Evolution Of Cenozoic Alkaline-rich Magma And Mantle Fluid From Western Yunnan Province And The Related Metallogenic Effect

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.