贵东岩体黑云母成分特征及其对铀成矿的制约

The Geochemical Characteristics of Biotites and their Constraints on Uranium Mineralization in Guidong Pluton

鲁溪岩体和下庄岩体为粤北贵东复式岩体的重要组成部分,两岩体构造背景相同,空间上紧密共生,时间上近乎同时生成,但鲁溪岩体不成矿,而下庄岩体赋存着大量铀矿床。这种差异的原因探究对深刻揭示花岗岩型铀矿床成矿机制具有重要的意义。对鲁溪岩体和下庄岩体中黑云母的电子探针、穆斯鲍尔谱和ICP-MS分析表明,两岩体黑云母的主量元素具有富铁贫挥发分的特点;微量元素特征类似,具明显的负铕异常;相对于鲁溪岩体,下庄岩体的演化程度和挥发性组分F含量较高,而温度和氧逸度较低。从鲁溪岩体到下庄岩体,由于演化程度升高,F含量增高,温度和氧逸度降低,使铀在花岗岩中的丰度升高,赋存方式由不利于成矿的类质同像替换演变为有利于成矿的晶质铀矿形式存在,这是鲁溪岩体不成矿而下庄岩体有大量铀矿床的主要原因。

The Luxi granite and the Xiazhuang granite are important parts of the Gui-dong composite pluton in the northern Guangdong provice.They have the same tectonic setting,similar spatial association and similar age.However,many uranium depostites are formed in Xiazhuang granite while none is found in the Luxi granite.A detail study on this difference is significant for understanding the metallogenic mechanism of granite-type uranium deposit.Biotite contains a lot of geochemical information about the magmatic crystallization.The results of the electrion microprobe,Mssbauer spectroscopy and ICP-MS analyses of biotites collected from Luxi and Xiazhuang granites indicated that these biotites have similar major element contents,rich in Fe but poor in volatile consitutents,and have similar trace element contents,strong negative Eu anomalies.The biotites of Xiazhuang granite are distinguished from those of the Luxi granite by higher evolution degree,more F content,lower temperature and lower oxygen fugacity.The evolution degree and F content increase and the temperature and oxygen fugacity decrease from Luxi granite to Xiazhuang granite may cause that the uranium content increases and that the existing state of uranium changes from isomorphism to uraninite,a mineralization unfavorable condition to a mineralization favorable condition.It is the main reason for that there are many uranium depostites in Xiazhuang granite while none is in the Luxi granite.