云南毛坪大型铅锌矿床成矿物质来源:原位S和Pb同位素制约

The sources of ore-forming elements of the Maoping large-scale Pb-Zn deposit,Yunnan Province: Constrains from in-situ S and Pb isotopes

位于扬子板块西缘的川滇黔铅锌集区是我国重要的贱金属生产基地之一。矿集区中部的毛坪大型铅锌矿床,累计探明铅锌金属储量超过300万吨,Pb+Zn平均品位12%~30%,局部达45%,是矿集区内第二大铅锌矿床。矿体呈似层状、透镜状或脉状集中分布于猫猫山背斜NW倒转翼及其倾伏端NE向层间断裂带内。主要发育3个矿体(群),其中I号矿体(群)赋存于上泥盆统宰格组白云岩中,II号矿体(群)赋存于下石炭统摆佐组白云岩中,III号矿体(群)赋存于上石炭统威宁组白云岩中。矿石主要由闪锌矿、方铅矿和黄铁矿等矿石矿物及白云石和方解石(少量石英和重晶石)等脉石矿物组成,具有块状、浸染状或脉状构造及粒状、交代、共边、胶状、集合体或碎裂结构。可见,该矿床后生成矿特征明显。纳米离子探针(Nano SIMS)原位S同位素组成分析结果显示,细粒草莓集合体状黄铁矿和胶状闪锌矿明显亏损34S,其δ34S值变化范围为-20. 4‰^-8. 7‰之间,具有典型生物成因S特征,暗示经历了海相硫酸盐细菌还原过程(BSR);而自形粒状黄铁矿和他形粒状闪锌矿的δ34S值变化范围为22. 1‰~25. 6‰之间,明显富集重S同位素,表明经历了海相硫酸盐热化学还原过程(TSR)。由于BSR和TSR过程主要受温度控制,因此,还原态硫离子的形成最可能是原地还原的,并先经历了相对低温的BSR过程,再经历了相对高温的TSR过程。飞秒激光剥蚀多接收器等离子体质谱(fs LA-MC-ICPMS)原位Pb同位素组成分析结果显示,方铅矿的Pb同位素组成相当均一,暗示其来源单一或混合均匀,对比显示其成矿金属主要由赋矿沉积岩提供,受到一定程度的下伏基底岩石影响。此外,方铅矿的原位Pb同位素组成有随着标高增加而升高的趋势,暗示成矿流体的运移方向很可能是向上的,且随着成矿流体的演化,富放射性成因Pb的赋矿沉积岩贡献了更多的成矿金属。综上,本文认为毛坪大型铅锌矿床是流体混合作用的产物,其深部可能具有良好的找矿潜力,这是因为富含金属元素的基底岩石对深部贡献更多。

The Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province,located in the western margin of the Yangtze Block,is one of the important bases for the production of base metals in China. The Maoping large-scale Pb-Zn deposit,occurred in the middle of this province,is the second largest Pb-Zn deposit. Its proven total Pb and Zn metal reserves exceed 3 Mt,and the average grade of Pb+ Zn is 12% ~ 30%,with a local reach of 45%. The ore bodies occur as a layered,lenticular or veined shape distributed in the NW inverted wing of the Maomaoshan anticline and its inclined end NE interlay fault zones. The Maoping deposit mainly consists of three ore bodies( groups),of which the No. I ore body( group) is hosted in the Upper Devonian Zhaige Formation dolostone,the No. II ore body( group) is hosted in the Lower Carboniferous Baizuo Formation dolostone,and the No. III ore body( group) is hosted by dolostone of the Upper Carboniferous Weining Formation. The ores are mainly composed of ore minerals,such as sphalerite,galena and pyrite,and gangue minerals,for example,dolomite and calcite with a small amount of quartz and barite. Those hydrothermal minerals have massive,disseminated or veined structures,and have granular,metasomatic,co-edge,colloidal,aggregate or fragmentation textures. Hence,the epigenetic ore-forming characteristics of the Maoping deposit are obvious. The Nano-SIMS in-situ S isotopic compositions show that the fine-grained strawberry aggregate pyrite and colloidal sphalerite had a significant depletion of34 S,and theirδ34 S values range from-20. 4‰ to-8. 7‰. Such sulfur isotope signatures have typical biogenetic sulfur characteristics,suggesting that they had undergone marine sulfate bacterial reduction process( BSR). On the other hand,the in-situ δ34 S values of granular pyrite and sphalerite vary from 22. 1‰ to 25. 6‰,obviously enriched in heavy sulfur isotopes,indicating that they had undergone a marine sulfate thermochemical reduction process( TSR). Since BSR and TSR are mainly temperature dependent,the formation of S2-in the Maoping deposit was local reduced and may first undergo a relatively low temperature BSR process,and then undergo a relatively high temperature TSR process. The fs LA-MC-ICPMS in-situ Pb isotopic ratios of galena show that Pb isotopic ratios of galena are relatively uniform,suggesting that the source of Pb is single or well-mixed. The comparison indicates that the ore-forming metals of the Maoping deposit were mainly derived from ore-bearing sedimentary rocks with a certain influence of underlying basement rocks. In addition,the in-situ Pb isotopic ratios of galena have a tendency to increase with the increase of the elevation,suggesting that the migration direction of the ore-forming fluid is most likely upward,and with the evolution of ore-forming fluids,the radiogenic Pb-rich sedimentary rocks contributed more ore-forming metals. Hence,this paper considers that the Maoping large-scale Pb-Zn deposit is the product of fluid mixing and its deep may have a good prospecting potential,as the metal-rich basements contribute more in deep.