云南临沧花岗岩中离子吸附型稀土矿床的成矿规律

Mineralization regularity of ion-adsorption type REE deposits on Lincang granite in Yunnan Province

近年来,在云南临沧某地花岗岩风化壳中新发现一中型离子吸附型轻稀土矿(iRee),是高海拔地区发现的又一个典型矿床,这对今后在高海拔地区找矿具有指导意义。通过研究此矿床,并结合岩体中已发现的iRee矿床,本文总结了此类矿床的成矿规律。研究区矿床的成矿母岩是黑云母二长花岗岩,似斑状、中粗粒结构,含稀土矿物主要是榍石、褐帘石、磷灰石、独居石、磷钇矿和锆石等;岩石具有高硅、高钾和低铁、低镁的特征,∑REE>150×10^-6,相对富集LREE。矿区内风化壳厚度在2~30m之间,发育完整的风化壳,自上而下可分为表土层、全风化层、半风化层和微风化层;全风化层是主要的含矿层;风化壳中∑REE的含量是基岩的3~5倍,稀土配分形式与基岩相似,局部见重稀土异常;矿区由北至南,地势由高到低,矿体埋深由深变浅,稀土品位逐渐增高;由西向东,稀土品位中间高、两边低,且东边高于西边。临沧花岗岩风化壳中iRee矿床或矿化点主要分布于岩体中—南段,均以LREE型稀土矿床为主;岩体处于中亚热带季风气候—热带雨林季风气候带;矿区主要发育中低山、丘陵地貌,区域上受次级断裂构造作用影响;成矿母岩多为中晚期岩浆岩产物,岩性为黑云母二长花岗岩。风化壳中∑REE含量受地形地貌的影响表现出:在平缓开阔的多层台地和起伏缓和的低山丘陵地貌下,山腰和山顶较为富集;中低山顶部宽缓的带状地貌,山腰至山顶较为富集。

An ion-adsorption type REE(iRee) deposit have been found on Lincang granite in Yunnan province. It’s a typical iRee deposit found on highland, which can be a model to find similar deposit on the similar background. Geochemical and mineralogical studies were conducted on this iRee deposit on Linchang granite in Yunnan province, in order to reveal the transport and adsorption of rare earth elements(REE) related to the ion-adsorption type mineralization. The parent rock is porphyry biotite monzogranite. REE are contained dominantly in allanite as well as in titanite, ilmenite, monazite, xenotime, apatite and zircon. REE pattern of the parent granite bigger than 150×10^-6, indicates enrichment of LREE relative to HREE and no significant Ce anomaly. Weathering crust is ranging from 2 to 30 m depth, which can be separated to humid layer, completely weathering layer and partly weathering layer, REE is rich in completely weathering layer. REE content of the weathering crust is 3~5 times to parent rock. REE pattern of weathering crust is parent rock pattern alike, part of which is showing high HREE. Land shape changed and the ore body change accordingly. REE contents showing on hillside richer than which on top and foot. REE rich in deep or shallow depends on the thickness of humid layer. The terrain high in the north and low in the south, on the contrary, REE contents showing low to high. From west to east, REE contents showing higher in the middle than in both sides, and REE in the east richer than in the west. iRee deposits are located in middle-south of Lincang granite, all showing LREE rich. This region extends across mid-subtropical monsoon climate zone and tropical Rainforest monsoon climate zone, Hilly, mountainous widespread, controlled by the secondary fault, influenced by multi-stage magmatic evolution, parent rock is biotite monzogranite. REE content is controlled by landform, rich in hillside, lower in foot of hill.