Tourmaline In The Maocaoping Vein Cu Deposit,Western Yunnan:Constraints On Fluid Source And Evolution

A large number of structurally controlled vein-type Cu deposits and prospects occur along the western border of the Lanping Basin,western Yunnan,China.Although many studies have been carried out,the evolution of ore-forming fluid during mineralization is still unclear,the source of ore-forming fluid remains controversial and the genesis of the deposits is still under discussion.Maocaoping is a newly-discovered vein Cu deposit containing abundant hydrothermal tourmaline,which has not been found in the other vein Cu deposits in this region.This paper studies the growth characteristics,chemical and B–H–O isotopic compositions of three stages of ore-related hydrothermal tourmaline in Maocaoping deposit,as well as the chemical and B isotopic compositions of magmatic tourmaline?tourmaline-G?from a neighboring peraluminous granitic intrusion that is contemporaneous with the Cu mineralization.The objective was to understand the control factors of tourmaline growth and composition and constrain the source and evolution of the CO₂-rich ore-forming fluid.The Maocaoping cupriferous veins?type V1 and V2?are hosted in mylonitized and metamorphosed Jurassic sedimentary rocks.Early-to late-stage hydrothermal tourmaline occurs as vein-related alteration halos?tourmaline-A?in altered marble or schist,as tourmaline-V1 in type V1 veins,and as tourmaline-V2 in type V2 veins.Tourmaline-A is anhedral,fine-grained,aligned parallel to the mylonitic foliation of host rocks.Tourmaline-V1 is euhedral,coarse-grained,and sub-parallel to the mylonitic foliation of host rocks.Tourmaline-V2 is euhedral,relatively fine-grained,and either randomly orientated or exhibit comb-growth texture.The growth characteristics of them reflect that the shear stress during the formation of tourmaline at Maocaoping was evolving from strong shearing,weak and no shearing.Tourmaline-A,-V1,and-V2 belong to the alkalic group and consist mostly of dravite,with?¹¹B values of–3.9‰to–1.1‰,–2.8‰to+0.5‰,and–6.3‰to–3.9‰,respectively.Tourmaline-G belongs to the alkalic group and consists of schorl,with?¹¹B values of–14.7‰to–12.2‰.The similarities in chemical composition between marble-and schist-hosted tourmalines and between tourmalines in veins and alteration halos?indicating their formation at different water/rock ratios?suggest that the compositions of the hydrothermal tourmaline at Maocaoping were controlled mainly by the chemistry of ore fluid rather than that of the host rocks.The similarities together with the lack of systematic variations in?¹¹B values in individual tourmaline grains imply that mixing of multiple fluids cannot be important during the mineralization.The positive shift in?¹¹B values from tourmaline-A to tourmaline-V1 is interpreted to result from continuous precipitation of tourmaline in a closed fluid system.Relative to tourmaline-A and-V1,tourmaline-V2 is Fe-rich,Al-poor,and ¹¹B-depleted.These differences may be explained by fluid phase separation and subsequent escape of the CO₂-rich vapor phase during the formation of type V2 veins,consistent with the observations that these veins are extensional,contain large amounts of carbonate minerals,and include fluid inclusions trapped in the vapor–liquid two-phase field.The earliest tourmaline-A records the initial ore fluid chemistry.The calculated?¹¹B,?¹⁸O,and?D values of the initial ore fluid in equilibrium with tourmaline-A at mineralization temperatures of 280 to 320°C are–0.3‰to+3.0‰,+10.7‰to+11.8‰and–110‰to–84‰,respectively.The?¹¹B values as well as the chemical compositions of tourmaline-A are much different from those of tourmaline-G,suggesting that the ore fluid at Maocaoping was not derived directly from the peraluminous granitic magma.The?¹⁸O values combining with the enrichment of CO₂ suggest that the ore fluid at Maocaoping cannot solely be derived from meteoric water or basinal brine but can directly come from a metamorphic fluid.However,the lower?D values of the ore fluid are atypical of a metamorphic fluid.The simplest explanation is that the fluid was derived from a metamorphic fluid that had obtained isotopically lighter hydrogen via reaction with“organic”compounds?e.g.,CH₄,H₂S?.Alternatively,the ore fluid sourced from the mixed metamorphic fluid and basinal brine,meteoric water,or degassed magmatic fluid.It implies that the vein Cu ore formation in the Lanping Basin is unlikely as a result of a sole magmatic or basinal fluid activity and it needs the involvement of metamorphic fluid/process.The significantly higher Fe³⁺content in late-stage tourmaline than that in early-stage tourmaline reflect that the pre-ore stage has relatively low oxygen fugacity than the ore stage.This,combined with the characteristics of ore-forming fluid and alteration,metal assemblages,and the lithology of host rocks in the other vein Cu deposits in the western Lanping Basin,suggest that a reduced,CO₂-buffered pH value fluid is responsible for Cu transportation for the vein Cu deposits.Fluid boiling and water/rock interaction may play important roles in the Cu precipitation.Comparison for S and Pb isotopic compositions indicates that Cu and S in vein Cu deposits may be derived from the VMS copper mineralization in the underlying Late Permian to Early Triassic arc volcanic rocks.