拉木伦钼成矿带西段,为一典型的中型石英脉型钼矿床。该钼矿床矿脉(体)主要产于燕山早期二长花岗岩-钾长花岗岩内NNW、NW向断裂构造体系之中,成矿作用过程经历了黄铁矿±辉钼矿+石英(Ⅰ)、辉钼矿+黄铁矿±黄铜矿+石英(Ⅱ)、黄铜矿+黄铁矿±闪锌矿+石英(Ⅲ)及石英±方解石(Ⅳ)4个阶段。系统的流体包裹体岩相学、包裹体组分析、包裹体显微测温研究表明,矿床初始成矿流体为高温、中低盐度(490~550℃,盐度(w(NaC1))2%~10%,50~62MPa)均匀的NaCl-H2O体系热液,δ18 OH2O-SMOW(2.21‰)及δDH2O-SMOW(-68.9‰)表明其主要来源于岩浆热液;成矿流体上升并不断汇聚于容矿断裂空间,伴随温度、压力降低(380~460℃,26~40MPa→360~420℃,25~30MPa)而进入两相不混溶区,流体开始发生沸腾→强烈沸腾作用,导致成矿元素Mo大量沉淀富集成矿,成矿晚期残余流体与大气降水混合(δ18 OH2O-SMOW为-2.41‰~2.51‰,δDH2O-SMOW为-110.1‰^-105.5‰),矿床属燕山早期中高温岩浆热液型钼矿床。

Nianzigou molybdenum deposit is located in the western part of the Xilamulun molybdenum metallogenic belt at the north margin of North China Craton,which is a medium-sized typical quartz vein type deposit.The ore veins mainly occur in the NNW,NW trending faults developed in the monzonitic granite-moyite intrusions of Early Yanshanian.The hydrothermal mineralization can be divided into four stages:pyrite±molybdenite+quartz(Ⅰ),molybdenite+pyrite±chalcapyrite+quartz(Ⅱ),chalcopyrite+pyrite±sphalerite+quartz(Ⅲ),and quartz±calcite(Ⅳ).The systematic study on petrography of fluid inclusions,fluid inclusion assemblages,and micro-thermometry shows that the primary ore forming fluids are of high temperature(490-550℃),low-medium salinity NaCl-H2O type solutions(10%-18%);theirδ18 OH 2O-SMOW(2.21‰)andδDH 2O-SMOW(-68.9‰)imply that they were originally from magmatic activities,and continuously up-rose and gathered into ore-controlling faults;later with the gradual decrease of temperature and pressure(380-460℃,26-40MPa→360-420℃,25-30 MPa),the state of immiscible two-phase appeared,and the fluid began to boil,which resulted in the deposition of large amounts of Mo;at the late stage of the fluid evolution,the residual solution mixed with meteoric water.Nianzigou Mo deposit is a medium-high temperature magmatic hydro thermal deposit in Early Yanshanian.