| The Laozhaiwan gold deposit in YUN NAN is a low-grade and large-reserves fine-disseminated gold deposit located at the Dian-Qian-Gui triangular space.Base on field work and laboratory study,we think the ore-body are strictly controlled by stratum and fracture fault,and gold ore-body is mostly distributing between F1 and F8 fracture,a few distributing by the east of F8 faultage;the gold ore-body's North-South direction is besieged by F6,F2 and F4 fractures.The host strata are clastic rocks of lower series of upper Devonian in Caledonian paleo-uplift zone.According to detailedly discussing mineralogy,rock structure and texture, finding out the Laozhaiwan gold mine includes three metallogenic stages in Hydrothermal period,which is the early ore-forming stage characterized by quartz-pyrite,thesecend ore-forming stage characterized fine vein quartz-pyritesericite stage,and the late ore-forming stage characterized by Stibnite-carbonate.Study of the compositions,densities,pressures,depth and sulfur, plumbum,hydrogen and oxygen isotopes of fluid inclusions in quartz of the hydrotherrnal ore-forming processes of the Laozhaiwan gold deposit indicates that the ore fluids belong to low-and medium-temperature(110～253℃),low-salinity (23.86～11.2 Wt%NaCl),low-density(0.97～1.08g/cm3)ones and the(SO42-) Cl-(Ca2+)Na+ hydrochemical type and formed in a shallow(0.21～1.14km) emvironment.The ore fluids mostly derived from the mixture which been exchanged from mantle differentiation fluids and meteoric water,surface water.Laozhaiwan gold deposit may be a meso-and epithermal gold deposit.The analysis of fluid inclusion composition indicates that gaseous composition are mainly H2O,CO2,including CH4,CO2 and N2,which shows ore-forming fluid comes from mantle,and is mixed by strata circulating fluid.The composition ofδ18OH2Oof fluid inclusion composition is-1.11‰～5.27‰,andδH2Ois-109‰～-90‰. On theδD-δ18O illustration,7 samples is outside magmatic water and metamorphic water isotopic region.Based on the above analysis,the thesis thinks ore-forming fluid comes from mantle differentiated fluid H and O isotope and formation water,meteoric water and surface water.The average distribution of sulfur isotope(δ34S‰)in Laozhaiwan is 5.35‰(The average distribution of sulfur isotope from mantle is 1～2‰.).The above indicates the first source of massive sulfide,especially pyrite and stibnite in diabase,is differentiated fluid from mantle,which is mixed by sulfur of marine sulfates(The cambrian and ordovician,located at the lower of host strata,have marine strata.)and make the value of mixed sulfur isotope positive.Based on enrichment ofδ18O in pyritized epidiabase of diabase dikes,this paper indicates deep fluid is mixed when going up;Meanwhile,the part of high Au content (10.15g/t～12.82g/t)in pyritized epidiabase of diabase dikes indicates deep fluid move along faulted and shattered zone,extracting ore-forming elements and mineralizer, and comes into being ore-forming fluid,which provides this region with ore-forming material source.Combining with the silicon isotope geochemistry and silica cathode luminescence research method to track the silica source have proved that mineralization silica is not provided by surrounding rocks,that is,the origin of gold deposit mineralization of Laozhaiwan is the same to silica,all come from the differential upper-mantle ore-forming fluid.In summary,this thesis thinks the metallogenic mechanism of Laozhaiwan gold ore deposit is as follow:First,fine grained clastic source bed of low Devonian is formed;Second,form parcel geothermal field during late magma hydrothermal activity, which made Au of first source bed activated again and formed a structure-magmahydrothermal circulation system;third,the system extracts ore-forming elements and mineralizer along the passage,and,further more,form ore-bearing hydrothermal solution including Au,As,Sb,S;the last,ore-bearing hydrothermal solution moves along fracture crushing belt of Laozhaiwan,going with transformation of physical chemistry condition,in faulted and shattered zone and the section of unconformity surface compound,gets across altered and metasomatic host strata and then come into being siticification and pyritized veinlet impregnation type gold deposits.
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