The Ore-forming Processes And Mineralization Of Honghuaerji Scheelite Deposit, Inner Mongolia

Honghuaerji scheelite deposit is located in the middle-northern section of Da Hinggan range metallogenetic zone and is the first discovered scheelite deposit within the area.Tungsten ore bodies mainly occur in the inner contact of granites.The ore-bearing granites intruded into the Ordovician Luohe Formation(O2-3lh)and consist of monzogranite and quartz monzonite.The metallogenic elements' zoning characteristics show that W beneath Mo in space.Wall-rock alterations are K-feldspathization,albitization,greisenization and silicification which are closely related to mineralization.The major ore type is quartz-vein type.According to the studies on isotopic geochronology,the zircon U-Pb ages of the ore-bearing granites are between 178.63 ± 0.71 and 179.18±0.49 Ma.Seven scheelite samples yield a Sm-Nd isochron age of 178.4 ± 2.9Ma.The Re-Os isochron age of seven molybdenite samples is 176.8 ±2.2Ma.The accurate dating results indicate that the ore-forming and rock-forming process occurred in Early Jurassic,i.e.Early Yanshanian.They are in accordance with the polymetallic metallogenic epoch in the region.The results of geochemical characteristics show that all of ore-bearing granites have high SiO2(63.09%?73.53%),K2O±Na2O(6.60%?8.74%),and slightly peraluminous(A/CNK = 0.97?1.08,A/NK=1.22?1.53).They are enriched in light rare earth elements,depleted in heavy rare earth elements(LREE/HREE=4.38?9.90,(La/Yb)N=3.96?12.28)with a medium negative Eu anomaly(?Eu=0.55?1.01).They are also enriched in Rb,Th,U,and K,depleted in Ba,Nb,Ta,Sr,P and Ti.The analytical results of major and trace elements show that these intrusive rocks belong to typical I-type granite and experienced a high degree of fractional crystallization.The Sr-Nd-Pb-Hf isotopic results show that the(87Sr/86Sr)i=0.70476?0.70525,?Nd(t)=+1.8?+3.1,selected K-feldspars' 206Pb/204Pb=18.263?18.276,207Pb/204Pb=15.507?15.512,208Pb/204Pb=37.994?38.018,zircon ?Hf(t)=+3.1?+14.4;Nd isotope depleted mantle model ages(TDM)between 665Ma and 901Ma,two-stage Hf model ages(TDM2)between 303Ma and 1028Ma.It indicates that the proto;ith of the granites may be derived from a Neoproterozoic basement.Fluid inclusion studies indicate that the inclusions in each mineralization stage are all gas-liquid two-phase aqueous inclusions.From early stage to late stage,along the decrease of mineralization temperature,the fluids salinity changed slightly.The temperature range of tungsten mineralization stage is between 290 ? and 395?,with the peak of 360??370?;the salinity varies from 0.88%to 7.02%NaCleqv,with the peak of 2.0%?2.5%NaCleqv.Combined with the analysis of Laser Raman Probe,the scheelite-forming fluid is a NaCl-H2O-hydrocarbon gas-H2 system with high-temperature and low salinity.The fluid related to scheelite mineralization may mainly derive from the parent magma fractional crystallization.The cooling of fluid is the main mechanism of scheelite precipitation in the fluid.The H-O isotopic features of the scheelite and quartz and the C-H-O isotopic features of carbonate suggeste that the ore fluids are mainly derived from the magmatic water.The Pb isotopic composition of the K-feldspar from the ore-related intrusive rocks and sulphides from the ore and the ?34S values of the sulphides indicate that the ore-forming materials may also originate from the magma.Fractional crystallization during the evolution of the Honghuaerji magma may probably contributed to the formation of the tungsten-rich magmatic-hydrothermal ore fluids;calcium may be derived from the decalcification effect of plagioclase in granites.Honghuaerji scheelite deposit and the ore-bearing granites are strictly controlled by the regional tectonic evolution.The regional tectonics analyses indicate that the protolith of the ore-bearing granites were probably related to the interaction between oceanic plate and upper mantle during Neoproterozoic;after the closure of Mongol-Okhotian marine trough,the compression tectonic setting resulted from the collision of plates triggered the partial melting of the protolith in Early Jurassic.