江西相山下家岭稀土矿风化球体中褐帘石的精细矿物学研究及其成矿指示意义

A detailed research on allanite in weathered sphere from the Xiajialing REE deposit in Xiangshan,Jiangxi,and its metallogenic significance

下家岭稀土矿位于江西相山火山岩盆地,为风化壳淋积型稀土矿床。在风化壳剖面各成分层中,球形风化层稀土元素最为富集。为了探索稀土元素在球形风化层的富集规律,本文利用偏光显微镜、扫描电镜、电子探针等多种手段,重点对球形风化层中风化球体的主要稀土矿物开展了精细矿物学的研究工作。研究发现原生褐帘石经改造后,发育了极其复杂的成分环带,衍生出多个富含稀土的“相”,根据硅、镧、铈等主要元素的含量差别,可初步划分为:高硅高镧富稀土相(SiO_(2):11.51%~19.77%,La_(2)O_(3):12.69%~17.54%,Ce_(2)O_(3):6.80%~8.49%,∑REE_(2)O_(3):46.14%~61.34%)、低硅高镧富稀土相(SiO_(2):0.58%~6.78%,La_(2)O_(3):21.09%~24.64%,Ce_(2)O_(3):4.45%~6.34%,∑REE_(2)O_(3):68.96%~81.52%)以及低硅高铈富稀土相(SiO_(2):0.45%~1.79%,La_(2)O_(3):12.89%~19.16%,Ce_(2)O_(3):20.66%~26.59%,∑REE_(2)O_(3):67.65%~77.6%)三种不同类型,这些相各自成分变化均较大,成分均一性较差。分析后认为,这种富稀土的特征,可能部分因为钍的放射性衰变使得富钍褐帘石发生蜕晶化并体积“肿胀”,“肿胀”后的褐帘石更容易让流体通过,同时α衰变还可以让矿物内部发生电离,可能会对带有电荷的稀土络合物产生吸附作用,从而造成稀土元素富集。不同富稀土相的形成,则可能是因为在风化作用的不同阶段,上部淋积下来的稀土种类不同导致。当低硅高铈富稀土相形成后,则会进一步被流体溶蚀分解,以供给下层稀土成矿。

The Xiajialing REE deposit is located in Xiangshan volcanic basin,Jiangxi Province.It is a weathered elution-deposited REE deposit.Among the layers of the weathering profile,the content of REEs in spherical weathering layer is the highest.In order to explore the enrichment law of rare earth elements in the spherical weathering layer,the microscope,the scanning electron microscope and the electronic microprobe were used in this study.The main REE minerals of the weathered sphere in the spherical weathering layer is focused on.It is found that after the REE-rich phase transformation of the allanite,an extremely complex compositional ring had developed,and many REE-rich“phases”had been derived.According to the content difference of silicon,lanthanum,cerium and other main elements,It can be preliminary divided into:the REE-rich phase with high silicon and high lanthanum(SiO_(2):11.51%~19.77%,La_(2)O_(3):12.69%~17.54%,Ce_(2)O_(3):6.80%~8.49%,∑REE_(2)O_(3):46.14%~61.34%),the REE-rich phase with low silicon and high lanthanum(SiO_(2):0.58%~6.78%,La_(2)O_(3):21.09%~24.64%,Ce_(2)O_(3):4.45%~6.34%,∑REE_(2)O_(3):68.96%~81.52%)and the REE-rich phase with low silicon and high cerium(SiO_(2):0.45%~1.79%,La_(2)O_(3):12.89%~19.16%,Ce_(2)O_(3):20.66%~26.59%,∑REE_(2)O_(3):67.65%~77.6%).These phases have large composition ranges and poor composition uniformity.After analysis,it is considered that this REE-rich feature may be partly due to the radioactive decay of thorium,which made Th-rich allanite undergo amorphization and volume expansion,and the expansive allanite was easier for the fluid to pass through.Theαdecay can also ionize the interior of minerals,which may adsorb charged REE complexes,resulting in the enrichment of REEs.The formation of different REE-rich phases may be due to the different types of REEs leached from the upper part at different stages of weathering.The REE-rich phase with low silicon and high cerium will be destroyed by fluid to supply REEs to the lower part for REE mineralization at last.