富稀土副矿物的风化特征及其对稀土成矿过程的影响--以广东仁居离子吸附型稀土矿床为例

Weathering characters of REE-bearing accessory minerals and their effects on REE mineralization in Renju regolith-hosted REE deposits in Guangdong Province

广东仁居石英闪长岩为燕山期钙碱性长英质岩浆岩,稀土总量为332~338μg/g。石英闪长岩中副矿物榍石的平均稀土总量约为10309μg/g,以其3.29%含量提供全岩52.7%的稀土,因此榍石在风化过程中的行为是离子吸附型稀土矿床中稀土富集成矿的关键。岩相学研究表明,仁居石英闪长岩风化过程中矿物风化顺序为:氟碳钙铈矿→(榍石→磷灰石)/(黑云母/角闪石→斜长石)→钾长石→磁铁矿→石英→锆石。由于榍石和磷灰石的抗风化能力与黑云母和斜长石相近,它们在半风化–全风化层发生溶解,所释放出的稀土元素易于被黏土矿物吸附,因此榍石和磷灰石分解是风化壳中离子吸附态稀土元素的主要来源。稀土元素主要富集于风化壳的全风化层上部5~15 m,稀土总量为504~813μg/g。由于原岩中稀土主要赋存于榍石之中,因此风化壳的稀土配分很大程度上继承了榍石的轻稀土富集特征。研究表明,热带–亚热带地区风化壳基岩中原生矿物的风化顺序和主要富稀土副矿物的抗风化能力是制约离子吸附型稀土矿床中稀土富集成矿的关键因素。

The Renju quartz diorites in Guangdong Province are Yanshanian calc-alkaline felsic magmatic rock with REE contents of 332 to 338μg/g.Titanite accounts for 3.29%of the bedrock mineral content,and has an average REE content of 10309μg/g,contributing to 52.7%of REE content.Thus,the weathering characters of titanite play a vital role in REE mineralization in the regolith-hosted REE deposits.Based on lithofacies observation,the mineral weathering sequence during the weathering of Renju quartz diorites is as follows:parasite→(titanite→apatite)/(biotite/amphibole→plagioclase)→K-feldspar→magnetite→quartz→zircon.As the weathering resistance of titanite and apatite is comparable to that of biotite and plagioclase,titanite and apatite are dissolved in the semi-weathered to completely weathered horizon,where the leaching REEs are adsorbed on clay minerals.This makes titanite and apatite the main source of ion-exchangeable REEs in the weathering crust.REEs are mainly enriched in the upper part of the completely weathered horizon at a depth of 5 to 15 m,with a content of 504 to 813μg/g.Because REEs in the bedrock are mainly enriched in titanite,the LREE enrichment characteristic in the weathering crust is mostly inherited from the fractionation pattern of titanite.In light of the above findings,in tropical and subtropical regimes,the weathering sequence of primary minerals and the weathering resistance of main REE-bearing accessory minerals are key factors constraining the REE mineralization in regolith-hosted REE deposits.