Mg-Fe同位素对东昆仑造山带夏日哈木和石头坑德含铜镍硫化物岩体地幔源区的制约

It has been discovered the largest number of magmatic Cu-Ni sulfide deposits within orogenic belts in China. Due to lack of effective geochemical tracers, the natural and geochemical features of the mantle source of magmatic Cu-Ni sulfide deposits within orogenic belts are still poorly understood, which are critical to realize the origin of metal sources of magmatic Cu-Ni sulfide deposits within orogenic belts. Since melt/fluid metasomatism could fractionate Mg and Fe isotopes of mantle peridotite during slab subduction, studies of Mg and Fe isotopes of Ni-Cu-bearing mafic-ultramafic intrusions may provide important constraints on these issues mentioned above. In this project, we will focus on investigation of the Mg and Fe isotopic variations in olivine and orthopyroxene from the Xiarihamu giant Cu-Ni deposit and Shitoukengde sulfide-bearing intrusion, East Kunlun Orogenic Belt. By combined results of mineralogy, petrology, and Sr-Nd isotopes, we attempt to compare and reveal the nature, geochemical features, and petrogenesis of mantle sources of these two intrusions. Furthermore, we will clarify the critical control factor(s) and geodynamics of large-scale mafic magma activities that is conductive to form magmatic sulfide deposits within orogenic belts. By doing this project, a better understanding on the precondition of formation of magmatic sulfide deposits within orogenic belts will be achieved, and also wider implication of Mg-Fe isotope geochemistry for investigations of igneous petrology and mineral deposits will be promoted.

我国是世界上造山带产出岩浆铜镍硫化物矿床最多的国家。由于缺乏有效的地球化学示踪手段，目前对造山带岩浆硫化物矿床地幔源区性质及地球化学特征的研究非常薄弱，严重制约了对该类矿床成矿物质起源的理解。鉴于造山带板片俯冲过程中熔体/流体的交代可能显著改造上覆地幔的Mg、Fe同位素组成，因此，硫化物含矿/矿化镁铁-超镁铁质岩体的Mg、Fe同位素研究将为探讨上述重要科学问题提供关键约束。本项目以东昆仑造山带夏日哈木超大型铜镍矿床和石头坑德含硫化物岩体为研究对象，以橄榄石和斜方辉石的Mg、Fe同位素研究为核心，结合矿物学、岩石学及Sr-Nd同位素分析，深入对比和揭示岩体地幔源区的性质、地球化学特征及其形成机制，阐明造山带有利于岩浆硫化物成矿的大规模镁铁质岩浆活动的关键控制因素和动力学过程，从而深化对造山带岩浆硫化物成矿前提条件的认识，并促进Mg、Fe同位素在岩石学和矿床学上的应用。

我国东昆仑造山带发现的超大型岩浆硫化物矿床—夏日哈木Ni-Co矿床打破了国际上流行的“造山带环境难以产出超大型岩浆硫化物矿床”传统认识。造山带环境如何短时间内产生大规模镁铁质岩浆活动是探讨造山带岩浆硫化物成矿的核心科学问题。本项目以东昆仑造山带夏日哈木超大型Ni-Co矿床为重点研究对象，辅以石头坑德含硫化物岩体，借助元素地球化学和Sr-Nd-Mg-O同位素地球化学示踪方法，阐明了夏日哈木矿床的地幔源区地球化学特征及其形成机制，厘清了阐明造山带有利于岩浆硫化物成矿的大规模镁铁质岩浆活动的动力学过程，取得了一些新认识。研究发现：（1）夏日哈木矿床的方辉橄榄岩（即基性程度高、“原始”的岩相）中斜方辉石具有高的Mg#值和Ni含量、低的δ26Mg（-0.49 ~ -0.34‰）和δ18O（+6.4 ~ +7.7‰）；全岩显示相对亏损的初始Sr-Nd同位素特征（Sri = 0.7067 ~ 0.7080，εNd(t) = +0.4 ~ +1.8）。这些地球化学特征一致表明该矿床的原始岩浆起源于受沉积碳酸盐交代的岩石圈地幔部分熔融；交代地幔的固相线温度通常比正常橄榄岩地幔低得多，受软流圈地幔上涌加热，二者相互作用诱发了大规模地幔部分熔融、短时间内产生巨量的镁铁质岩浆。“碳酸盐交代的岩石圈地幔与软流圈地幔相互作用”为超大型Ni-Co硫化物成矿奠定了关键的物质基础；（2）绝大部分斜方辉石岩、二辉辉石岩、辉长苏长岩（即基性程度较低、较为“演化”的岩相）富集Sr-Nd同位素，(87Sr/86Sr)i和εNd(t) 分别在0.7087 ~ 0.7118 和 -4.0 ~ -1.4之间，且其中斜方辉石的δ26Mg在-0.34 ~ -0.21‰之间，相对富集重的Mg同位素和O同位素（δ18O = +6.6 ~ +8.6‰），表明这些岩石是幔源原始岩浆在深部岩浆房内经历强烈的地壳物质同化混染形成的。这一结果暗示深部岩浆房地壳同化混染是控制岩浆达到S饱和、发生硫化物熔离的关键因素。（3）基于上述新发现，结合区域构造演化，构建了夏日哈木矿床原始岩浆的起源、地壳深部分异、以及浅部岩浆房演化全过程。这些新的认识发展了岩浆硫化物矿床成矿理论，为东昆仑造山带乃至全球造山带该类矿床的找矿勘探提供了重要的理论支撑。