The Relationship Of Magma And Mineralization In Chaobuleng Fe Polymetallic Deposit,Inner Mongolia

The Chaobuleng Fe polymetallic deposit locates at the eastern part of Li Zishan-DongUjimqin Banner metallogenic belt, Inner Mongolia. Mineralization and alteration occured alongthe contact zone between pluton and wall-rock. In wall rock, prograde skarn with agarnet+pyroxene+wollastonite±calcite assemblage or hornfels with a hornblend+biotite+plagioclase+quartz assemblage was produced by metasomatic alteration of marble ormud-sandstone. Sequencely, the Fe-oxide, consisting of specularite+maghemite+magnetite,with an epidote+calcite±hornblend±biotite halo, was formed by the retrograde alteration ofskarn or hornfels; An extend sulfide mineralization occurred during the latest retrogradealteration, with a chalcopyrite+pyrite+arsenopyrite+molybdenite+bismuthinite+fluorite+quartz+calcite assemblage, and overprinted the early mineralization or alteration; In the edge of pluton,potassic veins with a k-feldspar+biotite±garnet assemblage occurred during metasomaticalteration of granite，and retrograde epidote occurred. Sequencely, the sulfide vein with achlorite halo occurred and was superimposed on the former alteration. At last, an extentargillization ended up hydrothermal activity. The P-T condition indicated that this depositformed under a lower deep （1.7-3.5km）, high temperature environment with a complexhydrothermal-cooling trace（＞500-＜200℃）. The geology, paragenesis and P-T conditionsuggest that the Chaobuleng Fe polymetallic deposit was compatible with a shallow, proximalskarn deposit.Chaobuleng pluton can be divided into three granitic units, less porphyritic syeno-granite,containing enclaves more porphyritic syeno-granite,and more porphyritic syeno-granite.Thezircon U-Pb age of two samples of less porphyritic syeno-granite are138.1±1.1Ma（MSWD=9.4），140.6±1.1Ma（MSWD=4.2）；The zircon U-Pb age of two samples ofcontaining enclaves porphyritic syenite granite are138.6±0.91Ma（MSWD=4.9），137.4±1.2Ma（MSWD=5.6）；the U-Pb age of the enclave is138±1.8Ma （MSWD=6.7）; Moreporphyritic syenite granite, four samples of the zircon U-Pb age are133.9±1.2Ma （MSWD=7.8）,134.8±0.94Ma The （MSWD=5.3）,135.98±0.83Ma The （MSWD=3.6）.The Re-Os model age of five molybdenites the outside contact zone of the deposit are139.0±2.0Ma,140.9±2.1Ma,139.2±1.9Ma,140.5±2.0Ma, their time line age is140.7±1.8Ma （MSWD=1.12）; The Re-Os model age of the inner contact zone of two molybdenites are 135.0±2.1Ma,131.2±4.1Ma.The U-Pb ages of Zircon indicate that the time of magmatic activitywas near17Ma, Re-Os ages of molybdenite show that the mineralized hydrothermal activitytime was about9Ma, the Chaobuleng deposit formed in the long-term activities of themagma-hydrothermal system, similar to many magmatic hydrothermal mineralization systems inthe world. But compared to the geological features, the mineralization and the age of theChaobuleng deposit, the main mineralization stage of Chaobuleng deposit（iron oxide stage）occured in the early stage of magmatism, consistent with the time of less porphyritic syenogranite emplacement.The formation of Chaobuleng pluton was in the setting of post orogenic extension,belonging to the high temperature A-type granites, the Zr saturation temperature was800-900°C.The evolution of three granitic units is difficult to explain with the fractional crystallization ofthe same magma in the magma chamber, but the mixing of granitic magma （SiO₂=77%） andpartial mafic magma（SiO₂<63%, the diorite magma?） in the magma chamber can easily explainthe intrusive sequence, the whole rock chemical composition, plagioclase mineral compositionand the unbalanced texture within the granitic units. In the process of magmatism, the evolutionof each granitic unit of the magma was toward the low temperature and pressure, the eutectictemperature was as low as720-770°C, the pressure was0.5¹kbar. The chemical composition ofbiotite shows that the magma forming Chaobuleng pluton enriched in F-component （1.5-3.5%）.Combination of magma-mineralization age, the main mineralization stage, and magmatism inthe process of partial mafic magma injected into the granitic magma, these processes controlledthe mixing and evolution of magma units, and mineralization process.The occurrence of Fluorite mineralogy in the deposits indicates that ore-forming fluidenriched in F-component. The biotite in rich-F and the exsolved texture of fluid in Chaobulenggranite indicated that the ore-forming fluids was from F-rich granitic magma. There arecoexisting phases: the melt, magmatic-hydrothermal fluid and quartz. Such three phaseequilibrium is a good explanation for the resorption texture of quartz. Due to loss of volatile,residual melt was quickly quenched and crystallined into fine-grained matrix.