相山铀矿田黄铁矿微量元素、硫同位素特征及其地质意义

Sulfur isotope and trace element geochemical characteristics of pyrite in Xiangshan uranium orefield and its geological significance

相山铀矿田位于江西省境内的相山火山盆地中,是中国目前最大的火山岩型铀矿田。文章利用电子探针(EPMA)和激光剥蚀多接收电感耦合等离子质谱仪(LA-MC-ICP-MS)技术对矿田内几个典型铀矿床(居隆庵、河元背和沙洲矿床)中矿前期热液蚀变阶段形成的黄铁矿分别进行了微量元素及S同位素组成特征研究。研究结果表明,矿田内铀矿床中黄铁矿的Co/Ni比值主要介于2.00~6.00,支持其为热液成因。黄铁矿的δ34S值总体变化于+0.1‰^+16.2‰,但西部与北部铀矿床之间黄铁矿δ34S值存在显著差异:西部铀矿床(居隆庵、河元背)中黄铁矿δ34S值为+0.1‰^+8.4‰,介于矿田内新元古代基底变质沉积岩δ34S值(+7.9‰^+9.4‰)与壳源岩浆δ34S值(-5.0‰^+5.0‰)之间,暗示S可能来自基底变质沉积岩硫与围岩(流纹英安岩和碎斑熔岩)中硫化物的硫的混合;北部沙洲铀矿床中黄铁矿的δ34S值为+7.5‰^+16.2‰,与蒸发硫酸盐δ34S值相接近,表明硫的来源可能主要与矿田西北侧红盆内硫酸盐的热化学还原(TSR)相关,围岩(花岗斑岩)中的Fe2+在还原过程中发挥了重要作用。同时,热化学还原产生的H2S与围岩中的Fe2+进一步结合形成黄铁矿。铀成矿期含铀热液中的六价U(Ⅵ)与铀成矿前期形成的上述黄铁矿发生氧化还原反应,导致铀沉淀成矿。

The Xiangshan orefield in Jiangxi Province is the largest volcanic rock-hosted uranium orefield in China.This paper reports trace element content and sulfur isotope compositions of pyrites formed during pre-ore period by hydrothermal fluid in a few representative uranium deposits(Julong’an,Heyuanbei and Shazhou)analyzed by electron microprobe analysis(EPMA)and laser abrasion-inductively coupled plasma spectra(LA-MC-ICP-MS).Co/Ni ratios of pyrites are between 2.00 and 6.00,suggesting hydrothermal origin of these pyrites.δ34S values of pyrites in the Xiangshan orefield vary in a large range,from+0.1‰to+16.2‰,with significant differences between western and northern uranium deposits.Theδ34S values of pyrites in western uranium deposits(Julong'an,Heyuanbei)(+0.1‰^+8.4‰)vary betweenδ34S values of pyrites in the Neoproterozoic basement metamorphic sedimentary rocks(+7.9‰^+9.4‰)and those of crustal magmatic rocks(-5.0‰^+5.0‰).These data suggest that the sulfur that formed pyrite in these deposits originated from the basement rocks and magmatic wall rocks(cataclastic lava and rhyodacite).δ34S values of pyrites in the northern uranium deposit(Shazhou)(+7.5‰^+16.2‰)indicate that the sulfur of pyrites in this deposit likely originated from sulfate in the red strata.Thermochemical reduction of hydrothermal sulfate by Fe2+in the surrounding rock(granite porphyry)produced H2S which in turn reacted with remaining Fe2+to form pyrite.U(VI)in the later uranium-containing hydrothermal fluid reacted with Fe2+or pyrite in the altered rocks by previous hydrothermal fluids,facilitating uranium precipitation.In conclusion,sulfur of pyrites in the Xiangshan uranium orefield was multi-sourced,and pyrites formed in previous hydrothermal activities served as the major reductant during the uranium reduction and precipitation in the ore-forming hydrothermal fluid.