云南金顶超大型锌铅矿成因的铅同位素制约

Pb isotope constraints on the origin of the Jinding Zn-Pb deposit,Yunnan Province,China

金顶锌铅矿作为世界级的超大型矿床一直以来受到国内外学者的广泛关注,但对于其成矿类型及成矿物质来源存在较大争议。矿石铅同位素组成是探讨矿床成因,特别是成矿金属来源的重要手段。然而,由于实验技术和采样的原因,前人所报道的数据相差甚远,这也是矿床成因争论的主要原因。本文应用飞秒激光剥蚀多接收器等离子体质谱法首次获得金顶铅锌矿中方铅矿及闪锌矿原位铅同位素组成,结果显示铅同位素组成相对均一,^(206)Pb/^(204)Pb=18.480~18.032(平均值为18.436),^(207)Pb/^(204)Pb=15.688~15.588(平均值为15.668),208Pb/^(204)Pb=38.056~38.778(平均值为38.689),全部测点的μ值介于9.55~9.63之间,平均为9.60。矿石铅同位素组成在铅构造模式图上位于上地壳演化线与造山带演化线之间,表明其成矿物质具有壳源特征,其壳源特征亦得到其他地球化学证据(S、C、H、O、Sr、He同位素)的支持。结合前人研究及野外地质现象,认为这一铅锌矿床应属于MVT铅锌矿,为典型的后生矿床且与岩浆活动无关。对比兰坪盆地沉积岩及岩浆岩的铅同位素组成特征,可以推断其成矿物质来源主要为兰坪盆地沉积地层及其基底。

Ore metal source（s） of the large, world-class Zn-Pb deposits in Jinding is/are controversial and actiyely debated. Pb isotopes are important geochemical tracers for the sources of ore metals. Previously published Pb isotope data on ores and rocks from the orefield of the Jinding deposit exhibit wide variation, often leading to contrasting interpretations. In our study, in-situ Pb isotope analyses of galena and sphalerite from the Jinding deposit were carried out using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry （fs-LA-MC-ICPMS）. The results show that galena and sphalerite have similar Pb isotope compo- sitions with narrow ranges of variation： ^206pb/^204pb = 18.480-18.032, ^207pb/^204pb = 15.688-15.588 and ^208pb/^204pb = 38.056-38.778, with average values being 18.436, 15.668, and 38.689, respectively. The relatively homogeneous Pb isotope features and high p values （p = 9.55-9.63, average 9.60） indicate that the ore-forming material was derived from a single or well-mixed crustal source, which is also supported by geochemical evidence, such as S, C, H, O, Sr, He isotope studies. Combining the geological and geochemical characteristics, it can be inferred that the Jinding ore deposit is of epigenetic and amagmatic origin. Regarding the Pb isotope compositions and ore metal contents of the sedimentary and magmatic rocks in the Lanping basin, we suggest that the ore metals were derived mainly from the sedimentary rocks of the Lanping Basin, especially the sandstones, while the basement may also have contributed to the ore mineralization.