Fluid Evolution And Genetic Mechanisms For The Giant Dahutang Tungsten Deposit In Northern Jiangxi Province,South China

The Dahutang W-Cu-Mo deposit is a typical deposit in the tungsten metallogenic belt of northern Jiangxi,and the earliest super-large tungsten deposit discovered in northern Jiangxi.It has been estimated with more than 2 million tons of WO₃ along with 500000 tons of Cu and 20 000 tons of Mo.The ore field consists of three major sections:the south section,the north section,and the Dawutang section,consisting of Shimensi,Dalingshang,and Shiweidong ore deposits.The geological characteristics,metallogenic stages,and mineral assemblages of each ore section at Dahutang are similar.The tungsten mineralization is closely related to the Yanshanian multiple-stage?130-150 Ma?granites,dominated by veinlet-disseminated scheelite mineralization with some quartz-wolframite vein mineralization.In this paper,the ore-forming process in the Dahutang area will be discussed deeply through the comprehensive study of ore deposit geology,fluid inclusions,trace element geochemistry and isotope characteristics of minerals to discuss the properties and sources of ore-forming fluids,the migration and precipitation conditions of ore-minerals.These data provide evidence for explaining the ore-forming mechanism and elucidating the genesis of super-large ore deposits.At the same time,it will enrich the metallogenic theory and help guiding the prospecting and exploration of similar deposits.Petrographic observation and microscopic measurement of fluid inclusions in ore and gangue minerals show that both ore and gangue minerals were deposited in Na Cl-H₂O?CH₄±N₂±CO₂?hydrothermal systems with medium-to high-temperature of 190-440 ^oC,low salinity of 0.5-12 wt%Na Cl_eq and low density.CH₄ and N₂ in inclusions are dominant over CO₂,reflecting that the metallogenic system was in a reducing environment with low oxygen fugacity.CO₂ only appears in the sulfide stage after a large amount of tungsten has been deposited.The homogenization temperature of wolframite in the silicate-oxide stage?320-440°C?is generally higher than that of the coexisting quartz?200-300°C?,while most of the quartz in the late sulfide stage is 190-240°C.The homogenization temperatures of scheelite range from 280 to 350°C,while that of apatite range from 260 to 330°C.The salinity of the Dalingshang and Shimensi deposits in the north section decreased obviously in the late sulfide stage,but there were no obvious changes in the Shiweidong deposit in the south and the No.1 ore belt in the Dawutang section.The precipitation of metal elements in Dahutang hydrothermal system is accompanied by different fluid processes:there is no obvious addition of meteroic water in the formation of wolframite,scheelite,and apatite in the early stage,and precipitation of W occurred by the process of simple cooling;In Shimensi and Dalingshang deposits,more and more meteoric water was involved in the sulfide stage due to the development of structural fissures,and thus fluid mixing is an important mechanism for the precipitation of base metal sulfides such as Cu and Mo sulfides.Melt inclusions in granites and fluid inclusions in hydrothermal veins in Dahutang generally contain solid minerals of chalcopyrite,indicating that there is a considerable amount of Cu in residual melts and in hydrothermal fluids,which also proves the magmatic source of ore-forming materials.S-Pb isotopes of sulfides further prove that some samples in the sulfide stage contain depleted?³⁴S values?-14.4 to-0.9‰?,which may reflect the change of fluid environment due to the addition of late oxidizing meteoric water,causing an increase of oxygen fugacity.This is consistent with the results of fluid inclusions and H-O isotopes.The trace elements of scheelite and apatite,major elements and B isotopes of tourmalines show that fluid-rock interaction is an important mechanism for the precipitation of tungsten minerals,especially scheelite.In the process of fluid-rock interaction,plagioclase is decomposed to release a large amount of Ca,Eu and Sr into the W-rich fluid to form scheelite,apatite and other minerals.During the successive precipitation of scheelite and apatite,the REE patterns of these minerals change from diped type with negative Eu anomaly to flat type with positive Eu anomaly.As scheelite and apatite are preferentially enriched in middle rare earth elements?MREE?,the precipitation of them in the early stage can gradually deplete fluids in MREE,which changes the REE patterns of later-formed scheelite and other minerals.Tourmalines in the Dahutang deposits can be divided into DT,ST and VT types,with the VT type?VT-1 and VT-2?tourmalines from quartz veins recording the change of ore-forming fluid environment during the reaction between ore-bearing fluid and Neoproterozoic granodiorite surrounding rocks.The B isotope data of ST-type tourmaline in Neoproterozoic Shuangqiaoshan Group are obviously lower than that in nearby veinlets,reflecting different source attributes.In extensive wallrock metasomatism,the formation of wolframite,scheelite and sulfide minerals are contributed by materials in the surrounding rocks.By summarizing the basic geological and mineralogical characteristics,Sr,H,C,S isotopes of Dahutang and other large to super-large tungsten deposits,we found that water-rock reaction is widely developed in many tungsten deposits and is an important mechanism for changing fluid properties and causing tungsten mineral precipitation.