冈底斯成矿带西段鲁尔玛斑岩型铜(金)矿床的成矿物质来源研究

Sources of ore-forming materials of Luerma porphyry copper(gold) deposit,western Gangdise

西藏措勤县鲁尔玛铜(金)矿床为冈底斯成矿带西段新发现的斑岩型矿床,位于中拉萨微陆块的西部,矿体主要呈脉状和不规则状产于含矿斑岩顶部和接触带,矿床形成于晚三叠世(213Ma)。文章通过研究主成矿阶段硫化物的S-Pb同位素组成特征,并与晚三叠世含矿斑岩体Pb同位素组成进行对比,发现共生硫化物δ^34SV-CDT值变化范围较窄(δ^34SV-CDT=-2.38‰~1.75‰,极差为4.13‰,均值为-0.64‰),具有深源硫(地幔或下地壳)的特征;共生硫化物(黄铁矿、黄铜矿和毒砂等)Pb同位素(n(206Pb)/n(204Pb)、n(207Pb)/n(204Pb)、n(208Pb)/n(204Pb)分别为18.450~18.903、15.602~15.669、38.637~39.424)和晚三叠世含矿斑岩的Pb同位素(n(206Pb)/n(204Pb)、n(207Pb)/n(204Pb)、n(208Pb)/n(204Pb)分别为18.845~19.560、15.634~15.689、39.273~40.211)具有一致的线性演化曲线,显示出壳幔混合的特征,暗示鲁尔玛铜(金)矿的铅来自于晚三叠世的岩浆活动。研究认为,鲁尔玛铜(金)矿床形成于晚三叠世新特提斯洋壳向北俯冲的环境,成矿物质应源于新生地壳熔融形成的中酸性岩浆。

The Late Triassic Luerma copper (gold) deposit formed at ca. 213 Ma is located in Coqen County of southern Lhasa microterrane. The orebodies are hosted in the Late Triassic intermediate-acid intrusive rocks. In this study, the authors investigated the S and Pb isotopes from pyrite, chalcopyrite, arsenopyrite and ore-bearing porphyry to understand the genesis of the Luerma deposit. The δ^34Sv-CDT values in sulfide minerals range from -2.38‰ to 1.75‰ with a mean value of -0.64‰, which displays a deep sulfur source. Sulfide minerals in the Luerma deposit yielded the Pb isotope values of n(206Pb)/n(204Pb) ranging from 18.450 to 19.903, n(207Pb)/n(204Pb) ranging from 15.602 to 15.669, and n(208Pb)/n(204Pb) ranging from 38.637 to 39.424, consistent with values of of porphyry whose n(206Pb)/n(204Pb) ratios range from 18.845 to 18.560, n(207Pb)/n(204Pb) ratios range from 15.634 to 15.689, and n(208Pb)/n(204Pb) ratios range from 39.273 to 40.211. These Pb isotopes form a linear evolution curve, suggesting that fluids might have been derived from the Late Triassic magmatism. It is thus held that the oreforming materials in the Luerma deposit were mainly derived from the Late Triassic magmatism, which was likely related to the northward subduction of Paleo-Tethys oceanic crust.