Dolomite Typomorphy Of The Ore-hosting Hydrothermal Dolomites From The No.Ⅰ Orebody(group)at The Giant Maoping Zn-Pb Deposit In Northeastern Yunnan Province,SW China

Carbonate rocks are the main host rocks of the Mississippi Valley-type(MVT)Zn-Pb deposits,and are commonly associated with hydrothermal dolomitization and various sizes of ore-bearing hydrothermal dolomites(HTDs).The mineral composition of HTDs of the direct and indirect ore-hosting rocks is similar,so the precise identification of the mineral typomorphic characteristics of different types of dolomites of HTDs and the recorded information of Zn-Pb ore-forming fluids can provide an important basis for in-depth understanding of the ore formation of MVT Zn-Pb deposit and key guidance for related ores prospecting.The Maoping Zn-Pb deposit in northeast Yunnan Province,is a giant MVT Zn-Pb deposit in the Sichuan-Yunnan-Guizhou Zn-Pb metallogenic belt in the western margin of the Yangtze Massif.The confirmed Nos.I,II and III orebodies(groups)occur in the Upper Devonian dolomites and Carboniferous limestones along the core of the inverted anticline and its inverted lime of the upper wall of the NE-trending Maoping thrust fault,respectively,which constitute the main mining bodies.Amongst them,the ore-hosting dolomite of the No.I orebody(group)shows the spatial zonation of fine-mesocrystalline dolomite → medium-coarse crystalline dolomite →coarse-megachrystalline dolomite → lead-zinc sulfide orebody from the distal part to the proximal part of the main orebodies in turn,and their geological characteristics are relatively clear.Therefore,combined with the detailed geological observation of mining tunnels in different level,the mineral chemical typomorphic characteristics of different types of dolomites in HTDs are systematically analyzed,and the genetic correlations between hydrothermal dolomitization and ore-forming fluids evolution is determined.The main achievements and understandings are as follows:(1)The mineralization-and alteration-zoning of the No.I orebody(group)is characterized by dark gray-gray fine-medium crystalline dolomite zone at the distal part,gray-light gray medium-coarse crystalline dolomite zone at the middle,and light gray-gray white coarse-very coarse crystalline dolomite zone at the proximal part of the main orebodies,respectively.The direct ore-hosting rock is HTDs,and the indirect ore-hosting rock is the buried dolomites.(2)The ore-hosting HTDs of the No.I ore body(group)constitute a "hydrothermal dolomite" similar to some "intrusive body".According to the microscopic observation,it can be successively divided into dark brown hemideomorphic fine-mesocrystalline dolomite → dark brown-brown hemideomorphic fine-mesocrystalline dolomite → brown-light brown hemideomorphic coarse-extremely coarse dolomite from outside to inside of the orebodies.The number of intercrystalline pores and intrachrystalline pores increased successively,and the degree of recrystallization increased gradually,and the mineral growth zone was developed differently.Among them,the latter two are ore-bearing surrounding rocks related to mineralization,and the latter is more closely related.At the same time,from the far end of ore to the near end of ore,the coarse-grained dolomite particle proportion of ore HTDs increases to about 80%,while the mesocrystalline dolomite particle proportion decreases to about 15%.Moreover,the coarse-grained dolomite and the mesocrystalline dolomite of coarse-megachrystalline HTDs in the fine-mesocrystalline dolomite zone both account for 50%.The fine-mesocrystalline dolomite at the far end only shows growth zone around local pores,while the coarse-megachrystalline dolomite at the near end generally develops growth zone,showing obvious spatial heterogeneity and recrystallization.These characteristics indicate that the dolomite recrystallinity of HTDs gradually increases in response to ore-forming fluids from the far end to the near end,and its spatial heterogeneity may be caused by ore-forming fluids preferentedly migrating along the fissure channel of the preexisting fault and continuously occurring metasomatism with varying strength.(3)The mineral typomorphic characteristics of dolomite show that the buried dolomite in the indirect surrounding rock of No.I Zn-Pb body(group)is rich in Fe and Mn,with Fe/Mn ratio of 1.42～3.42,with high Zn and Pb contents and no Eu anomaly.Compared with the buried dolomite dolomite,the content of Fe and Mn in HTDs dolomite at the early stage of mineralization is slightly lower,the ratio of Fe/Mn is3.86～6.85,the content of Pb has no obvious change,the content of Zn is relatively reduced,and the content of Ga,Ge and Eu are higher negative anomalies.The dolomite of HTDs in the main ore-forming stage is Fe-poor and Mn poor,the ratio of Fe/Mn is 0.78～2.86,the content of Zn and Pb is significantly reduced compared with HTDs in the early ore-forming stage,and there is no obvious anomaly in Eu.The coarse-grained dolomite of HTDs at the late stage of mineralization is Fe-rich in Mn,Fe/Mn ratio tends to 1,with weak positive Eu anomaly.(4)The HTDs are tectonically driven ore-forming fluids that enter into each other along the fissure channels of preexisting faults and have different metasomatism with HTDs and buried dolomite surrounding rocks at earlier stages,thus controlling the rhythmical precipitation of HTDs dolomite and lead and zinc metal sulfide.The element composition and temperature of ore-forming fluids show an evolutionary trend of anisotropic fluctuation from morning to night.This genetic mechanism is of universal for all MVT Zn-Pb deposits.