Trace elemental and sulfur-lead isotopic variations in metamorphosed volcanogenic massive sulfide (VMS) mineralization systems: An example from the Keketale Pb-Zn(-Ag) deposit, NW China
Trace elemental and sulfur-lead isotopic variations in metamorphosed volcanogenic massive sulfide (VMS) mineralization systems: An example from the Keketale Pb-Zn(-Ag) deposit, NW China
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变质火山块状硫化物 (VMS) 矿化系统中的痕量元素和硫铅同位素变化：以中国西北部可克塔莱铅锌银矿床为例
DOI：
10.1016/j.oregeorev.2020.103685
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发表时间：
2020
影响因子：
3.3
通讯作者：
Wang Cheng-Ming
中科院分区：
文献类型：
--
作者：
Yu Peng-Peng;Zheng Yi;Wang Cheng-Ming
火山成因块状硫化物(VMS)后变形/变质作用对硫化物微量元素和硫铅同位素组成的影响尚不清楚。位于中国阿勒泰西北部泥盆系麦子火山-沉积盆地中的绿片岩至下角闪岩相变质可可塔勒铅锌(银)矿床，为解决上述问题提供了契机。可可塔勒矿化可分为两个成矿阶段，包括(1)来自海底热液成矿的原生条带状、块状和浸染型矿石；(2)矿石再活动，表现为强烈变形的矿石和横穿原生矿石的石英-多金属硫化物矿脉。硫化物微量元素数据表明，二期黄铁矿和磁黄铁矿的铜、锌、银、锑、铅含量较高，而一期闪锌矿和方铅矿的铜、锌、银、锑、铅含量较高。这种元素的再活化可能是由于快速的晶内扩散和随后的流体介导的释放，微量元素分别以颗粒的形式在同一颗粒和附近的矿物中重新析出。两个阶段的S-34增量值均呈三峰分布(-25.9~-22.7‰，-17.0~-10.6‰，0.6~2.1‰)。原生VMS成矿过程中形成的硫化物矿物对不处于硫同位素平衡状态，在变形变质过程中仅发生局部再平衡作用。此外，再活化硫化物脉方铅矿的铅同位素值与第I期方铅矿和黄铁矿的铅同位素值重叠。这些资料表明，可可塔勒矿床的区域变形和变质作用可近似为一个封闭的硫铅同位素系统。虽然方解石-石英脉中异常低的345值(最小为-25.9‰)的存在和方解石石英脉中硫化物独特的铅同位素比值证实了变质事件的晚期，但也有一些外来的铅和轻质硫来自矿田外。一期硫化物的原位S-34值变化很大(-17.0~+2.1ppm)，表明硫源来自于海相硫酸盐的细菌硫酸盐还原作用，可能有一定的岩浆输入。硫化物和矿化变质-沉积-火山岩的铅同位素在Pb206/Pb204与Pb207/Pb204图中呈线性分布。铅源可能来自长英质火山岩和镁铁质火成岩(和/或岩浆挥发物)，硫铅同位素可能来自浅层海水循环过程。
The effects of post-VMS (volcanogenic massive sulfide) deformation/metamorphism on sulfide trace element and sulfur-lead isotopic compositions remain unclear. The greenschist to lower amphibolite facies metamorphosed Keketale VMS Pb-Zn(-Ag) deposit, located in the Devonian Maizi volcanic-sedimentary basin in the Chinese Altay, NW China, provides an opportunity to resolve the abovementioned issue. Two mineralization stages are recognized at Keketale, including (1) primary banded, massive and disseminated ores derived from sea-floor hydrothermal mineralization and (2) ore remobilization, as represented by intensively-deformed ores and quartz-polymetallic sulfide ore veins that crosscut the primary ores. Sulfide trace elements data suggest that Stage II pyrite and pyrrhotite have higher contents of Cu, Zn, Ag, Sb and Pb, whereas contents of these elements are higher in Stage I sphalerite and galena than their Stage II counterparts. Such elemental remobilization was probably caused by the rapid intragrain diffusion and subsequent fluid-mediated liberation with trace elements re-precipitated as particles within the same grain and nearby minerals, respectively. The delta S-34(cDT) values in both stages show a trimodal distribution (-25.9 to -22.7 parts per thousand, -17.0 to -10.6 parts per thousand and 0.6 to 2.1 parts per thousand). Sulfide mineral pairs formed during primary VMS mineralization are not in sulfur isotopic equilibrium, and only localized reequilibration occurs during deformation and metamorphism. In addition, the lead isotopic values of galena from remobilized sulfide veins overlap with those of the Stage I galena and pyrite. These data indicate that regional deformation and metamorphism of the Keketale deposit can be approximated as a closed system in terms of sulfur-lead isotopes. Whilst the presence of the anomalously low delta 345 values (minimum -25.9 parts per thousand) and the distinct lead isotopic ratios of sulfides from the calcite-quartz veins argue for that in the late part of the metamorphic event, some exotic lead and light sulfur were sourced from outside the ore field. In situ delta S-34 values of Stage I sulfides vary widely (-17.0 to +2.1 parts per thousand), suggesting that the sulfur was originated from bacterial sulfate reduction (BSR) of marine sulfates with probably some magmatic input. Lead isotopes of the sulfides and mineralized meta-sedimentary-volcanic rocks display a linear distribution between the mantle and upper crust line in the Pb-206/Pb-204 vs. Pb-207/Pb-204 diagram. The lead source may be derived from both felsic volcanic rocks and mafic igneous rocks (and/or magmatic volatiles), and the sulfur-lead isotopes may result from a shallow-level seawater circulating process.