| The Xitieshan Pb-Zn deposit is located in the middle of the tectonic metallogenic belt in the northern margin of Qaidam basin in northwest China.Since its discovery in 1956,the accumulated proved reserves of Pb-Zn ore have reached 6.4 million tons,making it one of the three major lead and zinc deposits in China.Since the discovery and exploitation of the Xitieshan deposit,there has not been a consensus on the genesis of the Xitieshan deposit,Such as volcanic exhalative-hydrothermal overprinting deposit,hydrothermal metasomatic deposit,sediment-hosted Zn-Pb deposit(SEDEX),volcanis-associated massive sulphides deposit(VMS)and so on.In view of the genesis of the Xitieshan deposit,combining the previous research results and the work done in this study,starting from the basic geological features of the Xitieshan deposit,especially the ore-forming geological background,ore-bearing horizon,surrounding rock lithology combination,ore body characteristics,ore mineral composition,and the trace element composition of pyrite and the S isotope characteristics of sulfide,the genetic type of the deposit was discussed and the metallogenic model of the Xitieshan deposit was established.The Xitieshan deposit was ore-forming in the back-arc basin.Its surrounding rock was a shallow marine mafic to felsic volcanic-sedimentary rock of the Late Ordovician Tanjianshan Formation.The ore-bearing strata are the O3tna volcanic-sedimentary rock group and the O3tnb medium-basic volcanic clastic group.The ore minerals are mainly pyrite,sphalerite and galena,part of pyrrhotite and a small amount of colloidal pyrite,and a small amount of chalcopyrite locally.The gangue minerals are mainly quartz and calcite.According to macro-and micro-observation on ore fabric,three main types of pyrite from stratiform ore bodies had been identified,that is,colloidal pyrite(Py-?),anhedral pyrite(Py-?)and subhedral-euhedral pyrite(Py-?),which could represent primary depositional pyrite,low level of recrystallized pyrite and high level of recrystallized pyrite,respectively.Metallogenic elements,such as Cu,Pb,Zn,Co,Ni and so on,are enriched in Py-?and its Co/Ni ratio is gearter than 1.Among them,Ni,Co and As might be situated in lattice of pyrite as solid solution,while Cu,Pb,Zn and other elements might exist in lattice defect of pyrite in form of nanometer mineral inclusions.Metallogenic elements in Py-? are variable and obviously lower than which in Py-?,and yield lots of pumps in time-space resolved signal of LA-ICP-MS analysis,indicating their existence of nanometer mineral inclusions in pyrite lattice.Metallogenic elements in Py-? are extremely low and generally below detection limit,except for some spot with minor amount of trace elements.The Co/Ni ratio of Py-? and Py-? are mostly gearter than 1.The Co-Ni-As graphic analysis indicate that the pyrite of the Xitieshan deposit belongs to magmatic or volcanic hydrothermal pyrite.Based on their occurrence and fabric characteristics,we assume that the Py-? was formed in the process of the submarine-exhalative sedimentation with volcanic activities,while Py-? and Py-?were produced from Py-I by later metamorphic recrystallization in different degrees.The range of sulfur isotope composition of pyrite and sphalerite is small,?34SVCDT=+2.08?+5.81‰,with an average of +4.15‰,showing a small positive value(slightly higher than magma sulfur isotope),with obvious tower distribution characteristics.This also indicates that the ore-forming fluid of the Xitieshan deposit is derived from the deep magmatic fluid,and there is also a small amount of seawater sulfur mixed.The results of trace element of pyrites indicate that the original ore-forming fluids of the Xitieshan deposit were rich in ore-forming elements such as Fe,Cu,Pb and Zn.The ore-forming fluids rised along the exhalative conduct to the seabed sedimentary mineralization,and were later strongly metamorphosed by the Early Devonian green schist facies.The effect caused the original sedimentary sulfide to metamorphose and recrystallize.During the metamorphism,low-ionic potential elements such as Au,Cu,Pb and Zn had strong activity and reactivated migration with the metamorphic fluid.Due to the difference in activation and migration ability,elements such as Au,Cu,Pb,and Zn had migration and differentiation.Combined with the research results of chronological,geochemical and tectonic settings of volcanic rocks in the surrounding rocks of the Xitieshan deposit in recent years,the results of trace elements and sulfur isotope studies in the Xitieshan deposit,and comparating with the classical VMS deposits(Bathurst deposit,Wolverine deposit and Gacun polymetallic deposit),this study believes that the Xitieshan deposit should be a VMS type massive sulfide deposit.The Xitieshan deposit experienced a mineralization process and two post-mineralization transformation processes,namely 450-440Ma post-arc basin expansion and volcanic hydrothermal exhalative sedimentary mineralization,399Ma green schist facies metamorphic element reactivation migration,Indosinian and the Eocene two left-handed shearing superimposed tectonic migration,and finally formed nowadays the large spatial position difference of the Yuka oregenic gold deposit,the Luliangshan copper deposit and the Xitieshan Pb-Zn deposit. |
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