| Volcanogenic massive sulphide(VMS)deposit,also referred to as Volcanic Hosted Massive Sulfide(VHMS).The genesis for this deposit is not only related to the general theory of mineralization,but also involves the interaction of the lithosphere/hydrosphere/biosphere.Therefore,it has been a hotspot of international academia mineralization.In this paper,the most typical of VMS deposits——Sichuan Gacun auriferous and argeniferous polymetallic deposit was choosed.On the basis of geochemistry,zircon U-Pb age,Hf isotope geochemistry,in order to study the sources of ore-forming fluid,we developed quartz Li isotope analysis method,carried out a detailed study of Li-O isotope geochemistry of hydrothermal fluid inclusions in quartz,reshaped the Gacun magmatic-hydrothermal system activity time,defined the origin of magma evolution,got magmatic water/seawater ratio of ore-forming fluids,outline both of them variation in space,and determine the passage of hydrothermal activity.We determined the size range transport,magmatic water-seawater mixing intensity spatial variations,and the relationship between fluid evolution and mineralization,proposed metallogenic model of the deposit.The Gacun deposit is a typical massive sulfide deposit associated with Late Triassic seafloor calc-alkaline felsic volcanics(VHMS).In this paper,the latest rock geochemical data and zircon Hf isotopic data suggest,Yidun island arc is established on the basis of ancient continental crust from Ganzi-Litang oceanic subduction induced mantle melting of calc-alkaline basaltic magma,at the bottom of the crust may occur MASH process,andesitic magma formed,part of them became andesite,and part mixed with magma of ancient crustal melting produced,formed rhyolite that bearing the Gacun deposit.Zircon U-Pb age data defines Gacun magmatic-hydrothermal mineralization sequence of events:238Ma,arc magmatism led to the formation of andesite in the eastern of the deposit.233Ma,in arc zone(the western of the deposit),a large-scale bimodal magmatism formed rhyolitic volcanics,Gacun deposit exists in it.221Ma,volcanic eruptions tended to end,magma eruption lost momentum,subvolcanic intrusion occurred,a lava dome forming,which located under rhyolitic volcanic rocks that contained deposit.Meanwhile,as a "heat engine" driving hydrothermal convection cycle,217±1Ma,large-scale hydrothermal activity peaked.Consequently,the sulfide mineral deposited,Gacun VMS deposit produced.We determined the lithium isotope fractionation between qutarz and fluids,initially established a quartz-fluid Li isotope fractionation empirical formula:??7LiQuartz-fluid=-8.9382 ×(1000/T)+22.22(R2=0.98).We comprehensive ly analysis lithium-oxygen isotopic composition and spatial variation,and quantitatively estimate the ore-forming fluids in magmatic water-sea water mixing ratio,Li isotopes provides new evidence of magmatic water heavily involved in mineralization.All of the results improved water deposit metallogenic model of Gacun.This is the first study of lithium isotopes for deposits forming fluids. |
PDF Full Text Request