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Re-Os isotopic age dating of molybdenite separates from Elegen porphyry Mo (Cu) mineralized area, northwestern Alxa, western Inner Mongolia

NIE Feng-jun1, QU Wen-jun2, LIU Yan1, DU An-dao2 and JIANG Si-hong1(1 Institute of Mineral Resources, CAGS, Beijing 100037, China; 2 National Research Center of Geoanalysis, Beijing 10037, China)  
Located in the southernmost part of the Queershan-Wuzhuergashun terrane along the southern margin of the Siberian block, Elegen is a large-size porphyry Mo (Cu) mineralized area in Alxa, western Inner Mongolia. Porphyry Mo (Cu) mineralization occurs within Hercynian porphyritic granodiorite and granodiorite porphyry stocks as veins, veinlets and disseminated blocks. Re-Os isotopic age dating of five molybdenite separates from the major porphyry Mo mineralized zone gives an isochron age of (332.0±9.0) Ma and model ages ranging from 336.5 to 340.6 Ma, 338.3 Ma on average. As the Re-Os isochron age is in agreement with field geological evidence, and molybdenite is coexistent with chalcopyrite, it is held that the Re-Os isochron age represents the ore-forming time of porphyry Mo (Cu) mineralization within Elegen area. Combined with field geological observations and petrological evidence, it is suggested that the ore-forming materials might have been derived from a mixed source of depleted mantle- and crust-derived magma or fluids. It has been noted that the Re-Os isochron age of molybdenites from the porphyry Mo (Cu) mineralized zone in Elegen is located in the middle of the age range of those unpublished 40Ar/ 39Ar age data for K feldspar separates from monzogranite, tonalite and K feldspar granite bodies. Field geological observations, petrological studies and geochronlogical analyses show that in the hydrothermal environment of Elegen, 40Ar/ 39Ar age of K feldspar separates from the strongly altered intrusions may be partly or even significantly reset whereas Re-Os age records have preserved the primary mineralization event. The Re-Os isochron age of the five molybdenite separates in Elegen indicates that Early Carboniferous island-arc magmatism and relevant porphyry Mo (Cu) mineralization in the northwestern part of Alxa had been active before the Permian collision between the Siberian block and the Kazakhstanian-Beishan plate.
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