兴蒙造山带及华北板块北缘钼矿化——进展、规律、问题与成因初探

兴蒙造山带及其南侧受古亚洲洋南向俯冲所影响的华北板块北缘内各有1条显著的中生代斑岩钼成矿带,并在东西两侧首尾相连。文章综述了伸展环境下的斑岩钼矿床的研究进展,对兴蒙造山带及华北板块北缘内这2条钼成矿带的成矿背景、分布规律、矿床共生组合特点、成矿岩浆的属性、巨量金属和水的来源以及斑岩铜、钼矿化的异同等进行了总结,并从成矿岩浆源区塑造过程的角度初步探讨了巨型钼成矿带的形成特点。这2条钼成矿带在地质特征和区域矿床组合上非常相似,具有相似的启动时间和峰值时间,与区域内的斑岩铜矿化在时代上具有不共生的特点,矿化特征也与美国科罗拉多地区产出的高F型斑岩钼化类似,二者构成了统一的整体。元素地球化学对比研究显示,南、北2条钼成矿带的成矿岩浆与古生代斑岩铜成矿岩浆以及中生代的碱性岩浆均具有相似的特征,为脱水熔融的产物,并与古老陆下岩石圈地幔包体、新生代软流圈地幔玄武岩具有显著的差别。研究认为,南钼矿带的岩浆源区是亏损金属和水的古老岩浆源区在古生代洋片俯冲过程脱水交代改造后的产物,北钼矿带的岩浆源区是古生代洋片俯冲增生形成的富水源区。成(含)矿岩石Sr同位素研究显示,南、北2条钼成矿带成矿源区均启动于Rb/Sr比值较低的源区,受到上部高Rb/Sr比值地壳的混染;Nd同位素特征的对比研究显示,二者初始Nd值差别极大,但是Sm/Nd比值非常相似,显示放射性成因Nd的积累在三叠纪以后是一致的,也说明初始Nd值的差别是成矿源区塑造前所形成和继承的,同时也说明初始Nd值的差异可能掩盖了Mo成矿岩浆形成的真正原因。通过与世界范围内其他典型钼成矿带的对比研究,认为南、北2条钼成矿带成矿的岩浆源区位于陆下岩石圈地幔,古生代期间古亚洲洋向南、北两侧的俯冲在其形成过程中具有重要作用,主要体现在塑造富集型源区、水化造山带和增厚岩石圈等几个方面。俯冲改造、加厚并富集了水和大离子亲石元素的陆下岩石圈获得了地球化学上的不稳定性,在伸展构造环境(可能有多期伸展)驱动下,脱水熔融以达到稳定的趋势,在这个过程中,其化学成分将逐渐与古老陆下岩石圈地幔的化学成分趋于一致。因此,水化的陆下岩石圈地幔在伸展过程中的低程度批式脱水部分熔融,形成的富含金属和水的高分异型岩浆构成了成矿岩浆,并在岩石圈的不同尺度经过多阶段结晶分异-同化混染后,就位成为近矿岩浆房。陆下岩石圈脱水熔融的结束也意味着巨型热液钼矿化作用的结束,并决定了俯冲后巨型热液成矿带总的生命周期,这也与兴蒙造山带及华北板块北缘钼矿化(甚至其他热液型矿化)在早白垩世(约130 Ma)趋于减弱并熄灭的现象一致,也使得新生代的碱性岩浆岩不具有显著的脱水熔融特征。综上,笔者认为兴蒙造山带及华北板块北缘的斑岩钼矿化为一个统一的整体,属于古亚洲洋俯冲作用水化的源区在后期强烈伸展环境下部分熔融的产物,是古亚洲洋俯冲成矿作用的延续和发展,也是古生代塑造的富集型源区在中生代伸展构造驱动下的复合成矿作用。

Mo mineralization in Xing'an-Mongolian orogen and north margin of China craton:Review, question and a preliminary genetic model

The Xing''an-Mongolian orogen (XMO) is the eastern part of the Central Asian Orogenic Belt (CAOB). There are two Mesozoic Mo mineralization belts in the XMO and its adjacent areas, i.e., the northern and southern Mo mineralization belts. In this paper, the tectonic setting, distribution of Mo mineralization, deposits assemblage character, geochemical feature of ore-formation magma, source of metals and water for Mo deposits, and similarity between Mo and Cu mineralization in XMO and its adjacent areas are reviewed. Based on the authors'' and previous studies, this paper puts forward a genetic model for the two Mo mineralization belts with a focus on the source region of magma associated with the Mo mineralization. The two belts share similar geological features to high F type Mo deposits in Colorado mineralization belt, and the two belts also share similarity in their deposits assemblages, geochemistry of ore-forming magma, initiation and peak mineralization age. Thus, the authors propose that the two Mo mineralization belts share the same key genetic process, and constitute an integrated mineralization belt on the whole. Based on the comparative study of geochemical geology of the ore-forming magma for the two Mo mineralization belts, it''s proposed that the formation of the ore-formation magma is similar to the Paleozoic porphyry Cu deposits and Mesozoic alkaline magma in XMO and its adjacent areas, is a product of dehydration melting, and is different from the Proterozoic peridotite xenoliths in Hannuoba and the Cenozoic asthenospheric mantle rocks. The study indicates that the source for the Mo-mineralized belt in the XMO is the Proterozoic lithosphere depleted in metals, water and LILEs, but was metasomatized and enriched in metals during dehydration by subduction of an oceanic plate beneath the North China Craton during the Paleozoic. The source for the Mo-mineralized belt in the XMO is a juvenile lithosphere formed by ocean subduction under the Siberian plate during Paleozoic. The comparative Sr-Nd isotopic study of the belts indicates that the ore-forming magma was generated from a region in the lithosphere with a low Rb/Sr ratio, which is interpreted as the active magma end-member. The magma was subsequently contaminated with the introduction of a high Rb/Sr source in the shallow part of lithosphere, which is considered to be the passive magma end-member. Although there are significant differences in Nd isotopes for the ore-forming magma between the two Mo mineralized belts, their Sm/Nd ratios are virtually the same averaging 0.19~0.17. This indicates that the accumulation of radioactive Nd isotope was nearly the same between the ore-forming magmas of the two belts since the Triassic, and further indicates that the differences in initial Nd isotope values between the ore-forming magma of the belts was inherited from and formed in their source region before Mo mineralization was active. The difference of Nd isotopes between the two belts masks the true nature of the Mo mineralizing process in the two belts. Based on a comparative study of other Mo mineralization belts in the world, the authors hold that the source region for the Mo mineralization magma is located in SCLM. The Paleozoic oceanic subduction played a key role in the formation of the metal and water-fertile source region by re-hydrating, fertilizating (refertilizating) with ore metals and LILEs and by thickening the lithosphere. In spite of the fact that the resultant enriched, metasomatized and thickened lithosphere would have been chemically and lithospherically unstable, it had a re-stabilization trend by dehydration melting to make it chemically and lithospherically similar to the ancient and stabilized SCLM. A preliminary genetic model arguing that low degree of batch dehydration partial melting of metal-fertile and water and LILEs-enriched SCLM occurred under low heat flow environment in post-subduction tectonic setting is proposed for the formation of Mo mineralization magma, which would undergo significant crystallization differentiation, assimilation and contamination in the upper crust. The termination of dehydration melting of fertile SCLM means a termination of Mo mineralization, and the duration of dehydration melting defines the life span of giant hydrothermal mineralization in the Mesozoic post-subduction setting. This conclusion is consistent with the termination of Mo mineralization in Late Cretaceous (ca. 130 Ma) in XMO and its adjacent areas. Based on the study of this paper, the authors propose that the Mesozoic Mo mineralization in the XMO and its adjacent areas was related to a post-subduction tectonic setting of the Paleozoic Paleo-Asian oceanic plate.