沙特北部恩乌尔磷矿床磷块岩地球化学特征及沉积古环境分析

Geochemical characteristics and paleoenvironment analysis of phosphate rock in Umm wu'al phosphate deposits,northern Saudi Arabia

对沙特北部恩乌尔(Umm Wu'al)磷矿的磷块岩的常量、微量和稀土元素地球化学特征进行分析研究,探讨矿区内Arqah磷矿段中磷块岩的沉积古环境。分析结果表明,区内磷块岩主要由内碎屑、少量陆源碎屑、生物碎屑和填隙物等组成,以富含CaO、P2O5,少MgO、SiO2为特点。磷块岩中Ba含量高达350×10^-6,Sr含量高达1433×10^-6,表明磷块岩成矿与生物作用关系密切。磷块岩Sr/Ba比值1.96~21.20,反映出其是为正常沉积的,而U/Th比值2.83~21.83,均大于1,则反映出成矿过程中有热水作用参与。δU为1.79~2.0,均大于1,表明磷块岩形成于明显的缺氧沉积环境。磷块岩稀土总量较高(平均150×10^-6),经北美页岩标准化的稀土配分模式曲线呈现近乎水平,轻重稀土分异程度低,轻稀土相对亏损,重稀土富集的特征。LREE/HREE比值0.63~2.08,绝大多数大于1,(La/Yb)N平均为1.14,接近1,显示出磷块岩总体成矿古地理环境变化不明显,局部时段受海平面升降有所变化。δCe平均为0.28,小于1,负异常显著,进一步表明磷块岩形成于明显的缺氧的沉积环境。δEu平均为0.91,接近1,综合分析认为该区磷块岩形成于远岸的古地理变化不明显的缺氧沉积环境。

Geochemical characteristics of phosphate rock in constant, trace and rare earth elements were analyzed and studied in Umm wu’al phosphate deposits, northern Saudi Arabia. The sedimentary paleoenvironment of phosphate rock in Arqah phosphate rock block was discussed. The analysis results show that the phosphorite in the area is mainly composed of intraclast, a small amount of terrigenous clastic, bioclastic and fillings, which are characterized by rich CaO, P2O5, less MgO and SiO2. The Ba content in phosphorite rocks is as high as 350×10-6 and the Sr content is as high as 1433×10-6. The results show that there is a close relationship between phosphate rock mineralization and biological action. Phosphate rock Sr/Ba ratio 1.96~21.20, reflecting its normal deposition,while the ratio U/Th ranges 2.83~21.83, which is more than 1, reflects the participation of hot water during the process of mineralization. The value δU is 1.79~2.0, which is greater than 1, indicates that the phosphorite was formed in an obvious anoxic sedimentary environment. The total amount of rare earth in the phosphorite is higher(average 150×10-6). The standardized REE distribution pattern curve by North American shale is close to the level, the differentiation of light and heavy rare earths is low, the light rare-earth is relatively deficit, and the heavy rare earth is enriched. The ratio LREE/HREE is 0.63~2.08, Most are greater than 1,(La/Yb)N averaged 1.14, close to 1, It is shown that the paleogeographic environment change of phosphate rock is not obvious, the paleogeographic environment was changed by sea level fluctuation somewhat in local period. The average value of δCe is 0.28, less than 1. The negative anomaly is significant, which further indicates that the phosphorite was formed in an obvious anoxic sedimentary environment. The average value of δEu is 0.91, which is close to 1. The comprehensive analysis shows that the phosphorous block rocks in this area are formed in the anoxic sedimentary environment on the far shore where the paleogeographic changes are not obvious.