柴北缘青山造山型金-铅矿床成矿机制:矿石和黄铁矿组成约束

Metallogenic mechanism of the Qingshan orogenic Au-Pb deposit,northern Qaidam basin:Constraints from ore and pyrite composition

青山金铅矿床位于柴北缘滩间山矿集区内,是全球罕见的金-铅共/伴生的造山型矿床。矿体主要赋存于古元古代达肯大坂群片岩中,受到北西西向断裂控制,发育蚀变岩和石英脉两种矿石类型,矿石中硫化物较为发育,常见黄铁矿、褐铁矿、方铅矿、黄铜矿、闪锌矿等。黄铁矿作为该矿床的主要载金矿物,其地球化学特征可以指示金赋存状态,揭示成矿流体性质和矿床成因。为此,选取ZK2103和ZK3701钻孔中不同深度空间的矿石和围岩样品,分别进行成矿元素全岩测试和黄铁矿的电子探针分析工作。矿石和围岩稀土元素配分模式曲线都为右倾型,呈现明显的Eu负异常,未见Ce异常,说明成矿流体为变质流体且不含海水。矿石相对富集Au、Ag、Cu、Pb、Zn、As、Co、Bi、Sb、W、Ni、Sr、Cd、V元素,而Sn、Mo、Cr元素含量则相对偏低。矿石和围岩中黄铁矿的S/Fe值小于2.0,表现出亏S和富Fe的特征;黄铁矿的Au/Ag值均大于0.5,Co-Ni图解结果指示黄铁矿为热液成因。金主要以纳米粒子形式赋存,少量以固溶体形式存在。金在流体中以Au(HS)-2的形式运移,流体混合作用破坏了Au(HS)-2的稳定性,致使金的沉淀。高硫的成矿流体可以运移部分的铜、铅、锌。在金沉淀后,成矿流体温度和压力降低,致使铅开始沉淀。但由于该流体运移铅、锌的能力有限,青山金铅矿床铅矿体的规模较小。硫-铅同位素结果显示成矿物质来源于围岩,区域上金龙沟矿区和青龙沟矿区围岩中铅锌含量较低,所以未形成铅矿体。

Qingshan Au-Pb deposit,located in Tanjianshan ore concentration area,northern Qaidam basin,is a paragenetic or associated Au-Pb orogenic deposit that is very rare in the world.The ore of the deposit mainly exists in the schist of the Paleoproterozoic Darken Daban Group,whose distribution is controlled by the NW-trending fault.There are two types of ores,i.e.,the altered rock type and the quartz vein type.Sulfides,including pyrite,limonite,galena,chalcopyrite,sphalerite and so on,are well developed in the ores.Among them,the pyrite is the main gold-bearing mineral in the deposit,whose geochemical characteristics not only indicate the existent state of gold but also reveal the properties of ore-forming fluid and the genesis of deposit.Samples of different types of ores and country rocks together with pyrite in them at different depths in boreholes ZK2103 and ZK3701 were selected to analyze their whole rock compositions and ore-forming elements.The chondrite-normalized REE distribution model curves of the measured samples are right-lean-shaped with obvious Eu negative anomaly,but no Ce anomaly,showing that the ore-forming fluid is from metamorphic without any seawater.The ores are relatively enriched Au,Ag,Cu,Pb,Zn,As,Co,Bi,Sb,W,Ni,Sr,Cd and V elements,but depleted in Sn,Mo and Cr elements.The S/Fe values of pyrite in the ore and country rocks are less than 2.0,showing they are depleted in S but enriched in Fe.The Au/Ag value of pyrite is greater than 0.5,and the results of Co-Ni diagram all show that the pyrite is of hydrothermal origin.The gold mainly exists in the form of nanoparticles,with a small amount of gold existing in the form of solid solution.Gold was mainly transported in the form of Au(HS)-2.However,the mixing of fluids destroys the stability of Au(HS)-2,that is most likely the reason for gold precipitation.The sulfur content in the ore-forming fluid is high,as a result,a certain amount of Cu,Pb and Zn will be migrated with sulfur.After the gold precipitates,the reduction of temperature and pressure in the fluid leads to the Pb precipitation.Due to the limited ability of the fluid to transport Pb and Zn,the lead ore of Qingshan deposit developed in small scale.The results of S and Pb isotopes show that the ore-forming material comes from the strata,however,contents of Pb and Zn in the strata of Jinlonggou and Qinglonggou mining areas is low,so there is no lead ore formed in this area.