【作者】 彭惠娟；

【导师】 汪雄武；

【作者基本信息】 成都理工大学 ， 矿物学、岩石学、矿床学， 2011， 硕士

【摘要】 西藏甲玛铜多金属矿床位于冈底斯-火山岩浆弧东段,是冈底斯成矿带的重要组成部分。甲玛矿床为最具代表性的产于后碰撞伸展环境中的超大型斑岩-矽卡岩-浅成低温热液铜多金属矿床。矿床主要出露下白垩统林布宗组沙板岩和上侏罗统多底沟组灰岩,矿化发生于岩体与上侏罗统多底沟组接触带附近,以及和岩浆热液导通的多底沟组和林布宗组界线的层间破碎带。矿床中酸性斑岩的类型主要有花岗斑岩、花岗闪长斑岩、二长花岗斑岩、（石英）闪长玢岩。本文通过对甲玛铜多金属矿床矿物学特征的精细解剖,厘定了岩浆演化过程,查明了岩浆的温压变化,提出了岩浆混合作用的地球动力学背景,建立了岩浆混合模式;发现了斑岩岩浆-热液过渡阶段的证据,约束了初始出溶流体的性质及出溶时的物理化学条件;详细识别了热液演化过程中的地质记录,再塑了成矿流体演化过程;讨论了成矿物质来源、金属分离作用以及成矿物质沉淀的机制。研究表明,阴极发光是研究石英显微结构的有效技术方法。根据石英在阴极射线下所显示的生长结构差异,矿区斑岩体划分出3个世代,6种类型的石英斑晶。石英斑晶指示了两种主要的“不平衡”反应结构:石英斑晶重熔表面和石英钾长石外壳。石英斑晶显微生长结构表明原始岩浆经历过两次铁镁质岩浆混合作用。根据石英斑晶中Ti含量变化,得出在两次溶蚀前后,石英结晶温度分别增加了约110℃和80℃。此外斜长石斑晶反环带及其Ba、Sr、Fe等元素的浓度梯度、具镶边的眼球状石英斑晶、钾长石巨晶环斑结构、黑云母筛状结构等都有效的证明了铁镁质岩浆与长英质岩浆混合作用的存在。根据这些研究结果,初步构建了矿区岩浆混合作用模型并推测了岩浆混合过程。16Ma左右,岩石圈地幔拆沉,软流圈物质上涌导致正常下地壳部分熔融,产生含Mo的钾质岩浆。原始岩浆房内形成第I世代自形的高温石英晶体核。软流圈上涌,诱发含大量地幔组分的新生镁铁质下地壳部分熔融,产生含Cu、富水、高fO₂的埃达克质岩浆熔体,并与含Mo的长英质岩浆发生了第一次岩浆混合作用。在此过程中,早期石英核部溶蚀,形成高Ti的溶蚀表面。这些高fO₂的混合岩浆在浅部地壳发育大型岩浆房,并排泄出含Cu、Mo岩浆流体。在岩浆房中,石英晶体形成了均匀的生长环带和第II世代的石英斑晶核。地幔减薄和岩石圈拆沉直接引起地壳强烈伸展,形成垂直碰撞带的正断层系统和裂谷,使岩浆房内部压力急剧减小,岩浆快速侵位。由于岩浆房的突然腾空,诱发地幔物质上涌,造成了第二次基性岩浆的混入,造成第I世代和第II世代的石英斑晶边缘溶蚀。正是由于两次基性岩浆的加入,为成矿提供了大量的Cu、S。它们是形成甲玛超大型铜多金属矿床不可或缺的因素。矿床斑岩体矿物显微结构保存了许多岩浆-热液过渡阶段的信息,指示了成矿岩浆曾分离出大量岩浆流体。这些信息包括:石英斑晶具有强烈的溶蚀结构,蠕虫状石英斑晶中溶蚀湾切断石英第一阶段的生长环带;显微空腔将豆荚状或糖状结构石英与破布状黑云母（较少磷灰石,磁铁矿）,长石等相连接;斑岩中伴生萤石的存在,黑云母和角闪石中的高含量F,与矿化有关的岩浆含水约8wt.%,含氟1 wt.%。;斑岩体中流体包裹体与富挥发份熔融包裹体共存。岩浆挥发份作为主要的流体相被挤出岩浆并以气泡的形式聚集在岩浆房外壳顶部,并使已固结的矿物发生自交代。随着易挥发组分的增多,使气体压力大于束缚压力,导致岩浆外壳和邻近围岩突然破裂。岩浆流体沿着围岩裂隙流出并以热液的形式交代围岩。同时,由于减压造成岩浆热液的二次沸腾,使成矿元素沉淀。通过对甲玛岩体样品中脉体特征,石英显微结构以及微量元素含量的研究,划分出3种大类,5个期次的岩浆-热液流体。其中JMi无矿石英脉为岩浆-热液过渡阶段形成的P脉。金属硫化物主要沉淀于JMii和JMiv阶段。成矿流体生命周期长达5.2Ma,开始于岩浆房中的岩浆-热液流体的出溶,一直演化至成矿作用结束。含矿流体具有多期多阶段性,贯穿于整个岩浆-热液成矿系统。流体温度范围跨度极大,从823℃到130℃,即从斑岩型高温岩浆流体演化为浅成低温热液型流体。脉体石英中Al,Fe,K含量的变化指示了流体pH值的变化以及与钾化有关的蚀变特征。综上所述,甲玛矿床成矿流体中的金属元素和S来源于岩浆混合作用的幔源铁镁质岩浆;金属元素的分离得益于岩浆-热液过渡阶段富F流体的出溶。这一过程反复发生于岩浆房内部,并形成大量含矿流体。含矿流体中金属元素沉淀得益于岩浆流体的二次沸腾和酸性流体的中和作用。更多还原

【Abstract】 Jiama polymetallic copper deposit locates in east of Gangdese volcano - magmatic arc,which is an important component of the Gangdise metallogenic belt. Jiama deposit is the most presentative super-mall porphyry-skarn-epithermal polymetallic copper deposit, which is relating to post-collision.The stratums of deposit are mainly Limbuzong group of the lower Cretaceous and duodigou group of the upper Jurassic. The Ore body is located between the two stratums as like-layered. The main types of medium-acid porphyry in Jiama deposit are granite porphyry, granodiorite porphyry, monzogranite porphyry and （quartz） diorite porphyry.Through the intensive search for the minerals of Jiama polymetallic copper deposit, This paper has identified the magmatic evolution and the P-T alteration of magmatic, and proved the earth dynamics of the magma mixing, established.the process of magma mixing.We also have found the evidences of magmatic to hydrothermal transition, controlled over the physics and chemistry conditions of the exsolution fluid. Forthemove, we have dentified the geological records of hydrothermal evolutionary modification, established the evolutionary modification of ore-forming fluid. At last, we have discussed the processes of the origin, transport and precipitation of ore-forming matter.The results show that,Cathodoluminescence is an effective technical method on studying microstructure of quartz. According to the difference of growth patterns in quartz CL, there are three generations and six types of quartz phenocrysts which form the Jiama Porphyry Copper Deposit in Tibet . The microscropic structure of quartz phenocrysts indicate that primary magma have gone through two times of magma mixing events. After two resorption,the recrystallization temperature of quartz phenocrysts were raised by 110℃and 80℃respectively, according to the content changing of Ti in zoned quartz. In addition, element concentration steps in feldspar phenocrysts （ Ba, Sr, Fe）, plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the porphyry in the Jiama Cu-polymetalic deposits. Based on the result, the process of magma mixing has been established. At 16Ma, as a result of asthenosphere up swelling and lithosphere dismantling and subsiding, lower crust remelted and formed potassic magma containing Mo, forming primary magmatic chamber in where the first generation quartz crystals. During the post-collisional crustal extension period , shallow emplacement and fluid exsolution of the newly-born adakitic magma , resulting from the lower crust and rich in metals , water and high fO₂ , and mixed with primary magma which contain Mo , formed the shallow crustal magma chamber. Owing to the first mixing with mafic magma, resorption surfaces appeared around the core of quartz phenocrysts. And then, the quartz has experienced stable growth conditions, forming a steady growth zone with low CL contrasts. Gangdise orogen belts was in a extensional condition. At these condition, amount of normal faults have be made. The pressure of magma reservoir has reduced rapidiy, magmatic reemplacemented. Owning to the suddenly empty of magma chamber, the second magma mixing occurred.Information about the magmatic to hydrothermal transition is preserved in porphyry of Jiama copper-polymetallic ore deposit. Include: 1) quartz phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz cathodoluminescence banding. 2) Pods of saccharoidal quartz are connected by graphic quartz–alkali feldspar intergrowths and ragged biotite. 3) Fluorite as an accessory mineral in igneous rocks, high F in hydroxyl sites in igneous biotite and amphibole. 4) Fluid inclusions and silicate melt inclusions are present in the porphyry of Jiama. We present detailed petrographic observations of primary igneous features, combined with silicate-melt and fluid inclusion studies, and found physical models of exsolution. Metallic elements are preferentially partitioned into the associated magmatic volatile phase.As the volatile concentration increase, the vapor pressure leads to sudden failure of the carapace and adjacent wallrock– this occurs once the vapor pressure is greater than the confining pressure. Magmatic fluids escape as cracking of the wallrock continues; these fluids then hydrothermally alter the rock they pass through. At the same time, caused by the rapid decreased in pressure, hypersaline fluid was boiling again, and it have caused Cu-Mo sulfides to deposit from the fluid.The combination of scanning electron microscope–cathodoluminescence （CL）, and LA-ICP-MS microanalysis of Al, Ti, K and Fe in vein quartz has yielded results permitting a greater understanding of the complex mineralisation of the JiaMa porphyry-style deposits, These data demonstrate the relationship between quartz precipitation, dissolution and ore deposition as the mineralising fluid chemistry changed through time. Five major quartz generations are identified. Using the Ti-in-quartz geothermometer and fluid inclusion analysis, crystallisation temperatures for JMi to JMv is between 823°C and 130°C. The CL and trace element signatures of the JMi to JMv stages of the porphyries show similar features to those observed in porphyry-style deposits from other regions. This suggests that a common sequence of quartz crystallisation occurs during the formation of early veins in many porphyry copper systems.更多还原