湘南宝山矿床REE、Pb-S同位素地球化学及黄铁矿Rb-Sr同位素定年

REE,Pb-S Isotope Geochemistry,and Rb-Sr Isochron Age of Pyrites in the Baoshan Deposit,South Hunan Province,China

湘南宝山矿区的花岗闪长岩体、地层(灰岩)、方解石及黄铁矿具有相似的稀土元素地球化学特征,即轻稀土富集和Eu负异常。其中,黄铁矿表现为最低的稀土总量、明显的Eu负异常和较明显的Ce负异常,这反映了流体的稀土元素地球化学特征。认为成矿流体既有来自花岗闪长岩体的岩浆水参与,又有大气降水的作用。本次工作的矿床硫化物单矿物Pb同位素组成变化范围较小,在铅同位素构造演化图上均位于地壳演化线附近,反映的主要是地壳的信息。矿床硫化物S同位素组成与岩体及围岩地层基本一致,属于同源。通过对六个黄铁矿样品的Rb-Sr同位素分析,获得宝山矿床的Rb-Sr等时线年龄为174±7Ma,MSWD=0.55,黄铁矿的(87Sr/86Sr)i为0.70912±0.00016,高于花岗闪长岩体的锶同位素初始比值。黄铁矿Rb-Sr等时线年龄与花岗闪长岩体的单颗粒锆石U-Pb年龄十分一致,表明矿床的形成与岩体有密切的联系,花岗闪长岩体提供了矿床形成所需要的能量和部分流体,而地层也提供了部分成矿物质。

The rare earth element （REE） geochemistry of the granodiorites, strata （limestone）, calcites and pyrites from Baoshan deposit show the same REE patterns of LREE enrichment and negative Eu anomaly. In this study, the pyrites have the lowest REE concentration, high negative Eu and Ce anomaly. We consider the ore fluid consists of meteoric water and magmatic water. The Pb-isotopic compositions of the sulfides in this deposit show very narrow variations and plot near the upper crust evolution curve in Zartman^s Pb diagram, suggesting that Pb source mainly from the crust for this deposit. The S-isotopic compositions of the sulfides in the deposit are agreed with that of granodiorites and strata, suggesting that they have the same S sources. Six pyrite samples from the Baoshan deposit were selected for Rb-Sr isotope dating measurement in order to define the mineralization age of the deposit. It yields an Rb-Sr isochron age of 174.4Ma Ma （2a）, which is in accordance with their host granodiorite dated as 173.3±2.9 Ma by single-grained zircon U-Pb method. The initial 687Sr/ ^86Sr ratios of the pyrites are 0. 70919±0. 00029, which are higher than that of granodiorite. We suggest the genesis of the deposit is closely associated with the granodiorite intrusion, which provided the energy and media but was not the only main material source for the deposit.