江西相山铀矿田成矿物质来源的Nd、Sr、Pb同位素证据

Ore-Forming Material Sources for Xiangshan Uranium Ore-Field in Jiangxi Province: Evidence from Nd-Sr-Pb Isotopes

对相山大型火山岩型铀矿田中邹家山和沙洲铀矿床及其赋矿围岩 (碎斑熔岩及次花岗闪长斑岩 )进行了Nd、Sr、Pb同位素研究。结果表明 :成矿期萤石的εNd(t)值 ( - 6.7～ - 8.3)和初始87Sr/ 86Sr比值 ( 0 .7145～ 0 .72 0 7)与赋矿围岩的εNd(t)值 ( - 6.2～ - 9.4 )和初始87Sr/ 86Sr比值( 0 .712 1～ 0 .7192 )相似。在εNd(t) -t图上 ,成矿期萤石数据点的投影域与赋矿围岩的基本吻合 ,均落在相山元古宙基底演化域范围内。成矿期黄铁矿的铅同位素组成在2 0 6Pb/ 2 0 4 Pb - 2 0 7Pb/2 0 4 Pb关系图上呈线性分布 ,而火山岩的铅同位素组成位于此相关线低值一端。利用异常铅线的斜率及成矿年龄计算出富铀地质体的形成年龄为 144Ma ,这与赋矿围岩的成岩年龄 ( 135～ 140Ma)接近。因此 ,相山铀矿田成矿物质主要来自富铀的火山 -侵入杂岩 ,而火山 -侵入杂岩则是由类似于地表出露的元古宙基底变质岩部分熔融形成的。由此可见 ,相山铀矿田的成矿物质主要来源于地壳。

The Xiangshan volcanic rock related uranium ore field is the largest uranium ore field in China. The Zoujiashan uranium deposit (fluorite hyromica uraninite association) and the Shazhou uranium deposit (alkali feldspar alteration hemitite uraninite association) are the two representatives of the ore field. This paper presents Nd, Sr and Pb isotope compositions of these two uranium deposits and their wall rocks—the Xiangshan volcanic rocks. ε Nd (t) values (-6.7～-8.3) and initial 87 Sr/ 86 Sr ratios (0.7145～0.7207) of fluorite formed by the ore forming hydrothermal fluid are similar to those of the wall rocks (ε Nd (t) =-6.2～-9.4, initial 87 Sr/ 86 Sr=0.7121～0.7192). In the ε Nd (t) vs. t diagram, the plot area of data points for fluorite of ore forming stage is generally identical with that of the host rocks. Both of them fall within the evolution domain of Proterozoic basement in Xiangshan area. Pyrites from the ores have a linear correlation between 206 Pb/ 204 Pb and 207 Pb/ 204 Pb, while the wall rocks plot at the lower value end of this correlation line. From this line a reference age of 144Ma can be obtained, which is consistent with the age of the volcanic rocks (135～140Ma). This Nd Sr Pb isotope evidence suggests that the ore forming materials were derived mainly from the Xiangshan volcanic rocks. The volcanic rocks were derived mainly from the crust which was similar to the exposed basement. Therefore, the main ore forming materials were derived ultimately from the crust.