云南会泽铅锌矿田成矿物质来源:Pb、S、C、H、O、Sr同位素制约

Sources of the ore metals of the Huize ore field in Yunnan province:constraints from Pb,S,C,H,O and Sr isotope geochemistry.

云南会泽铅锌矿田是我国著名的超大型特富铅锌矿田之一,由相距3公里的矿山厂和麒麟厂两个独立的矿床组成,Zn+Pb 金属量超过五百万吨,矿石品位在25%至35%之间。为确定矿床成矿流体和成矿金属来源,本文系统研究了矿床的 Pb、S、C、O、H 和 Sr 同位素组成特征。矿石硫化物的铅同位素组成均一,^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb 和^(208)Pb/^(104)Pb 的变化范围分别为18.251～18.530,15.663～15.855和38.487～39.433,与围岩碳酸盐岩中浸染状黄铁矿一致,与碳酸盐地层相近,在^(208)Pb/^(204)Pb-^(206)Pb/^(204)Pb 图上显示明显的线性关系,表明铅同位素组成相近的碳酸盐围岩地层提供了成矿物质。矿石硫化物的δ^(34)S 变化范围为10.9‰～17.4‰,多数集中于13‰～17‰,表明还原硫主要来自地层中海相硫酸盐的还原,还原方式为热化学还原,下伏页岩、碎屑岩和泥质岩中的有机质在硫酸盐还原过程中发挥了重要作用。三种不同产状的脉石矿物方解石的碳氧同位素组成均一且没有明显差别,δ^(13)C 变化范围为-2.1‰～-3.5‰,δ^(18)O 为16.8‰～18.6‰。脉石矿物方解石中流体包裹体水的δD_(FI)为-50‰～-60‰,取温度为200℃计算包裹体水的δ^(18)O_(H_2O)值为7.0‰～8.8‰。碳、氧和氢、氧同位素研究结果表明,成矿流体为来自下部上升的变质流体,由于下伏页岩、碎屑岩和泥质岩中有机质的参与,成矿流体具有低的δ^(13)C和δD_(FI)值,在上升过程中与围岩发生了同位素交换。矿石中黄铁矿、闪锌矿和方解石的初始锶同位素组成(^(87)Sr/^(86)Sr)_i值的变化范围为0.714～0.717,赋矿围岩中未蚀变白云岩的初始锶同位素组成(^(87)Sr/^(86)Sr)_i值为0.7083～0.7093,明显低于蚀变白云岩(0.7106),表明成矿流体具有高的(^(87)Sr/^(86)Sr)_i比值。相对围岩碳酸盐岩而言,下伏地层中的页岩、碎屑岩和泥质岩往往具有高得多的^(87)Sr/^(86)Sr,因此,流经或者起源于这些地层的流体具有高的锶同位素比值。

The Huize ore field is one of the most famous high-grade Zn-Pb ore field in China. In this ore belt, the Kuangshanchang and Qilinchang deposits are the two largest Zn-Pb deposits with a Zn ＋ Pb reserve of 〉5Mr and ore grades of 25% to 35%. Pb, S, C, O, H and Sr isotope geochemistry is discussed in this paper in an effect to understand the sources of the ore metals and ore-fluids. The ^206Pb/^204pb, ^207pb/^204 Pb and ^208pb/^204 Pb values for the ores range from 18. 251-18.530, 15.663-15. 855 and 38.487-39.433, respectively, which are similar to those of the host carbonate rocks. The pyrites disseminated in the wall rocks have indistinguishable Pb isotope compositions with the ores, indicating that these rocks may have provided metals to the ore fluid. The δ^34S values of the ores show a total range from 10. 9‰ to 17.4‰, but most of them are between 13‰ and 17‰, which indicating that the sulfur should be derived from the in situ thermochemical reduction of sulfates from the sedimentary strata. There are three types of gangue calcite in the ores, and all of them share similar isotope compositions, whth δ^13 C-PDB values of -2. 1‰ to -3.5‰ and δ^18 O-SMOW values of 16. 8‰ to 18.6‰. The δDFI values of fluid inclusions in these calcites display a narrow range of - 50‰ to - 60‰ and the δ^18OH2O values calculated from δ^18O values of calcite range from 7.0‰ to 8.8‰, if their formation temperature is 200℃. These data indicate that the ore fluid was likely metamorphic water and had water-rock interaction and isotope exchange with shale, clastic rocks and mudstone underlain the ore-bearing strata during their ascending from the depth. Initial ^87Sr/^86 Sr values of the pyrite, sphalerite and calcite from the ores range from 0. 714 to 0. 717. The initial ^87Sr/^86Sr values of unaltered host dolomite rock （0. 7083-0. 7093） are lower than that of the altered dolomite （0. 7106）. The high initial ^87Sr/^86Sr values for the ore-forming fluids also suggest water-rock reaction and isotope exchange between the ore fluid and the shale, clastic rocks and mudstones or the fluid might have originated from these rocks.