Experimental Evidence For The Formation Of Jinchuan Ni-Cu(PGE) Sulfide Deposit,China

Transport of coexisting sulfide liquids and silicate melts in partially molten peridotite plays an important role in the sulfur cycle among Earth's various geochemical reservoirs and contributes to the redistribution of chalcophile elements within the upper mantle as well as the genesis of magmatic Ni-Cu sulfide deposits.Taking the world-class Jinchuan Cu-Ni deposit,China as an example,this study documents the topology and considers controls on the extraction of two immiscible liquids?sulfide and silicate melts?in the partially molten lithospheric mantle?the source of magmatic sulfide deposits?by using laboratory experiments,theoretical calculations,and X-ray synchrotron microtomography and combining with mineralogy and geochemical methods.The Jinchuan deposit that is the third largest Ni deposit in the world has been systematically studied by many geologists at home and abroad about the mineralogy,petrology,geochemistry and ore-forming processes of the deposit.However,it remains unclear how the distribution and transport of two immiscible liquids are and whether the crustal sulfur contributes to the formation of the Jinchuan deposit.Using Nano SIMS,this study conducted in-situ sulfur isotope analyses on pyrrhotite,pentlandite and chalcopyrite from some different types of representative ore samples in the Jinchuan deposit.The measured sulfur isotope(?³⁴S)in the disseminated,net-textured and massive sulfide ores mainly ranges from-4.5‰to 2.7‰,showing the characteristic of the mantle sulfur.In contrast,those hydrothermal sulfide ores display positive?³⁴S values?average value?2.05‰?,but negative values of?³⁴S?average value?-3.27‰?can be observed in hydrothermal-modified magmatic sulfide ores.The complementary characteristics of sulfur isotope values between hydrothermal ores and hydrothermal-modified magmatic sulfide ores imply that the hydrothermal process after the formation of the Jinchuan deposit can potentially enhance the migration of heavy sulfur isotope(³⁴S).Moreover,no obvious sulfur-bearing formation is found in the Jinchuan region.Therefore,although the magmatic homogenization of sulfur isotope occurred in magmatic sulfide ores may cover up sulfur isotope characteristics of the crust,the mantle sulfur is probably the main source of the Jinchuan deposit.However,the contribution of crustal material cannot be completely ruled out.Then,in the absence of the crustal sulfur,for the Jinchuan deposit with the massive accumulation of sulfide,what geological processes in the source of magmatic sulfide deposits can lead to such efficient accumulation of sulfide in the Jinchuan intrusion?To explore the issue,this study conducted three series of laboratorial experiments-hydrostatic partial melting experiments,deformation experiments and two-layer reaction experiments to systematically investigate the distribution and transport of coexisting sulfide and silicate melts in a partially molten peridotite.Under hydrostatic conditions,the measured dihedral angle at silicate melt-mineral-mineral junctions is 13.7-21.3°.Silicate melt is distributed along grain edges forming incompletely interconnected melt channels that disconnected by some dead ends at melt fractions?7-9 vol%.Application of theoretically predicted permeability(k?10^-14-10^-16m²)permits estimation of the extraction velocity of silicate melt of 0.7-11.1?m/day within a single interconnected melt channel.In the absence of silicate melt,isolated sulfide droplets?3.77 vol%?show a sulfide-olivine-olivine dihedral angle of 91.5-101.3°.However,in the presence of silicate melt,sulfide droplets?average size?2.53±2.14?m,1??are partially surrounded by silicate melt and stranded in triple junctions or melt pockets due to the limitation of the smallest dimension?0.3?m?of melt channels.Thus,the extraction of sulfide liquid is highly restricted by these dead ends and the smallest dimension of melt channels during porous flow of silicate melt.In contrast,during large-strain shear deformation?strain?1.6-2.5?,initially stranded sulfide droplets were elongated and extracted with silicate melt into liquid-rich sheets with a length of several hundred microns,constantly oriented at 14.3±4.5°to the shear plane and antithetic to the shear direction.The angle is lower than that?18-30°?of those sheets containing sulfide liquid only,indicating that silicate melt dominates liquid-rich sheets.Driven by stress,silicate melt-dominated liquid-rich sheets open the appropriately oriented grain boundaries between silicate minerals,thus providing an efficient pathway for the extraction of sulfide liquid during deformation.In addition,some two-layer reaction experiments were also conducted to explore the feasibility of reaction-driven sulfide migration along high-velocity silicate-melt channels.Reaction infiltration instability?RII?can drive the segregation of melt when a reactive fluid percolates through a dissolvable matrix.And,a positive feedback between melt flow and melt-rock reaction can lead to the channelized flow of silicate melt.With increasing duration from 12h to 72h,the formation of more silicate-melt channels?about?21 melt channels,the longest length?420?m?and the transport of more sulfide droplets?the largest size?10.7?m?into a depleted peridotite were observed due to the increase of the local permeability,and these sulfide droplets were usually distributed within melt channels.Based on the theoretical calculation,the migration velocity??0.09?m/s?of silicate melt in these melt channels is about two orders of magnitude higher than that(?10^-4?m/s)along the porous flow in the silicate matrix,thus,sulfide droplets in the liquid source can be entrained into melt channels by silicate melt with the high migration velocity.However,at a longer duration,the presence of some melt-channel relics implies that melt channels are temporary and ultimately closed when the reaction infiltration of silicate melt reached equilibrium in the depleted peridotite.Furthermore,theoretical calculations?high Da value?2.44*10⁴-1.15*10⁵>10³,low Pe value?0.66-1.63<10?indicate that both the reaction rate and diffusion rate of silicate melt are much larger than the transport rate in the system,that is to say,the RII of the system is suppressed.Moreover,the Zener pinning of sulfide enhances the RII,thereby facilitating the formation of temporary melt channels.Therefore,these two-layer reaction experiments demonstrate that sufficient silicate melt disequilibrium with solid phases in a liquid source potentially facilitate the formation of transient silicate-melt channels by enhancing the RII of the system and ultimately lead to the mechanical extraction of sulfide liquid along with these melt channels during reaction infiltration of silicate melt.Moreover,in the presence of the vapor phase,at a low pressure?0.5 GPa?,some incompletely interconnected melt channels are formed driven by the RII in the upper part of samples.Both theoretical analyses and experimental observations indicate that the vapor phase prefers to absorb on the surface of sulfide droplets forming some vapor-sulfide aggregates?the wetting angle between the vapor phase and sulfide droplets?99.7-101.4°,90%confidence interval?.Apparently,the density of vapor-sulfide aggregates is lower than that of sulfide droplets,contributing to the migration of sulfide droplets.Based on two-layer reaction experiments containing the vapor phase,longer duration,higher reaction temperature and higher vapor content can facilitate the formation of more vapor-sulfide aggregations,further enhancing the transport of more and larger sulfide droplets into melt channels.Therefore,the combination of the reaction infiltration instability and the bubble flotation is also an effective driving force to promote the physical migration of sulfide droplets.Because tectonic stable geological conditions are more conducive to the preservation of vapor-sulfide aggregates,for the Jinchuan deposit forming in a tectonically active continental rift zone,the contribution of differential stress and RII to the transport of sulfide is apparently greater than that of gas flotation.In conclusion,driven by deviatoric stress,RII and/or bubble flotation,sulfide droplets can be efficiently mobile through high strain domains of the upper mantle,contributing to the addition of chalcophile elements and the fertilization of the lithospheric mantle.This may be the first step of the sulfide ore formation.