贵州西部二叠系峨眉山玄武岩顶部古风化壳钪-铌-稀土矿化富集层的发现与意义

Discovery and significance of the Sc-Nb-REE-enriched zone in the paleocrust of weathering atop the Permian Emeishan basalt in the western Guizhou, China

二叠系峨眉山玄武岩顶部与上覆龙潭组不整合面广泛发育区域性古风化壳,该类古风化壳在川滇黔地区广泛分布,具有产出层位稳定,厚度大,多元素富集特点。目前,总体上对该类古风化壳的成矿和资源潜力研究程度不高,本次在水城—纳雍地区开展系统的岩石地球化学调查发现该层位具有明显的钪、铌、稀土矿化。矿化富集层中w(Sc_(2)O_(3))介于40×10^(-6)~133×10^(-6),平均73×10^(-6),w(Nb_(2)O_(5))介于30×10^(-6)~392×10^(-6),平均229×10^(-6),w(ΣRE_(2)O_(3))介于0.052%~0.214%,平均0.093%。钪、铌、稀土具有较明显的"分异"和"分层"富集特征,钪主要富集于中下部含铁质粘土岩系,形成于弱还原环境,铌、稀土主要富集于上部铝质、碳质粘土岩系,形成于氧化-还原转换环境,稀土元素为LREE富集型,在含碳质粘土岩、碳质页岩中富集程度最高,暗示有机质在稀土元素的迁移、富集过程中可能起到了重要的介质作用。稀土配分、La/Yb-ΣREE图解及Eu异常特征指示物源主要为峨眉山玄武岩,可能有少量中酸性火成岩的贡献。本文研究表明川滇黔地区的这类古风化壳中的钪、铌、稀土具有较大的资源潜力,未来有望成为古风化壳型(古风化-沉积型)钪、铌、稀土矿床的重要产出层,具有巨大的勘查、评价及综合研究价值。

The palaeocrust of weathering,widely developed within the unconformity between the Permian Emeishan basalt and the Longtan Formation in the Sichuan-Yunnan-Guizhou area,is characterized by stable layer,large thickness,and multi-elemental enrichments.However,very few studies have been carried out on its mineralization and resource potential.In this paper,it has been found that there are obvious Sc,Nb and REE enrichments in the palaeocrust of the Shuicheng-Nayong area based on the systematic petrogeochemical investigation.Samples of the enrichment layer have Sc_(2)O_(3) contents ranging from 40×10^(-6) to 133×10^(-6),with an average of 73×10^(-6),Nb_(2)O_(5) contents ranging from 30×10^(-6) to 392×10^(-6),with an average of 229×10^(-6),ΣRE_(2)O_(3)contents ranging from 0.052%to 0.214%,with an average of 0.093%.The Sc,Nb and REE enrichments show obvious characteristics of differentiation and stratification.The Sc enrichment mainly occurred in iron-bearing claystones in the middle and lower parts of the palaeocrustunder a weakly reduced environment.The Nb and REE enrichments occurred mainly in aluminous and carbonaceous claystones in upper part of the palaeocrust under a transition environment from oxidation to reduction.The LREE-enriched patterns and the highest REEenrichment in carbonaceous claystone and carbonaceous shale indicate that organic matter may have played an important role in processes of the migration and enrichment of REE elements.REE distribution patterns,La/Yb-ΣREE diagram andδEu characteristics indicate that REE could be mainly sourced from Emeishan basalts with possibly minor contribution from intermediate-acidic igneous rocks.This study shows that these paleocrusts of weathering in the Sichuan-Yunnan-Guizhou area may have relatively large resource potential of Sc,Nd and REE and could become important layers of the palaeocrust-type (palaeoweathering-sedimentary type) Sc,Nb,and REE deposit in future with great values of exploration,evaluation,and research.