甘肃毕家山铅锌矿床中镉的赋存状态及矿床成因:来自硫化物LA−ICP−MS微量元素和原位硫同位素的证据

Occurrence of cadmium and ore genesis in the Bijiashan lead-zinc deposit:Evidencefrom LA−ICP−MS trace elements and in-situ sulfur isotope of sulfide

甘肃毕家山铅锌矿床是西秦岭造山带西成矿田发现的首个铅锌矿床,累计探明铅锌资源量120×10^(4)t,Pb和Zn的品位分别为1.86%和5.42%。矿床中发育2类矿体,分别是赋存于灰岩中的灰岩型和赋存于千枚岩中的千枚岩型。2类矿体的矿石矿物组合均为闪锌矿+方铅矿±黄铁矿±黄铜矿。毕家山铅锌矿床镉的赋存状态尚不明确。通过激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)分别对灰岩型和千枚岩型矿石中的硫化物(闪锌矿、方铅矿、黄铁矿和黄铜矿)进行了微量元素及原位S同位素分析。研究结果显示,灰岩型和千枚岩型矿石中闪锌矿的Cd平均含量分别为2076×10^(-6)和1695×10^(-6),高于矿产工业一般标准(1000×10^(-6)),而方铅矿、黄铁矿和黄铜矿中的Ga、Ge、Se、In、Tl含量均低于矿产工业一般标准(100×10^(-6))。结合岩相学和LA-ICP-MS分析发现,毕家山铅锌矿床无Cd的独立矿物和以吸附形式存在的Cd,Cd主要以Cd^(2+)↔Zn^(2+)类质同象形式赋存于闪锌矿中,其次为Cd^(2+)+Fe^(2+)↔2Zn^(2+)。通过闪锌矿微量元素温度计获得灰岩型和千枚岩型矿石中闪锌矿的形成温度分别为198~254℃(平均值为227℃)和203~245℃(平均值为230℃),与前人通过流体包裹体和矿物对温度计的研究结果一致,表明其成矿温度为中温。此外,原位硫同位素分析结果显示,该矿床中灰岩型和千枚岩型矿石的δ34S平均值分别为+13.64‰和+17.72‰,表明硫来源于海相硫酸盐。综合闪锌矿微量元素数据、同位素地球化学、成矿温度、岩相古地理和矿床地质特征对比分析,推测毕家山铅锌矿床的成因类型属于SEDEX型。

The Bijiashan deposit is the first lead−zinc deposit discovered in the Xicheng ore field in the West Qinling Orogenic Belt(WQOB),with reserves of 1.2 million tons of lead and zinc,and grades of 1.86%and 5.42%,respectively.This deposit is characterisedby the presence of two distinct types of orebodies:limestone−type and phyllite−type,which are located within limestone and phylliteformations,respectively.The mineral assemblages present in both ore types include sphalerite,galena,pyrite,and chalcopyrite.Previous investigations have primarily concentrated on the geological characteristics,petrography,isotope geochemistry,and fluidinclusions associated with the deposit;however,the occurrence of cadmium(Cd)is still uncertain.In this study,laser ablationinductively coupled plasma mass spectrometry(LA−ICP−MS)was employed to analyse various sulfide minerals(sphalerite,galena,pyrite,and chalcopyrite)from both ore types.The analytical results indicate that the average Cd content in sphalerite from thelimestone−type and phyllite−type ores is 2076××10^(-6)and 1695××10^(-6),respectively,exceeding the general industry standard of 1000××10^(-6).Conversely,the concentrations of gallium(Ga),germanium(Ge),selenium(Se),indium(In),and thallium(Tl)in galena,pyrite,andchalcopyrite from both ore types are below the mineral industry standard of 100××10^(-6).Based on petrographic observations andLA−ICP−MS analyses,it is proposed that there are no independent Cd minerals present in the Bijiashan lead−zinc deposit.Instead,Cdis primarily incorporated into sphalerite through the substitution mechanism of Cd^(2+)↔Zn^(2+),with a minor substitution pathway ofCd^(2+)+Fe^(2+)↔2Zn^(2+).The estimated formation temperature for sphalerite in the limestone type and phyllite type ores are 198-254°C(mean of 227°C)and 203-245°C(mean of 230°C),respectively.These findings are consistent with previous studies utilising fluidinclusion thermometry,suggesting a moderate temperature for ore−forming mineralisation.In−situ sulfur isotope analysis revealsaverageδ34S values of 13.64‰and 17.72‰for the respective ore types,indicating a marine sulfate source.Drawing upon the evidencefrom LA−ICP−MS trace element data of sphalerite,petrography,geological characteristics of the deposit,isotope geochemistry,andmetallogenic temperature,we propose that the Bijiashan lead−zinc deposit is classified as a sedimentary−exhalative(SEDEX)deposit.