东天山图拉尔根铜镍硫化物矿床PGE和Re-Os同位素特征及其对成矿的约束

PGE and Re-Os isotope geochemistry and their constraints on mineralization of the Tulaergen Ni-Cu sulfide deposit in the East Tianshan

中亚造山带南缘东天山地区是我国战略资源Ni的重要产区,图拉尔根矿床是该地区众多铜镍矿床中以岩体高矿化比例为特色的矿床。图拉尔根杂岩体由晚石炭世基性岩体(辉长岩、角闪辉长岩)和早二叠世超基性岩体(二辉橄榄岩、角闪橄榄岩)组成,矿体呈透镜状、“悬浮”状产出于超镁铁岩体的中部和中上部,矿化类型为块状/半块状、海绵陨铁状、稠密/稀疏浸染状、星点状、斑杂状以及少量珠滴状矿化。不同类型矿石∑PGE变化较大(10.8×10^(-9)~208×10^(-9)),除块状矿石样品外,其他类型矿石(S<15%)S与PGE、IPGE与PGE元素之间均显示良好正相关,表明PGE含量变化主要受控于R因子(100~5000)变化;块状矿石的上述元素间呈现离散现象,亦反映单硫化物固溶体分异的影响。母岩浆中Pt和Pd的显著亏损以及矿石中Cu和Pd的强烈解耦,表明图拉尔根母岩浆经历了深部硫化物的熔离。Pd/Ir比值揭示的高镁玄武质岩浆性质和含水地幔源区可能的低程度部分熔融,表明源区硫化物残留也可能造成PGE的亏损。图拉尔根矿床的^(187)Os/^(188)Os比值和γ_(Os)值显著高于造山带橄榄岩地幔和全球典型岩浆铜镍矿床,反映其成矿母岩浆受到地壳物质强烈混染;Os-Nd同位素端元混合模拟显示有限的硅铝质地壳混染难以解释高^(187)Os/^(188)Os比值,图拉尔根矿床的Os同位素信息主要受到地壳硫化物改造,因此外源硫加入是图拉尔根铜镍成矿的关键因素之一。

The East Tianshan located at the southern Central Asian Orogenic Belt is an important area of China s strategic resource of Ni.The Tulaergen deposit,one of the numerous Ni-Cu deposits in this area,is characterized by a high proportion of mineralization in the intrusion.The Late Carboniferous mafic intrusion(gabbro,hornblende gabbro)and the Early Permian ultramafic intrusion(lherzolite,hornblende peridotite).The lenticular and suspended ore bodies are located in middle and upper parts of the ultramafic intrusion,consisting of massive/semi-massive,net-textured,dense/sparsely disseminated and blotchy sulfide ores.The∑PGEs of different ore types show significant variations(10.8×10^(-9)~208×10^(-9)).Except for massive ores,other types(S<15%)show positive correlations between S and PGE,between IPGE and PPGE,indicating that the PGE variations are mainly controlled by the R factor(100~5000).Such positive correlations are not observed on massive ore types,reflecting the influence of differentiation of monosulfide solid solution.The significant depletion of Pt and Pd in the parent magma and the decoupling of Cu and Pd indicate that the Tulaergen parent magma has experienced sulfide segregation in depth.The high-Mg basaltic magma and the possible low degree of partial melting of the water-bearing mantle source,indicate that sulfide retention in the source may also cause the PGE depletion.The ^(187)Os/^(188)Os ratios andγ_(Os) values are significantly higher than the orogenic peridotite mantle and the global typical Ni-Cu deposits,suggesting that the parent magma was strongly contaminated by crustal materials.This limited siliceous crustal contamination revealed by Os-Nd isotope mixing calculation cannot well explain the high ^(187)Os/^(188)Os ratios,indicating that the Os isotopic signature of the Tulaergen deposit is mainly modified by crustal sulfides.Therefore,addition of crustal sulfur is one of the critical factors for the Tulaergen Ni-Cu sulfide mineralization.