内蒙古拜仁达坝银多金属矿床二氧化碳不混溶成矿作用研究

摘要

通常流体的沸腾、混合及与围岩的反应被认为是成矿元素沉淀的3种有效途径.报道了一种较少见的由二氧化碳引起的成矿元素沉淀方式,即二氧化碳不混溶成矿作用.拜仁达坝是一超大型岩浆热液型银多金属矿床,银储量1442 t.对该矿床中与多金属硫化物共(伴)生方解石典型样品进行了碳、氧同位素、流体包裹体均一温度-盐度和稀土元素的系统测试分析.结果显示,方解石发育富气相和富液相两类包裹体,其富气相包裹体均一温度集中在360～380℃,富液相包裹体为280～340℃,它们盐度集中在6％～9％NaCl~v;方解石具正Eu异常(Eu/Eu*=2.64～5.88)且富集轻稀土元素[(La/Yb)N=4.74～63.91],表明其为热液成因;方解石碳和氧同位素平均值分别为-13.2‰和12.85‰,依此进行二氧化碳不混溶建模,其结果与实测情况吻合.研究结果表明,在拜仁达坝矿床的成矿元素沉淀过程中,二氧化碳的不混溶起着一定成矿作用.%Generally,fluid boiling,fluids mixing and fluid-wall rock interactions are effective processes responsible for the precipitation of ore minerals.In this study,an unusual process is reported,which is caused by CO2.It is called CO2 immiscibility.Bairendaba is a super-large,magmatic hydrothermal deposit with silver reserve of 1442 Ton.This study is based on the calcite of associating with polymetallic sulfide from Bairendaba.There is a system analysis including carbon and oxygen isotopes,fluid inclusion homogenization temperature and salinity,and rare earth elements.The result showed that the calcite has two kinds of inclusions,gas rich phase and liquid rich phase.The gas rich inclusions homogenization temperature is 360-380 ℃,and liquid rich inclusions homogenization temperature is 280-340 ℃,both salinity concentrated in 6％-9％ NaCleqv.The calcite has positive Eu anomaly (Eu/Eu* =2.64-5.88) and enrichment of light rare earth elements [(La/Yb) N =4.74-63.91],which indicates it is precipitated from a hydrothermal fluid.Carbon and oxygen isotope average values of the calcite are-13.2％ and 12.85％,respectively.A CO2 immiscibility model is suggested and the result fits with the real situation.The results indicated that to some extent,CO2 immiscibility seems to be responsible for the precipitation of ore minerals in the Bairendaba deposit.