浪都夕卡岩型铜矿床中石榴子石稀土元素地球化学研究

REE geochemistry of garnets from the Langdu skarn copper deposit.

文中对浪都矿床夕卡岩中石榴子石进行了主量和稀土元素研究。分析结果表明研究区石榴子石为钙铁榴石-钙铝榴石固溶体系列,成分变化于Ad_(87)Gr_(13)-Ad_(92)Gr_8之间,以钙铁榴石为主。与世界上很多夕卡岩矿床中石榴子石REE配分模式截然不同,研究区石榴子石稀土元素总量较低、配分模式表现为轻稀土富集,重稀土亏损,并且具有明显的Eu正异常。研究显示,浪都矿床钙铁榴石是在水/岩比值较高的环境下快速形成,其与流体之间并没有完全达到REE平衡。岩浆热液中REE的配分模式、表面吸附可能为制约石榴子石REE含量及配分模式的主要因素。Eu^(2+)(r=1.25 A)与其他REE^(3+)相比具有更大的离子半径,更容易被吸附在石榴子石晶体表面,可能是形成浪都矿床中石榴子石Eu正异常的主要原因。

This study focuses on major and rare earth element compositions of garnets from the Langdu skarn copper deposit.Electron microprobe analyses show that the garnets from the Langdu deposit belong to grossular-andradite solid solution ranging in composition from Ad_（87）Gr_（13）to Ad_（92）Gr_8.The garnets from the Langdu deposit are characterized by much lower∑REE content and LREE-enriched and HREE-depleted pattern,with a significantly positive Eu anomaly,which is obviously different from those of the majority of skarn deposits worldwide.The present study indicates that the andradites from the Langdu deposit grew rapidly and were not in complete equilibrium with the ore-forming fluids,indicative of high W/R ratios.Under such conditions the incorporation of REE in andradites was controlled by crystal kinetics（surface adsorption and occlusion）and the REE pattern of the magmatic fluid.Eu（2＋）（r=l.25 A）has a bigger ionic radius than the other REE（3＋）, making it easier to be adsorbed by garnet crystals,which could have been the dominant control on the strongly positive Eu anomaly in garnets of the Langdu deposit.