镁铁—超镁铁质岩与花岗岩-伟晶岩“小岩体成大矿”对比——以昆仑成矿带夏日哈木和大红柳滩超大型矿床为例

Comparison of Mafic-ultramafic and Granite-pegmatite "Small Intrusion Forming Large Deposit"——Taking Xiarihamu and Dahongliutan Super-large Deposits in Kunlun Metallogenic Belt,China as Examples

夏日哈木矿床和大红柳滩矿床是分布在我国昆仑成矿带上的两个超大型矿床,分别形成于早泥盆世(411 Ma)古特提斯构造裂解环境和晚三叠世(204~202 Ma)古特提斯构造闭合碰撞背景。前者是与幔源镁铁—超镁铁质岩有关的岩浆熔离型镍钴硫化物矿床,后者是与壳源花岗岩-伟晶岩系统有关的高度岩浆分异演化的花岗岩-伟晶岩型锂铍稀有金属矿床,较小的岩浆岩和伟晶岩脉内蕴藏大规模的金属元素矿体,“小岩体成大矿”是两个矿床的共同特征。对比结果表明:夏日哈木矿床是地幔部分熔融形成的基性岩浆,经深部熔离和岩浆分异作用上侵-贯入而成,大红柳滩矿床则是含泥质岩层地壳由于深部加热部分熔融形成的中酸性岩浆,经高度分异演化形成的含矿伟晶岩侵位所致,大规模的岩浆是关键。地幔橄榄石、辉石中的Ni、Co等元素即使全部溶解进入岩浆,仅依靠结晶分异也不可能造就Ni、Co等元素具有工业价值的富集,只有在橄榄石、辉石结晶前发生大规模的硫化物液相-硅酸盐熔体之间的不混溶(熔离)作用,才可使本来有限的Ni、Co大量聚集在硫化物液相中而形成巨大富集;而上地壳的泥质岩熔融实验表明,即使全部熔融也仅有(100~200)×10^(-6)的Li集中,只有大规模的泥质地壳岩层熔融形成大规模的中酸性岩浆,才可能使有限的Li、Be在岩浆高度分异演化过程中聚集在高温热液流体端元,最终得以超常富集。由此可见,大规模的岩浆是两类矿床形成金属元素超常富集的先决条件,而后成矿元素必须集中于有限特殊物性的较小体积里。有限体积的超常富集是以更多岩浆中金属元素的贫化或亏损为前提的。因此,“小岩体成大矿”是重大突变地质事件的产物,只有重大地质事件使壳-幔物质交换造就大规模岩浆作用,并经历非平衡的极端地质作用,才可能使较小的岩体蕴藏巨大的金属矿产富集。以岩浆镍钴硫化物矿床成矿特点为依据提出的“小岩体成大矿”理论认识,与酸性岩浆有关的岩浆热液矿床的成矿与找矿研究中亦不断得到了证实。尽管其成矿作用有所不同,但所蕴含的科学范式则有共同之处。通过表层结构的对比,挖掘其深层结构普遍原理,可以丰富“小岩体成大矿”理论的科学内涵。

Xiarihamu and Dahongliutan deposits,which are two super-large deposits distributed in Kunlun metallogenic belt,China,are formed in the Early Devonian structural cracking environment of Paleo-Tethys(411 Ma) and the Late Triassic closed collision background of Paleo-Tethys(204-202 Ma).The former is magmatic immiscible Ni-Co-sulfide deposit related with the mantle source mafic-ultramafic rocks,the latter is granite-pegmatite type lithium beryllium rare metal deposit related with the evolution of high magmatic differentiation of crust source granite-pegmatite system,and smaller magmatic rock volume contains large-scale metal elements,of which “small intrusion forming large deposit” is the common characteristics of these two deposits.The results show that Xiarihamu deposit is formed by intrusion-injection of the mantle partial melting of basic magma through deep liquation and magmatic differentiation,and Dahongliutan deposit is formed by intrusion of high differentiation evolution of pegmatites through the subductive muddy rock crust due to the deep heating melting formation of acidic magma,of which large-scale magma is the key.Ni and Co in olivine and pyroxene of mantle,even all dissolve into the magma,only relying on crystallization differentiation could not make Ni and Co enrichment of industrial value,only large-scale sulfide liquid phase-silicate melt between immiscibility(liquation) before olivine and pyroxene crystallization can make originally limited Ni and Co gathered in sulfide liquid phase and form a huge enrichment;and on the upper crust of argillaceous rock melting experiment,even all melting but only Li(the mass fraction is(100-200)×10^(-6)) could cause enrichment,so only large-scale mud crust layer melt forming large-scale acidic magma can make the limited Li and Be in the process of magma highly differentiation evolution,and gather in high temperature hydrothermal fluid,eventually to extraordinary enrichment.It can be seen that large-scale magma is the premise of the extraordinary huge enrichment of metal elements in the two types of deposits,and the basis is that it must be concentrated in a limited and small volume of special physical properties.The extraordinary enrichment of limited volume is based on the dilution or depletion of metal elements in more magma.Therefore,“small intrusion forming large deposit” is the product of major mutation geological events,only the major geological events can make the large-scale magma created by the exchange of crust and mantle material,and experience non-equilibrium extreme geological effects,and make the final small intrusion rich in huge metal minerals.Based on the mineralization characteristics of magmatic Ni-Co-sulfide deposits,the theory of “small intrusion forming large deposit” is proposed,and the mineralization and prospecting studies of magmatic hydrothermal deposits related to acid magma have been proved continuously.Although its metallogenic function is different,the scientific paradigm has something in common.Through the comparative study of the surface structure,the general principle of its deep structure is explored,so as to enrich the scientific connotation of the theory of “small intrusion forming large deposit”.