克拉通边缘岩石圈金属再富集与金-钼-稀土元素成矿作用

克拉通是大规模成矿的重要构造环境,其边缘产出了众多世界级规模的金、钼、稀土元素矿床。然而,克拉通如何控制巨型矿床的形成与分布尚不十分清楚。文章基于作者和前人的研究成果,探讨了扬子和华北克拉通岩石圈早期金属富集与后期金属活化问题。在全球范围,多数克拉通在其形成之后长期保持稳定,但部分克拉通(如华北、扬子)在克拉通化之后又经历了早期(元古代)增生与晚期(中生代—新生代)改造。在克拉通化及其之后,处于克拉通边缘的大洋岩石圈或克拉通块体间的有限洋盆发生板片俯冲,释放出含金属组分(REE、Cu、Au)的富CO2流体,交代亏损的大陆岩石圈地幔(SCLM),并使之发生交代和金属再富集。俯冲诱发的弧岩浆在大陆下地壳底侵可形成新生下地壳,伴随着少量硫化物的堆积而发生金属(Au、Cu)再富集。由于克拉通相对稳定,新生下地壳在进变质脱水过程中仍能保存部分金属,释放的(含Au)变质流体很可能被封存或固结在地壳的某个部位。在克拉通破坏改造期,软流圈上涌改变克拉通SCLM热结构并诱发其部分熔融,产生富REE的碳酸岩熔体和富水的基性岩浆(如煌斑岩)。前者在浅部地壳侵位并出溶成矿流体,形成碳酸岩型REE矿床;后者在深部地壳脱挥发分(H2O+CO2),诱发新生下地壳重熔和含Au硫化物(和/或含Au流体囊)活化,形成富Au岩浆系统或流体系统。这些深地壳熔/流体沿克拉通边界或岩石圈不连续运移至上部地壳,岩浆系统直接出溶成矿流体,形成以斑岩体为中心的斑岩型Au矿,含Au富CO2流体流沿断裂网络系统活动并沉淀金属,形成石英脉型和蚀变岩型Au矿。伴随克拉通破坏改造,克拉通边界断裂或基底断裂重新活化,并诱发古老下地壳熔融,产生含Mo岩浆系统。这个理论框架不同于已有的造山带成矿理论模式,它解释了克拉通边缘异常富集Au、Mo、REE矿床及其成矿规律,可用于类似克拉通地区的成矿预测。

Metallic refertilization of lithosphere along cratonic edges and its control on Au, Mo and REE ore systems

The cratons are one of the most important geodynamic settings for ore mineralization, numerous world-class Au, Mo and REE deposits commonly form along its egdes and in its interior. However, what controls on the formation and distribution of such world-class deposits remain unclear. This paper, based on our and previous studies, discusses the processes of the early metallic refertilization and later remobilization in the Yangtze and the North China Cratons. It is well known that most cratons in the world have been stable after cratonization, but some of them (e.g., the Yangtze and the North China Cratons) have underwent early-stage (Proterozoic) lithospheric accretion and late-stage (Mesozoic-Cenozoic) significant reworking. After cratonization, subduction of oceanic lithosphere beneath the cratons would release metal-bearing (REE, Cu, and Au) CO₂-rich fluids, which would intensively metasomatized the sub-continental lithosphere mantle (SCLM), led to metallic refertilization. Underplating of the subduction-related arc magmas on the base of cratonic crust would form a juvenile lower crust or magmatic cumulate zone, which was associated with Au and Cu accumulation as sulfides at the base of the crust. Such metallic refertilization along the craton edges has been preserved since the Proterozoic during prograde metamorphism, largely due to relatively stability of the cratons, while the Au-rich fluids liberated during high-grade metamorphism could be trapped as "solid capsules" in some places of the crust. During cratonic destruction and reworking, partial melting of the refertilized SCLM, triggered by upwelling of asthenosphere, produced the REE-rich carbonatites and hydrous mafic melts (e.g. lamprophyre), ascending upwards into to the upper crust. The carbonatitic melts emplaced at shallow crust rapidly exsolved highly-oxidized, REE-rich fluids, finally forming the carbonatite-associated REE deposits. The volatiles (CO₂+H₂O) escaping from the hydrous mafic melts at deep crust trigger remelting of sulfide-bearing Au-rich juvenile lower crust and remobilization of gold-bearing fluid capsules, leading to the formation of gold-rich magmatic systems or hydrothermal fluid systems. Porphyry Au deposits formed by fluids exsolved from gold-rich crust-derived magmas, while gold-bearing CO₂-rich fluids transported along fault network systems produced quartz-vein and altered-rock type Au deposits. As results of the cratonic destruction, remelting of an ancient lower crust, caused by reactivating of translithospheric faults along cratonic edges or in the cratonic basement, produce the molybdenum-bearing magma system at the cratonic edges. This concept framework, presented here, intend to explain enormous enrichments of giant Au, Mo, and REE deposits along cratonic edges, which distinguishes from the metallogenic theories for ore systems in orogenic belts, and has potential implication for mineral exploration in similar cratons.