相山铀矿田西部地区深部多金属矿化成矿年代与成矿流体演化:Rb-Sr同位素体系的制约

The timing and ore-forming fluid evolution of deep polymetalic mineralization in western Xiangshan uranium ore field, South China: Constraints from Rb-Sr isotope systematics

江西相山铀矿田科学深钻3号孔在深部-700 m发现大量铅锌多金属矿化脉,垂向上呈"上铀下多金属"的分布特征。本文选取深部多金属矿脉主成矿阶段(S3)自形闪锌矿样品6件和不同阶段的毒砂、黄铁矿、方铅矿、方解石等样品12件,以及围岩全岩样品17件,进行了Rb、Sr同位素组成研究。结果表明:(1)由闪锌矿Rb-Sr等时线法确定的相山铀矿田深部多金属矿化形成于121. 0±3. 5Ma,与围岩火山岩存在较大时差,可能与晚于围岩的深部次火山有关。根据穿插关系,多金属矿化略晚于碱性交代铀矿化,但明显早于酸性交代铀矿化;(2)多金属矿化脉体中金属矿物的Rb和Sr含量分别介于0. 041×10^-6~1. 38×10^-6和2. 35×10^-6~23. 11×10^-6之间,Sr同位素初始比值(87Sr/86Sr),变化较大,介于0. 706114~0. 718814之间,平均值为0. 713579,暗示相山铀矿田深部多金属矿化的成矿物质主要来源于地壳。初始流体Sr同位素值(0. 718665)明显高于成矿时赋矿围岩(流纹英安岩为0. 714581,碎斑流纹岩为0. 714417)的Sr同位素组成,表明多金属成矿流体和物质并非来自围岩火山岩;(3)由早到晚阶段的(87Sr/86Sr)i呈明显降低的演化趋势,表明成矿流体演化过程中受到大气降水的不断稀释作用。相山矿田的铀矿和深部多金属矿化同形成于华南中生代板内伸展构造背景。

The Xiangshan uranium ore field,the largest hydrothermal uranium ore field in China,is hosted in Early Cretaceous felsic volcanic rocks. In recent years,through deep drilling exploration,a discovery of hidden lead-zinc-silver mineralization in deep(-700 m) was achieved in west of Xiangshan uranium ore field. In this paper,the deep polymetallic mineralization was studied in detail in the aspects of Rb-Sr isotope of ores and host rocks. Six sphalerite samples taken from main mineralization stage was selected for Rb-Sr isochron dating to determine the age of the deep polymetallic mineralization,and combining twelve metallic minerals samples from different mineralization stages and seventeen host-rock samples,the source and evolution of ore-forming fluids are studied. The results shows that:( 1) A Rb-Sr isochron defined by six sphalerite samples yields an age of 121. 0 ± 3. 5 Ma( MSWD = 2. 1),with initial Sr isotopic composition(87 Sr/86 Sr)i= 0. 714988 ± 0. 000028,which indicates that the deep polymetallic mineralization with a near formative age of deep sub-volcanic rocks formed during the Early Cretaceous. And the interpenetration relationship between ploymetallic and uranium mineralization shows that the former is slightly later than that of alkaline-metasomatic uranium mineralization and obviously earlier than that of late acid metasomatic-uranium mineralization in Xiangshan ore field.( 2) The Rb contents range from0. 041 × 10^-6 to 1. 38 × 10^-6 and the Sr contents range from 2. 35 × 10^-6 to 23. 11 × 10^-6,and the(87 Sr/86 Sr)i( initial Sr isotope ratios) range from 0. 706114 to 0. 718814,with an average of 0. 713579. Both Rb and Sr isotopic geochemical characteristics imply that ore-forming fluids of Xiangshan deep polymetallic mineralization mainly originated from the crust. Moreover,the Sr isotopic value of initial ore-forming fluid is obviously higher than that of host rocks,which suggests that polymetallic ore-forming fluids and materials are not derived from host volcanic rocks. Furthermore,the(87 Sr/86 Sr)idecreased significantly from early stage to late stage,indicating that the ore-forming fluid was continuously diluted by meteoric precipitation during the evolution process. The origin of the deep polymetallic mineralization and uranium deposit may be related to Mesozoic intra-plate extension tectonics of South China.