The Nannihu-sandaozhuang Mo(W) Deposit Typical Research

The Nannihu-Sandaozhuang molybdenum deposit is the largest porphyry-skarn type deposit in the eastern Qinling region.In this paper,we report new LA-ICP-MS zircon U-Pb age data of granitoids(including granitic vein)and ICP-MS molybdenite Re-Os isotopic age data of the samples from the mining area.The obtained formation ages of granitoids are 145.2±1.5?146.7±1.2Ma,and the model ages of 5 molybdenite samples from stockwork in the Nannihu mining area and 5 molybdenite samples from disseminated skarn in the Sandaozhuang mining area are 143.4±2.0?146.5±2.3Ma with isochron age of 146.0±1.1Ma.These new geochronological data show explicitly that the granitoids and the ore formed simultaneously revealing its genetic relationship.Further more,these new data have confirmed the existence of magma-thermal event at about 158Ma revealed by former dating work,and also this is the first time to reveal that the eastern Qinling area once undergone another magma-thermal event at about 175Ma.The published weighted average age data are of distinct deviation from the real intrusive age of the Nannihu magmatic body,which is resulted from 1)unhomogeneous origin of zircons in magmatic rock and 2)unrepresentative dating of zircon group.Based on the observation on the field geology,the microscopic and backscattered electron(BSE)images,a two stage formation processe of the skarn from the Nannihu-Sandaozhuang Mo(W)deposit is recognized.On the first stage,the fluid reacts with marble to form wollastonite,andradite,grossularite,hedenbergite and diopside.As the fluid outward,the dark limestone is replaced by diopside skarn or wrigglite diopside skarn.On the second stage,the fluid spreads into the wall rock along fractures to form new skarn superimposing on the earlier skarn.According to these observation,the skarn minerals such as garnet and pyroxene could be divided into two generations.The first generation of garnet(Grt?)(Gro3-82And15-96)is spot-like in wollastonite marble or skarn,and the first generation of pyroxene(Px?)(Di18-86Hd13-70Jo0-13)could intergrows with Grt? in wollastonite marble or skarn and with plagioclase(Ab55-70An30-44)in wrigglite diopside skarn.The second generation of gamet(Grt?)(Gro23-58And37-74)and the second generation of pyroxene(Px?)(Di0-68Hd28-84Jo3-16)superimposed on Grt?and Px? with smaller space area.The mineralization of molybdenite and scheelite has commenced at the very beginning of skarnization and throughout the whole skarn formation process.The local reducing environment,induced by the reaction between the fluid,with adequate concentration of Mo and W elements,and the wall rock may be the trigger of metal precipitation.The hydrothermal ore-forming process of the Nannihu-Sandaozhuang Mo(W)deposite can be divided into four stages,from early to late,including(1)skarn and hornfels stage,(2)Quartz-K-feldspar and retrograde stage,(3)Quartz-sulfides stage and(4)quartz-carbonate stage.According to our study on homogeneous temperature,salinity and chromatographic analysis of fluid iclusions and Carbon,Hydrogen and Oxygen isotope from different stages.We argue that the ore-forming fluid sourced mainly from magmatic system and subordinately from the carbonate strata with minor organic matter.The alleged mospheric water probably contribute little to the ore-forming.From stage(1)to(3)then to(4),the ore-forming experienced three times of phase unmixing consisting of immiscibility between melt and volatile-rich hydrothermal fluid,unmixing between hydrothermal fluid and volatile-rich water,and phase separation of CO2 rich gas from water caused by suddenly decompression.