Ore Genesis Of The Luziyuan Zn-pb Polymetallic Deposit In The Southern Baoshan Block,sanjiang Region(SW China): Constraints From Mineral Chemistry And Isotope Geochemistry

The super-large Luziyuan Zn-Pb polymetallic deposit is located in the southern Baoshan block,central-southern Sanjiang Tethyan Metallogenic Domain(SW China).The Luziyuan deposit contains over 4.2 million tonnes(Mt)of contained Zn + Pb metal reserves at 5.09% Zn and 2.40% Pb,and 301 Mt of Fe ore resources at 30.02%TFe,which is the largest and most representative Zn-Pb deposit in the Baoshan block and shows big ore potential at depth with ongoing exploration.Despite decades of study,its genesis is still under debate.Uncertainties remain on whether the deposit is strata-bound/sedimentary-reworked,SEDEX-related or skarn-type,but all of which lack systematic geochemical evidence.The PhD study is based on robust geological survey and detailed laboratory work on the rock/ore petrography,which identified three alteration/mineralization stages in the Luziyuan metallogenic processes:(I)pre-ore(garnet,clinopyroxene,pyroxenoid);(II)syn-ore(magnetite,actinolite,chlorite,epidote,quartz,carbonates,fluorite,mica,sulfides);and(III)post-ore(calcite,quartz,fluorite,chlorite).The alteration and mineralization zoning pattern of the metallogenic system evolution at Luziyuan is also manifested by mineral assemblage change from(deep-level)garnet-clinopyroxene(-pyroxenoid-actinolite)calcic-skarn Fe-Cu mineralization ?(medium-level)pyroxenoid(-clinopyroxene-actinolite)mangano-skarn Zn-Pb-Fe-Cu mineralization ?(shallow-level)Zn-Pb mineralization in the(chloritized)marble.This matches with a decreasing temperature trend away from the concealed intrusion,and the time-space zoning is consistent with typical distal Zn skarn deposits.Systematic sampling and in-situ trace elemental and isotopic analyses were conducted on the rocks and ores from different locations and metallogenic stages at Luziyuan.The major findings include:(1)the skarn minerals are enriched in Zn-Pb-Fe-Cu,suggesting that they are ore-related;(2)calcite from the pre-ore,syn-ore,and post-ore stage shows positive Eu anomaly,no Eu anomaly,and negative Eu anomaly,respectively.This suggests the ore fluids may have changed from oxidizing to reducing along with their evolution pathway;(3)the magnetite has relatively low Al,Ti,Sn,Ga,V,and Cr contents,and falls into the hydrothermal skarn field in the Ni/(Cr+Mn)vs.Ti+V and Ca+Al+Mn vs.Ti+V,Ti vs.Ni/Cr,V vs.Ti,and V/Ti vs.Fe(wt.%)discrimination plots.This further supports a skarn-type origin;(4)the sphalerite is enriched in Mn,Fe,and Co and In-depleted,similar to typical skarn-type sphalerite(mostly Co > 200 ppm,Mn > 1000 ppm),but clearly different from that of SEDEX,VMS(enriched in In,Sn,Ga)and MVT(enriched in Ge,Cd,Tl,and As);(5)C-H-O-S isotopes indicate that the ore fluids were mainly sourced from deep magmatic-hydrothermal fluids mixed with meteoric waters,with input from the carbonate wallrocks and seawater sulfates(possibly through thermochemical sulfate reduction);(6)Pb and Sr isotopes indicate that the ore metals were mainly sourced from the Early Cretaceous granitic magmatic-hydrothermal fluids interacted with upper crustal(wallrock)materials;(7)sphalerite Zn-S isotopes and magnetite Fe-O isotopes indicate that the ore metals were mainly sourced from magmatic-hydrothermal fluids.The deposit geology and geochemical features indicate that the Luziyuan deposit should be classified as a distal skarn Zn-Pb-Fe-Cu deposit,which constitutes a magmatic-hydrothermal mineral system surrounding its ore-causative deeply concealed granitic pluton.