研究生知识库

冷水箐岩浆Cu-Ni硫化物矿床和高家村岩体位于扬子地块西缘，就位在早新元古代的盐边群变质火山-沉积岩中。冷水箐矿床主要赋存于4个小的镁铁质-超镁铁质岩体中，含21万余吨金属镍，是扬子地块西缘目前发现的最大的新元古代岩浆Cu-Ni硫化物矿床。其中101号岩体含16万吨金属镍，主要岩性依次为二辉橄榄岩、橄榄二辉岩、辉长岩等，矿化主要集中在岩体底部的超镁铁质岩中。高家村岩体作为该区域目前发现的最大的新元古代镁铁质-超镁铁质岩体，主要由外带的中性-镁铁质岩（闪长岩、闪长辉长岩和辉长岩等）和内带的超镁铁质岩（纯橄岩和橄榄岩等）组成。其中内带含五层岩浆Cu-Ni硫化物矿化，每层约2~4 m厚，镍品位为0.6~1.5 wt%。本论文在前人研究的基础上，对冷水箐101号含矿岩体（钻孔ZK37801和ZK37802），和高家村内带（钻孔ZK3902）以及外带（野外露头）进行系统采样，通过年代学、矿物学、主微量元素、亲铜元素（PGE和Se）、以及全岩的Sr-Nd-Os-S同位素研究，讨论冷水箐和高家村岩体的岩石成因和矿床（矿化）成因，为未来Ni矿资源勘探提供方向，并结合扬子西缘新元古代的区域地质背景，讨探扬子西缘的大地构造背景在新元古代的演化。高家村岩体外带中角闪辉长岩（GJC1603）的SIMS锆石U-Pb谐和年龄为824 ± 6 Ma，与冷水箐矿床形成于同一时代（~820 Ma）。冷水箐矿床和高家村岩体中最原始橄榄石的Fo（100Mg/(Mg+Fe), molar）分别为85和86 mol%，表明了它们的母岩浆均经历了中等的结晶分异作用。其橄榄石的Ca含量均<1000 ppm，与岛弧型镁铁-超镁铁质岩体中橄榄石组份一致。而且这两个岩体中的单斜辉石和包裹在橄榄石中尖晶石的组份也均与岛弧型堆晶成因的镁铁-超镁铁质岩体中矿物组份相似，明显不同于金川和其他裂谷/地幔柱型堆晶成因的镁铁-超镁铁质岩体中矿物组份。依据校正过的橄榄石与硫化物之间的Fe与Ni交换关系，计算得到冷水箐和高家村岩体中岩浆硫化物系统的氧逸度为QFM +1。冷水箐和高家村岩体都表现出中等程度的轻稀土富集和明显的Nb-Ta亏损。冷水箐矿床的εNd(t)和(87Sr/86Sr)i分别在+1.1 ~ +5.8之间和在0.7038 ~ 0.7050之间，高家村岩体的εNd(t)和(87Sr/86Sr)i分别在+1.3 ~ +4.5之间和在0.7044 ~ 0.7059之间。通过Sr-Nd同位素模拟计算，显示出冷水箐和高家村岩体受到的地壳混染均十分微弱（< 5 wt%），暗示其明显的Nb-Ta亏损并不能通过单独的地壳混染来解释，而是原始岩浆自身的性质。这些证据都表明了冷水箐和高家村岩体成因与岛弧岩浆系统有关。通过对百分百硫化物中PGE含量和R因素的模拟，得到冷水箐母岩浆含0.25 ppb Pd，0.006 ppb Ir；高家村母岩浆含0.12 ppb Pd，0.02 ppb Ir，明显比峨眉山苦橄岩的PGE含量低将近2个数量级，但却与大部分岛弧成因的岩浆Cu-Ni硫化物矿床中母岩浆的PGE含量相似。造成PGE亏损的原因可能是由于地幔的低程度部分熔融导致硫化物残留或岩浆在上在升过程中经历了硫化物熔离。依据不同类型矿石的Ni、Cu及PGE特征和硫化物熔离及分离结晶的模拟计算，结果表明了冷水箐矿床和高家村岩体中矿石的PGE含量主要受R因素和MSS的结晶分异控制。冷水箐矿床矿石的含有广泛的δ34S ( -4.0 ~ +1.3 ‰)，高的S/Se (4,800 ~12,500) 和高的γOs(t) (115 ~ 320)，高家村矿石中也含有高的δ34S ( +2.3 ~ +3.3 ‰)和S/Se (4,100 ~5,100)，均暗示有地壳硫的加入。此外，通过MELTS软件对岩浆进行结晶分异模拟，结果表明单独的结晶分异作用不能导致岩浆硫化物饱和。这些证据都暗示了地壳硫的加入是导致岩浆硫化物饱和的最主要因素。同时，依据Sr-Nd同位素暗示的低程度地壳同化混染和其直接接触的围岩并不含硫化物，它们说明了岩浆在深部经历了地壳硫化物的选择性同化混染。薄片中也发现液滴状硫化物包含在橄榄石，甚至铬尖晶石中，说明了硫化物饱和发生在橄榄石和尖晶石结晶之前或同时。最后，综合扬子西缘新元古代岩浆岩的岩石组成、基性-超基性岩的时空分布和地球化学特征，暗示了年龄>820 Ma的岩体更符合岛弧岩浆系统成因模型，而与地幔柱成因模型明显不相符。结合岩浆Cu-Ni硫化物矿床的成矿因素和本文的研究成果，暗示了高家村岩体深部极有可能赋存大型的冷水箐型岩浆Cu-Ni硫化物矿床。

The Lengshuiqing magmatic Ni-Cu ore deposit and the Gaojiacun intrusion, located in the western margin of the Yangtze craton in China, was emplaced into the early Neoproterozoic metamorphosed volcanic-sedimentary rocks, granites and diorites. The Lengshuiqing deposit is hosted in four small mid-Neoproterozoic mafic-ultramafic intrusions. It is the largest Neoproterozoic magmatic Ni-Cu ore deposit in the western margin of the Yangtze craton in China, with 210 thousand tons of metal nickel. Of which the 101 intrusion contain 160 thousand tons of metal nickel. The intrusions are dominated by lherzolite, olivine websterite, websterite and gabbro, with sulfide mineralization associated with ultramafic rocks in the lower parts of the intrusions. The Gaojiacun intrusion is the largest Neoproterozoic mafic-ultramafic intrusions in the western margin of the Yangtze craton. It can be divided into the outer zone and inner zone. The outer zone is mainly characterized by olivine-free mafic-intermediate rocks, including gabbro, hornblende gabbro, and diorite. The inner zone is dominated by olivine-bearing mafic-ultramafic rocks, including dunite, lherzolite, and troctolite, other than a few clinopyroxenite. Five layers of sulfide mineralization with 2-4 meter and 0.6 to 1.3 wt % Ni grade in the Gaojiacun ultramafic portion were discovered in the latest drill cores. The samples used in this study were collected from drill cores of the Lengshuiqing 101 intrusion (ZK37801, ZK37802) and Gaojiacun inner zone (ZK3902) and outer zone (field outcrop). We report additional data such as chronology, mineralogy, systematic mineral compositions (Cr-spinel, olivine and clinopyroxene), new PGE data, and complementary whole-rock chemical and Sr-Nd-S-Os isotopic compositions. These data, together with geological constraints, are used to evaluate the competing geodynamic models (subduction versus mantle plume) that have been proposed for the associated basaltic magmatism, as well as the fundamental controls on the genesis of this type of magmatic Ni-Cu ore deposit. The implications of the results from this study for Ni exploration at both global and regional scales are assessed. Finally, combined with the regional geological background, the tectonic setting of the western margin of the Yangtze in the Neoproterozoic is also discussed.The ages of the host intrusions of the Lengshuiqing deposit were dated at ~820 Ma. In the paper, the Gaojiacun outer zone is determined using a SIMS, dating at 824 ± 6 Ma (hornblende gabbro, GJC1603) The most forsterite contents of olivine crystals in the Lengshuiqing and Gaojiacun intrusions are 86 mol% and 85 mol%, respectively, indicating that their parental magma experienced moderate degrees of fractional crystallization. Olivine crystals in the intrusions are depleted in Ca (<1000 ppm), a common feature for arc-type mafic-ultramafic intrusions worldwide. Moreover, clinopyroxene and Cr-spinel in the Lengshuiqing and Gaojiacun intrusions are remarkably similar to those in arc-type olivine-clinopyroxene cumulates elsewhere in the world, whereas significantly different from the rift-related Jinchuan intrusion. Based on the Fe-Ni exchange coefficient between olivine and sulfide liquid, the estimated oxidation state for the Lengshuiqing deposit and Gaojiacun intrusion are close to QFM + 1. Whole rock samples from these intrusions are all characterized by moderate light REE enrichments relative to heavy REE, moderate to significant degrees of negative Nb-Ta anomalies. The Lengshuiqing deposit has positive εNd(t) from +1.1 to +5.8, and low (87Sr/86Sr)i from 0.7038 to 0.7050. The Gaojiacun intrusion has positive εNd(t) from +1.3 to +4.5, and low (87Sr/86Sr)i from 0.7044 to 0.7055. The isotope data indicate that the parental magma for these intrusions experienced only minor degrees of crustal contamination that cannot fully account for the observed negative Nb-Ta anomalies in the rocks. The data imply that the parental magmas were already depleted in Nb and Ta prior to crustal contamination. Taken together, the data support the view of an arc setting for the Lengshuiqing and Gaojiacun mafic-ultramafic intrusions. Like most other arc-type magmatic sulfide deposits, the PGE tenors of the Lengshuiqing deposit are very low. The estimated initial contents of PGEs in the parental magma are 0.25 ppb Pd, 0.006 ppb Ir for the Lengshuiqing and 0.12 ppb Pd, 0.02 ppb Ir for the Gaojiacun, which are two orders of magnitude lower than those in PGE undepleted picrites of continental flood basalt provinces such as the Emeishan and Siberian traps. Their PGE tenors are mainly controlled by the variable R-factors and moderate degrees of sulfide liquid fractionation on cooling. 34S, γOs(t) and S/Se of the sulfide ores in Lengshuiqing deposit are from -4.0 to +1.3 ‰, from 115 to 320, and from 4,800 to 12,500, respectively. 34S and S/Se of the sulfide ores in Gaojiacun intrusion are +2.3 to +3.3 ‰, and from 4,100 to 5,100, respectively. These values are significantly different from typical mantle values, and indicate that addition of external sulfur played a major role in triggering sulfide saturation in the magma. According to the MELTS simulation and calculation for the S content at sulfide saturation in the fractionated magma, alone the fractional crystallization process cannot result in sulfide saturation in magma. Therefore, the addition of crustal sulfur played the most important role in sulfide saturation. Rounded sulfide inclusions in some olivines and even spinels indicate that sulfide saturation could take place before/during olivine and spinel crystallization.Together with the rock composition, spatiotemporal distribution and geochemical characteristics of all reported Neoproterozoic magmatic rocks in the western margin of the Yangtze, we believe that these rocks (>820 Ma) are more in line with the genetic model of the island arc magma system, whereas obviously do not conform to the genetic model of the mantle plume. Considering the ore-forming factors of the magmatic Cu-Ni sulphide deposit, we believe that the Gaojiacun intrusion has the possibility of hosting a large-scale arc-type magmatic sulfide deposit at depth.