超慢速扩张环境下超镁铁质岩系统的热液硫化物成矿机理以及启示

The metallogenic mechanism and enlightenment of hydrothermal sulfide from the ultramafic-hosted hydrothermal systems at ultra-slow spreading ridge

西南印度洋63.5°E热液区是在超慢速扩张洋脊发现的首个超镁铁质岩热液系统。对取自该区的热液硫化物样品进行了系统的矿物学和地球化学分析,矿物学分析结果表明:该热液区硫化物为富Fe型高温硫化物,且经历了较深程度的氧化蚀变,大量中间态的Fe氧化物充填在硫化物矿物间的孔隙及内部解理中;这些硫化物相以白铁矿为主,其次是等轴古巴矿和少量铜蓝,缺乏黄铁矿、闪锌矿。据推断,该区的热液成矿作用分为4个阶段:低温白铁矿阶段→高温等轴古巴矿阶段→自形白铁矿阶段→后期海底风化阶段(少量铜蓝以及大量的Fe的羟氧化物)。与之相对应,地球化学分析结果表明这些硫化物的Fe含量较高(31.57%~44.59%),Cu含量次之(0.16%~7.24%),而Zn含量普遍较低(0.01%~0.11%);微量元素较为富集Co(328×10^(-6)~2 400×10^(-6))和Mn(48.5×10^(-6)~1 730×10^(-6))。该区硫化物中较高含量的Fe、Co与超镁铁质岩热液系统相似,明显高于镁铁质岩热液系统。独特的热液硫化物矿物学特征和元素组成可能与该区普遍出露的地幔岩、橄榄岩蛇纹石化作用以及拆离断层的广泛发育的环境有关。

The 63.5°E hydrothermal field was the first ultramafic-hosted hydrothermal system on the super-slow spreading ridge-South West Indian Ridge(SWIR).In this study,we report the detailed mineralogical and geochemical characteristics of hydrothermal metal sulfides recovered from this are.The results show that the mineral assemblage were highly weathered,characterized by a large number of intermediate Fe-hydroxides filling in the vugs or fractures.Marcasite is the main mineral(there are two generations marcasite),followed by the isocubanite,and a small amount of covellite,sphalerite and pyrite were not identified.Four distinctive mineralization stages have been identified:(1)low-temperature stage consisting largely of marcasite;(2)high temperature stage distinguished by isocubanite;(3)low-temperature stage consisting mainly of euhedral marcasite stage;(4)weathering stage characterized by minor Cu-sulfides(covellite),mass Fe-ox hydroxides.For the Fe-sulfide,there only observed marcasite,which could be due to the strong reducing environment resulted from the serpentinization of peridotite.Geochemical analysis indicates that the hydrothermal sulfides relative enrichment of Fe(31.57%-44.59%),a small amount of Cu(0.16%-7.24%),however,the Zn(0.01%-0.11%)content is relatively low.As for trace elements,enrichment of Co(328×10-6-2 400×10-6)and Mn(48.47×10-6-1 730×10-6)content.The high content of Fe and Co is comparable to results from seafloor massive sulfides collected from other ultramafic-hosted sites,significantly higher than that of collected from other mafic-hosted hydrothermal systems.The unique mineral and elements characteristics of hydrothermal sulfide in coincidence with the widely exposed mantle rock and the extensive development of detachment faults at this field.