云南离子吸附型稀土矿成矿规律

The metallogenic regularity of ion-adsorption type REE deposit in Yunnan Province

云南的离子吸附型稀土矿(iRee)主要分布于滇西南一带,临沧岩体是近年来找矿进展最大、关注度最高的地区。与南岭地区iRee矿床最大的区别是矿床海拔均在1000m以上,属于高原低山丘陵地貌环境下形成的风化壳型矿床;成矿岩体沿"三江"断裂旁侧展布;岩性以二长花岗岩和钾长花岗岩为主,多呈中粗粒结构;成岩锆石U-Pb年龄集中分布于208~240Ma和52~80Ma两个区间;按轻重稀土在基岩中的配分,可将基岩分为LREE型和HREE型;前者主要富集轻稀土,后者重稀土相对富集;局部富集HREE的LREE型基岩形成的风化壳,是目前云南发现重稀土异常的主要类型,有可能成为今后寻找HREE矿床的主要目标。含稀土矿物存在与否对REE成矿的贡献具有举足轻重的意义;除斜长石、黑云母和角闪石等造岩矿物中含少量REE外,含稀土副矿物是基岩REE的重要来源;基岩交代作用类型和强度不同决定了含稀土矿物的丰富程度和矿化强度;如早期交代作用主要产生铈硅磷灰石、铈萤石和氟碳铈矿等含LREE的矿物;随着交代作用的增强,HREE的活化作用增强,则产生如高钇钍石、钇萤石和氟碳钇矿等矿物;在风化作用下易破碎解离的含稀土矿物如榍石、褐帘石和氟碳酸盐矿物等是矿床中Ree^3+的主要来源。含矿风化壳的物质组成可分为碎屑矿物、黏土矿物和副矿物三类,黏土矿物是风化壳中Ree^3+的主要载体;完整的花岗岩风化壳从表土层→全风化层→半风化层→微风化层都有发育,全风化层是主要的含矿层;根据稀土在风化壳中的分布特征,可分为"弓背式"、"喇叭式"和"波浪式";因表土层厚度的不同,又可分为"深潜式"和"表露式"。Ree^3+在风化壳中的迁移与富集除受控于基岩、地貌、环境等因素外;持续反复的风化作用也是影响Ree^3+富集成矿的重要因素。

The Ion-adsorption type rare earth(iRee) deposits in Yunnan Province are mainly distributed in the southwest where the altitude is higher than 1000 m, and thus are different from the Nanling range in China. The main bedrock types are monzogranite and moyite distributed along the Sanjiang fault. Zircons from the parent rock of iRee deposits yield a concordant U-Pb age ranging from 208~240 Ma to 52~80 Ma. Based on the LREE/HREE ratio, the bedrocks are divided into two types: light rare earth(LREE: La-Eu) type and heavy rare earth(HREE: Gd-Yb+Y) type. The LREE type rocks with partially enriched HREE deserve more attention as they may become an important resource for the ion-adsorption type of HREE(iHREE) deposit. REE-bearing minerals play a decisive role towards contribution to REE in parent rocks. There are few REE in plagioclase, biotite and amphibole, whereas REE-bearing accessory minerals contribute abundant REE to REE-rich rocks. The type and intensity of metasomatism in the bedrock determines the categories of REE-rich minerals. Most of the LREE-rich minerals such as allanite, apatite, monazite, britholite, bastnaesite andcerfluorite tend to be enriched in the early stages. The HREE-rich stage builds stronger complexes with fluoride and carbonate relative to LREE at ambient temperature, thereby increasing the concentration of the HREE and crystalized minerals like xenotime, yttrocrasite, yttrofluorite and Bastnaesite-(Y). Dissolution of different minerals in the weathering crust would result in a different type of deposit. The former, previously enriched in LREE, tend to subsequently be enriched in HREE. The weathering crust can be divided into detrital, clay,and accessory minerals. REE-bearing clay is the ore. It is generally accepted that the REE were mobilized and fractionated during intense weathering of the REE-rich granite in low altitudes where the continuous and repeated weathering under warm and humid conditionsplayed an important role.