Study On Prospecting Model And Metallogenic Prediction Of Hidden Lead-zinc Deposit In Xicheng Ore Field

Xicheng ore field is one of the important producers of Pb-Zn and other nonferrous metal in China,which is located in the west of the Devonian Pb-Zn metallogenetic belt,Qinling orogen.It is between the Shangdan suture zone and the Mianlue suture zone,the Dangchang county in the west,the Fengtai Pb-Zn ore field in the east,the Huangzhuguan fracture in the north and the Rentushan-Jiangluo fracture in the south.Based on detailed study of typical Pb-Zn deposits,directed by the theory of metallogenetic system,this paper focus on summarizing the influence of mineralization construction and structure alone with hydrothermal reformation to the lead-zinc deposition at first,then establish the model of ore-occurrence and ore-prospecting.At the second place,we made a deep research on the respective relations between the mineralization of lead-zinc and the metallogenetic basin dynamic context,the sedimentary formation,the reformation and the magmatic activities during the basin-mountain evolution,after that we generalize the metallogenetic conditions and ore-controlling factors of these Pb-Zn deposit.Above all,we conduct the metallogenetic prediction aim at the deep-seated orebodies on the base of geophysics and geochemistry data.The main results yielded as follow:1.The representative Guojiagou Pb-Zn deposit,Luoba Pb-Zn deposit and Shuiguanzi Pb-Zn deposit are chosen to summarize the characteristics of lead-zinc mineralization and metallogenetic mechanism of the Xicheng ore field.We conclude that lead-zinc orebodies in Xicheng belt are mainly stratiform,stratiform-like and saddle-form which occur in the interface of the wall rocks of the Devonian Xihanshui Formation,Wujiashan Group and Anjiacha Formation.Orebodies are hosted by two kinds of wall rocks,one is clastic rock intercalated with argillite and thin-bedded carbonatite,the other is carbonatite intercalated with micritic limestone,bioclasticlimestone and siliceous rock.They all show a clear characteristic of stratabound deposits'.Among them,the stratiform lead-zinc ores contribute a principal part,the stratiform-like ores occupy a smaller proportion and the saddle reef is the product of the reformation of post-ore tectonic movements and hydrothermal fluid,which holds similar characteristics with MVT ore deposit.Thus we think these deposits went through two phases of mineralization: the sedimentary phase and the hydrothermal reformation phase.2.Study on fluid inclusions in the regional lead-zinc ore deposits shows that the fluid of exhalative-sedimentary phase was heated groundwater and the fluids of superpose-reformation phase were a mixture of magmatic and meteoric waters.The ore-forming fluids are characterized by medium-low temperature,medium-salinity,low-density,medium-pressure,acidic and weak-oxidizability,belonging to a Ca²⁺(Mg²⁺/Na⁺)-SO₄^2-?Cl^-?system.The high values for sulfides of ores are interpreted to be derived from TSR of seawater sulphate whereas the Pb isotope compositions of ores indicates that the lead is derived from the underlying basement Bikou Group,the Liziyuan Group and the Devonian sequence.Carbon,oxygen and strontium isotopic data of hydrothermal calcite from the ores can be attributed to the sedentary sequence.Moreover,the geochemistry data of the siliceous rock which spatially associated with the ore-forming are interpreted to be a hydrothermal exhalative origin,it indicates that there were hydrothermal exhalative events at the early mineralization stage in the Devonian sedimentary basin.Thus the origin of those deposits are considered to be sedimentary exhalative deposit which superimposed by magmatic hydrothermal reformation.3.The Devonian sedimentary basin has evolved from a passive continental margin extensional basin in the Silurian to a foreland basin in the compressive setting.It controlled the formation and spatial-temporal distribution of early stage exhalative sedimentary stratiform orebodies,and the orebodies are occur in the interface of carbonatite and clastic rock and also the extensional section.Based on that,we construct the prediction model of “strata + interface + trap structure” on the scale of ore district.4.The research on the U-Pb age dating of detrital zircons in difference Devonian sequences shows that: the sedimentation of Devonian basin in the Xicheng orefield is no early than the Silurian,the source of basin sediments are mainly from the north Qinling and a small part is from the south Qinling terrane or the north margin of the YangtzBlock.So we propose that the closure time of the Shangdan ocean is the late Silurian,at that moment,the south and the north Qinling have already experienced closure and the sedimentary basin have transformed into post-collision orogenic foreland basin other than extensional rift basin.5.Based on the GIS technique alone with comprehensive analysis of ore-controlling factors and various favorable ore-forming factors in Xicheng ore field,we established the evidence weight model which serves the comprehensive mineralization prognosis factors as main evidence layer.As a result,three levels of ore forming prospective area,including 5 I level,6 II level and 5 III level,are classified.6.Based on the pattern of the mineral occurrence of “interface” and “hinge zone of the fold” in the ore district,we have delineated the morphology of the interface of the carbonatite and clastic rock,and have also identified an EW-trending anticline by the EH4 technique.After implement the drilling verify on the hinge zone of the fold,a deep-seated orebody was found below the depth of 350 m in Guojiagou ore district.Up to now,this deposit has already possessed a total Pb+Zn metal reserves of 3 million tonnes,and Ag metal reserves of over 1000 tonnes.