Genesis Of UST At Qulong, Tibet: Implication For Formation Of Giant Porphyry Cu Deposit

Porphyry deposit is one of the most important deposits in the world,mainly occurred in island arc and continental margin arc environment.In recent decades,geologists found porphyry deposits also can be output in continental collision environment.There are still many unclear issues in the metallogenic model of porphyry deposits when describe magama-hydrothermal process.Such as where were the ore-forming fluids from?Was there a metal pre enrichment process in the lower magama chamber?The key of solving these scientific problems is the special texture——unidirectional solidification structure?UST?,which formed during the magama-hydrothermal transfer process.Qulong porphyry Cu-Mo deposit is the largest one of Gandese porphyry deposits belt.We found continuous five meters UST samples in Qulong drill hole.Based on the detailed SEM-CL of UST,we carried out the fluid temperature measurement,single inclusion composition analysis,quartz trace elements research,sulfide trace elements research,oxygen isotope studies and rutile situ LA-ICP-MS U-Pb dating.Through the comparison and analysis of the results in order to answer the above issues.Compared the composition of quartz and the properties and physicochemical conditions of fluid inclusions in different UST layers,from the top to the bottom there were no obvious changing,therefore the fluid that formed UST was from the lower magma chamber and the physical conditions of initial fluid exsolution was T>575?,P>200Mpa,this is the further proof of the exitence of the lower magma chamber.The individual fluid inclusion composition analysis results show that the Cu/Mo ratio?304?of fluid inclusions is approximately equal to Cu/Mo ratio?300?of the deposit metal reserves,suggests the exsolved fluid from the lower magma chamber carring the metal and formed the deposit.First time obin the oxygen isotope composition of UST quartz,the?¹⁸Oresult is+6.2‰to+7.6‰,show the UST is the exsolution result of magma fluid,without the addition of atmospheric water.For the first time we found the sulfide melt inclusions in Qulong UST quartz layers,we carried out the situ composition anlysis of the sulfide melt inclusions and found it significantly different from the hydrothermal sulfide.The existence of sulfide melt inclusions in UST quartz layers suggests the lower magma chamber occurred sulfur saturation,the pre enrichment of metals and sulfur in lower magma chamber is likely the key factor of forming the super large Qulong porphyry Cu-Mo deposit.According to dating result of rutile in situ LA-ICP-MS U-Pb is as the same as the dating result of molybdenite in Re-Os dating.This give us a new thinking of choosing dating minerals.If the rutile has clear symbiotic relationship and genetic relationship with the metal sulfur like pyrite,chalcopyrite,or molybdenite,we can use the rutile dating age to represent the ore-forming age.This can solve the problem that some deposit has no molybdenite or little molybdenite that hard to pick out.