| The tectonic-magma-fluid activity in Sanjiang Area is very strong and frequentwith a long-term activity. In the late Paleozoic, regional tectonic movements aremainly tension and faulting; from the late Hercynian to Indosinian, tectonicmovements are extrusion-based activities; in the period of Yanshanian, tectonicmovements change from extrusion to tension; and in the time of Himalayan, tectonicmovements show mainly thrusting movements. Entering the collision orogenic stage,it can be divided into the main collision period (65~41Ma), the late collision period(40~26Ma) and the later the collision period (25~0Ma). Especially in the late collisionperiod, mineralization is very strong and concentrates on the structural transformationzone of the eastern edge of Tibet plateau. Mineralization peak is mainly between30~40Ma. Among the four major mineralization events related to these collisions,shear zone type Au mineralization events associated with large shear system form afamous Ailaoshan large gold belt. Among them, Laowangzhai gold mine develops inintracontinental transformation orogenic environment, is controlled strictly bystrike-slip-nappe-shearing action and uniform deep geological process, and has theclose relationship with the activity of magma and fluid derived from mantle andcrust-mantle caused by asthenosphere upwelling.Cenozoic Laowangzhai gold deposit is one of large scale gold deposits onAilaoshan gold mineralization belt, develops in the joining parts between Indian plateand Yangtze plate, and lies in extreme unstable transitional tectonic unit and deep faulttectonic strong activity zone. This zone is not only beneficial totectonic-magmatic-deep fluid activities, providing ore-forming material, but also isgood for building a good ore-bearing environment, which is conducive tomineralization. Based on researches carried out, this paper is focusing on further studyand discussion for cenozoic Laowangzhai gold deposit by modern metallogenictheories, analytical techniques and methods. It puts forward the model of crust-mantlematerial mixing and overlapping mineralization initiated and promoted by the effctand evolution of mantle fluid. The main research achievements are following:(1)According to the petrographical identification for surrounding rock,mineralized rock, mineral and vein body, it suggests that with various alterations (silicification, carbonatizations, sulfidization, silica fume petrochemical anduralitization) and multiphase alteration overlapping, there is black opaque materialdevelopping along cleavage crack, fracture and intergranular part of minerals in rockand ore. By the component analysis of energy spectrum and electron probe and theabservation of scanning electron microscope, it shows the black opaque material ismainly an ultramicroscopic cryptocrystalline aggregate material which is composed ofsilicate, quartz, sulfide and carbonate uneven mixed melting or separately dissolvedand is rich in scheelite, rutile and specularite (magnetite). These differentmicrocrystalline minerals show the coexistence of precipitated eutectic structure andliquation intergrowth relationship. The ultramicroscopic petrographical featurereflects along with the changes of environmental physical-chemical conditions anddepth in the process of mantle fluid, the properties of fluids evolve from magma fluidto supercritical fluid, and then to the crustal hydrothermal fluid. It is the effect andevolution of mantle fluid which causes the mixing and overlapping of crust andmantle materials and then promote the mineralization. So the black opaque material asa microscopic tracer for the effect and evolution of mantle fluid is an importantgeochemistry power and material source which promotes the gold mineralizing in thedeep geological process.(2)The water in ore-forming fluid principally has the nature of magmatic water;the ratio of3He/4He in ore-forming fluid inclusions is between0.1608~3.47Ra whichlies somewhere between the eigenvalues of crust and mantle; the ratios of20Ne/22Neand21Ne/22Ne are respectively in9.816~12.64and0.0267~0.0481wich is close to themantle ratio; the ratios of40Ar/36Ar and38Ar/36Ar respectively change in307.5~681.7and0.1769~0.2132which accordingly lie between the ratios of AIR and MORE.Compared from the ratios of the atmosphere,128~136Xe/130Xe of samples all showssurplus characteristics, and especially the excess of129Xe/130Xe and136Xe/130Xereveals that there is probably mantle-derived Xe participating or joining inore-forming fluid. The ore-forming fluid is rich in Cl-, HCO3-, CO2, CH4, S2-and HS-;ore-containing fluid overall shows alkalinity; and C and S isotopic composition ofdifferent mineral has the typical characters of mantle source. These analysis resultsshow that auxiliary agent and mineralizing agent of gold in the orefield are mainlyCO2, CH4, HCO3-, Cl-, S2-and HS-, the ore-forming material and fluid come mainlyfrom deep mantle. In the mineralized alteration process, the effect and evolution ofmantle fluid cause crust-mantle material mixing and overlapping, and are thenfavorable for mineralization. (3)Metallogenic fluid inclusions have mainly NaCl-H2O and CO2-H2O types.Hydrothermal solution in the metallogenic stage belongs to the fluid with mid-lowtemperature (140.0~260.0℃), low salinity (4.650~4.495﹪) and mid-low density(0.918~0.926g/cm3). Gold in the ore-forming fluid migrates mainly in AuS-, Au(HS)2-,AuAsS32-and AuSbS32form in relative reductive environment. And then under thecondition of the reduction of environmental temperature, pressure, salinity, PH value,oxygen fugacity and sulfur fugacity, it parses and happens to mineralization, andthereby forms the gold deposit.(4)On Ailaoshan gold ore belt in Western Yunnan, the ore-forming time(20~50Ma) of Laowangzhai gold deposit and the diagenetic age (28~43Ma) oflamprophyre in very close coexistence with gold mineralization are both concentratedon the Himalayas period. At this time with the emplacement of a large number ofalkali-rich intrusive rocks and lamprophyre magma derived from mantle, theAilaoshan fault belt develops tension tectonic movement. This indicates that theformation of the gold deposit is related to the extensional tectonic movement, theactivity of mantle-derived magma and the effect of mantle fluid which happen on theHimalayas period in the orefield.(5)The bringing in and bringing out of major elements in rocks and ores mainlyshows the increase of silication and alkalis in the mineralized alteration process.Different rocks and ores are obviously rich in LILE, HFSE and LREE, and at thesame time have the characteristics with relative high ratio of87Sr/86Sr（>0.7050）, lowratio of143Nd/144Nd（<0.512638） and high ratio of206Pb/204Pb（17.8~19.1）as wellas the characteristics with reverse evolution of Ce and Eu abnormity. This is not onlya reflection of enriched mantle source area itself but also an important symbol ofcrust-mantle material mixing and overlapping mineralization. The value of δ30Si insilicon-rich metallogenic fluid and native metallogenic silicified quartz is between-2.4‰~-0.1‰, the value of δ30Si in mineralized alteration porphyry, granite porphyry,lamprophyre and alkali-rich porphyry changes between0.4‰~0.0‰, and the value(0.0‰~0.4‰) of δ30Si in lamprophyre and alkali-rich porphyry is consistent with thevalue (0.3‰~0.5‰) of δ30Si in mineralized alteration porphyry and mineralizedalteration sandstone, but the δ30Si value of the latter is bigger. s of silicon isotopedynamics, it can be speculated that silicon-rich ore-forming fluid may be originatedfrom the primitive mantle and alkali-rich magma may be originated from metasomaticenriched mantle. And alkali-rich porphyry may be come under metasomatism ofsilicon-rich ore-fo rmingfluid to a certain extent. With silicon-rich ore-forming fluid migrating from porphyry towards surrounding rock, the properties of fluidaccordingly transfer from mantle source to crust source.(6)The formation of Cenozoic Laowangzhai gold deposit is closely related todeep geological process and the gold deposit is the result of crust-mantle materialmixing and overlapping mineralization caused and promoted by the effect of mantlefluid. Comprehensive study suggests that oblique convergence and oppositesubduction of Indian continent and Yangze massif result in the development oflarge-scale strike-slip fault, thrust nappe，shearing action and the effect of deepmantle-derived fluid in Jinsha River and Ailao Mountains. In this process, theprimitive mantle gradually transits and evolves to metasomatic enriched mantle, andpartial melting occurs at the same time. This also forms silicon unsaturated ultrabasic,basic and alkaline magma and ore-bearing mantle fluid with magma property whichare mixing but not mixed melting and constitute multiphase magma. Meanwhile, thebottom invasion of mantle-derived multiphase magma leads to hotspot rising andinduces deep crust partial melting, which makes silicon saturated felsic magma.Furthermore, it produces dissolved commixture with mantle-derived multiphasemagma of bottom invasion. At last, they further constitute multiphase magma ofmantle and crust mixed source. While accompanying with multiphase magma risingemplacement and crystallizing diagenesis along deep fracture, ore-bearing mantlefluid forms ultrabasic and basic magmatic rocks, lamprophyre, syenite, quartzporphyry and granite porphyry. With the depth, temperature, pressure and oxygenfugacity of environment changing, the properties of fluid evolve from magma to crusthydrothermal, which leads to the coexistence of liquation intergrowth structure andprecipitated eutectic structure and the association of sulfate and sulfide in blackopaque material. At the same time, ore-bearing mantle fluid as a kind of fluid throughmagma can carry out synchronously metasomatizing different crystalline rock derivedfrom mixing and alteration into mine. It can be separated from magma and rock massunder the conditions of having channels as well and then enter the secondary fracturesor favorable interface where gold ore body lies. It interacts with surround rock strataand other rock mass constantly, which inevitably promotes crust-mantle material andenergy strongly exchanging, mixing and overlapping mineralization, and therebyforms the large-super scale gold deposit. |
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