Study On Metallogenesis Of Porphyry Deposits In Lesser Xing'an Range And Its Adjacent Areas,NE China

Lesser Xing'an range and its adjacent areas are located in the eastern section of the giant Xing-Mongolia orogenic belt and experienced superposition and transformation of the Palaeo-Asia ocean tectonic domain,the Mongol-Okhotsk ocean tectonic domain and the Pacific tectonic domain since the Phaneozoic,causing frequent and intense tectonic-magmatic activities.The history of the geodynamic evolution of the lesser Xing'an range and its adjacent areas has been systematically summarized.During the initial Phanerozoic(~540Ma),a continental ocean with limited expansion was formed between the North China Plate and the Siberia Plate.Erguna ? Songnen and Jiamusi-Xingkai blocks with precambrian metamorphic crystalline basements and crystallized in different ages with their own independent evolutionary histories scattered among the ocean.Da Xing'an range and the lesser Xing'an range-Zhangguangcai range are the Hercynian orogenic belts around the Songnen block.Whereby the Northeast region is divided into six tectonic units:the Eerguna block?the Songnen block?Bureya-Jiamusi-Xingkai block and the Da Xing'an range orogenic belt,lesser Xing'an range-Zhangguangcai range orogenic belt,and the Wandashan orogenic belt.By studying the geochronology and geochemistry of the Duobaoshan granodiorite and the Xiaoxi'nancha diorite,the tectonic setting of the studying area in the Paleozoic was discussed.During the Caledonian period(510~476Ma),the Northeast China was in the process of structural transformation.The Erguna,Songnen and Jiamusi-Xingkai blocks were converted from passive continental margins to active continental margins.The oceanic basins subducted and the blocks began to convergent.The Da Xing'an range orogenic belt was formed in the middle Hercynian(330~290Ma)by the closure of the inter-continental ocean basin between Erguna and Songnen blocks.In the late Hercynian period,the lesser Xing'an range-Zhangguangcai range orogenic belt was formed by the collision between Songnen and Jiamusi-Xingkai blocks with the closure of the Palaeo-Asia ocean(270~250Ma).With the closure of the Palaeo-Asia ocean,the Northeast China was gradually controlled by the evolution of the Mongolia-Okhotsk tectonic domain and the Pacific tectonic domain in the Mesozoic.The geochronological and geochemical studies of granitoids from the lesser Xing'an range-Zhangguangcai range show that in the Early-Middle Jurassic(178~170Ma),the studying area has been under the influence of the subduction of the paleo-Pacific plate.After experiencing a strong compression and thickening in the early Mesozoic,detachment,extension and thinning of lithosphere occurred in the early Cretaceous,resulted in a large scale of tectonic magmatic activities and mineralization.Based on the field works and laboratory studies,we are approaching a new understanding on the ore-controlling conditions and genesis types of the typical deposits in the studying area.The Xiaoxi'nancha copper-gold ore deposit is a superimposed deposit rather than the porphyry deposit considered by predecessors.There are two periods of copper and gold mineralization,including late Hercynian stockworks type in diorite and late Yanshanian hydrothermal veins type,and the first of which is the main one.The copper-gold mineralization in the late Hercynian was controlled by the diorite rock.The mineralization is concentrated in the upper part of the diorite body,forming stockworks type copper-gold mineralization along the fractures developed in the rock body.The wall rock alterations are weak and do not have the characteristics of alteration zones.The single supercritical fluid that has dissolved from the deep magma chambers has undergone boiling due to the sudden decrease in pressure,forming a magmatic hydrothermal fluid with high temperature,high salinity and high oxygen fugacity.The ore-forming fluid migrates upward along the cracks in the diorite body,the temperature gradually decreased,and the fluid properties are converted from oxidative to reductive.The metal material was largely unloaded and precipitated,eventually forming the stockworks type copper-gold mineralization at the top of the diorite body.The Hercynian mineralization occurred in the Middle Permian.The Yanshanian mineralization is a hydrothermal vein copper-gold deposit controlled by fault structures,mainly composed of sulfide quartz veins and the banded polymetallic sulfide veins,and the mineralization occured in the Early Cretaceous.The molybdenum deposits formed in the Early-Middle Jurassic such as Chang'anpu and Luming molybdenum deposits in the lesser Xing'an range have the following characteristics:(1)The deposits are produced in granite;(2)The wall-rock alterations are weak and do not have obvious alteration zones;(3)The ore body is mainly stockworks veins;(4)The depth of the ore body was more than 3km.Such deposits do not have the geological characteristics of typical porphyry deposits and should be the pluton type calc-alkaline molybdenum stockworks deposits,which more conforming with the actual geological characteristics of the deposits.Considering the metallogenic era and the tectonic setting of the typical deposits,the metallogenetic series in the studying area is re-divided :(1)Porphyry copper-goldmolybdenum metallogenetic series located in the island arc formed by the subduction of the Palaeo-Asia ocean in the early Paleozoic;(2)Stockworks in diorite type copper-gold metallogenetic series located in the island arc formed by the subduction of the Palaeo-Asia ocean in the late Paleozoic;(3)The stock type and the pluton type calc-alkaline molybdenum stockwork metallogenetic series formed in the subduction of the Pacific ocean plate in the early Mesozoic;(4)Porphyry and hydrothermal veins types copper-gold metallogenetic series formed in the extensional environment related to the collapse or delamination of the thickened lower crust in the early Cretaceous.