研究生知识库

金川超大型Ni-Cu-(PGE) 硫化物矿床是世界上第三大在采岩浆硫化物矿床，Ni开采量仅次于俄罗斯Noril’sk-Talnakh和加拿大Sudbury矿床，其成因研究备受关注。金川超镁铁质岩体位于华北地块西南缘的龙首山隆起带中，通常以断裂为界将金川超镁铁质岩体分为四个部分。其中，Ⅰ号和Ⅱ号岩体岩相完整并赋存金川矿床最主要的矿体。现有的研究认为金川超镁铁质岩体的母岩浆起源于被地幔柱加热的、异常富集的岩石圈地幔，其构造背景为活动大陆边缘或裂谷环境。由于缺乏有效的示踪手段，目前对金川岩体富集地幔源区性质和地球化学特征的认识非常薄弱。近年来随着原位微区技术的成熟发展，单矿物微量元素在解决上述问题时提供了新的研究思路。针对上述问题选择岩浆早期结晶的铬尖晶石和橄榄石作为研究对象，利用电子探针（EPMA）和激光—等离子体质谱（LA-ICP-MS）原位分析了金川超镁铁质岩体中橄榄石和铬尖晶石的元素地球化学组成，厘清了矿物元素变化的规律及其控制因素，进而尝试约束金川超镁铁质岩体的地幔源区性质及其地球化学特征。橄榄石元素地球化学研究结果显示：Ni（1029~2528 ppm）与Co（98~197 ppm）、Zn（45.4~118.3 ppm）与Mn（1553~2204 ppm）具正相关关系。Ni、Cr（34~158 ppm）与Fo（80.7~86.0 %）呈负相关关系，Zn/Fe和Mn/Zn值与Fo无相关性。在原始地幔橄榄石多元素标准化图中，金川Ⅰ、Ⅱ号岩体的橄榄石显示出相似的配分模式，表现为Cr、V、Ni和Co 的负异常，不相容元素Zr、Y、Ti、Sc和Ca的正异常等特征。二者相似的元素标准化配分模式表明金川Ⅰ、Ⅱ号岩体具相似的母岩浆性质，是同一岩浆通道系统内的产物。与铬尖晶石的共结影响着橄榄石中Cr、V、Al、Ti元素的含量；硫化物熔离使橄榄石亏损Ni、Co元素；与粒间熔体的反应会对Mn、Zn、Ca元素造成影响。另一方面，金川Ⅱ号岩体相较于Ⅰ号岩体具偏低的Cr、Al、V含量，暗示Ⅱ号岩体的母岩浆经历了更高程度的演化。金川铬尖晶石分析结果显示其具相对高的Cr#（47.9~98.2）以及TiO2（0.5~6.7 wt%）、Zn（1186~10726 ppm）含量，较低的Mg#（3.9~23.8）和Ni（134~1632 ppm）含量。金川Ⅰ、Ⅱ号岩体铬尖晶石微量元素标准化到MORB铬尖晶石显示出相似的配分模式，表现为Ti、Zn、Mn的正异常和Sc、Ni的负异常。随着岩浆的演化，铬尖晶石中Al2O3、Ga含量增加，Cr2O3、Zn、Sc含量减少，Mn、V变化很小。热液蚀变作用使铬尖晶石富集Zn、Mn元素，早期的硫化物熔离以及深部橄榄石的分离结晶可以使铬尖晶石亏损Ni、Co等元素。Ti 与Fe3+/(Fe3++Al+Cr) 的负相关不仅是与间隙熔体反应所致，更是岩浆演化的结果；同时Ti与元素组合Zn+Mn+Co具正相关对应着铬尖晶石中出熔钛铁晶石的程度，暗示了金川母熔体可能是高钛的。纯橄岩中铬尖晶石能够近似代表相对原始的铬尖晶石成分，与之平衡的熔体相较于MORB熔体富集Ga、Ti、V，亏损Sc。上述橄榄石和铬尖晶石的地球化学组成为探讨金川超镁铁质岩体的地幔源区性质及其成因提供了有益约束。橄榄石高的Mn/Zn值（>13）和低的Zn/Fe值（<11）指示金川岩体母岩浆可能起源于橄榄岩地幔的部分熔融，而非前人认为的辉石岩地幔源区。铬尖晶石强烈的Sc负异常与源区存在石榴子石的残余相关，暗示母岩浆起源较深。铬尖晶石MORB标准化微量元素配分特征与夏威夷拉斑玄武岩和层状岩体中的铬尖晶石相似，同时在Al2O3-TiO2构造环境判别图上，金川初始铬尖晶石的成分落入与地幔柱相关的堆晶岩的范围内，表明地幔柱在形成金川岩体中起到了至关重要的作用。

The giant Jinchuan magmatic sulfide deposit in China is the third largest mining deposits in the world, the production of nickle is second only to the Noril’sk-talnakh deposit in Russia and the Sudbury deposit in Canada. Although many research have been done, there still exist some debates in the genesis of deposit. The Jinchuan ultramafic intrusion is located in the Longshoushan terrane of southwest edge of the North China block. The Jinchuan intrusion generally was divided into four parts by the fault, the parts of Ⅰ and Ⅱ have major ore bodies and full-developed lithofacies. It is generally believed that Jinchuan intrusion originated from the enriched lithospheric mantle heated by mantle plume and belongs to the active continental margin or rift environment. Due to the lack of effective tracer methods, the study is relatively weak on the nature and geochemical characteristics of the enriched mantle in Jinchuan intrusion. With the mature development of in suit microzonation technology in nearly years, the new ideas have been provided to solve the above problems by using trace elements in minerals. In this study, the trace elements of olivine and Cr-spinel in Jinchuan intrusion were analyzed in situ by electron probe microanalysis (EPMA) and laser-plasma mass spectrometry (LA-ICP-MS) in order to clarify the variation rules and controlling factors in trace elements, and to try to constrain the mantle source characteristics of the Jinchuan intrusion.The results of the geochemical study of elements in olivine show that Ni (1029~2528 ppm) has positive correlation with Co (98~197 ppm), as well as Zn (45.4~118.3 ppm) and Mn (1553~2204 ppm); Ni and Cr (34~158 ppm) correlate negatively with Fo (80.7~86.0%), Mn/Zn and Zn/Fe show no obvious correlation with Fo. The primitive mantle olivine-normalized trace element patterns of the Jinchuan intrusion show that the olivine of intrusions Ⅰ and Ⅱ have the same trace elements characteristics, which display negative Cr, V, Ni and Co anomalies and enrichment of Zr, Y, Ti, Sc and Ca. The multi-element patterns of the Jinchuan olivine imply the intrusions Ⅰ and Ⅱ have similarity parental magma, which were producted in the same magma channel system. The Cr-spinel which cocrystallization with the olivine make effects on elements in Cr, V, Al and Ti. The contents of Ni and Co in olivine are influenced by the process of sulfide segregation. The reaction between olivine and trapped liquid have an effect on elements in Mn, Zn and Ca. The lower content of Cr, Al and V in olivine of the intrusion Ⅱ compared with the intrusion Ⅰ imply that the parental magma of the intrusion Ⅱ was more evolved. The analytical results in the Cr-spinel show that higher Cr# (47.9~98.2), TiO2 (0.5~6.7 wt%), Zn (1186~10726 ppm) contents and lower Mg# (3.9~23.8), Ni (134~1632 ppm) contents. Compared with Cr-spinel in MORB, Cr-spinel in intrusions Ⅰ and Ⅱ display negative Sc, Ni anomalies and enrichment of Ti, Zn and Mn. With the evolution of magma, the content of Al2O3 and Ga in Cr-spinel increased, the content of Cr2O3, Zn and Sc decreased, and the content of Mn and V changed little. Hydrothermal alteration makes Cr-spinel enrichment Zn and Mn, the early sulfide segregation and olivine crystallization in a deep-seated staging chamber make Cr-spinel depleted elements of Ni and Co. The negative correlation between Ti and Fe3+/(Fe3++Al+Cr) is not only caused by the trapped liquid reaction effect, but also the result of magma evolution. Meanwhile, the positive correlation between Ti and Zn+Mn+Co corresponds to the degree of exsolution ulvospinel and suggesting that the original parent magmas could be high Ti. The Cr-spinel in dunite can approximately represent the relatively original Cr-spinel composition. Parental magmas of the Cr-spinel in dunite have higher Ga, Ti and V, and lower Sc concentrations than MORB melts.The geochemical characterical of olivine and Cr-spinel discussed above provides a useful constraint for the exploration of the mantle origin and genesis of the Jinchuan ultramafic intrusion. Higher Mn/Zn (>13) ratios and lower Zn/Fe (<11) ratios in the olivine indicate that the magma of Jinchuan intrusion likely originate from partial melting of peridotite mantle possibly, instead of pyroxene mantle sources proposed by predecessors. The strong negative Sc anomaly of Cr-spinel is related to garnet residue in the mantle source and indicating a deeper origin of magma. MORB standardized mult-element distribution of Cr-spinel are similar to the Cr-spinel in Hawaii tholeiite basalt and layered mafic intrusion and the Cr-spinel of Jinchuan intrusion falls into the fileds of the plume-related cumulates in the diagram of Al2O3-TiO2, which indicating that mantle plume plays a crucial role in the form of the Jinchuan intrusion.