Research On The Geochemical Features And Genesis Of The Changba Lead-Zinc Deposit In Gansu

The Changba-Lijiagou lead-zinc deposit,located in the Xicheng basin of the West Qinling orogenic belt,is the largest lead-zinc deposit in Devonian of Qinling lead-zinc metallogenic belt.Through the study on the sources of ore-forming fluid and the material component,the transport mechanism and the depositional mechanism of ore-forming materials,determining the ore-forming age,for the determination of ore-forming processes and the genesis of deposit,and geological exploration of Changba lead-zinc deposit,still has important significance of theoretical guiding for the regional geology.This paper carried out a field geological survey,collected samples,identified and analyzed the ray of samples.By analyzing and testting the stable isotope C-S and rare gas isotope He-Ar of the pyrite in ore,preliminarily analysis the sources of the ore-forming fluids and material.Combining with the previous research results of H,O,Pb isotope,REE and the fluid inclusions,the following conclusions are achieved:(1)The ore bodies of Changba lead-zinc deposit mainly occur in Anjiacha group of Middle Devonian among marble and quartz schist.The ore body in accordance with the strutigraphic occurrence,with the output of rock is layered and integrated,sedimentary characteristic-s significantly.Fracture,fold structure and the specific stratigraphic control orebody significantly.The main metallic minerals are pyrite,sphalerite,galena,chalcopyrite,arsenopyrite amount.Gangue minerals are calcite,secondary,quartz,barite,biotite.(2)Pyrite ?34S change from 19.3‰ to 30.6‰,the average is 25.7‰,indicated that the required sulfur of mineralization derived from marine sulfate.The ?13C ratio of the fluid-inclusions in pyrite change from-6.1‰ to-14.6‰,mean is-8.7‰.The initial determination is that ore-forming fluids comes from metamorphic fluid,affected by magmatic activity.The40Ar/36Ar ratio of the fluid-inclusions in pyrite is between 294.04-580.6,the average is 438.78,and with the argon isotopes of atmosphere water-saturated(40Ar/36Ar=295.5)very close,far away with radiogenic argon of crust and mantle,the ratio of 3He/4He is 0.0137-0.0758Ra,consistent with the 3He/4He of crust-fluid source.(3)Comprehensive geological and geochemical characteristics shows that Changba lead-zinc deposit belongs to the type of SEDEX deposit.Metal ore-forming materials mainly derived from the deep portion of substrate layer,Devonian marine sulfate provided ore-forming materials sulfur.Ore-forming fluid is a mixed fluid,mainly came from the underground thermal brine,related to rainfall and Devonian seawater,and deep metamorphic-fluids in deeper crust,with the participation of magmatic hydrothermal,and the major of the ore-forming fluid comes from crustal fluid.In the tectonic setting of stretched and cleaved in Devonian,the hot brine rises from deep crust,along regional growth faults,effusive to the closure-semi-enclosed water basins limited mixing with sea water,enriching the initial forming-minerals.In Indo-early Yanshan,accompanying with tectonic regime of contraction and extrusion,study area occurred large-scale regional metamorphism,Changba lead-zinc deposit occured hydrothermal alteration,enriched more ore-forming elements,thus forming a huge scale industrial deposits.