榍石和石榴子石U-Pb定年和微量元素对矽卡岩型W矿成矿时代和成矿条件的指示:以皖南逍遥钨多金属矿床为例

U-Pb dating and trace element of titanite and garnet as indicators of the age and ore-forming condition of skarn W deposit:A case study of the Xiaoyao W polymetallic deposit in southern Anhui,China

逍遥矿床是安徽南部新发现的大型钨多金属矿床,其精细的岩浆-热液时限及成矿作用仍不明确。本文对其开展了石榴子石和榍石的U-Pb年代学和微量元素分析,发现逍遥矿床发育多阶段榍石(岩浆榍石和热液榍石),岩浆榍石与石英、磁铁矿、长石共生,而热液榍石与石英、方解石、辉钼矿、黄铜矿关系密切。逍遥矿床中石榴子石和榍石中均具有一定的U含量,其中石榴子石中U含量较低(平均为15.9×10^(-6)),岩浆榍石和热液榍石的U含量较高(平均值大于100×10^(-6))。石榴子石和榍石U-Pb同位素定年结果显示成岩和W多金属矿化均形成于150Ma左右,指示W多金属矿化与花岗闪长岩密切相关。岩浆榍石、钾长石、石英、磁铁矿矿物组合指示逍遥成矿岩浆具有氧化性强和富水的特征。逍遥钨矿中的石榴子石以钙铁榴石为主且富集W、Sn特征,指示早期流体为氧化性流体。此外,热液榍石中HFSE元素的高度富集表明其成矿流体为富碱、富F的热液系统,为钨、铅、锌、铜、银、钼多金属的富集提供了有利热液条件。综上所述,石榴子石和榍石能够有效指示矽卡岩W矿的成矿时代和成岩成矿条件。

The Xiaoyao deposit is a newly discovered large-scale tungsten-polymetallic deposit in southern Anhui,China,and its precise magmatic-hydrothermal timing and ore-forming processes are still unclear.In this study,analyses of U-Pb dating and trace element of titanite and garnet from the Xiaoyao deposit were carried out.Multi-stage of titanite(including magmatic titanite and hydrothermal titanite)were identified.Magmatic titanite coexist with quartz,magnetite,and feldspar,while hydrothermal titanite was closely related to quartz,calcite,molybdenite,and chalcopyrite.Both garnet and titanite in the Xiaoyao deposit contain a certain amount of U,with an average U content of 15.9×10^(-6) in garnet and higher U content(>100×10^(-6))in magmatic and hydrothermal titanites.U-Pb isotopic dating of garnet and titanite shows that both the magmatism and W polymetallic mineralization occurred at~150Ma,confirming the genetic relationship between granodiorite and mineralization.The mineral assemblage of magmatic titanite,K-feldspar,quartz,and magnetite indicates that the Xiaoyao causative magma was derived from an oxidized and H_(2)O-rich environment.Garnet from Xiaoyao is dominated by andradite and enriches in W and Sn,indicating that the early ore-forming fluids were oxidized.In addition,the high enrichment of High Field Strength element(HFSE)in the Xiaoyao hydrothermal titanite indicates that the hydrothermal fluid was formed in an alkaline and F-rich system,providing favorable hydrothermal conditions for tungsten,lead,zinc,copper,silver,and molybdenum polymetallic mineralization.In summary,garnet and titanite can effectively indicate the ages and characteristics of skarn-type W mineralization.