绿松石低温生长过程中Cu同位素特征及其对产地溯源的意义

Copper Isotopic Variation of Turquoise in Low-Temperature Growth Process and Its Significance for Origin Traceability

绿松石(CuAl_(6)(PO_(4))_(4)(OH)_(8)·4(H_(2)O))是重要的表生宝石矿物,已有研究采用绿松石主量元素Cu的同位素进行产地溯源,但是溯源机理不明确.此外,绿松石生长过程复杂,该过程是否造成Cu同位素组成的显著变化还不甚清楚,这限制了其产地溯源的应用.湖北省竹山县是全球最大的宝石级绿松石产出地,采用MC-ICP-MS对竹山县条带状绿松石的生长条带进行高精度Cu同位素测试.结果表明:绿松石δ^(65)Cu值较高,然而不同条带没有明显变化(δ^(65)Cu=10.99‰~11.54‰).绿松石含矿热液沉淀过程分馏有限(<1‰),这指示绿松石Cu同位素显著变化主要发生在含矿热液的形成过程.原生硫化物的δ^(65)Cu=0±1‰,显著低于绿松石样品测定值,推测引起含矿热液Cu同位素显著分馏的主要原因是原生硫化物发生氧化作用.全球典型绿松石矿区的数据整体与该结果一致,表明绿松石Cu同位素组成主要受源区环境的控制;同一矿区中形成的绿松石,即使经历周期生长产生条带状结构,其Cu同位素组成基本一致.该研究深入探究了Cu同位素示踪绿松石矿区的机理,同时也加深了对低温过程Cu同位素分馏的认识.

Turquoise(CuAl_(6)(PO_(4))_(4)(OH)_(8)·4(H_(2)O))is an important supergene gemstone mineral.The isotope of Cu,its major component,has been increasingly used to trace the origin of such gemstone.However,the tracing mechanism is not clear.Besides,it is still unclear whether Cu isotope composition changes significantly or not during the complex growth of turquoise,limiting the reliable application of trace ability via Cu isotope.In this work it used MC-ICP-MS instrument to measure the Cu isotopic compositions of banded turquoise,which were obtained from Zhushan County,the largest gem-grade deposit of turquoise in the world.The results show that the values of Cu isotopes are very high,however,they are remarkably identical in different bands of the turquoise sample(δ^(65)Cu=10.99‰-11.54‰).The Cu isotope fractionation,in the precipitation of ore-bearing hydrothermal solution,thus has been limited(<1‰).Instead,the significant fractionation of Cu isotope would occur mainly during the formation of ore-bearing hydrothermal solution.The Cu isotope composition of primary Cu sulfides lies usually within the range of 0±1‰,significantly lower than the value measured in this study,suggesting that the oxidation of the primary sulfide in the source area is the major factor for the significant fractionation of Cu isotope in the ore-bearing hydrothermal solution.The results are consistent with the data of typical turquoise deposits in the world,which confirms that the Cu isotopic composition of turquoise is mainly controlled by the source environment.Even though the turquoise has experienced periodic growth and produced banded structure,the Cu isotope of turquoises formed in the same mine hardly change.This work has explored the mechanism of tracing the origin of turquoise deposits via Cu isotope,and better understanded Cu isotope fractionation in low-temperature hydrothermal process.