华南白垩—第三纪地壳拉张与铀成矿的关系

THE RELATIONSHIP BETWEEN URANIUM METALLOGENESIS AND CRUSTAL EXTENSION DURING THE CRETACEOUS—TERTIARY IN SOUTH CHINA

华南是中国最重要的铀矿产区之一。按赋矿围岩的不同 ,该区主要产出花岗岩型、火山岩型和碳硅泥岩型 3类铀矿床。铀矿区都分布有比铀成矿超前形成的富铀岩石 ;铀矿床成矿热液中的水主要为大气成因地下水 ,成矿温度约为 1 2 0～ 2 5 0℃ ,成矿热液的δ1 3 C值主要为 - 4‰～ - 8‰ ,表明幔源CO2 参与了成矿作用 ;矿床的N(3 He) /N(4He)为 0 .1 0～ 2 .0 2Ra,显示成矿热液中大量幔源He的存在。这些铀矿床的成矿时代与赋矿围岩的岩性和时代无关 ,都集中在该区地壳受到强烈拉张因而断陷盆地广泛发育并伴有幔源基性岩浆活动 (基性脉岩、玄武岩 )的白垩—第三纪。研究表明 ,白垩—第三纪导致了地幔与地壳表层沟通的地壳拉张 ,把该区 3大类型的铀矿床串联成了一个有机的整体 :(1 )地壳拉张通过控制向大气成因的贫CO2 热液提供铀成矿必不可少的幔源CO2 ,而与铀成矿发生联系 ;(2 )同一机制形成的富CO2 热液浸取同一或不同铀源岩石中的铀并在不同围岩中成矿 ,形成了按赋矿围岩划分的各种矿床类型 (花岗岩型、火山岩型和碳硅泥岩型 )。

South China is one of the most important uranium producing areas in China. In terms of their host rocks, the uranium deposits in this region can be divided into three types, i.e., granite type, volcanic type and carbonaceous-siliceous-pelitic type. Water in ore-forming hydrothermal fluids responsible for the deposits was predominantly meteoric underground water. The ore-forming temperatures were approximately within the range of 120～250℃; the δ^(13)C values of ore-forming fluids varied mainly between -4‰ and -8‰, which indicates that the CO 2 in ore-forming fluids was mantle-derived; the N(~3He)/(N(~4He)) of the Xiangshan uranium deposit is 0.10～2.02 R a, which shows there existed a lot of mantle-derived He in the fluids. In the area of uranium mineralization are commonly recognized pre-ore U-rich rocks. The uranium deposits, having nothing to do with the lithologic and chronological characters of the host rocks, formed in the Cretaceous—Tertiary during which the crust in South China experienced intensive extension with associated normal faulting, basin formation and mantle-derived magmatic activity (basic dykes and basalts). The authors suggest that it was the crust extension during the Cretaceous—Tertiary that integrated the three types of uranium deposits into an interrelated entirety: (1) crustal extension promoted uranium mineralization by supplying mantle-derived ΣCO 2, which was indispensable for uranium mineralization, to the ΣCO 2-poor meteoric hot water; (2) these ΣCO 2-rich hydrothermal fluids extracted uranium from the same or different uranium-source rocks, and then deposited uranium in different types of host rocks, giving rise to the formation of different types of uranium deposits classified in terms of their host rocks (e.g. granite type, volcanic type and carbonaceous-siliceous-pelitic type).