东秦岭钼矿带成矿特征及其与美国克莱马克斯-亨德森钼矿带的对比

东秦岭钼矿带是中国最主要的钼矿带,钼矿呈近东西向展布。钼矿以斑岩型为主,从南到北,钼矿带钼矿大体有斑岩Cu-Mo矿、斑岩Mo矿、斑岩Au-Mo矿分带的趋势,与从俯冲带到克拉通边缘斑岩Cu矿、斑岩Cu-Mo矿、斑岩Mo矿依次发育的分带现象相似,表明钼矿的形成与扬子地块向华北地块俯冲有关。根据钼矿Re-Os年龄资料统计钼矿分为～220Ma、～140Ma和～110Ma三期,其成矿动力学背景分别为碰撞造山、碰撞造山后伸展和中国东部岩石圈减薄。钼矿流体包裹体均一温度介于83℃～424℃;平衡盐度介于0.61%～42.5%。流体包裹体水的δD介于-100‰～-40‰,δ18OH2O介于-4.3‰～8.7‰;且从成矿早期到晚期流体包裹体水的δD和δ18OH2O分别变小,表明钼矿的成矿流体主要来源于岩浆,后期有大气水的加入。东秦岭钼矿的铅同位素为206Pb/204Pb=17.12～17.89、207Pb/204Pb=15.23～15.70、208Pb/204Pb=37.57～39.10,与区域下地壳铅同位素一致;小斑岩体的Sri=0.705～0.714,δ18O=7.2‰～12.1‰,与I型花岗岩的锶、氧同位素相一致,表明钼矿的成矿物质主要来源于下地壳。东秦岭钼矿带的钼资源总量占中国钼资源的51%以上,美国克莱马克斯-亨德森钼矿带(Climax and Hender-son)的钼资源总量占美国钼矿资源的42%以上,美国和中国的钼资源在世界上的排名分别为第一和第二位,两钼矿带是世界钼资源高度集中的两个区域。克莱马克斯-亨德森钼矿带位于美国中西部、美洲克拉通西缘;钼矿主要形成于33～18Ma,稍晚于拉腊米(Laramide,75～54Ma)陆内造山运动;钼矿形成于碰撞造山后伸展环境。东秦岭与克莱马克斯两钼矿带相比:1)两钼矿带都位于克拉通边缘;2)两钼矿带的钼矿化都形成于陆内碰撞造山之后的伸展环境,与成矿有关的岩体都为花岗斑岩小岩体;3)两钼矿带钼矿的辉钼矿平均丰度分别为0.073%～0.140%和0.171%～0.264%,东秦岭钼矿的丰度明显较低;4)两钼矿带钼矿的辉钼矿成矿温度分别为300～400℃和460～600℃,东秦岭钼矿明显较低,反映与其成矿有关的岩浆的侵位深度较浅。通过两钼矿带间的综合对比得出:克拉通边缘经历陆内碰撞造山作用后在伸展环境下有利于斑岩钼矿的形成;与钼矿有关的小斑岩体岩浆的侵位深度影响钼矿中辉钼矿的丰度,岩浆的侵出深度越深其钼矿的辉钼矿品位越高。

Characteristics of Mo Deposits in the Eastern Qinling and Comparison with Those in Climax-Henderson

The eastern Qinling molybdenum belt is the main molybdenum metallogenic regions in China,which is located in the southern margin of North China Craton.Molybdenum deposits generally occur in nearly E-W direction along the tectonic lineation,most of which occur as porphyry-type deposits.There is a trend of porphyry Cu-Mo,Mo and Au-Mo deposit zoning from south to north,similar to the mineralization zoning of porphyry Cu,Cu-Mo and Mo in subduction zone.It indicates that the molybdenum mineralization was correlated with the northward subduction of the Yangtze Block beneath the North China Block.According to Re-Os chronology data of molybdenite from Mo deposits in the eastern Qinling,three episodes of molybdenum mineralization are recognized:～220 Ma,～140 Ma and～110 Ma.The formation time for Mo deposits are corresponding to those of intra-continental collision,post-collision extension and rift extension,respectively.Homogenization temperatures of fluid inclusion in Mo deposits range from～83℃ to 424℃ and salinities from 0.61% to 42.5%(NaCl equivalent).Oxygen(δ18OH2O=-4.3～8.7‰) and hydrogen(δD=-100～-40‰) isotope data on fluid water of hydrothermal minerals indicate a dominant derivation from magmatic fluid and post-magmatic meteoric water(e.g.post-mineralization alteration).The features of Mo mineralization and alteration of wall rocks indicate that the Mo mineralization may have been enriched by fluid differentiation from the small granite porphyry hosts.The lead isotopic compositions(206Pb/204Pb=17.12～17.89,207Pb/204Pb=15.23～15.70 and 208Pb/204Pb=37.57～39.10) of Mo deposits are identical with that of lower continental crust,while strontium(Sri=0.705～0.714) and oxygen(δ18O=7.2‰～12.1‰) isotopic compositions of granite porphyry are akin to that of I-type granites.So,the above results of Mo deposits and small granite porphyry stocks indicate a derivation of lower continental crust for Mo deposits.Mo resource in metallogenic belt of the eastern Qinling accounts for more than 51% of the total in China,while it in Climax & Henderson up to 42% of the total in America.Mo resource in America and China ranks No1 and No2 in the world,respectively,indicating a high enrichment in the eastern Qinling,in China and Climax & Henderson in the middle-western of America.Climax & Henderson Mo deposits were formed at 33～18 Ma,a slightly later than the Laramide(75～54Ma) intro-continental orogenic events.Molybdenum mineralization occurred in extensional setting.Some cognitions can be attained through the comparison between the eastern Qinling and Climax & Henderson as follows:1) The two regions are both located at the margin of cratons;2) Molybdenum mineralization were both occurred in post-collision extension system;3) The abundances of molybdenites in the eastern Qinling(0.073%～ 0.140%) are lower than that of Climax & Henderson(0.171%～0.264%);4) Temperatures for molybdenum mineralization are lower in the eastern Qingling(300℃～400℃) than that in Climax & Henderson(460℃～600℃),indicating a shallower intrusion of the related magma and also support assumation of an origin from the margin of craton.Thickened crust caused by collision of continents and relative extension after collision are beneficial to mineralization of molybdenum.During the formation of the small granite porphyry stocks,the intrusive depths of granite magma influence the grade of Mo deposits.The deeper the intrusive levels were,the higher the grade of Mo deposits was.