粤北仁化棉花坑铀矿床成矿热液演化及其对成矿过程的约束

The evolution of ore-forming fluid and its constraint on mineralization process in Mianhuakeng uranium deposit,northern Guangdong,China

本文以棉花坑(302)铀矿床成矿期紫黑色萤石、浅粉红色方解石及赤红色微晶石英等含铀脉石矿物及与之共生的黄铁矿为研究对象,采用流体包裹体热力学、群体成分分析及黄铁矿微量元素分析等方法,对成矿流体演化特性及对成矿过程的指示与约束开展了研究。研究表明,成矿期的萤石、方解石、微晶石英中流体包裹体类型以富液相两相Na Cl-H2O型为主,平均均一温度分别为185. 8℃、177. 0℃、140. 4℃,平均盐度分别为2. 24%Na Cleqv、1. 36%Na Cleqv、1. 75%Na Cleqv,矿床流体具有中低温低盐度特征;计算出平均成矿压力分别为39. 5MPa、38. 0MPa、30. 1MPa,平均成矿深度分别为1. 5km、1. 3km、1. 1km。流体包裹体群体成分显示成矿流体中富含K^+、Na^+、Ca^2+等阳离子和HCO3^-、F^-、SO4^2-等阴离子及CO2、H2O等气相成分。这些脉石矿物为成矿期不同阶段沉淀的产物,随着成矿流体温度、压力逐渐降低,流体存在演化分异和不混溶现象,流体内的∑M^+/∑M^-逐渐升高,矿物沉淀按先析出萤石、其次方解石、最后微晶石英的顺序进行。成矿期黄铁矿Y/Ho平均比值变化显示,矿物沉淀过程逐渐改变了成矿流体性质,使得Zr/Hf、Nb/La、Co/Ni等稀土、高场强元素平均比值逐渐变小,还原性的成矿环境也会发生轻微波动;铀元素在流体演化的最晚阶段才大量与微晶石英一同沉淀,沉淀出的黄铁矿U/Th平均比值逐渐升高。

In this paper,the evolution of ore-forming fluid and its constraint on ore-mineralization process in the Mianhuakeng( No. 302) uranium deposit has been studied by means of thermodynamics and composition of fluid inclusions in various syn-ore gangue minerals( such as purple black fluorite,light pink calcite and red microcrystalline quartz) and the trace elements of symbiosis pyrite.The results show that the inclusions of syn-ore fluorite,calcite and microcrystalline quartz were mainly liquid-rich two-phase Na Cl-H2O with the average homogeneous temperatures of 185. 8℃,177. 0℃ and 140. 4℃ and average salinity of 2. 24% Na Cleqv,1. 36%Na Cleqv,1. 75% Na Cleqv respectively. That was to say,the No. 302 deposit had the characteristics of hydrothermal mineralization at low temperature and low salinity. In addition,the average ore-forming pressure was 39. 5 MPa,38. 0 MPa and 30. 1 MPa,and the average ore-forming depth was 1. 5 km,1. 3 km and 1. 1 km respectively. The composition of fluid inclusion groups has been showed that the ore-forming fluids were rich in K^+,Na^+,Ca^2+cations,and HCO3^-,F^-,SO4^2- anions as well as gaseous components such as CO2 and H2O. It was considered that these gangues were the products of precipitation in different stages of the syn-ore period,there were evolutionary differentiation and immiscibility in the hydrothermal syn-ore process with the decreasing of temperature and pressure of the ore-forming fluid and increasing of ∑M^+/∑M^-in the inclusions. Mineral precipitation was accompanied by the sequence of gangue mineral formation of fluorite,calcite and microcrystalline quartz. The change of Y/Ho average ration of pyrite associated with syn-ore period showed that the properties of the ore-forming fluid has been changed during the mineral precipitation process,which made the average ratio of REE and HFSE such as Zr/Hf,Nb/La,Co/Ni gradually decrease,and made the reductive syn-ore environment undergo slight microwave movement,uranium element would be precipitated with microcrystalline quartz in large quantities at the latest stage of fluid evolution,the U/Th average ratio of the precipitated pyrite was increased gradually.