利用铀镭平衡系数计算第四纪以来的铀成矿年龄

Using uranium-radium equilibrium coefficients to calculate uranium mineralization ages since Quaternary period

中文关键词:铀镭平衡系数  铀成矿作用  成矿年龄  砂岩型铀矿  鄂尔多斯盆地南部

keywords:uranium-radium equilibrium coefficient  uranium mineralization  mineralization age  sandstone type uranium deposits  southern Ordos Basin

中文摘要:砂岩型铀成矿作用常具有多期多阶段性，与锆石、磷灰石等硅酸盐矿物不同，铀矿物的铀铅体系封闭温度低，元素易迁移。因此，铀成矿定年一直是砂岩型铀矿研究的难点。近年来，随着铀矿勘查的深入，在新近系乃至第四系等年轻地层中发现了工业铀矿体，表明存在非常年轻的铀成矿作用。如何准确厘定如此新的铀成矿作用时限，是亟待解决的问题。铀镭平衡系数是铀矿勘查过程中必须测量的物理参数，对准确估算铀资源量具有重要意义。作为放射性平衡系数的一种，从铀矿形成直至铀镭达到平衡，铀镭平衡系数是时间的函数。因此，在一定的前提条件下，利用铀镭平衡系数，可准确计算铀成矿时限。文章经过严密的数学推导，建立了铀镭平衡系数与铀成矿年龄的函数关系。利用鄂尔多斯盆地南部镇原地区白垩系砂岩中铀矿化样品的铀镭平衡系数（K_p=0.69,n=62），计算得到的铀成矿年龄为0.127 Ma，与全岩铀铅等时线年龄(（0.38±0.02）Ma)接近。推测后者可能代表铀成矿的起始时间，前者代表铀成矿的结束时间。此外，国内外诸多砂岩型铀矿的铀镭平衡系数均小于1，表明存在一次大范围的晚更新世以来的铀成矿作用。

Abstract:Uranium mineralization of sandstone type uranium deposits often exhibits multi-period and multi-stages. In addition, different from the silicate minerals such as zircon and apatite, uranium minerals have low closure temperatures which means U and Pb are easier to migrate. Therefore, uranium mineralization dating has always been a difficult issue in the study of sandstone type uranium deposits. In recent years, uranium exploration has discovered industrial uranium ore bodies in young strata, such as the Neogene and even the Quaternary, indicating the existence of very young uranium mineralization. How to accurately determine the time limits of such young uranium mineralizations is urgent to be solved. Uranium-radium equilibrium coefficient is a physical parameter that must be measured during uranium exploration with great significance for accurately estimating the uranium resources. As a type of radioactive equilibrium coefficient,uranium-radium equilibrium coefficient is a function of time from the formation of uranium ore until uranium-radium reaches equilibrium. Therefore, under certain conditions, the uranium-radium equilibrium coefficient can be used to accurately calculate the age of uranium mineralization. This study has established a functional relationship between the uranium-radium equilibrium coefficient and the age of uranium mineralization through mathematical derivation. The uranium-radium equilibrium coefficients（K_p=0.69, n=62） of uranium ores from the Cretaceous sandstones in the Zhenyuan area, southern part of the Ordos Basin was used to calculate the uranium mineralization age of 0.127 Ma, which is close to the U-Pb isochron age of whole rocks(（0.38±0.02）Ma). 0.38 Ma may represent the beginning of the uranium mineralization,while 0.127 Ma may represent the end of the uranium mineralization. It is worth mentioned that uranium-radium equilibrium coefficients of many sandstone type uranium deposits in China and abroad are abviously less than 1,which indicates the explosion of a regional uranium mineralization since the Late Pleistocene.