内蒙古克什克腾旗小东沟斑岩型钼矿床成岩成矿机制探讨

本文选取内蒙古克什克腾旗的小东沟斑岩型钼矿区作为研究区,它位于西拉沐伦钼矿带的西南部.对小东沟岩体进行了主微量元素、SHRIMP锆石U-Pb定年等地球化学方面的研究;对岩体中的钾长石和含矿矿物辉钼矿进行了普通铅同位素分析.小东沟岩体具有高硅富钾、REE含量低、Zr含量高、无负铕异常、高ε_(Nd)(t)、低Sr_i等特点,指示岩浆起源于加厚新生下地壳的熔融,同时具有高温、快速熔融、快速析离逃离源区的特点;小东沟岩体的SHRIMP锆石U-Pb定年结果为142±2Ma,对应140Ma左右主应力场由南北向转为东西向的构造体制大转折时期,这个时期大兴安岭处于伸展的构造环境下,底侵作用发育,地幔物质可以添加到下地壳熔融形成的岩浆中;对岩体钾长石和辉钼矿进行的普通铅同位素分析显示前者具有从造山带-地幔过渡的特征,后者则显示有地幔特征,说明成岩、成矿物质来自两个不同的源区,下地壳的岩浆和地幔含矿流体发生了混合.通过以上的分析,结合水-岩浆物理反应的实验结果,本文提出小东沟斑岩型钼矿床的成矿模式:大兴安岭地区在早白垩世伸展的构造背景下,地幔含矿流体加入受底侵作用加热的下地壳中使之熔融形成岩浆,随后更多的地幔含矿流体进入岩浆房,促使其迅速析离逃逸出源区,二者一起上升侵位.在岩浆温度较高时,地幔舍矿流体和岩浆大致以较稳定的液态不混溶的状态共存;当岩浆和流体侵位到较浅深度时,压力下降,温度降低,晶体增多,流体-岩浆体系变得不稳定,岩浆被流体分割成许多很小的岩浆团.流体中H~+、K~+和各种成矿元素的存在使之必然会和岩浆或冷却后形成的岩石发生蚀变反应,并晶出成矿矿物,形成现在我们看到的具有浸染状矿化现象的小东沟斑岩型钼矿床.

Petrogenetic and metallogenic machenism of the Xiaodonggou porphyry molybdenum deposit in Hexigten Banner, Inner Mongolia

The Xiaodonggou porphyry molybdenum deposit located in the Xarmoron molybdenum metallogenic belt is chose as the research area. The Xiaodonggou pluton is rich in silicon, potass, zirconium, and low in REE. It has no or small negative Eu anomaly and shows an isotopic composition high in ε_(Nd) (t) and low in Sr_i, indicating its magma origining from the melting of juvenile thicken lower crust. In the meanwhile, it contained the features of high temperature, quick melting, quick segregation. The SHRIMP zircon U-Pb dating gives a result of 142 ± 2Ma, corresponding to the big tectonic transition period of 140Ma, when the major stress field changing from NS to WE. At that time, the Da-Hinggan Mountains area was under an extensive tectonic background, underplating proceeded and mantle materials could add into the magmas forming in the lower crust. The common lead analysis of the pluton orthoclases and molybdenites shows that the former transfer from orogen to mantle and the latter come from mantle, demonstrating that the rock and ore-forming materials of the deposit have different sources, magma from the lower crust mixing with mantle fluid. From the above analysis, with the physical experiment results of the water-magma reaction, we propose the following model for the Xiaodonggou porphyry molybdenum deposit. In the Early Cretaceous period, the Da-Hinggan Mountains area was under an extensive tectonic background, the adding of mantle fluid containing ore materials into heated lower crust made it melt to produce magmas. Following more mantle fluid got into the magma room and urged the magma to escape from the source quickly. The fluid and magma uplifted together, when they arrived at shallow depth, the fluid-magma became unstable and the latter was broken into many small agglomerates with fluid connecting them in the interspaces. Because of the H~+ , K~+ and various elements existing in the fluid, it would reacted with the magma and the rock through alteration and ore minerals crystallized out, forming the Xiaodonggou porphyry deposit with disseminated mineralization phenomenon.