Zhaowei Zhang

东昆仑夏日哈木是全球造山带背景发育的最大规模的岩浆镍钴硫化物矿床, 但其构造背景、岩浆作用及成矿机制等尚未形成共识, 这也制约了区域内镍钴资源找矿突破。本文系统分析了东昆仑造山带构造背景演化及岩浆活动, 发现以夏日哈木镁铁-超镁铁质岩浆活动与区域内古特提斯洋裂解关系密切, 且呈现出多期次脉冲的特点。通过夏日哈木矿床的矿物学、矿相学连同地球化学研究, 揭示该矿床经历了4期成矿作用, 分别为早期岩浆成矿期、晚期岩浆成矿期、岩浆期后热液成矿期和构造-变质热液期。其中, 早期岩浆成矿期是矿床的主要成矿阶段, 形成了规模巨大的浸染状矿石; 而晚期岩浆成矿期则以富矿石为主, 矿石品位高, 显示出巨大的成矿潜力; 此外, 岩浆期后热液成矿期和构造-变质热液期则改造了早期矿石, 形成了变余海绵陨铁状矿石和富铜矿体。基于此, 认为夏日哈木矿床的成矿过程与多期岩浆活动密切相关, 其成矿特征反映了形成环境的构造演化过程。研究发现成矿岩体呈高MgO和低SiO₂的特征, 且富集大离子亲石元素(LILE)和轻稀土元素(LREE), 亏损相容元素(Nb、P、Ti、Zr、Hf)和高场强元素(HFSE)的地球化学特征, 以及Sr-Nd同位素组成显示富集趋势, 推断夏日哈木岩浆源区起源于亏损地幔, 混入了辉石岩地幔组分, 而这种地幔辉石岩是通过大陆地壳俯冲过程中析出的熔/流体交代橄榄岩地幔转化而成。夏日哈木岩体的母岩浆表现为高镁玄武质岩浆, 早期成矿期母岩浆的MgO含量为10.93%, 晚期成矿期母岩浆为10.35%, 显示出大陆裂谷玄武岩与岛弧钙碱性玄武岩的混合特征。夏日哈木岩体的原始地幔部分熔融程度较高, 早期成矿期为24.8%, 晚期成矿期为21.7%, 较高的部分熔融程度使得Ni、Co等元素进入岩浆, 是夏日哈木矿床富集Ni、Co的关键因素。模拟计算表明, 夏日哈木岩体由N-MORB加入约30%下地壳和10%~20%富镁碳酸盐物质演化而成。岩石圈减薄, 软流圈地幔减压熔融并强烈混染, 下地壳和富Mg碳酸盐物质的混染导致S急剧饱和, 最终发生巨量硫化物熔离, 形成超大型Ni-Co硫化物矿床。是在原特提斯构造体制消亡、古特提斯开始裂解的构造背景多期岩浆活动与镍钴成矿作用的结果。

镍、钴属于新能源金属,在高端制造业、合金材料、新能源电池等领域具有不可替代性,需求量逐年陡增。但国内镍钴矿产资源原料极度短缺,对外依存度已高达93%和99%,潜在资源安全威胁凸显,加强国内镍钴矿产资源勘查已迫在眉睫。本文系统分析了我国镍钴矿产资源成矿特征,无论是成矿类型,还是构造背景特征,与全球均存在显著不同。全球60%的镍资源主要赋存于红土型镍钴矿床中,40%的镍资源则储存在岩浆镍钴硫化物矿床内,而我国镍资源的93%赋存在岩浆镍钴硫化物矿床中。世界上60%的钴资源来自沉积型铜钴矿床、23%的岩浆型镍钴矿床、15%的红土型镍钴矿床及2%的热液型含钴矿床,而我国钴资源45%赋存于岩浆镍钴硫化物矿床内、40%的钴资源赋存在与热液相关矿床中,受赋存状态及开发利用条件限制,这部分赋存在热液矿床中的钴资源尚不能有效利用。可见,我国镍钴资源关键来源是岩浆型镍钴矿床。对比我国岩浆镍钴硫化物矿床成矿特征及地质分布特点,本文提出我国岩浆镍钴硫化物矿床独特的资源禀赋条件,重要结合带旁侧滞后发育的铁质系列镁铁-超镁铁质侵入岩是镍钴资源赋存的关键空间,明确勘查工作的重点方向。另外,分析认为,加强钴矿赋存状态研究,创新破解开发利用条件限制,是实现钴资源增储上产的关键路径。这为指导镍钴资源勘查、助力找矿新突破提供了重要依据。

The Baixintan mafic-ultramafic intrusion in the Dananhu-Tousuquan arc of the Eastern Tianshan orogenic belt is composed of lherzolite, olivine gabbro, and gabbro. Olivine gabbros contain zircon grains with a U-Pb age of 276.8 ± 1.1 Ma, similar to the ages of other Early Permian Ni-Cu ore-bearing intrusions in the region. The alkaline-silica diagrams, AFM diagram, together with the Ni/Cu-Pd/Ir diagram, indicate that the parental magmas for the Baixintan intrusion were likely high-Mg tholeiitic basaltic in composition. The Cu/Pd ratios, the relatively depleted PGEs and the correlations between them demonstrate that the parental magmas had already experienced sulfide segregation. The lower CaO content in pyroxenites compared with the Duke Island Alaskan-type intrusion and the composition of spinels imply that Baixintan is not an Alaskan-type intrusion. By comparing the Baixintan intrusion with other specific mafic-ultramafic intrusions, this paper considers that the mantle source of the Baixintan intrusion is metasomatized by subduction slab-derived fluids’ components, which gives rise to the negative anomalies of Nb, Ti, and Ta elements. Nb/Yb-Th/Yb, Nb/Yb-TiO₂/Yb, and Th_N-Nb_N plots show that the Baixintan intrusion was emplaced in a back-arc spreading environment and may be related to a mantle plume.

The Eastern Kunlun Orogenic Belt (EKOB) has a complex geological structure and diverse magmatic activities, which are closely related to the Qaidam Basin and the Tethys tectonic evolution. There are at least 3 stages mafic-ultramafic rocks occurred in the Early Paleozoic in EKOB. The first stage is the Later-Silurian to Early Devonian, represented by the giant Xiarihamu super large magmatic Cu-Ni deposit, containing about 1.18 million metric tons (Mt) of nickel with average grades of 0.65% Ni, 0.013 % Co, and 0.14% Cu, and its age of ore-forming pyroxene peridotite is 411 Ma; The second stage is the Early Carboniferous, represented by the large Shitoukengde magmatic Cu-Ni sulfide deposit, and its ore-forming age of the olivine websterite is 334 Ma; The third stage of mafic-ultramafic rocks occurred mainly during the Middle-Late Triassic, represented by Xiaojianshan, Lalinggaoli, and Kaimuqi complexes, and no economical ore bodies have been found in this period. We summarized the difference between the ore-bearing and the non-mineralized mafic-ultramafic rocks in the EKOB. The olivine of the ore-bearing complexes contains higher MgO and SiO₂ content but lower FeO and CaO contents, and the clinopyroxene of ore-bearing complexes contains lower FeO and CaO contents. Crustal sulfur contamination is key to the formation of the giant Xiarihamu Ni deposit, and crustal sulfur contamination degree of the giant magmatic Ni deposit is higher than that of large Ni deposit. The above indicators could guide the exploration and evaluation of similar deposits in the EKOB.

The Xiarihamu Ni deposit is located in the East Kunlun orogenic belt in the northern part of the Qinghai-Tibet plateau. The regional tectonic setting, lithology, and mineralogy suggest that the Xiarihamu Ni deposit formed in a postcollisional setting. We use a slab break-off model to explain the simultaneous appearance of the Xiarihamu magmatic Ni sulfide deposit and the high-pressure eclogite exhumation in the East Kunlun orogenic belt. In situ δ5³⁴S_V-CDT value of pyrite from the country rock (granitic gneiss) of the Xiarihamu complex is 11.2‰, whereas in situ δ5³⁴S_V-CDT value of sulfides from the Xiarihamu I complex range from 2.4 to 7.7‰, with an average of 4.5‰. Crustal sulfur most likely accounts for approximately 40 to 60% of the total sulfur content in the Xiarihamu magma. Our modeling shows that the Xiarihamu parental magma may have achieved sulfide saturation and experienced approximately 0.005% sulfide segregation before complex emplacement. Based on previous studies and the platinum group element data in this study, we believe that the Xiarihamu magma was sulfide saturated when olivine began to crystallize. The zones with rapidly declining olivine Fo and clinopyroxene Mg^# values correspond well with the locations of orebodies, implying that the crystallization of olivine or clinopyroxene played a role in the sulfide saturation. The whole-rock Ni content of the peridotite is inversely related to the Cr^# and FeO contents in spinel, and the whole-rock Ni content of the websterite is negatively correlated with orthopyroxene Mg^#; thus, during the crystallization of spinel and orthopyroxene, more sulfide formed. Our calculation shows that the decrease in sulfur content at sulfide saturation (SCSS) during fractional crystallization depends mainly on a decrease in temperature, and the change in the melt composition is a secondary or minor factor. Although the contribution of fractional crystallization to total sulfur content may be only approximately 3.3 to 6.0%, an increasing concentration of sulfides were accumulated at the latest crystallization in the host rock because of the incompatibility of sulfur. Recognizing this phenomenon is important for ore exploration. We logged 11 drill cores meter by meter and prepared 110 thin sections from across these 11 drill cores. The lithofacies distribution and crosscutting phenomena have been clearly determined. Combined with systematic geochemical data on the cross section, this paper summarizes the mineral crystallization sequence in different locations of the Xiarihamu I mafic-ultramafic magma chamber and depicts a six-stage model for sulfide saturation during the consolidation process of the Xiarihamu complex.