Geological Character And Ore Genesis Of Cu-polymetallic Deposits Surrounding Yangxin Batholith

Edong district is one of the seven ore districts in Middle and Lower Yangtze River metallogenic belt.Fe and Cu is the main mineralization type in Edong district with subordinate Mo,Au,Pb,W,Zn mineralizaiton.Most deposits are skarn and porphyry deposits,which are mainly related to the Yanshanian magmatism.Ore-associated and ore-barren rocks were selected in this study.Detail petrographic study,bulk rock major and trace element composition,Sr-Nd isotope analysis and in-situ major and trace element of minerals analysis were carried on these samples.The geological characters,ore materals source and ore-forming fluid source are also studied on three Cu polymetallic deposits.The rocks are classified into stock and baltholith based on the exposure area.The zircon U-Pb age of ore-related stocks and baltholith are>140 Ma and<140 Ma respectively.The ages of ore-barren stocks are also<140 Ma.The S type two mica granite has a similar age?142.1±3.2Ma?with the Ruanjiawan quartz mondiorite.Except the two-mica granite,the selected magmatic rocks in Edogn district belong to high-K calc-alkaline series and metalumious rocks.All the rocks are enriched in LREE than HREE.In the spider diagrams,all the rocks are enriched in LILE and depleted in HFSE.And obvious Pb and Sr positive anomaly and Nb,Ta and P negative anomaly can be observed on the diagrams.These rocks have small Sr and Nd isotope variations which is similar with the early Cretaceous rocks.These characters may imply that the ore-associated and ore-barren rocks originated from delaminated lower crust and underlying lithospheric mantle.The delamination leads to the upwelling of the asthenosphere.The mixture of lower crust,lithospheric mantle and asthenospheric materials with different degree produce the basic,intermediate and felsic rocks in Edong district.The petrographic study,BSE images and major elements of the amphibole in these rocks imply the existence of magma chamber at⁵ km both for ore-related and ore-barren rocks.Tonglvshan stock is selected to study the magma evolution in deep magma chamber.The modeling shows that magma underwent 40%fractional crystallization of amphibole,plagioclase and apatite before the emplacement.Copper and chlorite enrichment occurred in the residual magma after undergoing fractional crystallization.Because the ore-barren stocks and ore-related baltholith has similar age?<140Ma?,apatite of these rocks are used for exploring the mineralization potential.The study shows that Cl,S,Li in ore-related rocks are higher than in ore-barren rocks.Primary fluid inclusions are also found in most ore-barren apatites but rarely in ore-associted apatites.These features show that the ore-barren rocks have fluid exsolution in deep magma chamber,which depleted the Cl,S,Li,Cu and Au in the magma.The ore-barren and ore-associated rocks in Edong district have similar age and bulk rock composition.As a result,age and bulk rock composition are not usefull propesting indicatiors in Edong district.Although high oxygen fugacity and shallow emplacement depth are favorable for mineralization,these two factors are similar in ore-barren and ore-associated rocks in Edong region.This study proved that the magma that has fractional crystallization history but no fluid exsolution at deep position is favourable for mineralization.Furthermore,Cl,Li and S in apatite can be used as usefull propesting indicators.Fluid inclusion in prograde stage in Fuzishan deposit is mainly composed of halite bearing fluid inclusion,then is vapor rich and fluid rich two phases fluid inclusions.The homogenization temperature of this stage vary from 405? to>550?.The salinity of halite bearing fluid inclusion,vapor rich and fluid rich two phases fluid inclusions are 28.8-42.5wt%NaCl equiv.,3.7-13.5wt%NaCl equiv.and 17.3-33.1 wt%NaCl equiv.repectively.The petrographic and microthermometric study imply the fluid-boiling process in prograde stage.The fluid inclusion is mainly two phase incluision in retrograde stage,quartz-sulfide and carbonate stage whose temperature and salinity decrease gradually.The S isotope of Fuzishan deposit vary significantly,with value of-3.2 to 14.6‰.In the early stage,S isotope value is close to 0,but the values already show the sign of contamination of gypsum in the surrounding rocks.The contamination gradually increase as the sulfide deposition going on.As a result,the ore-forming fluid and ore material are mainly magmatic source.The gypsum contamination is obvious in the quartz-sulfide stage.The coexistence of halite bearing and two phase fluid inclusion can be widely observed in prograde stage of Fujiashan-Longjiaoshan deposit.The fluid in this stage is mainly NaCl-KCl-H₂O system with no CO₂.They have high temperature?>550??and salinity characters.The salinity can be divided into two groups with 60-70 wt%NaCl equiv.and 15-20 wt%NaCl equiv.respectively.The fluid inclusion in retrograde stage has similar type and distribution pattern with the prograde stage.The fluid in this stage is mainly NaCl-KCl-H₂O system.The temperature?350-400??and salinity are silightly lower than in prograde stage.The salinity can also be divided into two groups like in prograde stage.Scheelite deposited only in retrograde stage,although fluid boiling happened both in prograde and retrograde stage.Quartz-sulfide stage is mainly composed of two phase fluid inclusion and independent CO₂ phase bearing fluid inclusion.Because of the decrease of temperature and sanlinity or meteoric water addation,fluid immiscibility happened at this stage.Sulfide and scheelite deposition mainly happened at this stage.The ore-forming fluid is mainly magmatic fluid in the prograge stage,but has the addition of meteoric wate in the following stage.The sulfide crystallized at the early stage has S istope ratio close to 0‰,but decreas to-21.3^-6.8‰at the late stage which is usually in the surrounding rocks.The?¹³C_V-PDB-PDB ratio is-9.1^-6.9‰in the early quartz-sulfide stage,but-14.7^-6.4‰in the late quartz-sulfide stage.The C and S isotope imply that the ore-forming fluid only comtaminate reduced materials in surrounding rocks at very late stage.The fluid inclusion is mainly liquid rich two phase inclusion in prograde stage in Ruanjiawan deposit,with surbodinate halite bearing fluid inclusion and vapor rich two phase inclusion.The homogenization temperature is 510? to>550?.The salinity of fluid in this stage is 13.0-22.6 wt%NaCl equiv..In retrograde stage,most fluid inclusion is also liquid rich two phase inclusion.But the coexistence of vapor rich two phase inclusion and halite bearing inclusion is also common.The temperature of fluid in this stage is 336-473?.The salinity can be divided into two groups,which is34.1-42.5 wt%NaCl equiv.and 9.0-17.4 wt%NaCl equiv.respectively.Fluid boiling happened at the retrograde stage which also produce scheelte deposition.Two phase and independent CO₂ phase bearing fluid inclusions take most part of the fluid inclusion in quartz-sulfide stage.The fluid immisciblity in this stage induce that the deposition of sulfide and scheelite.In prograde stage,?¹⁸O_V-SMOW and?D_V-SMOW of the fluid is 10.9-12.4‰?average 11.6‰?and-105^-99‰?-102‰?respectively.The?¹⁸O_V-SMOW and?D_V-SMOW of retrograde stage fluid is 8.6‰and-97‰repectively.The?¹⁸O_V-SMOW and?D_V-SMOW of quartz-sulfide stage fluid is 5.2-5.4‰?average 5.3‰?and-91‰^-88‰?average-89.5‰?repectively.The?¹⁸O_V-SMOW and?D_V-SMOW of carbonate stage fluid is 2.5‰and-99‰repectively.The ore-forming fluid is mainly magmatic water with some mixture of meteoric wate in the late stage.The S isotope of early sulfide vary from-2‰to 4‰.But the value of sulfide in surrounding rocks has a large variation of 3-18‰.This proved that the contamination of gypsum in the surrounding rocks is only significant in late stage.