黔东南扒草金矿微量和稀土元素特征及其意义

Trace element and rare earth element characteristics of Bacao gold deposit,Southeastern Guizhou province and the significance

扒草金矿产于黔东南下江群浅变质岩系中,是近年来新发现的金矿床。文章对其赋矿地层、蚀变围岩、不同成矿期石英脉的微量和稀土元素进行了研究。研究结果表明,赋矿地层与蚀变围岩的微量、稀土元素特征无差异。石英脉的稀土、微量含量低于地层和蚀变围岩,但其稀土元素配分模式、微量元素配分模式总体上与地层和蚀变围岩相似;所有样品在稀土元素三组分图中的投影均处于同一位置。不同成矿期石英脉对比发现,成矿早期石英脉的稀土配分模式与地层及蚀变围岩一致,而晚期石英脉的稀土元素配分模式发生了一些变化。即从早期到晚期,石英脉中W元素含量迅速降低,而V、Cr元素含量升高,特征值δTb、δTm由正异常变为负异常;微量元素曲线呈现地层、蚀变岩中的Sr均表现为负异常,而石英脉中大部分表现为Sr的正异常,少部分表现为负异常;石英脉的w(Y)/w(Ho)值与区域变质岩相似,但部分样品偏离区域变质岩的范围。总体认为,成矿物质主要来自于浅变质岩系,成矿流体主要为变质流体,但在晚期可能有其它流体的加入;石英脉富含LREE的同时富集部分HFSE,w(Th)/w(La)、w(Nb)/w(La)、w(Hf)/w(Sm)值均小于1,表明成矿流体为Cl->F-的流体体系;特征值δEu负异常、δCe无异常和w(Th)/w(U)值大于1.25,表明扒草金矿的成矿环境为还原环境。

The Bacao gold deposit is a newly discovered gold deposit in recent years in low grade metamorphic rock series of Xiajiang group at southeastern Guizhou province.This paper studies the trace elements and REE of host strata,altered rocks and different mineralization periods＇ quartz veins of the deposit.The results show the same characteristics of the host strata with those of the altered rocks and quartz veins.The trace elements and REE content of quartz veins is lower than those of the host strata and altered rocks but their distribution patterns similar.Projection of three components of REE for all samples falls in the same position.Comparison of the trace elements and REE content of different mineralization periods＇ quartz veins reveals that the early quartz vein has the same REE distributed pattern with the host strata and altered rocks and that of the late has some differences.W content decreases rapidly from the early to late quartz veins.V,Cr content increases.The eigen values ofδTb,δTm are from positive anomalies to negative anomalies.Most of the quartz veins are characterized by positive Sr anomalies but a little by negative Sr anomalies.The w（Y）/w（Ho）value of the quartz veins are generally similar to that of the regional metamorphic rocks.A few of the value deviate from scope of the metamorphic rock.In summary the ore materials are derived from the low grade metamorphic rock.The ore fluid is dominated by metamorphic fluid and is incorporated with other fluid in the late stage and the quartz vein is rich in LREE and HFSE.w（Th）/w（La）、w（Nb）/w（La）、w（Hf）/w（Sm）values are all less than 1indicating that the fluid is of Cl^-〉F^-fluid system.Negative anomaly ofδEu,no anomaly ofδCe and w（Th）/w（U）〉1.25 is the manifest of reduction condition under which the Bacao gold deposit formed.