黔西新华大型磷矿磷块岩稀土元素地球化学及其成因意义

Rare Earth Elements Geochemistry and Genesis of Xinhua Large-size Phosphorite Deposit in Western Guizhou

利用ICP_MS测试方法系统分析了黔西织金新华磷矿磷块岩的稀土元素组成,同时还测定了贵州遵义和湖南张家界等华南下寒武统黑色岩系中磷块岩的稀土元素含量。结果显示①新华磷块岩稀土元素总量(ΣREE)较高,变化较大,为164.23×10-6～1395.01×10-6,平均642.54×10-6,轻重稀土分异明显,LREE/HREE值为5.04～6.52,平均5.69,而华南其他地区磷块岩的ΣREE和LREE/HREE分别为156.69×10-6～637.41×10-6(平均431.75×10-6)和3.17～6.95(平均4.37);②新华磷块岩的δCe为0.26～0.53,华南其他地区磷块岩的δCe为0.30～0.66,均具明显的Ce负异常,表明它们均为典型海相沉积磷块岩,主要形成于陆缘海环境,且新华磷块岩形成的深度较深;③大多数样品具不明显的Eu正异常,少部分出现较高的Eu正异常,δEu介于0.98～2.40,显示这些磷块岩的形成过程中可能有较高温(>200℃)、强还原热液的加入;④新华磷块岩中HREE相对亏损,且多数样品LaN/NdN>1,而δY>1.5,δY与δCe之间不存在相关性,说明磷块岩遭受了较强的后期风化淋滤作用,深埋成岩作用基本未改变其REE组成。

Zhijin County in western Guizhou is one of the most important phosphorite_producing areas in China. To further investigate the genesis of the Xinhua phosphorite deposit in this area, the authors systematically analyzed REE composition of this deposit by ICP_MS method. For comparison, phosphorites from Lower Cambrian black rocks in such areas of South China as Zunyi of Guizhou and Zhangjiajie of Hunan were also analyzed. Some conclusions can be reached from the analytical data: ① The Xinhua phosphorite possesses quite high and rather varied total rare earth elements (ΣREE), which vary between 164.23×10 -6 and 1395.01×10 -6 , averaging 642.54 ×10 -6 , and marked differentiation between light and heavy rare earth elements. The ΣLREE/ΣHREE values vary between 5.04 and 6.52, while the ΣREE and ΣLREE/ΣHREE values of the Zunyi and Zhangjiajie phosphorites are 156.69×10 -6 ～637.41×10 -6 (431.75×10 -6 on average) and 3.17～6.95 (4.37 on average) respectively. ② Phosphorites from Xinhua, Zunyi and Zhangjiajie all show remarkable negative Ce anomalies. Besides, the Xinhua phosphorite possesses lower δCe, varying between 0.26 and 0.53 (0.35 on average), while δCe of phosphorites from Zunyi and Zhangjiajie are 0.30～0.66, averaging 0.47, suggesting that phosphorites are exclusively typical marine sediments mainly deposited in ancient pericontinental seas, and the depositional position of the Xinhua phosphorite might be relatively deep. ③ Most phosphorites from Xinhua, Zunyi and Zhangjiajie possess indistinct positive Eu anomalies, with only a few samples assuming marked anomalies. δEu vary between 0.98 and 2.4, suggesting that hot (>200℃) and strong reductive hydrothermal fluids might have been involved in the formation of phosphorites. ④ The Xinhua phosphorite is relatively depleted in HREE, whereas La N/Nd N and δY of most samples are higher than 1 and 1.5. Besides, there exists no correlation between δY and δCe, implying that phosphorites were subjected to quite strong weathering and leaching processes, and deep buried rock_forming processes have not remarkably changed the REE composition. ⑤ Phosphorus in the phosphorites might have mainly been derived from weathering and leaching of granitic continents and alkaline basalts around the ancient pericontinental seas.