Research On Characteristics Of Geology, Geochemistry And Metallogenic Mechanism Of The Jurassic Molybdenum Deposits In The Mid-East Area Of Jilin

The Mid-East area of Jilin province is an important polymetallic metallogenic district since theDaheishan Mo deposit was discovered, a number of deposits have been discovered in the region, includingthe Jidetun, Dashihe, Fuanpu, Yixintun, Liushengdian, Dongfeng medium scale Mo deposits, as well as sixsmall sized Mo deposits and more than30occurrences in recent ten years. More than200million tons ofproven reserve had been confirmed until2008, it makes the study area to be the second-richestconcentration of Mo resource in China, a fact that attracted a large amount of interest to research in themineralization of this area. In order to further reveal the Mo mineralization regularity and resourcepotential, this study focuses on the geological characteristics, fluid property, geochemistry andgeochronology to deeply reveal that diagenesis and mineralization age, metallogenetic specialization,tectonic settings, petrogenetic-metallogenic mechanism, and then establish metallogenic model anddynamical model. The main advance achievements from this study are as followings.According to geological features of endogenic Mo deposits in the region are mainly divided intoporphyry type, contact-metasomatic hydrothermal type and medium temperature hydrothermal quartz veintype. The orebodies for porphyry Mo deposits are hosted by monzonitic granite （porphyry） or granodiorite（porphyry）, which are lenticular and irregular forms in shape. Wall-rock alteration includesK-feldspathization, silicification, sericitization, epidotization and carbonation. The mineralization can bedivided into a quartz–pyrite stage （I）, a quartz–pyrrhotite-pyrite stage （II）, a quartz–molybdenite stage （III）,a quartz–polymetallic sulphides stage （IV）, and a quartz–carbonate phase （V）. The medium temperaturehydrothermal quartz vein type deposits are distributed in granite fracture, and the orebodies are vein formin shape. Wall-rock alteration includes silicification, sericitization, K-feldspathization. The mineralizationcan be divided into a quartz stage （I）, a quartz–molybdenite-pyrite stage （II）, and a quartz–carbonate phase（III）. The contact-metasomatic hydrothermal type deposits are hosted by contact zone between granites andthe Paleozoic stratum, and the orebodies are vein and lenticular forms in shape. Wall-rock alterationincludes chloritization, epidotization, sericitization, carbonation and silicification. The mineralization can be divided into a dry skarn stage （I）, a wet skarn stage （II）, a quartz–molybdenite stage （III）, aquartz–polymetallic sulphides stage （IV）, and a quartz–carbonate phase （V）. Three types of deposits haveobvious relativity in space-time, and often constitute porphyry or medium temperature hydrothermal quartzvein type-contact-metasomatic hydrothermal type mineralization system.The mineral fluid inclusions reveal that fluid inclusions from porphyry deposits mainly are aqueoustwo-phase inclusions and minor daughter minerals bearing polyphase inclusions. The homogenizationtemperatures are＞420～400℃,360～350℃,340～230℃,220～210℃and180～160℃, respectively.Salinity are＞41.05%～9.8%NaCleq,38.16%～4.48%NaCleq,35.78%～4.49%NaCleq,7.43%NaCleqand7.8%～9.5%NaCleq, respectively. The mineral fluid inclusions from contact-metasomatichydrothermal deposit mainly are aqueous two-phase inclusions and minor CO₂bearing three-phaseinclusions. The homogenization temperatures are＞337～280℃,260～200℃and190～101℃,respectively. Salinity are＞18.5%～9.3%NaCleq,20.5%～9.98%NaCleq and22.9%～7.33%NaCleq,respectively. The mineral fluid inclusions from medium temperature hydrothermal quartz vein deposit areaqueous two-phase inclusions. The homogenization temperatures are＞330～300℃,280～170℃and160～120℃, respectively. Salinity are＞12.07%～8.81%NaCleq,9.86%～3.37%NaCleq and9.21%～4.63%NaCleq, respectively.Combining with temperature measurement, gas phase composition for fluid inclusions and hydrogenand oxygen isotope characteristics for group inclusions indicate that the initial ore-forming fluid for theporphyry Mo deposits is CO₂-H2O-NaCl magmatic fluid of CO₂-bearing with minor CH₄, N₂and H₂S, andparticipation of meteoric waters in the late ore-forming stage. The medium temperature hydrothermalquartz vein Mo deposit is H2O-CO₂-NaCl multiphase fluid, and participation of meteoric waters in the lateore-forming stage. The initial ore-forming fluid for the contact-metasomatic hydrothermal Mo deposit ischaracterized by medium to high temperature, medium to high salinity H2O-CO₂-NaCl fluid ofCO₂-bearing with minor CH₄and N₂, and influxing of meteoric waters in the ore-forming stage. We candraw a conclusion that the porphyry Mo deposits undergo strongly fluid immiscibility in the mineralizationprocess, while mixing of fluid happened in the evolution of fluid in the contact-metasomatic hydrothermaland medium temperature hydrothermal quartz vein Mo deposits.Studies on diagenetic and metallogenetic epoch show that Mo mineralization in the study area mainlyoccurred in200～165Ma. Porphyry Mo deposits mineralization occurred in186～167Ma, and the relatedmagmato-thermal event for the Porphyry Mo deposits are closely related to the magmatic evolution of themonzonitic granite （porphyry） and granodiorite （porphyry）. The metallogenesis occurred in the late stageof magmatic evolution （189¹67Ma）. The medium temperature hydrothermal quartz vein Mo depositoccurred in176.4Ma, and the mineralization is associated with the Middle Jurassic monzonitic granite.The contact-metasomatic hydrothermal Mo deposit occurred in196.6Ma, and the mineralization is associated with the Early Jurassic magmation, carbonate rocks and clastic rocks.Element geochemical characteristics for granite complex associated with mineralization shows thatmonzonitic granite （porphyry） and granodiorite （porphyry） associated with mineralization of porphyrydeposits have high-Si, K, Al and alkali-rich, and belong to quasi-aluminous/weakly peraluminous high-Kcalc-alkaline series（SiO₂=62.59～73.5%, Na₂O=2.61～5.38%, K₂O=3.03～5.74%, K₂O/Na₂O=0.81～2.17, A/CNK=0.92～1.22, A/NK=1.14～1.64）, enrichment of LREE（∑REE=100.42～154.27ppm，LREE/HREE=10.91～16.52，（La/Yb）N=1.49～14.56）, weak or negligible Eu anomalies （Eu=0.85～1.08）,enrichment of the large ion lithophile element （LILE） and depletion of the high field-strength element（HFSE）. Monzonitic granite associated with mineralization for medium temperature hydrothermal quartzvein Mo deposits have high-Si, K, Al and alkali-rich, as well as weakly peraluminous high-K calc-alkalineseries （SiO₂=76.47～76.6%, Na₂O=3.15～3.17%, K₂O=5.35～5.42%, K₂O/Na₂O=1.68～1.72,A/CNK=1.01～1.02, A/NK=1.12～1.13）, obvious LREE enrichment, negative Eu anomalies（∑REE=91.43～99.22ppm, LREE/HREE=14.9～15.32,（La/Yb）N=4.68～7.94, δEu=0.5）, enrichmentof LILE and depletion of the HFSE. Granodiorite associated with mineralization for contact-metasomatichydrothermal Mo deposits have high-Si, K, Al and alkali-rich, as well as quasi-aluminous/weaklyperaluminous calc-alkaline series （SiO₂=63.98～71.14%, Na₂O=4.02～4.76%, K₂O=2.11～4.29%,K₂O/Na₂O=0.52～0.95, A/CNK=0.99～1, A/NK=1.21～1.81）, LREE enrichment（∑REE=128.24～159.48ppm, LREE/HREE=12.8～15.41,（La/Yb）N=17.26～24.01）, weak or negligible Eu anomalies（δEu=0.99～1.04）, enrichment of the LILE and depletion of the HFSE.Sr-Nd-Pb isotopic geochemistry compositions for granite complex associated with mineralizationindicate that monzonitic granite （porphyry） and granodiorite （porphyry） associated with mineralization forthe porphyry Mo deposits have (⁸⁷Sr/⁸⁶Sr)iratios range from0.70404to0.70554, εNd（t） values range from-0.9to2.4.²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios are18.4576～19.2028,15.5623～15.6144,38.2591～38.8874, respectively. Granodiorite associated with mineralization for the contact-metasomatichydrothermal Mo deposits have (⁸⁷Sr/⁸⁶Sr)iratios range from0.70413to0.70474, εNd（t） values range from3.3to3.6.²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios are18.5199～18.6146,15.5698～15.5805,38.3686～38.4782, respectively. Monzonitic granite associated with mineralization for the mediumtemperature hydrothermal quartz vein Mo deposits have （87Sr/86Sr）iratios range from0.70656to0.70721,εNd（t） values range from1.6to1.8.206Pb/204Pb,²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios are18.6488～18.6631,15.5884～15.5922,38.6708～38.6762, respectively. The three types of granites have low value of initialSr and high value of initial εNd（t）, which indicate that the granite is the product of crust-mantle mixing.Combing with diagenetic and metallogenic epoch, Sr-Nd isotope, ore-bearing fluid and provenanceproperty, we further confirm that metallogenic thermal source for porphyry type, contact-metasomatichydrothermal type and medium temperature hydrothermal quartz vein type Mo deposit is derived from basaltic magma derived from partial melting of secondary lithospheric mantle, and incentive is subductionof the Pacific Palte. There are two kinds of mineralization process for porphyry Mo deposits. The first isthat the magma formed by melt of the lower curst, which induced by underplating of the Early Jurassicbasaltic magma, and then the lower crust melt and form magma chamber through underplating, and thenformed ore-forming fluid by magmatic fractional crystallization. The ore-forming fluid rises with reducingof temperature and pressure, and boiling, which lead to unload ore-forming elements and formedbrecciated and veinlet disseminated orebodies.The other is that magma chamber formed by underplating of the Middle Jurassic basaltic magma, anda few basaltic magma mixing with the lower curst formed ore-forming fluid. The ore-forming fluid riseswith boiling and formed brecciated and veinlet disseminated orebodies. The magma related to thecontact-metasomatic hydrothermal Mo deposit is derived from basaltic magma underpalting the lowermafic crust in the Early Jurassic, and ore-forming fluid formed by crystal fractionation of magma chamber.The ore-forming fluid mixed with meteoritic water, and metasomatism carbonate rocks during rising, andthen formed the hydrothermal contact metasomatic Mo deposit. The mineralization process of the mediumtemperature hydrothermal quartz vein Mo deposit is that magma chamber formed by underplating of theMiddle Jurassic basaltic magma, and a few basaltic magma mixing with the lower curst formedore-forming fluid. The ore-forming fluid rising along fracture and mixing with meteoritic water, which leadtransform of physical and chemical conditions. Thus molybdenum precipitated and formed the mediumtemperature hydrothermal quartz vein Mo deposit.