粤北左坑离子吸附型稀土矿床含矿岩体地球化学特征与稀土元素迁移‒富集机理

Geochemical Characteristics of the Granite Bedrock and REE Migration and Enrichment Mechanism of the Zuokeng Ion-adsorption REE Deposit,Northern Guangdong

广东省新丰县左坑大型稀土矿床为典型的华南离子吸附型矿床,位于佛冈复式岩体的北部,已查明稀土矿石量17152万吨,总稀土氧化物(TREO)资源量约18万吨,离子相稀土氧化物(SREO)资源量约11万吨。对该矿床成矿母岩黑云母花岗岩进行主量和稀土元素分析发现,其岩石性质和地球化学特征与佛冈花岗岩主体类似,同属于高硅、富碱、贫钙镁的铝质A型花岗岩。对6个钻孔的SREO含量、TREO含量、离子相稀土氧化物析出率、地理位置、成矿母岩稀土含量等信息进行对比研究后发现:①基岩稀土含量相似的情况下,产出于山顶的矿体上方风化壳厚度越大,形成矿体所需要的离子相稀土氧化物析出率越小;②稀土元素在成矿母岩浆中完成初始富集后,进而在风化过程中完成二次富集,还可能在剥蚀过程中随地表水下渗迁移至风化壳中渗透系数更小的位置完成第三次富集。因此,在离子吸附型稀土矿床中影响稀土元素垂向分布模式的因素除了风化壳中水体pH值和水化学类型外,还需考虑静水面与矿体的相对位置和渗透系数的共同作用。

The Zuokeng large-scale rare earth deposit in Xinfeng County,Guangdong province,is a typical ion-adsorption type deposit in South China.It is located in the north of the Fogang batholith.The proven reserves of rare earth ores are 171.52 million tons,with total rare earth oxide(TREO)and ion-adsorption rare earth oxide(SREO)reserves of about 0.18 and 0.11 million tons,respectively.The biotite granite bedrock is characterized by high silicon,alkali-rich,calcium-magnesium-poor,and belongs to aluminum A-type granite,similar to those of the other granites from the Fogang granite.On the basis of detailed geological,hydrogeological,and geochemical analyses of six representative drill cores,it can be concluded that:(1)For bedrocks with similar REE contents,the ore grade of ion-adsorption REE is positively correlated with the burial depth of the ore body in the weathering profile.(2)Enrichment of ion-adsorption REE in the ore body is mainly controlled by the initial enrichment during the magmatic and hydrothermal processes of the bedrock and the secondary enrichment related to the weathering of REE-bearing minerals,formation of clays,and ion-adsorption in the weathering crust.Besides,infiltration and downward movement of surface water may also play a role in REE migration and enrichment,particularly for the location with a smaller permeability.In addition,the pH value,hydro-chemical type,and static water table of the groundwater,and permeability coefficient of the weathered products may also affect the vertical distribution patterns of rare earth elements in ion-adsorption deposits.