相山西部牛头山铅锌矿化体成矿物质来源:原位硫同位素的制约

Study on In-Situ Sulfur Isotope Compositions of Sulfides: Implication for the Source of Pb-Zn Mineralized Body of Niutoushan in the Xiangshan Area

近年来,在相山铀矿田的西部牛头山地区深部发现了铅锌矿化体,其成因机制不明.为探讨牛头山铅锌矿化体物质来源,开展了硫化物原位硫同位素分析研究.根据硫化物矿物之间的充填和包裹关系判断,铅锌矿化体金属硫化物形成的先后顺序是:黄铁矿形成最早,方铅矿和闪锌矿次之,细脉状黄铜矿形成最晚.利用LA-MC-ICP-MS技术对矿化体中几种金属硫化物分别进行了系统的原位硫同位素分析.结果显示:黄铁矿、闪锌矿、方铅矿、细脉状黄铜矿的δ34S值介于-4.8‰^+5.4‰之间,各硫化物矿物之间硫同位素未达到完全平衡分馏,利用黄铁矿δ34S值得到的矿化流体δ34SΣS值(总硫同位素组成)近似为+3.7‰,与共生矿物对(闪锌矿-方铅矿)图解法得到的闪锌矿和方铅矿沉淀时矿化流体的δ34SΣS值(+3.2‰)相近,表明形成牛头山铅锌矿化体的矿化流体δ34SΣS值大约为+3.7‰,为岩浆硫.结合前人的岩浆岩年龄数据,我们判断该铅锌矿化体金属硫化物的硫可能主要来自次火山岩相花岗斑岩岩浆热液.同一薄片中闪锌矿δ34S值高于共生的方铅矿,表明两者硫同位素基本平衡,利用共生矿物对(闪锌矿-方铅矿)硫同位素温度计计算得出平衡温度为197~476℃,与前人通过脉石矿物流体包裹体得到的铅锌矿化流体温度基本一致.相山火山盆地与相邻的北武夷黄岗山、梨子坑等产铅锌矿的火山盆地具有相似的成矿条件及成矿物质来源,使相山火山盆地具有良好的铅锌多金属找矿前景.

In recent years,the Pb-Zn mineralization was discovered in the Niutoushan area in the west part of the Xiangshan volcanic basin.The genetic mechanism of Pb-Zn mineralization is still unclear.In order to reveal the source of the Pb-Zn mineralization,in-situ sulfur isotope analysis using laser altered-inductively coupled plasma spectra(LA-MC-ICP-MS)of sulfides is carried out.Paragenetic and crosscutting relationship between sulfide minerals formed by the hydrothermal fluids suggest that the earliest precipitated mineral was pyrite,followed by galena and sphalerite,and the chalcopyrite in fine vein shape was formed at the latest stage of the hydrothermal fluids.The analytical results of this study indicate that theδ34S values of metal sulfide minerals(pyrite,sphalerite,galena and fine-vine chalcopyrite)range from-4.8‰^+5.4‰.In term of sulfur isotopes,not all the sulfide minerals are in completely isotope equilibrium.Theδ34SΣS(total sulfur isotope)value of the mineralized fluid calculated from theδ34S values of pyrite at its formation temperature is+3.7‰,which is basically consistent with hydrothermalδ34SΣSvalue obtained from theδ34S values of the paragenetic mineral pair(sphalerite-galena).Therefore,theδ34SΣSvalue of the mineralization fluid(+3.7‰)in the Niutoushan Pb-Zn mineralization indicates that the mineralization fluid was magmatic in origin.Combined with the published dating data of the magmatic rocks of the Xiangshan volcanic basin,the sulfur isotope data of this study suggest that hydrothermal fluid of the Pb-Zn mineralization may have been mainly derived from the subvolcanic magma of the granitic porphyry.The sulfur isotope values of the sphalerite minerals were higher than those of the paragenetic galena in the mineralized bodies,indicating sulfur isotope equilibrium between these two minerals.The temperatures calculated by using the sulfur isotopic compositions of these two minerals are between 197℃and 476℃,which is consistent with the published temperatures from fluid inclusions.The metallogenic conditions and sources of ore-forming materials of the Niutoushan Pb-Zn mineralization in the Xiangshan volcanic basin are similar to those of Pb-Zn ore deposits in the Huanggangshan and Lizikeng volcanic basins in Northern Wuyi area,which hints promising prospects for Pb-Zn deposit prospecting in the Xiangshan volcanic basin.