The Ore-forming Fluid And Metallogenesis Of W-Sn Polymetallic Deposits In The Nanwenhe-song Chay Dome Areas,Southestern Yunnan Province

Nanwenhe-Song Chay dome in southeastern Yunnan,situated in the junction of Cathaysia,Yangtze and Indosinian plates,or the interaction range between Tethys and Pacific tectonic domain,is a famous W-Sn-polymetallic mineralization area.With the purpose of fluid and metallogenesis in this paper,we selected the Nanyangtian scheelite deposit,and the Dulong Sn-polymetallic deposit as the subjects.Based on the previous studies,mineral assemblages and relationships,we constrain on compositions,properties,sources,and evolutions of the fluids from different stages,through the thermodynamics and compositions of fluid inclusions,as well as the mineral chemistries and H-O-S isotopes of minerals in different stages（such as plagioclase,garnet,scheelite,magnetite,pyrite,and chalcopyrite）.Finally,we discuss the ore-forming processes,variations,and significances between the Nanyangtian scheelite deposit and the Dulong Sn-polymetallic deposit.The main understandings are as follows:1.The Nanyangtian Scheelite deposit is typically characterized by the development of bedded skarns with km-scale,mostly existing with feldspar-quartz vein rocks,which constitute the most important skarn-type and feldspar-quartz vein-type ores,and generally parallel to the tectonic laminae of various types of schist and gneiss,with consistent folding deformation.Based on mineral assemblages,the ore-forming stages can be divided into the NYT-I stage for plagioclase(NYT-I_plfor short),NYT-II_astage for thermal contact metamorphic siliceous alteration or recrystallized quartz(NYT-II_a-Silfor short),NYT-II_bstage for calc-silicates(NYT-II_b-Pyro,NYT-II_b-Grt)and scheelites(NYT-II_Grt-Sch,NYT-II_Pyro-Schfor short),NYT-III stage for plagioclase（or tourmaline）-bearing quartz vein rocks(NYT-III_Pland NYT-III_Turfor short)and scheelites(NYT-III_Pl-Sch,and NYT-III_Tur-Schfor short),NYT-IV stage for metal sulfides such as pyrrhotite and pyrite(NYT-IV_Po（Py）for short),and NYT-V stage for the disorderly carbonates that be possibly related to the Cretaceous Laojunshan granite.Furthermore,scheelites were mainly formed in the NYT-II_band NYT-III stages.There formed a eutectic texture between scheelite and calc-silicates such as pyroxene and garnet,and a poikilitic texture with tourmaline,quartz and early plagioclase.2.The studies of fluid inclusions and H-O-S isotopes from the Nanyangtian Scheelite deposit show that the ore-forming fluids are classified as the simple Na Cl-H₂O type,which may be derived from the high degree fractional crystallization and extreme evolution process of high f O₂（oxygen-fugacity）magnetite-series（or I-type）granitic magma in the deep.With the rising of the fluids,more metamorphic water and less atmospheric water were mixed in.Concretely,the early（NYT-I）fluids,which can be used as the supercritical fluids（water-rich melts?）of an oxidized and acidic environment with a high temperature（T>400°C）,high pressure（p>300MPa）,high salinity(4-12 wt%Na Cl_equiv),have both fluid and melt properties with H₂O-,F-rich and are conducive to the extractions and migrations of W.Compared with the early fluids,the fluids of middle stages（from NYT-II to NYT-III）show a slightly decreased oxygen fugacity（f O₂）and increased acidity（p H）,which are favorable to the formations of low f O₂,moderate oxidized or reduced skarns and the migrations and precipitations of Ca and W.The late（NYT-IV）fluids,with lower temperature,pressure,salinity,as well as relatively low f O₂and relatively high p H,mainly precipitate metal sulfides ang quartz.From the early to late stage,the fluids show an overall oxidized,acidic fluid with slowly decreased f O₂and continuously increased p H.Furthermore,the reduced T and increased p H of the fluids were the critical factors for the precipitations of scheelite and other minerals.3.From the NYT-I to NYT-IV stage of the Nanyangtian Scheelite deposit,the mineral chemistries of plagioclase,garnet,pyroxene,and scheelite also constrain and support the trends of a slowly decreased f O₂and continuously increased p H.Overall,the early（NYT-I）supercritical fluids（water-rich melts?）could transform into a relatively closed fluid system with the lower water/rock ratio（W/R）or water flux in the NYT-II stage and a relatively open fluid system with higher W/R or water flux in the NYT-III stage.（Tourmaline）feldspar-quartz vein rocks(NYT-III_Pl,NYT-III_Tur)and corresponding types of scheelite(NYT-III_Pl-Sch,NYT-III_Tur-Sch),which may be direct products of and coexisted peraluminous（A-type）and partially alkaline（B-type）residual melts from the highly evolved granitic-magmas,can be analogous to aplite-pegmatite dykes;and W-skarn(NYT-II_Grt,NYT-II_Pyro)and skarn-type scheelite(NYT-II_Grt-Sch,NYT-II_Pyro-Sch)are the products of early exsolution fluids or reactions between exsolution fluids and wall rocks.Meanwhile,the different behaviors of Mo,Sr,and W between the residual melts and exsolution fluids during the further evolutions,as well as the"competition"of F-,Ca-bearing apatite（Ap）,topaz（Toz）,tourmaline（Tur）,garnet（Grt）and other accommodated HREEs or LREEs minerals,both of which directly form the four types of scheelite with different Mo,Sr,and W correlations and REEs partitioning patterns.In addition,due to the extremely low REEs content of the precipitated feldspar and coexisted pyroxene,the NYT-II_Pyro-Schscheelite can represent the ore-forming fluids that better reflect the REEs characteristics of the deep source.4.Dulong Sn-polymetallic deposit is typically characterized by a large-scale bedded skarns alteration,mineralization and mineral zonations from the high to low temperature and the proximal to the distal end of the contact zone,all of which have closely spatial and temporal distributions and relationships with the Laojunshan granitic complex.Based on different mineral assemblages,the ore-forming phases can be divided into the dry skarn stage（DL-I for short）of garnet+diopside,the wet skarn stage（DL-II for short）of actinolite+tremolite+wolframite+magnetite,the oxidation stage（DL-III for short）of cassiterite+iron sphalerite+molybdenite+pyrrhotite+chalcopyrite,the quartz sulfide stage（DL-IV for short）of sphalerite+galena and carbonate mineral stage（DL-V for short）.The five stages are characterized by transitional zones from high-temperature minerals to low-temperature minerals and from oxidation minerals to sulfide minerals.Furthermore,the magnetites of DL-II include the early saccular（or banded）magnetite（I-Mt for short）in the skarn and the later vein magnetite（II-Mt for short）.The paragenetic minerals of I-Mt and II-Mt respectively include diopside+garnet and Fe-bearing sphalerite+chalcopyrite.5.Studies of fluid inclusions and S isotopes from the Dulong Sn-polymetallic deposit shows that the ore-forming fluids are classified as CO₂-N₂-Na Cl-H₂O type,which were mainly derived from the low f O₂ilmenite-series granitic magma in the slightly shallow part of the crust.The initial ore-forming fluids,as the products from highly crystallizations,differentiations,exsolutions of the granitic magma,have typical properties such as high temperature,high salinity,and abundant volatiles of N₂,CO₂,F^-,and Cl^-.With the ore-forming fluids ascending,depressurizing,and cooling,there were generally the boiling or unmixing phenomenons and forming an open fluid system with reduced,alkalinity,fast flow and high water/rock ratio.The changes in F,Cl,and S contents not only reflect and control the forms of extraction,migration,and enrichment of elements in different stages,but also represent the rock-and ore-forming processes that transformations from the metasomatisms with strong material exchange to the filling processes dependent on T,p H,f O₂,and f S₂,or the changes from fluid chemistries to fluid physics.In this transformation process,the continuously decreased f O₂and the significantly increased f S₂should be the key factors to form a large-scale Sn-polymetallic deposit.6.The mineral chemistry of magnetite from the Dulong Sn-polymetallic deposit shows that the major and trace elements change regularly from I-Mt to II-Mt.The II-Mt,which could better represent REEs composition of the ore-forming fluid,inherited REEs distribution trend and negative Eu anomaly of Laojunshan granite,which indicates that magnetite and Cretaceous Laojunshan granite have the same material source.Morever,the magnetite of DL-II and the large-scale metal sulfides,such as Fe-bearing sphalerite,chalcopyrite,pyrrhotite,pyrite and galena,were the diferent stage products of the same metallogenic process,which were enriched by hydrothermal replacement and filling,and belongs to the magmatic-hydrothermal skarn genesis.Moreover,the fluid from I-Mt to II-Mt,was characterized by a slightly higher f O₂and a gradually lower higher temperature（～300°C）,while the large-scale metal sulfides were formed in the low-moderate temperature and reduced fluid environment with continuously declining f O₂and temperature.7.During the Indochina orogeny,as the crust thickening and remelting in the deep environment,partial oxidized I-S granitic magmas were formed,which gradually evolved toward the residual melt of coexisted peraluminous（A-type）and partially alkaline（B-type）by continuous crystallization and differentiation.Subsequently,the Nanwenhe-Song Chay dome began to uplift,and the residual melts and exsolution fluids then rose and cooled,and continued to ascend in p H.The early exsolution fluids or reactions between exsolution fluids and wall rocks formed the moderately oxidized and low-T W-skarns,correspondingly,the skarn+quartz type scheelite.Overall,the mineralization may be at the distal end of the magmatic-hydrothermal system,while the range of Na-rich plagioclase precipitating may be closer to the central or proximal part,all of which also predicts that it is still possible to form skarns and ores at depth.In addition,scheelites contain Mo in various degrees,especially higher Mo（～260 ppm）within the NYT-II_Pyro-Schscheelites,reflecting the possible existence of high-f O₂and high-Mo granitic magma to easily migrate Mo⁶⁺at depth,which also implies the potential for Mo mineralization in depth.Besides,the Sn-and W-rich characteristics within the Nanyangtian garnets also indicate that there may be prospects for finding Late Triassic Sn mineralization in the deep or peripheral part.During the Indosinian-Yanshanian period,with the large-scale lithospheric extensional thinning and upwelling of the asthenosphere,the shallower crust underwent repeated melting and fractional crystallization,controlled by the alternating extension and extrusion,the continual leftward-shearing,and finally formed in the partially reduced,S-type Late Cretaceous Laojunshan granitic complex,and exsolution fluids with abundant volatiles of F,Cl,S.With the fluids depressurizing and cooling,F-,Cl-,and S-bearing complexes were decomposed,and mineral zonations from high to low temperature and from oxides to sulfides were developed from the proximal to the distal end in turn,all of which constitute the super-scale magmatic-hydrothermal skarn system.