The Study Of Ion-Adsorbed Type Of Rare Earth Deposits In Weathering Crust From South Jiangxi And North Guangdong Provinces

First discovered in the late of 1960's in Longnan County, Jiangxi Province, the Zudong ion-adsorbed rare earth deposit is proved to be a new type of rare earth, especially yttrium, resources of China. Later on, this new type of deposits with variety of REE distribution patterns is widely discovered in several provinces of south China and shows significant economic values in industry.The investigation area of this paper is focused on the southern part of Jiangxi and the northern part of Guangdong Provinces where, in tectonic sense, overlap both the post-Caledonian uplifts and the Hercynian-Indosinlan depressions. Since the late of tertiary, warm and humid climate and gentle hilly toporgraphy have provided favourable conditions for development of weathering crust of monosiallitization type. The weathering -crust is composed of three zone s from top to bottom: (1) sell zone, 0.5 to 2 m in thickness, consisting of kaolinite-halloysite(7A)- gebbslte-goethlte in assemblage; (2) weathered zone which is about 3 to 20 m in thickness has halloysite(7A) and kaolinite as the main clay minerals;(3) sub-weathered zone composed of halloysite(7A), kaolinite, some montmorilonite and significant amount of quartz and feldspar fragments. The abrasion pH values are increased from 4.5 at the top of the profile to 6.3-6.5 at the bottom.Rare earth ore bodies, 3 to 10 m in thickness, occur in layer-like form in weathering crust, mainly in weathered zone, of various granitic rocks or intermidiate- basic igneous rocks. The average grades range from 0.08-0.2% RE₂O₃ wiht 0.7% as the highest and 0.04% as the lowest. Most of REE can be extracted by leaching solutions, e.g. NaCI, (NH₄)₂ SO₄, etc. The leaching percentages range from 50 to 90%.The REE distribution patterns of ores are inherited from parent rocks, although there occur slight fractlonatlons among individual REE during weathering process. According to∑Ce₂O₃ /∑Y₂O₃ ratio and Y₂O₃ and Eu₂O₃ percentage in total RE₂ O₃ of ores, the deposits can be divided into five categories. The frequency of the size of deposits obeys logarithmic index distribution withλ= 0.54.In general, REE contained in rock-forming minerals of parent rocks provide only 5 to 50% contributions to the total amount of REE in whole rocks. Accessory minerals, especially rare earth minerals, play most important roles in determining not only REE fractlonatlons during endogenic process, but also REE remobillzatlon in weathering process during which an empirical sequence for susceptibility of rare earth minerals is as follows: carbonates with fluorites, sillicates, niobates and tantalates, arsenites and phorsphates.In endogenic environments, there are mainly five ways for REE fractionations: (1) partial melting; (2) fractional crystallization; (3) thermogravitational diffusion; (4) hydrothermalism and (5) differentiation of metamorphism and migmatism. The (1) and (5) lead to enrichment of LREE and the (2) and (3) are the most important processes for HREE concentration in magma. The(4) has both effects according to the features of the involved hydrothermal fluids. To highlight the variety of mechanism for REE fractionations, three examples are discussed in detail. Based on the data of tectonic setting, Pb and Sr isotopic compositions and population types of zircon, the relationship between genesis of granitic rocks and evolution of REE is also discussed. It ks concluded that local phisycochemical environment of magma is a key constraint to REE fractionations.Through the lnvestegations of the variations of chemical compositions, mineral components and REE distribution patterns in weathering profiles, combined with REE contents and light stable isotopic compositions of oxygen and hydrogen of spring waters and kaolinite veins, it is proved that REE undergo migration and enrichment from soil zone to weathered zoneThe experiments of repeated extraction and leaching process under different pH conditions suggest that the adsorption of REE in clays has strong dependence on pH values. The measurements of C.E.C. and distribution coefficients of clays reveal that REE mineralization in weathering environment is a disequilbrium process. The dynamics of REE enrichment is disscused tentatively by using dissipative structure thoery.In the last part of the paper, the topographic conditions for development of weathering crust in this area are investigated and a map of 1:500,000 scale showing the relationship between the distribution of rare earth deposits/occurences of ion-adsorbed type and the topographic divisions is compiled by using the relief amplitude (R). It is suggested that the dissected hilly lands and less dissected highlands with R values ranging from 100 to 200 meters are the most favourable area for the ore-formatlon.