Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-type Porphyry Mo Mineralization
Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-type Porphyry Mo Mineralization
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中国中东部黔峨冲岩浆热液演化：大别型斑岩钼矿化洞察
DOI：
10.1093/petrology/egac013
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发表时间：
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影响因子：
3.9
通讯作者：
Dan Kaibo
中科院分区：
文献类型：
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作者：
Zhang Daohan;Wei Junhao;Nadeau Olivier;Shi Wenjie;Dan Kaibo
大别型斑岩钼矿床是近年来发现的一种新的斑岩钼矿床类型，但与成矿有关的岩浆在该类型存款形成中的作用，以及与已被广泛研究的高潮型斑岩钼矿床在成因过程上的区别等问题仍存在一些疑问。本文对大别山前鹅冲巨型存款矿床中的矿物包裹体和熔体包裹体进行了研究，旨在加深对成矿岩浆的性质及其在大别斑岩型钼矿床成因中的作用的认识。该系统的岩浆和热液演化重建主要是基于石英托管的硅酸盐熔体和矿物包裹体的分析，与现场和岩相学的关系，以及以前发表的U-Pb和Re-Os地质年代学。前鹅冲与矿有关的岩浆由早到晚包括石英斑岩脉、流纹斑岩脉、花岗斑岩（岩株和岩脉）和一个新发现的深源二长花岗斑岩岩株。根据TitaniQ热压法，这些岩石来自约16-19 km深（500-600 MPa）的岩浆房，并在720°C至690°C的温度下经历了近等温减压至约7 km（200 MPa）。根据熔融包裹体的微量元素组成，结合已发表的全岩Nd同位素组成，认为早期岩浆经分馏结晶作用由石英斑岩演化为流纹斑岩，从而形成晶体糊。这种粘稠的晶体糊状物随后被重新熔化以产生花岗斑岩，并被注入来自不同来源的熔体以产生二长花岗斑岩。前鹅冲熔融包裹体中Mo含量低（2-8 ppm），>5 wt. %的H2O，以及几乎没有F（≤ 0.26wt. %），反对形成大型斑岩型钼矿床必须由成矿岩浆富集钼的要求。瑞利分馏模型表明，在石英和花岗斑岩中的钼的浓度增加，通过分馏的石英，辉长岩，黑云母，磁铁矿，钛铁矿和钼随后耗尽熔体通过流体出溶，专门在流纹岩和二长花岗斑岩，作为岩浆上升和减压的结果。这表明，除了与矿相关的花岗斑岩外，流纹岩和二长花岗斑岩也对热液成矿有贡献。这项研究证实，斑岩钼矿床的形成并不依赖于异常高浓度的钼在与矿有关的岩浆，而是需要有效地提取钼从大量的岩浆，与正常浓度的钼。与高潮型矿床不同的是，成矿流体的多个脉冲是从对流的浅岩浆房中输送出来的，大别型矿床中的钼成矿是通过成矿流体的聚集实现的，这些成矿流体来自连续侵位的、相对较深的侵入体。虽然大别山型和高潮型钼矿床分别与大别造山带的I型和A型花岗岩有关，但这两种岩浆都是俯冲扬子陆壳部分熔融的产物，是挤压向伸展构造环境的转换控制了大别造山带不同类型的斑岩钼成矿作用。
Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian’echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian’echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian’echong, all melt inclusions have low concentrations of Mo (2–8 ppm), >5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen.