Insights Into Evolution Of Metallogenic Fluid At The 26°S Hydrothermal Field On Southern Mid-Atlantic Ridge

The Mid-Atlantic Ridge,as a slow-spreading ridge,is characterized by few vents,a stable tectonic environment,a long ore-formation history,and massive sulfide deposits on the seafloor,which are rich in metals such as Cu,Zn,Pb,and precious metals.China has found 12 hydrothermal sites between 12°S and 30°S,showing prosperous mining prospects and significant economic value.The three-stage five-year plan has completed prospect investigation,general exploration,detailed exploration,and delineated anomalous mineralized areas.During this time,Chinese researchers completed the abandonment of 75%of the area and locked the mineralized areas,laying the foundation for applying for mining licenses.The 26°S hydrothermal field（SMAR 26°S）,also known as the Xunmei hydrothermal field,was first discovered by Chinese Cruise in 2011.A large number of sulfide chimneys and sulfide debris were observed in this field.There are no discharged fluid and fluid inclusions which can be directly analyzed due to only inactive chimney samples and melt inclusions were found in this site,which hinders the research on the evolution of hydrothermal fluids.Overwhelming researchers host an opinion that the formation of SMS is controlled by the seawater-rock interaction during the dual convective circulation of hydrothermal fluid,whereas several lines of evidence verify magma and magmatic volatiles contribution to the formation of polymetallic sulfides in the SMS.In addition,previous researchers ignored the integration of the petrographically different sulfides,mineral assemblages,geochemical characteristics of the fluid,and the fluid evolution during chimney growth,but mainly focused on analyzing the entire black smokers.Therefore,our study focuses on one critical scientific problem,i.e.,insights into the evolution of metallogenic fluid in SMAR 26°S hydrothermal field,including the paragenetic sequences of sulfides,the migration of metals,the source of ore-forming fluids,the variations in the physicochemical condition,and the assessment of the systematics of trace element variations during chimney growth,carrying out a comparative study of petrography（SEM）,geochemistry（EPMA and LA-ICP-MS）and isotopes（SIMS）for basalts,black smokers and sulfide debris.SMAR 26°S located at 26°12’on a smooth topographic high within the center of the asymmetrical 26°S segment.This site is primarily basaltic in composition（N-MORB）.Melt inclusions can sometimes be seen in feldspar phenocrysts of basalts.The results show that polymetallic sulfides,such as chalcopyrite,pyrite and magnetite,were adhering to the bubble wall of melt inclusions in phenocrysts of basalts,indicating that ore-forming metallic elements and sulfur derived from the volatile-rich melt.These ore-forming elements could enter into the volatile phase and transport/precipitate during the magma degassing process.The inactive black smokers and sulfide debris collected from the researched area are predominantly composed of Fe sulfides（pyrite and marcasite）,followed by chalcopyrite and amorphous silica,as well as only trace amounts of sphalerite and isocubanite.The inactive black smokers display concentric mineral zones of different mineral assemblages.From the exterior part to the central conduit,it can be divided into the porous outermost chimney wall with low temperatures（zone A）,the porous intermediate zone with medium to low temperatures（zone B）,the dense interior zone with medium to high temperatures（zone C）,and the high-temperature central zone（zone D）.The petrographically different pyrites precipitated throughout the entire chimney.Pyrite nodules（PyⅠ）,dendritic pyrite（PyⅡa）and banded colloform pyrite（PyⅡb）precipitated in the lowest-temperature stage;to a higher temperature,concentric colloform pyrite（PyⅡb）and porous massive pyrite（PyⅢa）were dominant sulfide in the intermediate stage;massive pyrite（PyⅢb）and coarse-grained euhedral pyrite（PyⅣ）were the production of the high-temperature stage.According to the structure of chimneys and paragenetic sequences of sulfides,the hydrothermal activities are divided into the main hydrothermal stages and late mature stages,where the main stages experience the initial low-temperature Fe-Zn-rich stage,medium/low-temperature Fe-rich stage in the intermediate zone,to inner medium/high-temperature Fe-rich stage,and then to central high-temperature Cu-rich stage.Fine-grained euhedral pyrite,fine-grained chalcopyrite with dissolved boundaries and isocubanite-chalcopyrite intergrowths,which distributed in the open-pore spaces between early formed minerals,are formation products of the Cu-Fe-rich late mature stage.Based on the chemical reaction equation and research on the thermodynamic equilibrium constant p H-f O₂,it is speculated that the hydrothermal fluid is an acid fluid with low f O₂ in a reducing environment,and the sulfur in the system exists in the form of H₂S.The hydrothermal fluid evolved from acid to weakly alkaline,resulted in the precipitation of sphalerite.Chalcopyrite precipitation indicates that the late ascent fluid is alkaline with low oxygen fugacity.According to the textural relationships and microanalytical data of all Cu-Fe-S phases,the early stage of the hydrothermal system in the researched site is formed at around～300℃.All evidence advocates the SMAR 26°S hydrothermal field as both an immature short-living system and high mature system,which show the different sulfide association.The chimney composed of medium-low temperature mineral assemblages represents a relatively lower maturity and is characterized by nodules,dendrites,colloform pyrites,sphalerites,and isocubanite-chalcopyrite intergrowths.The chimney sample,which is consisted of high-temperature mineral assemblages,with complicated textures and morphology as well as coexistence of multi-stage sulfides,represents a higher maturity.High maturity chimneys are typically formed by coarse-grained euhedral pyrite and coarse-grained chalcopyrite with dissolved boundaries.In general,the hydrothermal system evolved from low temperature,low maturity,weakly acidic and low f O₂ fluids to high temperature,higher maturity and weakly alkaline fluids.In-situ S-isotope compositions of morphologically different pyrites from the SMAR 26°S hydrothermal field integrated with S and Cu isotopic values in chalcopyrite reflect a systematic evolution of hydrothermal fluids and the dominance of magmatic fluids.Theδ³⁴S values of sulfides（-1.27 to+5.25‰）indicate mixing of magmatic fluid,seawater,and volatile-rich magmatic fluid.The negativeδ³⁴S values for chalcopyrite indicate that magmatic volatiles can be used as a carrier for metallogenic materials.The positiveδ³⁴S values for pyrites are a proxy for a mixture of seawater and hydrothermal fluid to different extents.The unsteady increase inδ³⁴S from the interior conduit to the exterior wall and gradually increase in sulfur contribution from seawater sulfate（0 to 10.7%,and to 18.3%）are related to isotopic interaction between previously precipitated sulfides and hydrothermal fluid,and a greater contribution of seawater sulfate sulfur,which is caused by increase in permeability or porosity across the chimney.Theδ⁶⁵Cu values of chalcopyrite vary from+0.17 to+0.48‰,indicating the contribution of magmatic volatiles and the fluid mixing.The systematic variations of sulfur isotopes for pyrite,negative sulfur and Cu isotopic composition of chalcopyrite further imitate the significant contribution of magmatic fluid in SMS,which is consistent with the evidence from melt inclusion from basalts.All these features indicate that magmatic degassing and phase separation during the evolution of basalt magma cause metal elements preferentially incorporated into volatiles,thus providing the necessary material source for massive sulfide mineralization.In order to eliminate the dismutation of different sulfide minerals on trace elements,in-situ analysis of morphologically different pyrites formed at various paragenetic stages enables the interpretation of fluid evolution during chimney growth.It was confirmed for the first time that magmatic fluid dominance to the ore-forming process.The behavior of trace elements is strongly associated with the various physicochemical conditions and multiple metallogenetic processes,where the enrichment of trace elements is seen more frequently at the initial stage,whereas depletion of them is dominant at late stages.The outermost chimney wall enriched in Zn,Cd,Tl,Ag,Pb,Sb,Mn,Mo,and V is induced by mixing of seawater and hydrothermal fluid;the intermediate zone enriched in Cu,Au,Te,and Bi are influenced by magmatic volatile influx;the interior zone contains high Co and Se,which is controlled by magmatic fluid.The variation of trace elements in different paragenetic stages of pyrite reveals that the hydrothermal system might have evolved from low-temperature low-chloride liquid-dominated fluids to vapor-dominated magmatic fluids,and then to high-temperature fluids（magmatic fluids）,indicating the metallogenetic fluid might mainly be derived from magmatic fluid and vapor-dominated fluid with less leached fluid from water-rock interaction.Systematic variations in trace element compositions of hydrothermal fluids induced by different physicochemical conditions can also be well revealed by the law of normal distribution of trace elements.In conclusion,the metallogenetic model for the SMAR 26°S differs from the traditionally held view of seawater-rock interaction,that is under the premise that hydrothermal fluids are affected by water-rock reaction and seawater-fluid mixing,the magmatic fluids and volatile-rich magmatic fluids formed during degassing can be considered as the principal sources of ore-forming fluids,constituting the dominance of the hydrothermal systems.