黔北务—正—道地区新木—宴溪铝土矿含矿岩系底部稀土元素富集机制

The REE enrichment mechanism in the bottom layer of ore-bearing rocks of the Xinmu-Yanxi bauxite deposit in the Northern Guizhou,China

通过对黔北务—正—道地区新木—宴溪铝土矿样品主量和稀土元素的系统分析,探讨了黔北铝土矿稀土元素富集特征以及底部层位中稀土元素的富集机制。研究认为:1)介质pH值以及剖面上铝土矿化程度是稀土元素富集的关键影响因素,含矿岩系底部由于地下水活性减弱以及碳酸盐岩基岩的影响,pH为中性到碱性,有利于稀土元素聚集,且铝土矿稀土元素含量随着风化程度的降低而增加;2)铝土矿底部的更高含量的粘土矿物是造成铝土矿含矿岩系底部稀土元素富集的重要因素之一,REE主要的寄主矿物磷灰石也更易于在含矿岩系底部保存,而铝土矿底部自生稀土矿物的存在也是造成含矿岩系底部稀土元素富集的因素之一;3)针铁矿对稀土元素尤其是轻稀土元素发挥了重要的吸附作用,新木-宴溪铝土矿底部层位Fe含量相对较高,且含矿岩系底部接近中性到偏碱性的pH值,更有利于针铁矿的形成。4)稀土元素富集与氧化还原环境有关。该矿床底部部分样品Ce明显负异常,说明弱氧化的沉积环境可能有利于稀土的富集。样品的δCe与(La/Nd)N近似呈弱的负相关关系,表明轻稀土的富集可能与介质的氧化还原条件有关,在弱氧化的条件下,轻稀土越富集。

Based on a systematic analysis of major and rare earth elements in rocks and ores of the Xinmin-Yanxi bauxite deposit in the Wuchaun-Zheng’an-Daozhen area of the Northern Guizhou,we have discussed enrichment characteristics and mechanism of the REE in the bottom layer of ore-bearing rocks of the Xinmin-Yanxi bauxite deposit in this paper.It is believed that the pH value of medium and the degree of bauxite mineralization in profile of the deposit are the key factors affecting the enrichment of rare earth elements.Due to the reduction of activity of underground water and the influence of carbonate rock,the bauxite-bearing rocks have pH values varying from neutral to alkaline,which is beneficial to the REE enrichment in the bottom part of the ore-bearing rock series and the increase of REE contents in bauxite ores with the decrease of weathering degree of bauxite ores.The relatively high REE content of clay minerals in the bottom part of ore-bearing rock series is one of important factors for the enrichment of REE.The major REE hosting mineral of apatite can be easily preserved in the bottom part of the ore-bearing rock series.In addition,the presence of authigenic REE minerals in the bottom part of the bauxitic layer could also one of important factors for the enrichment of REE in the bottom part of the ore-bearing rock series.Goethite has played important role for adsorption of the REE especially the LREE.The relatively high Fe contents of the bottom part of bauxite layer of the Xinmu-Yanxi bauxite deposit and the weakly alkaline to alkaline pH values for the formation environment of the bottom part of the ore-bearing rock series are more favorable to the formation of goethite.The REE enrichment is associated with the redox environment.The obvious negative Ce anomalies of some samples in the bottom part of ore-bearing rock series of the Xinmu-Yanxi bauxite deposit indicate that an weakly oxidized sedimentation environment could be favorable to the REE enrichment.The weakly negative correlation between δ Ce and(La/Nd)N values of samples indicates that the enrichment of LREE may be related to the redox condition of the medium of sedimentation,with the relatively high enrichment of LREE in the bauxite-bearing rock series under the weakly oxidized condition.