华北板块北缘中段含铜镍铂镁铁超镁铁岩地球化学特征
Geochemistry of Cu-Ni(PGE) Sulfide-bearing Mafic-ultramafic Rocks on the Middle Segment of the North China Plate
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摘要: 内蒙古中部发育一条近东西向的含铜镍铂镁铁-超镁铁质岩带, 宽30 km, 长约400 km。该含铜镍铂镁铁-超镁铁质岩带位于华北板块北缘深大断裂两侧, 横跨两个构造单元:宝音图岩浆弧和狼山-白云鄂博台缘坳陷。研究区所处地理位置相对偏僻, 研究程度较浅, 近年来发现的多处铜镍矿床(点)均分布在该岩带内, 逐渐引起地质学者的关注。本文主要通过分析、总结各含矿岩体地球化学数据, 对该含铜镍铂镁铁-超镁铁质岩带开展岩石地球化学特征研究。区内岩体形成时代主要集中在314~269 Ma, 应属华力西晚期产物。主量元素化学组成表明, 该含铜镍铂镁铁-超镁铁质岩主要为拉斑玄武岩系列, 部分为钙碱性系列。辉长岩、辉石岩稀土配分模式分两类:轻、重稀土分馏不明显的相对平坦型和轻稀土相对富集、重稀土比较平坦的右倾型(LaN/YbN为1.49~20.08), Eu为正异常或负异常(δEu为0.56~1.66);辉橄岩稀土元素配分呈轻稀土相对富集、重稀土比较平坦的右倾型(LaN/YbN为5.56~13.57), Eu为负异常(δEu为0.57~0.83)。岩石稀土配分模式总体表现出轻稀土相对富集, 重稀土相对平坦的右倾型, 稀土总量由辉长岩→辉橄岩呈规律的降低。各类岩石微量元素也各具特色, 总体表现出亏损高场强元素Nb、Ta、Zr。岩浆在演化过程中发生了一定程度的同化混染作用。此外, 各岩体表现出高MgO、低碱的特征, 具有较高的Mg#、m/f比值, 显示较好的含矿潜力。
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关键词:
- 地球化学 /
- 镁铁-超镁铁岩 /
- Cu-Ni(PGE)矿床 /
- 内蒙古中部
Abstract: A Cu-Ni(PGE) ore-bearing mafic-ultramafic rocks belt is exposed in central Inner Mongolia, EW extending about 400km long and 30km wide. This belt is located on both sides of the fracture of the North China Craton and stretches across two tectonic units: Baoyintu magmatic arc and Bayan Obo marginal rift. As a remote geographical position, the research is relatively weak in this area. In recent years, many Cu-Ni(PGE) deposits (ore occurrences) including Ebutu, Dabuxun, Tepogeritu, Wengeng, Kebu, Huanghuatan and Xiaonanshan have been discovered in this belt, raising the concern of geologists gradually. Based on the systematical summary and analysis of geochemical data, the author mainly carried out the research on geochemical characteristics of the Cu-Ni(PGE) ore-bearing mafic-ultramafic rocks belt. The ore-bearing intrusions are formed in the range of 314—269Ma, indicating that they should be the products in late Variscan era. Most of rocks belong to tholeiitic series, and parts of rocks are calc-alkaline series in petrochemical composition. Gabbros and pyroxenites show two types of REE patterns: relatively flat REE patterns with inconspicuous fractionation of LREE and HREE, and rightward oblique linear REE patterns with LREE enrichment in relative to HREE(LaN/YbN is 1.49 to 20.08), and exhibit positive Eu anomaly or negative anomaly(δEu is 0.56 to 1.66); Pyroxene peridotites also show LREE-enrichment(LaN/YbN is 5.56 to 13.57) and HREE relatively flat, right-inclined REE distribution pattern, and display negative Eu anomaly(δEu is 0.57 to 0.83). Overall, all rocks show rightward oriented linear REE patterns with LREE enrichment in relative to HREE and exhibit a regular ΣREE decrease from gabbros to pyroxene peridotites, and are characterized by negative HFSE (Nb, Ta, Zr) anomalies. According to these geochemical characteristics, it may be inferred that all of the ore-bearing intrusions in this belt may come from the same source of magma, and magmatic assimilation occurs in the evolution. With high MgO and low-alkaline, the intrusions show high Mg# value and high m/f ratios, which indicates good prospecting potential. -
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