摘要
金沙江—哀牢山富碱侵入岩带是我国西部的一条呈北西向延伸、规模宏大的新生代钾质火成岩带。已有研究指出,青藏高原之下富含深部(源)流体;根据稀有气体同位素的研究,证明青藏高原的主体部分受到来自软流圈物质流的顶托,其中地幔流体作用不可避免。自晚新生代以来,金沙江—哀牢山缝合带受印度—亚欧板块碰撞和与此相伴的青藏高原整体快速抬升的影响,断裂由挤压转为拉张,显示具裂谷特点的台缘坳陷,出现断陷盆地,并伴随地幔上拱和岩浆喷发,尤其是富碱岩浆和地幔流体沿深大断裂带上侵,由此发生的构造—岩浆活动及深部地质过程,为在缝合带及邻近地区广泛发育富碱侵入岩和其中深浅来源不同的各类岩石包体,及与之相关的多金属矿床的形成提供了有利的地质—构造背景条件。本文通过深入研究以六合岩体为代表的富碱岩浆和地幔流体作用,创造性应用透岩浆流体和地幔流体成矿理论,重点分析解剖以马厂箐大—中型Mo-Cu-Au矿床、金顶超大型Pb-Zn矿床为典型代表所揭示的深部过程与流体作用的系列成矿效应,综合阐述富碱岩浆的形成和运移—含矿地幔流体作用与演化—壳幔混染叠加成矿三者之间的内在联系。取得的主要成果如下:1.根据六合岩体霓辉正长斑岩及其深源包体的岩相学鉴定发现,伴随交代蚀变,寄主岩石和各类包体中普遍发育沿粒间和矿物裂隙或解理缝贯入或穿插的呈网状和细脉浸染状分布的富铁微晶玻璃,及其以独立岩石形式产出的富铁熔浆包体,进一步利用电子探针、扫描电镜和能谱分析确认,其物质组成以超微晶硅酸盐和石英为主,含有碳硅石、含铬自然铁等地幔标型矿物,其中超微晶金属和非金属矿物之间呈熔离结构交生。综合研究认为,富铁微晶玻璃和富铁熔浆包体反映了地幔流体的熔浆性质及其与富碱岩浆不混溶的特征,是地幔流体作用现实微观踪迹物质的两种表现形式;该物质作用于寄主岩石和深源包体,引发各种交代蚀变作用,如角闪石化、硅化和绿泥石化等,并导致结晶蚀变矿物组合总体上表现为暗色矿物由辉石→角闪石→黑云母→绿泥石的退变序列。2.富碱斑岩和包体及其典型矿床岩(矿)石的地球化学特征表现出极大的相似性。其稀土和微量元素配分模式基本一致,即表现为LREE富集、具无或弱的负Ce和负Eu异常;蛛网图呈大致类似的“驼峰”型,总体表现大离子亲石元素相对富集,且具明显的Ta-Nb-Ti负异常而显示受到古俯冲带流体交代特点。类似的Sr、Nd、Pb同位素组成显示源区为EMⅡ型富集地幔,并与含石榴石相地幔岩石的低程度部分熔融有关。由此表明,滇西新生代富碱斑岩和包体的形成及其相关多金属系列成矿统一受控于深部地质过程与地幔流体作用。3.锆石U-Pb定年显示,六合岩体中花岗岩包体成岩年龄(39.2±2Ma)与主岩(霓辉正长斑岩)和马厂箐多金属矿床中赋矿斑状花岗岩年龄(36.17±0.36Ma)基本一致。基底变质锆石的206Pb/238U平均加权年龄为108.4±4.4Ma,与角闪石化金云石榴透辉岩中角闪石Ar-Ar年龄102.87±1.19Ma基本吻合,代表地幔流体交代矿物的结晶年龄,表明地幔流体交代作用过程可能在白垩纪前后延续一个相当长的时期。4.结合富铁熔浆包体和不同部位富铁微晶玻璃及石英包晶中发现高钾富硅碱玻璃包裹体和矿物流体包裹体中出现的不混溶相态,以及稀有气体和铅同位素显示富硅碱质流体来自与富碱岩浆同源的EMⅡ型富集地幔等的综合研究表明,富碱侵入岩中硅不饱和与硅过饱和两类岩石的成因联系和岩浆起源及其演化关系表现为:前者直接源于富集地幔的富碱岩浆的结晶产物,后者源于由富碱岩浆的底侵作用和与之伴随的地幔流体作用引发地壳深熔形成长英质岩浆,再与幔源岩浆和地幔流体一定程度混合形成的富碱长英质岩浆的结晶产物。5.从区域上看,与富碱岩浆共同运移,并与之互不混溶的地幔流体作用及其演化表现为,由六合岩体→马厂箐岩体→金顶矿床,其流体的超微观物质组成由以含铬自然铁、碳硅石等地幔标型矿物为特征→以富铁硅酸盐矿物为主→硫化物、碳酸盐、硅酸盐及黄铁矿与石膏伴生为标志,表现伴随其参与成矿过程中,引发交代蚀变并导致壳幔混染,流体属性由熔浆流体→超临界流体→液相流体转化,也正是这一流体作用和演化过程,促进了滇西地区新生代不同矿种在不同部位、不同围岩和温压条件下伴随不同程度壳幔混染叠加成矿。6.应用并引伸透岩浆流体成矿理论和地幔流体作用释义,可以认为:本文论证的地幔流体微观踪迹物质(富铁微晶玻璃和富铁熔浆包体)与富碱(长英质)岩浆共存,并共同运移,但两者由于组成和性质的差异而互不混溶;当富碱(长英质)岩浆和地幔流体系统封闭较好,地幔流体则伴随岩浆的结晶过程对富碱岩石进行同步自交代蚀变,在岩体内或其深部形成矿床,构成正岩浆成矿类型,如马厂箐斑岩型Mo矿;若在此成岩成矿过程中发生构造扰动,则地幔流体进入岩体与围岩接触带或紧邻接触带的地层围岩中进行交代蚀变成矿,构成接触带成矿类型,如马厂箐矽卡岩型Cu(Mo)矿和地层围岩中的构造破碎蚀变岩型金矿。若岩浆和流体运移的深大断裂体系发育,环境处于相对开放,则地幔流体伴随富碱岩浆的成岩过程而脱离岩浆沿分支断裂通道或拆离滑脱带进入远离岩体的不同地层岩石中,伴随与地壳岩石相互作用而引发交代蚀变,进而导致壳幔混染叠加成矿,构成远程热液成矿类型,如产出于沉积碎屑岩系中的金顶超大型铅锌矿床。
Alkaline-rich intrusive belt along the Jinshajiang-Ailaoshan is one of the largestCenozoic potassium igneous rocks belts which is stretching from northwest tosoutheast in Western China. It has been pointed out that lower Tibetan is rich in deepfluids. And the3He/4He shows that the main body of Tibetan Plateau is uplifted by theasthenospheric material flow which includes mantle fluid. Since Cenozoic era, as aresult of the Indian-Eurasian plate collision and uplift, the nature of faults istransformed from extrusion to pull-apart and the platform edge is depression whichinduces the formation of faulted basins. And thus the tectonic-magma active and deepgeologic process provide a favorable condition to the mantle upwarp and magmaeruption, especially alkaline-rich melt and mantle fluid moving along the deep faults.Based on the studies of alkaline-rich intrusive rock from Liuhe County, YunnanProvince and metallogenic theory on trans-magma and mantle fluid, the main attentionis put on the Machangqing large-middle Mo-Cu-Au deposit and Jinding super-largePb-Zn deposit in order to reveal the metallogenic effect in deep geologic process andthe relationship among the formation and transportation of alkaline-magma, evolutionof ore-forming mantle fluid and crust-mantle overlapping mineralization. The researchfruits are listed as followed:1.Petrologic studies of the aegirine syenite porphyry and deep-sourced xenolithssuggest the existence of Fe-rich glass. The Fe-rich glass develops in the space betweenmineral grains and along the cleavages. The electron microprobe, scanning electronmicroscope and energy spectrum confirm the Fe-rich melt xenoliths are mainlycomposed of ultra-microlite silicates and quartz, as well as moissanite and Cr-bearing native iron. The moissanite and Cr-bearing native iron are considered to be mantlesourced, while the ultra-microlite metal and non-metal substances shows unmxingtexture. The Fe-rich glass and Fe-rich melt xenoliths are the products of meltcharacteristic of mantle fluid and unmixing features of alkaline-rich magma, as well asthe two manifestations of microscopic traces of mantle fluid. As a result of theinteractions between mantle fluid and host rocks, amphinolization, silication andchloritization developed as well as the degrading succession of dark minerals frompyroxene→amphibole→biotite→chlorite.2.Geochemical similarity among alkaline-rich magma, deep xenoliths and typicalpre deposits is noticeable. REE patterns and spider diagram of trace elements are almostthe same. LREE are enriched, while the negative Ce and Eu abnormalities are notsignificant. The spider diagram of trace elements is like a “camel back”. Large ionlithophile elements (LILE) are relatively enriched, while the depletion of Ta-Nb-Tisuggests the fluid metasomatism from palaeo-subduction zone. Sr, Nd and Pb isotopegeochemistry compositions show that the mantle fluid origins from EM Ⅱmantle whichis related to the low degree partial melt of the garnet mantle rocks. The formation ofalkaline-rich intrusive rocks and related deep xenolths, as well as the metal ore depositsare controlled by the deep geologic process and mantle fluid.3.Geochronologic studies of zircon U-Pb reveal that granite xenoliths from Liuheaegirine syenite porphyry formed at39.2±2Ma, which is similar to their host rocks andore-bearing porphyritic granites (36.17±0.36Ma) from Machangqing large-middleMo-Cu-Au deposit.206Pb/238U weighted average age of metamorphic zircon frombasement is108.4±4.4Ma, which is close to the amphibole Ar-Ar ages (102.87±1.19Ma)of amphinolized phlogopite-garnet canaanite.206Pb/238U weighted average age ofmetamorphic zircon and amphibole Ar-Ar age of amphinolized phlogopite-garnetcanaanite stand for the crystallized time of mantle fluid metasomatic minerals andimply that mantle fluid is active since Cretaceous Era.4. Combined with the noble gas isotope and Pb isotope of alkaline-rich intrusiverocks, immiscible phase of high potassium silicon-alkaline-rich glass inclusions andfluid inclusions from quartz peritecticum reveal that the silicon-alkaline-rich fluidorigins from EMⅡ mantle. Alkaline-rich intrusive rocks can be divided into two types,the one is silicon-undersaturation type, and the other one is silicon-oversaturation type.The former is the direct product of alkaline-rich magma derived from enriched mantle;the latter is the hybrid product of alkaline-rich magma and felsic magma. Thealkaline-rich magma under-plating leads to the crustal anatexis, during which process, the felsic magma formed.5.Judging from the regional geologic backgrounds, from Liuhe aegirine syeniteporphyry→Machangqing large-middle Mo-Cu-Au deposit→Jinding super-large Pb-Zndeposit, compositions of mantle fluid change from ultra-microlite glass containingmoissanite and Cr-bearing native iron→Fe-rich silicates→sulfides, carbonates,silicates, and pyrite and gypsum. The composition changes reveal the nature of mantlefluid varies during the evolution process, from melt→supercriticalfluid→hydrothermal fluid. No other than this mantle fluid process induces theformation of Cenozoic mineralization in Western Yunnan Province.6.Applied and explicated the metallogenic theory on trans-magma and mantlefluid, it is believed that microscopic traces of mantle fluid (Fe-rich glass and Fe-richmelt xenoliths) and alkaline-rich magma (felsic magma) move synchronously, howeverunmixing due to the difference of compositions and nature. When the fluid and magmais well trapped, the alkali-rich porphyry is synchronically self-replaced and alterated bythe mantle fluid in the process of alakali-rich magma crystallizztion, and theotrthomagmatic deposit formed in magma body or its deep-seat, and the typical depositis Machangqing porphyry Mo-deposit; if tectonic perturbation happens, the mantlefulid would enter into the contact zone between magmatic rocks and wall rocks or strataclose to the contact zone, leads the formation of contact-metasomatic type deposit.The typical deposit is Machangqing porphyry Cu-deposit, which exists in skarn-marblezone, while porphyry Au-deposit exists mainly in atrsta rocks; if the deep faults are welldeveloped, and environment is relatively open, the ore-forming mantle fluid will flowfar from alkali-rich magma along the branch fractures and enter into different strata androcks, just as epithermal deposit form, which is Jinding supper-large Pb-Zn deposit.