黔东南油麻坳金矿床含金石英脉钨富集机理

Tungsten enrichment mechanism of gold-bearing quartz veins in the Youma’ao gold deposit,southeastern Guizhou Province,China

黔东南油麻坳金矿床产于下江群清水江组低绿片岩相浅变质岩系中,具有典型造山型金矿特征。与其他造山型金矿一样,含金石英脉中以W为代表的高温亲石元素富集机理尚不清楚。通过岩相学特征分析,应用电感耦合等离子体质谱(ICP-MS),对油麻坳金矿床含金石英脉、近矿围岩及赋矿地层样品的岩石学特征、微量元素和稀土元素系统研究。结果表明,含金石英脉中除Cu、Pb、Zn、As等主要成矿元素外,亲石元素W也相对富集,并以极少数白钨矿单颗粒形式赋存;稀土元素分布特征为轻稀土富集、重稀土亏损的右倾型配分模式,δEu异常不明显,δCe呈弱负异常。R型聚类分析表明,W、Mo等成矿元素与变质作用密切相关的Rb、Sr、Ba、Ta、Zr、Sc、V、Sn等上地壳分散元素和弱矿化元素具有成因联系。综合分析认为,钨富集机理为较低温压条件下,成矿流体携带的W与局部高含量钙质矿物结合,以白钨矿形式赋存于含金石英脉中,随后期变质作用加强,W被化学活动性流体携带进入近矿围岩,当蚀变达千枚岩化时,石英脉体中W几乎全部进入近矿围岩及赋矿地层中。

Host rocks of the Youma ’ao gold deposit in southeastern Guizhou are mainly low greenschist facies metamorphosed rock series in the Neoproterozoic Qingshuijiang Formation of the Xiajiang Group. The Youma’ao gold deposit is a typical orogenic gold deposit. So far,the enrichment mechanism of high-temperature lithophile element of W in gold-bearing quartz veins is unclear. The petrography and inductively coupled plasma mass spectrometry( ICPMS) are used to determine trace elements and REE contents of the gold-bearing quartz veins,wall-rocks and orehosted strata rocks collected from the Youma’ao gold deposit. The results show that the lithophile element W is also relatively enriched besides the enrichment of main ore-forming elements( e. g. Cu,Pb,Zn,and As,etc.) in the gold-bearing quartz veins. There are slightly right-declined REE patterns,with enrichment of LREE,no obvious Eu anomaly,and weakly negative Ce anomaly. The R-type cluster analysis shows that W,Mo of ore-forming elements have close genetic relationship with Rb,Sr,Ba,Ta,Zr,Sc,V,Sn of upper crust disperse elements or weak mineralized elements. Above all,the W enrichment mechanism is believed that the W in the W-bearing fluid was firstly combined with the local high-content calcareous minerals under low temperatures and pressures to have formed the scheelite particles in the quartz-bearing vein. Secondly,with the increase of the metamorphism intensity,the W was reactivated by the chemically active fluid and then gradually transported into the wall-rock. Thirdly,at the stage ofphyllitization,almost all of the W which was activated and removed from the quartz veins was then transported into the wall-rock and ore-bearing strata.