江西冷水坑Ag-Pb-Zn矿田闪锌矿矿物化学特征及赋矿铁锰碳酸盐层成因背景制约

Mineral chemical characteristics of sphalerite and genetic constraints of iron and manganese carbonate in Lengshuikeng Ag-Pb-Zn ore deposit, Jiangxi Province, China

冷水坑Ag-Pb-Zn矿田发育2类矿体,即花岗斑岩体内大脉状、细脉浸染状矿体和火山旋回内层状矿体,引起众多学者重视。但与大脉状、细脉浸染状矿体有关的花岗斑岩与层状矿体的关系、层状矿体赋矿层铁锰碳酸岩层成因背景、2类矿体间金属硫化物的成分差异等问题尚未引起足够关注。本文选取冷水坑矿田内金属储量最大的下鲍矿床,对一条横穿花岗斑岩及层状矿体的剖面进行了重点考察研究,对其元素含量及比值的变化特征进行深入讨论,进而对2类矿体内闪锌矿矿物化学进行了对比研究。花岗斑岩及层状矿体野外接触关系清晰,未出现渐变特征。二者元素含量及比值差异明显,花岗斑岩具有更高的Al、Ba、Sr、K、Na含量,铁锰碳酸盐岩层则具更高Ag、Fe、Mn、Zn、Pb、Mg含量。铁锰碳酸盐岩层元素化学特征表明其沉积成因,并未经历规模较大的热液交代作用,形成背景为火山-沉积有关的陆相深湖盆。对大脉矿体及层状矿体内发育的闪锌矿进行了电子探针分析,闪锌矿总体w(S)32.05%~33.81%,平均33.02%;w(Zn)53.51%~63.79%,平均59.38%;w(Fe)变化范围较大(2.02%~12.42%),2种矿体间差异显著,层状矿体中闪锌矿w(Fe)含量2.02%~7.69%,平均4.17%,大脉状矿体闪锌矿w(Fe)8.02%~12.42%,平均9.49%。层状矿体赋存于铁锰碳酸岩层内,但矿体内闪锌矿Fe、Mn含量却低于大脉状矿体中的闪锌矿。综合分析认为,2种矿体的Fe、Mn、Pb、Zn元素在成矿相关岩浆体系内,可能经历了不同物理化学条件(流体成分、温度、压力、p H值等)的影响,造成成矿相关岩浆体系的差异。

The Lengshuikeng Ag-Pb-Zn ore deposit contains big vein ore bodies, veinlet-dissemination ore bodies and bedded ore bodies, which has attracted the attention of many many researchers. However, the relationship between two ore body types, especially the relationship between granite porphyry veins and bedded ore bodies, as well as the genetis of the iron and manganese carbonate rock formation in which the bedded ore bodies occur, and the reasons for differences in sulphide compositions between the two ore bodies remain ambiguous. This paper focuses on the Xiabao Ag-Pb-Zn ore deposit, which is the largest in the Lengshuikeng ore deposit. Samples were taken along a section that crosses the granite porphyry veins and the bedded ore body. Subsequently, the chemical characteristics of sphalerite in both ore bodies were analysed by using electron microprobe analysis. It is not the gradual feature in field contact area between granite porphyry and bedded ore body. The chemical content and ratios of various elements show obvious differences between the two ore bodies. Granite porphyry veins have higher Al, Ba, Sr, K and Na content, whereas bedded ore bodies occurring in iron and manganese carbonate have higher Ag, Fe, Mn, Zn, Pb and Mg content. The Iron and manganese carbonate rock dispays chemical characteristics indicating sedimentary origin without having undergone larger-scale hydrothermal metasomatism. These findings indicate that the cause of formation is likely a volcanic-sedimentary deep-lake-basin related to the continental facies. The microprobe analysis also examined other chemical characteristics of sphalerite samples from both ore bodies. Sulphur content of sphalerite ranges from 32.05% to 33.81% with a mean of 33.02%, while the corresponding Zn content ranges from 53.51% to 63.79% with a mean of 59.38%. Iron content showed a larger range(2.02%―12.42%), exposing significant difference between the two ore bodies. The Fe content of sphalerite in the bedded ore body ranges from 2.02% to 7.69% with a mean of 4.17%. However, sphalerite in the big vein ore deposit has a much higher Fe content, ranging from 8.02% to 12.42% with mean of 9.49%. Although the bedded ore body occurs in the iron and manganese carbonate rock, the sphalerite has a much lower Fe and Mn content than that found in the big vein ore body. The elements Fe, Mn, Pb and Zn within the magmatic system with respect to the mineralization process, may have been influenced by different physical and chemical conditions(e.g. temperature, pressure, p H, and so on) resulting in mineralization differences related to the magmatic system.