Characteristics and Metallogenic Mechanism of Mo-W and Pb-ZnAg Deposits in Nannihu Ore Field,Western Henan Province

The Nannihu Mo-W-Pb-Zn-Ag ore field in west Henan Province is tectonically located in the east part of Jinduicheng-Nannihu MoW polymetallic metallogenic belt of south margin of the North China Craton.There occur porphyry-skarn Mo-W,skarn polymetallic pyrite and hydrothermal silver-lead-zinc vein deposits exhibiting zoning outward porphyries.The ore-forming fluids show that these deposits are in the same metallogenic system and the hydrothermal solution is mainly derived from primary magmatic water in the early stage and from the mixture of the primary magmatic water and meteoric water in the late stage.Sulfur and lead isotope components show that the ore substances and related porphyries are derived from the lower crust.The large-scale MoW-Pb-Zn-Ag mineralization occurred at about 140 Ma according to the Re-Os isotopic dating.Based on the study of Mesozoic tectonic evolution,we propose that the large-scale mineralization was the consequence of lithospheric extension during the late stage of the tectonic regime changing its main stress from NStrending to EW-trending.The metallogeny is as follows: during the subducted Yangtze continental slab broke-off possibly in south margin of the North China Craton in Middle-Late Jurassic,the underplating of the mafic magma near the boundary between the mantle and the crust caused by upwelling of the asthenosphere through slab window,drove partial melting of the thickened lower crust,forming the ore-bearing granitic magma.The ore-bearing granitic magma intruded along intersection of the NNE-trending and EW-trending faults and emplaced at the shallow places,attributing the formation of the granitic porphyries,the Mo-W, polymetallic pyrite and Pb-Zn-Ag mineralization.