Study On Metallogenesis Of Copper Deposits In Naerhong-Tianhexing Region And Its Adjacent Area In Southeast Jilin Province,NE China

The Naerhong-Tianhexing region in the southeast Jilin Province is tectonically located in the eastern segment of northern margin of North China Carton and the most southern margin of Xing'an-Inner Mongolia Orogenic Belt(XIMOB).This area has undergone formation of the Archean crystalline basement of the North China Carton,the evolution of "Liaoji " ocean in the Proterozoic,Paleo-Asian Ocean tectonic domains in the Paleozoic-Early Mesozoic,and the overprint or transformation of the Circum-Pacific tectonic domain in the Mesozoic-Cenozoic which eventually form the complex tectonic regime in the area.This paper has obtained the formation age of Archean complex in Huadian-Longgang area of Longgang block to be 2574 ~ 2504 Ma by chronological study of tonalite,granodiorite and biotite plagioclase gneiss.The rock series are supposed to be formed by the partial melting of heat,young and subducted Archean oceanic crust.Combined with regional research data,the formation of the Liao-ji activity belt may be closely related to the subduction system,and this belt may have experienced active continental margin setting during 2.2 ~ 2.0Ga and sedimentation during 2.0 ~ 1.9Ga,metamorphism in ~ 1.9Ga and post-orogenic extension during 1.89 ~ 1.82 Ga,among which,the magmatism intermission during 2.0 ~ 1.9Ga may be due to the high-rate,high-angle and cold subduction effect of the old oceanic crust.According to the chronological study of syncollisional granite porphyry in Hunchun area,the limit for final closure of the Paleo-Asian Ocean along the eastern segment of Xar Moron-Changchun-Yanji suture zone is constrained to be ~ 247 Ma with the scissor pattern from west to east.The occurrence of the Indosinian A-type granite(~ 215 Ma)in the Naerhong area may indicate that the post-orogenic setting after the closure of Paleo-Asian Ocean is still dominated in the Late Triassic and is not affected by the southern subduction of Mongolian-Okhotsk Ocean.The effects of Circum-Pacific tectonic domain on the southeastern Jilin province are two peaks of subductions in the Early Jurassic and Early Cretaceous.The first subduction was represented by calc-alkaline volcanic association of 190 ~ 173 Ma in the eastern part of Jilin and Heilongjiang Province.The second subduction resulted in strong regional lithospheric thinning(114 ~ 103Ma)and formed A-type granite(114 ~ 103Ma)which indicated the existence of regional extension in Naerhong-Tianhexing Region.In this paper,the Changfagou and Tianhexing deposits are selected to study the field geological characteristics and geochemical characteristics.It is suggested that the Changfagou Copper Mine is typical porphyry ore deposit,the ore body is obviously controlled by quartz porphyry which commonly occur as stocks,and the alteration and mineralization are distributed around the inside and outside contact belt of rock mass.The ore-forming fluids are H2O-NaCl-CO2 system with high temperature and high salinity.The mechanism for molybdenum precipitation is immiscibility or boiling and the copper precipitation is probably related to addition of atmospheric water and temperature decrease.The Tianhexing copper-molybdenum deposit is a magmatic-hydrothermal superimposed deposit with magmatic mineralization in the late Paleo-proterozoic and porphyry mineralization in the late Yanshanian.The magmatic mineralization is controlled by metagabbro or metadiabase which is formed by little proportion of the melting of the mantle source.The porphyry mineralization mainly occurs in second stage of the granite porphyry intrusion or near the contact zone where the change of pressure and sulphur content in the fluid may be important factors to the copper precipitation in Tianhexing deposit.The geochemistry and Sr-Nd-Hf isotopic analysis of the late Yanshanian ore-bearing and non-ore-bearing porphyry in the two mining areas suggest that the ore-bearing and ore-free porphyry are both fractionated I-type granite,and both two types of granites are originated from the same magmatic source but with different degree of fractional crystallization.Moreover,it is supposed that the higher degree of differentiation,the more disadvantageous formation of copper deposit.The study on the regional copper mineralization shows that the absence of fluid addition during subduction in the late Indosinian period is not conducive to the formation of porphyry-skarn type hydrothermal deposits such as Changfagou copper and Linjiang copper deposits,but is favorable for magmatic liquation deposits as the mantle material are characterized by high water content and strong oxidation which affect the liquation of sulfide from the magma source.The large-scale molybdenum deposits in early Yanshanian are mainly formed in the Early Jurassic(189 ~ 187Ma)and the Middle Jurassic(174 ~ 167Ma),respectively corresponding to the compressive regime at the beginning of first subduction of the Paleo-Pacific ocean and the intermittent extension after the subduction.The study on the source of ore-forming material in the late Yanshanian porphyry copper-molybdenum deposit shows that the ore-forming material has the characteristics of crust-mantle mixed source.The molybdenum is mainly derived from the crust,the copper may come from the copper-rich upper-mantle source of the North China Craton and the ore-forming fluids also behave as mantle-derived C-H-O fluids.The two stages of porphyry metallogenesis in the Early-Middle Jurassic and Early Cretaceous to Late Cretaceous correspond to the two peaks of subduction of the Paleo-Pacific Ocean plate in the northeastern China.However,they have obvious difference: the first metallogenesis mainly result in formation of porphyry-hypogenic-fine stockwork type deposits which is dominated by hypogenic-fine stockwork type with molybdenum mineralization;the second metallogenesis mainly result in formation of porphyry type deposits which is dominated by copper mineralization and formed in the geological setting of lithospheric thinning and are closely related to the angle,direction and velocity of the subducted Paleo-Pacific ocean plate.Based on the spatial distribution,ore-controlled factors and mineralization of ore deposits and tectonic units in the area,the southeastern part of Jilin Province can be divided into Central Jilin continental margin metallogenic belt,Eastern Jilin(Yanbian)volcanic metallogenic belt and Southern Jilin(Laoling)metallogenic belt.The Liaoning and Jilin province,northern North Korea and even northeastern part of North China Craton all exist magmatic mineralization in the late Paleoproterozoic.The favorable location for mineralization of magmatic copper-nickel sulfide deposit in Indosinian should be the Central Jilin continental margin metallogenic belt which is the intersection of the northeast and the eastwest fault zone.The hypogenic-fine stockwork type deposits should be the main objective for molybdenum(copper)exploration in Central Jilin continental margin metallogenic belt.Moreover,the acid intrusive rocks(not porphyries)of the calc-alkaline ore parent rock series play an important instructive function for finding this type of deposits and the related exploration work should be strengthened in the northern part of ancient suture.The Southern Jilin(Laoling)metallogenic belt exist copper-rich upper-mantle source,thus the distributed location of ancient subduction zone and large-scale Precambrian basaltic rocks should be the favorable place for exploration of porphyry copper(molybdenum)deposits and the hypabyssal to super hypabyssal calc-alkaline acid intrusion should be specially explored.