Mineralization Of VMS-type Deposit Within The Changning-Menglian Suture Zone,yunnan Province,SW China

Changning-Menglian suture is a important suture zone which is located in the south of Sanjiang orogenic belt,SW China.A series of VMS deposits related to oceanic island basalt(OIB)and oceanic ridge basalt(MORB)were formed in the process of oceanic basin expansion,such as Laochang Pb-Zn-Ag deposit and Tongchangjie Cu deposit.The comparative study of two types of deposits in different mineralization environments plays an important role in deepening the understanding of the mineralization of the Paleotethys' VMS-type deposit,but this work has been lacking.Based on system analysis of regional geological characteristics and important magmatic event,this paper is dedicated to study of field geological,volcanic rock elements and isotopic geochemistry,ore-forming chronology,fluid inclusion thermodynamics and C-O-S-Zn isotopic composition in Laochang deposit and Tongchangjie deposit,trying to restrict Changning-Menglian suture zone Paleotethys' space-time frame,find out ore-forming fluid characteristics and metallogenic material source.Try to extract the similarities and differences of metallogenic factors in Laochang deposit and Tongchangjie deposit,and establish the metallogenic model.The Laochang deposit reserves in Carboniferous-Permian volcanic rocks-marine carbonate sequences.Laochang basalts have typical characteristics of OIB elements and isotopes,and the U-Pb age of basalts is 320.7±2.6Ma,which is similar with the isochron age of galena and sphalerite in massive ore bodies(308±25Ma).It is proved that early carboniferous exists the mineralization occurs in the same period of diagenesisThe research of fluid inclusion in Laochang deposit shows that there are no boiling inclusion and three phase inclusion.The laser Raman shows that Laochang's fluid inclusion don't have CH4 rather than CO2.The metallogenetic temperature of massive ores is 113-240? and salinity is 6.0~14.4 wt%NaCl eqv.By contrast,the metallogenetic temperature of stockwork ores is 254-440?,and salinity is 3.9?19.8wt%NaCl eqv.The ore-forming fluid of Laochang have a low-middle temperature,low-middle salinity.The reducing of metallogenetic temperature from the lower stockwork mineralization to the upper massive orebody reflects the process of high temperature metallogenetic hydrotherm rised to seafloor along the venting channels and cooled down with seawater.By contrast,the salinity did not decrease significantly during the process,which may be related to the continuous addition of magma fluid.The result of C-O isotope shows that stockwork ores ?13C is-11.8‰?3.87‰ and?18O is 8.12‰?14.20‰.By contrast,the massive ores ?13C is-6.17‰-2.71‰ and ?18O is 10.74‰?13.15‰.The ?13C of stockwork ores is closer of mantle carbon than the?13C of massive ores,which shows that from stockwork ore bodies to massive ore bodies the mantle carbon decrease.It is supposed to be the mixed source of mantle carbon and marine carbonate.The stockwork ores ?18Ofluid is 3.11‰?9.19‰,and the massive ores ?180fuid is-0.09‰?2.32‰,which shows that stockwork ores ?180fluid is closer to magmatic hydrothermal,and the massive ores ?18Ofluid is closer to seawater.It is supposed to be the mixed of magmatic hydrothermal and seawater.The ?34S values of various types are almost same,range from-1.43‰?1.32‰,shows it's mantle source characteristics.The Zn isotope result shows the source of Zn is basalt.Therefore,it is speculated that Laochang deposit was formed by magmatic degassing.The minerals of Laochang deposit is Pb-Zn-Ag,shows that the Pb element in Laochang deposit needs to be supplied from other source.Therefore,it is speculated that heat driven leaching marine carbonate rocks and volcanic rock and formed hydrothermal solution enriched with Cu,Pb and Zn.Finally,the unloading of ore-bearing hydrothermal fluid on the seabed leads to the formation of massive sulfide deposits.Therefore,it is believed that Laochang deposit is the transitional cause of VMS-SEDEX.Tongchangjie deposit reserves in Carboniferous-Permian ophiolite of Tongchangjie.Tongchangjie basalts are MORB-type.The research of fluid inclusion in Tongchangjie deposit shows that there are no boiling inclusion and three phase inclusion.The metallogenetic temperature of massive ores is 110-189? and salinity is 4.0?10.2 wt%NaCl eqv.By contrast,the metallogenetic temperature of stockwork ores is 201-351?,and salinity is 5.1?15.5wt%NaCl eqv.The ore-forming fluid of Laochang have a low-middle temperature,low-middle salinity.The reducing of metallogenetic temperature from the lower stockwork mineralization to the upper massive orebody reflects the process of high temperature metallogenetic hydrotherm rised to seafloor along the venting channels and cooled down with seawater.The ?34S ranges from-0.16‰?3.31‰,shows it's mantle source characteristicsIn summary,the VMS deposits in the Changning-Menglian suture zone are all formed in the early carboniferous.The ore-forming fluid is characterized by the mixing of magmatic hydrothermal fluid and seawater.The ore-forming materials of Laochang deposit have the characteristics of mantle source and magmatic degassing heat driven leaching marine carbonate rocks.The metallogenic materials of Tongchangjie deposit have the characteristics of mantle source.