REE concentration processes in ion adsorption deposits: Evidence from the Ambohimirahavavy alkaline complex in Madagascar
REE concentration processes in ion adsorption deposits: Evidence from the Ambohimirahavavy alkaline complex in Madagascar
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DOI：
10.1016/j.oregeorev.2019.103027
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发表时间：
2019-09-01
影响因子：
3.3
通讯作者：
Nason, Peter
中科院分区：
文献类型：
--
作者：
Estrade, Guillaume;Marquis, Eva;Nason, Peter
离子吸附矿床是稀土元素吸附在粘土矿物表面的矿床，是目前世界上重稀土元素（Gd-Lu）的主要来源。离子吸附沉积物中稀土元素的富集被认为是一个占主导地位的表生过程，其中易降解的稀土矿物（如稀土氟碳酸盐）分解并释放稀土元素，然后吸附到风化物质中的粘土矿物表面。在这里，我们提出的数据，从新生代Ambohimirahavavy碱性复杂的马达加斯加，进一步约束控制的形成和重稀土富集过程中离子吸附deposits.The红土风化剖面描述的碱性火成岩，包括SiO2不饱和和过饱和岩性的目的。后者包括稀土矿化的过碱性花岗伟晶岩和花岗脉岩。风化矿物学包括三水铝石、粘土矿物和少量铁、锰羟基氧化物。X-射线衍射和红外光谱表明，在所有网站的粘土部分主要是高岭石和埃洛石（7埃和10埃）。用硫酸铵（0.5 M（NH 4）（2）SO 4，pH = 4;除去保留在固体表面上的弱吸附金属并溶解碳酸盐）提取表明，在不同剖面和沿着同一剖面，可浸出的REE含量是不均匀的，范围为5 ~ 2300 mg/kg总REE，重REE含量为1 ~ 32%。浸出与氯化镁（0.5 M MgCl 2，pH = 6，去除只有弱吸附的稀土元素）释放类似的总稀土元素浓度，这表明大多数可浸出的稀土元素吸附到矿物表面。在大多数红土剖面中，可浸出稀土元素的量随着深度的增加而不断增加。回收率取决于稀土元素的原子序数，一般从La到Lu下降，除了Ce总是low.In Ambohimirahavy复杂的，原岩的性质是控制的红土风化剖面中容易浸出的稀土元素的量的主要因素。水文和地形是次要因素。最有利的原岩包括SiO2-不饱和火山岩和蚀变泥岩与花岗岩岩墙。主要的原生稀土矿物为钠长石类矿物（纯渗析）和褐帘石-（Ce）。在局部花岗伟晶岩脉中，晚期岩浆流体的自交代作用通过将易风化的花岗伟晶岩矿物转化为不风化的锆石，抑制了离子吸附矿的形成。
Ion adsorption deposits, in which the rare earth elements (REE) occur adsorbed onto clay mineral surfaces, currently provide the world's dominant supply of heavy REE (Gd-Lu). Concentration of REE within ion adsorption deposits has been proposed to be a dominantly supergene process, where easily degradable REE-minerals (e.g. REE-fluorcarbonates) break down and release REE that are then adsorbed onto clay mineral surfaces in the weathered material. Here we present data from the Cenozoic Ambohimirahavavy alkaline complex in Madagascar, with the aim of further constraining controls on the formation and HREE enrichment processes in ion adsorption deposits.The laterite weathering profiles described here are developed on alkaline igneous rocks, including both SiO2-undersaturated and oversaturated lithologies. The latter group includes REE mineralised peralkaline granitic pegmatites and granitic dykelets. The weathering mineralogy includes gibbsite, clay minerals and minor Fe and Mn oxyhydroxides. X-ray diffraction and infrared spectroscopy show that the clay fraction in all sites is dominated by kaolinite and halloysite (7 angstrom and 10 angstrom). Extraction with ammonium sulfate (0.5 M (NH4)(2)SO4, pH = 4; removes weakly adsorbed metals retained on the solid surface and dissolves carbonates) indicates that, in the different profiles and along the same profile, the leachable REE content is heterogeneous, ranging from 5 to 2300 mg/kg total REE, with 1 to 32% heavy REE. Leaching with magnesium chloride (0.5 M MgCl2, pH = 6; removes only weakly adsorbed REE) releases similar total REE concentrations, suggesting that most of the leachable REE are adsorbed onto mineral surfaces. In most of the laterite profiles, the amount of leachable REE continuously increases with depth up to the saprock. Recovery rates depend on the REE atomic number, generally decreasing from La to Lu except for Ce which is invariably low.In the Ambohimirahavavy complex, the nature of the protolith is the main factor controlling the amount of easily leachable REE in the laterite weathering profile. Hydrology and topography are secondary factors. The most favourable protoliths include SiO2-undersaturated volcanic lithologies and altered mudstone with granitic dykelets. The main primary REE minerals include agpaitic minerals (eudialyte) and allanite-(Ce). Locally in granitic pegmatite dykes, autometasomatism by late magmatic fluids inhibits formation of ion adsorption ore by transforming easily weathered agpaitic minerals into unweatherable zircon.