滇西北甭哥金矿床岩浆作用与矿床成因研究

发布时间：2019-02-08 18:55

【摘要】：甭哥金矿床产于青藏高原东缘玉树-义敦弧南端。甭哥矿区发育超钾质正长岩有别于与俯冲相关的钙碱性中酸性岩。义敦弧岩浆带多发育斑岩型Cu矿床,甭哥金矿作为少数已知的独立Au矿床与区域上斑岩型成矿系统相区别。本文通过对甭哥矿区正长岩体及金矿床地质特征的研究,查明了岩浆起源及演化特征,约束了义敦弧内不同类型岩浆同时空共生的动力学背景;同时详细解剖了金矿床地质特征,约束了成矿流体、成矿物质来源及演化过程,建立了其成因模型。本次研究主要取得以下成果与认识:甭哥地区发育粗面岩(220.1±2.7Ma)、黑云辉石正长岩(218.4±4.1Ma)和伟晶正长岩(210.4±5.2Ma),三者为同期岩浆活动产物。正长岩岩石地球化学及Sr-Nd同位素特征表明,正长岩岩浆起源于受俯冲洋壳/流体交代的富集地幔的低程度部分熔融,同时可能混入少量软流圈地幔岩浆。正长岩浆在演化晚阶段经历高程度分离结晶作用,伟晶正长岩的显微晶洞构造是高程度结晶分异作用下形成的富CO2挥发分成矿流体出溶的地质证据。正长岩形成于俯冲带靠近克拉通一侧拉伸扩张机制下,软流圈地幔上涌并诱发受俯冲物质交代的富集地幔物质低程度部分熔融上升侵位形成。成矿阶段石英脉中流体包裹体、不同成矿阶段黄铁矿的SHRIMP微区原位硫同素和LA-ICP-MS微量元素研究表明,岩浆出溶的成矿流体为CO2-H2ONaCl体系,并将金以Au(HS)2-络合物的形式从岩浆中萃取出来。流体在从深部向上运移过程中,在低压环境下“沸腾”,发生富H2O相和富CO2相分离。在相分离过程中,CO2及H2S逃逸降低了流体总硫含量及Au的溶解度,当流体与围岩发生水-岩相互作用时,H2S与岩体中含铁矿物相互作用,导致黄铁矿的沉淀,金以包裹体的形式赋存在早阶段黄铁矿中。变形作用使早阶段黄铁矿形成了脆性压裂角砾,局部发生重结晶作用。伴随着变形过程,早阶段黄铁矿中以独立形式存在的Au-Te-Pb-Sb-Bi包裹体在变形活动中被破坏,Au及伴生元素Pb、Sb、Te和Ag发生活化迁移,并在晚期成矿流体中“均匀消化”,均一的以晶格金形式分布于晚期“泡沫状”黄铁矿中。综上所述,甭哥金矿床是与侵入岩有关的金矿床。
[Abstract]:The Bago gold deposit occurs at the southern end of Yushu-Yidun Arc on the eastern margin of the Qinghai-Xizang Plateau. The ultrapotash syenite is different from subduction-related calc-alkaline intermediate-acid rocks in the Bago ore area. Porphyry type Cu deposits are often developed in the Yidun arc magmatic belt. As a few known independent Au deposits, the Bago gold deposit is distinguished from the regional porphyry metallogenic system. Based on the study of geological characteristics of syenite and gold deposit in Bago ore area, the origin and evolution characteristics of magma are found out, which constrains the dynamic background of different types of magma in Yidun arc. At the same time, the geological characteristics of gold deposit are dissected in detail, the ore-forming fluid, ore-forming material source and evolution process are restricted, and the genetic model is established. The main achievements and understanding of this study are as follows: coarse rock (220.1 卤2.7Ma), gabbro syenite (218.4 卤4.1Ma) and pegmatite syenite (210.4 卤5.2Ma) developed in Bago area, which are the products of magmatic activity in the same period. The petrogeochemistry and Sr-Nd isotopic characteristics of the syenite indicate that the syenite magma originated from the low-degree partial melting of the enriched mantle derived from subduction oceanic crust / fluid metasomatism and may be mixed with a small amount of asthenospheric mantle magma. The microcrystalline cave structure of pegmatite syenite is the geological evidence of rich CO2 volatilization formed by high degree of crystallization differentiation into ore fluid dissolution. The syenite formed in the extensional and spreading mechanism of the subduction zone near the craton side and the asthenospheric mantle upwelling induced the low-degree partial melting upwelling emplacement of the enriched mantle material which was metasomatized by the subduction material. Fluid inclusions in quartz veins in metallogenic stage, in situ sulfur isotin and LA-ICP-MS trace elements in SHRIMP microregion of pyrite in different metallogenic stages indicate that the ore-forming fluid dissolved in magma is CO2-H2ONaCl system. Gold is extracted from magma in the form of Au (HS) 2-complex. During the upward migration of fluids from deep to deep, the separation of rich H 2O phase and rich CO2 phase occurs at low pressure. In the process of phase separation, CO2 and H _ 2S escape reduced the total sulfur content of fluid and the solubility of Au. When water-rock interaction occurs between fluid and surrounding rock, H _ 2S interacts with iron-bearing minerals in rock mass, resulting in the precipitation of pyrite. Gold occurs in early stage pyrite in the form of inclusions. Deformation resulted in brittle fracturing boulders and local recrystallization of pyrite in early stage. Along with the deformation process, the Au-Te-Pb-Sb-Bi inclusions in the early stage of pyrite in independent form were destroyed in the deformation activities, and the Au and associated elements Pb,Sb,Te and Ag were activated and migrated. In the late ore-forming fluid, it is "evenly digested" and distributed uniformly in the late "foam" pyrite in the form of lattice gold. To sum up, the Banguo gold deposit is a gold deposit related to intrusive rocks.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P618.51

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