西藏甲玛铜多金属矿床S、Pb、H、O同位素特征及其指示意义

西藏甲玛铜多金属矿床是中国近年来发现的特大型铜铅锌多金属矿床之一,其产出的环境和形成机理为国内外矿床学家所关注。对甲玛铜多金属矿床中代表性岩(矿)石样品进行了S、Pb、H和O同位素分析,并从成矿系统中"源"的角度对其变化规律和成因意义进行了探讨。研究结果表明,甲玛铜多金属矿床的围岩和矿石中δ34S值变化于-4.9‰～0.5‰,在硫同位素直方图上呈塔式分布,成矿热液δ34SΣS在0值附近,与矿区内斑岩体的δ34S组成(-0.2‰～-0.7‰)十分接近。表明了矿石中硫的来源单一,主要来源于岩浆。矿石铅同位素变化范围较大,明显分为两组:第一组样品富放射性成因铅,其206 Pb/204 Pb变化范围为18.603～18.752,207Pb/204Pb变化范围为15.610～15.686,208 Pb/204 Pb变化范围为38.910～39.135;第二组样品具有低放射性成因铅特征,其206 Pb/204 Pb变化范围为18.130～18.270,207 Pb/204 Pb变化范围为15.470～15.480,208Pb/204Pb变化范围为38.140～38.850。各同位素比值相对稳定,变化范围较小。将含矿斑岩的岩石铅与矿石铅进行综合投图,两种类型的铅并非单阶段正常铅,而是混合铅,有放射性成因铅的加入。可能存在不同的源区或在演化过程中有不同源区物质的混入。氢氧同位素研究结果显示,氢同位素的来源主要为深部的花岗岩体,而氧同位素由于后期大气降水增多、水/岩比值升高,导致含矿石英脉中δ18 OH2O降低。因此推断甲玛铜多金属矿床成矿流体早期以深源流体为主,随着成矿过程的演化,大气降水所占的比例也越来越大。

Isotopic characteristics of S,Pb,H and O of Jiama copper-polymetallic ore deposit,Tibet and their significance

Jiama copper-polymetallic deposit,discovered in recent years,is one of the giant Cu-Pb-Zn polymetallic ore deposits in China.Its geological setting and metallogenic mechanism caused serious concern from the economic geologists.This paper analyzed S,Pb,H and O isotope of the representative rock or ore from Jiama copper-polymetallic deposit,and discussed the regularity of its variation and genesis.The investigation indicates that the δ34S values of the wall rocks and the ores show a total range from-4.9‰ to 0.5‰ in Jiama copper-polymetallic deposit.It is the tower distributing in the sulfur isotope histogram of δ34SΣS.The δ34SΣS values of hydrothermal ore-forming solution fluctuate around zero point,which are similar to that of the porphyry bodies.It indicates that the sulfur is of single source and mainly from magma.Lead isotopes of the ores vary over a wide range obviously in two groups.In the first group,the samples are characterized by high radiogenic lead.206Pb/204Pb ranges from 18.603 to 18.752,207Pb/204Pb ranges from 15.610 to 15.686 and 208Pb/204Pb ranges from 38.910 to 39.135.In the second group,the samples are characterized by low radiogenic lead.206Pb/204Pb ranges from 18.130 to 18.270,207Pb/204Pb ranges from 15.470 to 15.480 and 208Pb/204Pb ranges from 38.140 to 38.850.The isotopic ratios are relatively stable.Plotting together the lead isotope data both of porphyry rock and ore,it is found that the two types of lead are not the single-stage normal lead,but the mixed lead.It means that there are different sources or different source materials joined in the evolution processes.The result of hydrogen and oxygen isotopes indicates that hydrogen isotope is mainly from deep granite mass,but δ18OH2O values decreased in mineralized quartz veins owing to the increase of meteoric water and water/rock ratios.Therefore this paper concludes that ore-forming fluid was mainly originated from deep-derived fluid for the early period of metallogenesis and the meteoric water was introduced later.From the view point of "source" of the metallogenic system,this paper discusses the sources of ore-forming materials and ore-forming fluids of Jiama copper-polymetallic deposit.