Study On Metallogenic Overprinting And Resources Of The Dongyaozhuang Gold Deposit,Wutai Mountain,Shanxi Province,China

The Hengshan-Wutai-Fuping region is an important gold region of the North China Craton,due to its location on paleo-continental margin,the complexity of the tectonics and openness of the gold host systems make it diffcult to figure out the time,processes and sources of those Archean terrane hosted gold deposit,which would be barriers to understand the genesis of such type of Au deposits and their further exploration.The Dongyaozhuang?DYZ?Au deposit,located on center-south part of the Wutai Mountain,hosted in Arhean volcanic-sedimentary series?greenschist faces?.We did detailed study on its filed geology,mineral assemblages and fluid inclusions,analyzed chemical composition of the fluid inclusions and estimated their trapped P-T conditions;for chlorites and pyrites in various stages,we analyzed their mineral chemistry to investigate problems such as ore foming conditions,time,overprinting processes and sources.We finally got results and conclusions as follows:?1?By integrating field work with petrography and mineragraphy,we clarified the mineralization stages of the DYZ gold deposit.At least two mineralization events were revealed with field geology and mineral assemblages:the first episode features deformed quartz-pyrite veins filling in the regional foliation?MI,Paleoproterozoic?and can be divided into two stages?MIa and MIb?based on whether the mineralization is accompanied by tourmalinization.The mineral assemblages are mainly quartz+carbonate+pyrite+chalcopyrite?+tourmaline for MIb?;both are cut by deformed barren quartz veins,which represent the final stage of MI.The second episode features undeformed stockworks/veinlets with a mineral assemblage of quartz+carbonate+pyrite+chalcopyrite+arsenopyrite±native gold?MIIa?,crosscutting MI;these ore bodies are also cut through by undeformed barren carbonate and/or quartz veins?MIIb?,which represent the end of metallogeny.?2?Dating the ore forming age by pyrite Re-Os isotopesPyrites of four samples from the MII yield an isochron age of 1237±6Ma(MSWD=4.8,¹⁸⁷Os/¹⁸⁸Os_i=0.110+0.050),which is interpreted as the actual age of gold emplacement and imply the potential connectation with the Mesoproterozoic tectonic-heat event?s?.Whereas those elder model ages such as 1838Ma and2257-2637Ma,may be responses to different tectonic-heat event?s?in ancient time.?3?Constrained physichalchemical conditions of two stages of mineralization with fluid inclsuions,geothermometers?chlorite and biotite?.Types of fluid inclusions from various stages are similar with each other,they uniformly show low salinity?0–12.6 wt%NaCl equiv?CO₂-bearing aqueous solution.By using the results of chlorite geothermometer as the trapping tempteratures,the physicalchemical conditions of MI and MII are constrained to be:MI:320⁴40?,2.2-3.7kbar,QFM-NNO?fo₂,biotite?;MII:200??or lower?³20?,0.8¹.5kar??4?By integrating the major and minor elements of pyrites with C-H-O-S,we investigated the sources of the DYZ gold deposit.Major and minor elements of pyrites reveal:1)MII is characteristiced by,relative to MI,more concentrated in As,Bi,Sb,Pb,Au,Sr,Cu,Ag,Rb,Ba,Zr,W,Mn;2)pyrite of MI has multi-sources;3)pyrite of MI have both vocanic and hydrothermal genesises,whereas MII pyrite are mainly hydrothermal pyrite;5)a dissolution-reprecipitation process was found on pyrite in MII,which is companied by Au remobilization-enrichment-precipitation.C-H-O isotopes from different stages reveal:the fluid of both MI and MII are metamorphic fluids;S isotopes indicate MI and MII share similar material?e.g.Au and S?origin,and the Mass-independent fractionation(?³³S)indicate the source?s?of the MI and MII would have a close link with Archean-Paleoproterozoic crust?especially sediments?,for example,materials of MI would come from dehydration of paleo-sedimentary??2.32Ga?+vocanic rocks;while the source of the MII may have connections with metamorphic fluids triggered mantle uppwilling or even mantle plume during 1.3-1.2Ga,which containminated by Archean-Paleoproterozoic sedimentary materials when upwilling.?5?Invistigate mineralization overprintingBy synthetically considering the crosscutting relations of veins,characteristics of mineragraphy,overprinting of Re-Os age,discrepance of trapped pressure of fluid inclusions in a mineral scale as well as similarity of trace elements between part MI and MII would eventually indicate that the Dongyaozhuang gold deposit is a orogenic gold deposit formed with different mineralization events at different period of techtonic-heat background.