秦岭造山带南缘荞麦冲金矿岩浆岩特征及成矿作用研究

发布时间：2018-03-20 00:01

  本文选题：秦岭南缘　切入点：荞麦冲金矿　出处：《中国地质大学(北京)》2015年硕士论文　论文类型：学位论文

【摘要】：秦岭造山带南缘存在大片的早古生代基性火山岩,荞麦冲金矿的含矿正长斑岩脉体呈北西向侵入基性火山岩地层中。前人在对荞麦冲金矿进行元素地球化学、岩石化学、同位素、包裹体、稀土元素地球化学等方面的研究中,证实了金矿成矿过程中的主导作用是燕山期的脆-韧性剪切变形变质作用,得到一组模式年155Ma可能反应矿床的成矿年龄。通过对该正长斑岩锆石的LA-ICP-MS U-Pb定年、元素地球化学以及Hf同位素研究,探究正长斑岩脉体的侵入时代、岩石成因和大量基性岩的成岩构造环境。结合大别山造山带地质背景特征,可将脉体的侵入时代可以分为两期,锆石加权平均年龄值分别为(415±7.3)Ma、(444±5)Ma,前者属于早泥盆世时期,后者为寒武纪到早奥陶世时期。所测锆石Th/U比值基本集中在1附近,结合稀土特征,典型的Ce正异常、Eu负异常,重稀土富集,可推测本次试验锆石为岩浆锆石。地球化学特征上显示含较高的Si O2含量(75%),里特曼指数δ=1.59,高Al低Ti的特点,Eu和Sr具有明显负异常,为钙碱性脉体。正长斑岩脉体的Hf同位素检测结果得到εHf(t)全部为正值,其变化范围较大(3.4~11.8),说明岩浆的主要来自亏损地幔,其中大部分集中在6~11范围内,个别εHf(t)值偏小(4),推断岩浆上侵过程中混入少量地壳物质,从而显示壳幔混合特征。在基性岩的地球化学特征研究中,稀土总量很高(∑REE=149.62×10-6~321.55×10-6),轻稀土强烈富集型,呈现出高度分异的特点,样品的大离子亲石元素和高场强元素都较MORB富集,显示铁镁质围岩具有类似OIB的特征,岩浆源区可能来源于软流圈地幔,或许与地幔柱有关。锆石具有洋壳锆石与大陆锆石的双重特征,样品13QMC-011和13QMC-013的锆石二阶段模式年龄的值,变化范围分别是913~1284Ma和749~1521Ma,为中元古代到新元古代时期。锆石的εHf(t)值变化范围分别为4个ε单位和8.4个ε单位,显示了其壳幔混合的源区特征。两期正长斑岩岩脉的εHf(t)均为正值,指示了该区从大陆裂解构造环境逐步转变为有限洋盆环境。在早古生代时期,区域上为板内裂解的构造环境,软流圈地幔上涌冷凝形成基性岩地层。在志留世末期,脉体完成侵入,将地壳深部的Au元素带入地壳表层,为地层Au的预富集以及后期剪切成矿作用提供了Au元素的物质来源。此后该区域处于洋盆环境,形成了一些沉积建造。在加里东末期,洋壳环境发展结束,扬子板块再次向北发生俯冲。在三叠纪时期,与华北板块发生碰撞。在侏罗纪时期,随着碰撞作用的增强,区内再次发生岩浆热液作用,为正长斑岩脉体中Au的活化成矿提供了热。正长斑岩脉体形成时代的厘定,确定了该区金源层形成时代,金源层是后期金成矿的首要物质源,这对矿床的形成具有重要意义。
[Abstract]:In the southern margin of the Qinling orogenic belt, there is a large area of early Paleozoic basic volcanic rocks, and the ore-bearing syenite veins of the buckmaichong gold deposit are in the NW direction of intrusive basic volcanic strata. The predecessors are doing elemental geochemistry and petrochemistry of the buckmaichong gold deposit. Studies on isotopes, inclusions and geochemistry of rare earth elements have proved that the dominant role in the metallogenic process of gold deposits is the brittle ductile shear deformation metamorphism in the Yanshanian period. A series of model years 155Ma may reflect the metallogenic age of the deposit. By studying the LA-ICP-MS U-Pb dating, element geochemistry and HF isotopes of the porphyry zircon, the intrusive age of the syenite porphyry vein has been explored. Based on the geological background characteristics of the Dabie orogenic belt, the intrusive age of the vein can be divided into two periods. The weighted mean zircon age is 444 卤5Ma.The former belongs to the early Devonian period. The latter is from Cambrian to early Ordovician. The measured Th/U ratio of zircon is concentrated in the vicinity of 1. In combination with the characteristics of rare earth, the typical ce positive anomaly is negative anomaly of EU, and the heavy rare earth is enriched. It can be inferred that the zircon in this test is magmatic zircon. The geochemical characteristics show that there is a high Sio _ 2 content of 75%, the Rittmann index 未 _ (1.59), and the characteristics of high Al and low Ti are obviously negative anomalies of EU and Sr. It is a calc-alkaline vein. The HF isotopic detection results of syenite porphyry are all positive, and the range of variation is larger, indicating that the magma mainly comes from the depleted mantle, most of which is concentrated in the range of 60.11, and the magma is mainly derived from the depleted mantle, and most of the magma is concentrated in the range of 6N11, and the magma is mainly derived from the depleted mantle. Some 蔚 Hfft values are relatively small, and it is inferred that a small amount of crustal material was mixed in the process of magmatic transgression, thus showing the characteristics of crust-mantle mixing. In the study of geochemical characteristics of basic rocks, the total amount of rare earth elements is very high (鈭，

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