Geology And Geochemistry Of Granites In Luanchuan Ore District And Their Implications For Mineral Exploration

The Luanchuan ore district forms central part of the East Qinling metallogenic belt in central China,where a large number of Mo-W-Pb-Zn deposits and granites were formed in late Mesozoic.In this study,we compare Mo-bearing intrusions with the ore-barren Laojunshan intrusion to investigate the difference between them and factors controlling metallogenic potential of granites.The results of geology and geochemistry are as follows:(1)the plutons in Luanchuan ore field are characterized by granite porphyry and porphyritic monzogranite.(2)Mineral chemistry of biotites from the intrusions exhibit similar features of high Si and Mg,and low Al and Fe.They are characterized by magnesian biotite.But biotites from Nannihu and Shangfang are more affected by fiuld.(3)Geochemically,all intrusions are characterized by high-Si,high-K,and alkalis,together with moderate-ASI,exhibiting I-type granite features.The rocks,especially the ore-related plutons also show enrichment in LREEs and LILE.Nannihu and Shangfang plutons experienced higher degree of differentiation.(4)Zircon grains from the ZYK intrusion yielded a U-Pb age of 149.6±2.4 Ma.The zircon grains show ?Hf(t)values and two stage model ages(TDM2)in the range of-16.8 to-19.7 and 1998 to 2156 Ma respectively.(5)Age data show that the ore-related intrusions in Luanchuan ore district cluster into three magma pulses: 160–154 Ma,150–141 Ma,and 136–130 Ma.The Re–Os model ages of the molybdenite from the Porphyry-Skarn type Mo deposits bracket their emplacement in the range from147 Ma to 139 Ma.However,the ore-barren Laojunshan batholith formed in an extension-related geodynamic setting.By investigating features of plutons and related Mo deposits,we evaluate the controlling factors of metallogenic potential of granite.The results are as follows:(1)The source of magma and petrogenesis semay be main factors determining whether to form large-scale metallogeny.Both of ore-bearing plutons and ore-barren batholith originated from partial melting of aged crust with the involvement of mantle derived material.But the source of ore-bearing plutons have less mantle materials than the ore-barren intrusion.The difference may control whether to form large-scale Mo mineralization.The genesis of ore-bearing granites and ore-barren batholith are also different.(2)The physico-chemical conditions of magma,compositional feature of rocks and shape and scale of pluton affect the size of ore deposits.Nannihu and Shangfang pluton,with large scale Mo mineralization,show higher temperature and oxygen fugacity and lower solidification pressure.And the pluton which experienced higher degree of compositional evolution and have higher content of K,Si and heat production elements favor the formation of large-scale Mo deposits.At last,the pluton with smaller exposed area and lower erosion intensity may have larger-scale deposit.(3)The types of wall rocks are also the key factors to determine the associated minerals(W or Fe)in Mo deposits.Based on the comparison between Huangbeiling pluton other ore-bearing plutons in Luanchuan ore district,it is known that Huangbeiling has high metallogenic potential of Mo-W deposits.The following exploration actions should be taken around the Huangbeiling pluton.