砂岩型铀矿成矿期与非成矿期黄铁矿的微区原位Pb同位素识别特征  

In-situ Micro-scale Pb Isotope Identification Characteristics of Metallogenic and Non-metallogenic Pyrites in Sandstone-type Uranium Deposits

作　　者：张效瑞 吴柏林[1] 雷安贵[2] 杨松林[2] 姚璐航 庞康[1] 包志安[1] 王苗 郝欣 刘明义 李琪 林周洋 ZHANG Xiaorui;WU Bailin;LEI Angui;YANG Songlin;YAO Luhang;PANG Kang;BAO Zhian;WANG Miao;HAO Xin;LIU Mingyi;LI Qi;LIN Zhouyang(State Key Laboratory of Continental Dynamics/Department of Geology,Northwest University,Xi’an 710069,China;Research Institute of Exploration and Development of Liaohe Oilfield,Panjin 124010,China)

机构地区：[1]大陆动力学国家重点实验室,西北大学地质学系,陕西西安710069 [2]辽河油田勘探开发研究院,辽宁盘锦124010

出　　处：《岩矿测试》2022年第5期717-732,共16页Rock and Mineral Analysis

基　　金：陕西省自然科学基础研究计划(重点)项目(2022JZ-18);辽河油田公司科技项目(2021KJ-07-3);国家基础科学人才培养基金(XDCX2020-08);陕西省重点研发计划项目(2021KW-28)。

摘　　要：砂岩型铀矿中含大量不同形态、不同阶段的黄铁矿。仅凭矿相学对黄铁矿产状及电子探针对黄铁矿形态的观察难以准确地判别成矿期、成矿前及成矿后形成的黄铁矿。而成矿期黄铁矿是铀矿床成因和形成过程的重要信息载体,对其准确识别具有特别重要的意义。以往国内外研究采用激光剥蚀电感耦合等离子体质谱(LA-MC-ICP-MS)方法分析Pb同位素,但该方法对于低含量Pb样品分析精度较低且较难获得^(204)Pb数据。本文对铀矿石中的黄铁矿利用微区原位的手段进行更加精准的飞秒级质谱(fs-LA-MC-ICP-MS)的Pb同位素测试,发现大量黄铁矿存在Pb同位素异常,从中可能区分出成矿期与非成矿期的黄铁矿。经U-Th-Pb放射性衰变原理分析并结合黄铁矿矿相学特点可以发现,矿相学镜下明确是成矿期的黄铁矿,其^(206)Pb/^(204)Pb比正常克拉克值大十几倍甚至数十倍,^(207)Pb/^(204)Pb稍有异常,而^(208)Pb/^(204)Pb基本不变。矿相学中产状呈草莓状,以及铀矿物围绕其生长但未有穿插关系的非成矿期黄铁矿,其^(206)Pb/^(204)Pb正常;矿相学镜下难以确定形成阶段的、与铀矿物没有任何接触关系的黄铁矿,其Pb同位素没有明显的规律性。这些结果证明了利用Pb同位素异常来判断黄铁矿形成阶段的准确性。因此,利用黄铁矿微区原位Pb同位素差异,适当配合矿相学形态和产状观察,可较为精准地识别出成矿期黄铁矿。BACKGROUND:Sandstone-type uranium deposits contain a large number of pyrites of different shapes and stages.It is difficult to accurately discriminate the pyrite formed before,during or after the metallogenic period solely by observation of pyrite morphology by mineralogy and electron probe microanalysis.Pyrites during the metallogenic period are important information carriers for the genesis and formation process of uranium deposits,and their accurate identification is of great significance.Previous studies both domestically and internationally have used the LA-MC-ICP-MS method to analyze Pb isotopes,but this method has low analytical precision for low-content Pb samples and it is difficult to obtain ^(204)Pb data.OBJECTIVES:To identify metallogenic and non-metallogenic pyrites by in situ micro-scale Pb isotopes.METHODS:Femtosecond laser ablation multi-collector inductively coupled plasma-mass spectrometry(fs-LA-MC-ICP-MS)was used to determine the lead isotope of pyrite in uranium ores.RESULTS:Under the mineralogy microscope,it is clear that the pyrite is related to mineralization and its ^(206)Pb/^(204)Pb ratio is more than ten times or even dozens of times larger than the normal Clark value.^(207)Pb/^(204)Pb ratio is slightly different,and ^(208)Pb/^(204)Pb ratio is constant.The occurrence of strawberry-shaped pyrites,and non-metallogenic pyrites with uranium minerals growing around them but not interspersed,have normal a ^(206)Pb/^(204)Pb ratio.Pyrites without any contact relationship have no obvious regularity in its Pb isotopes.CONCLUSIONS:In-situ micro-scale Pb isotopic difference of pyrites was combined with appropriate observation of mineralogy morphology and occurrence,resulting in pyrites in the metallogenic period being more accurately identified than previously.

关 键 词：成矿期黄铁矿 微区原位分析 Pb同位素异常 U-TH-PB 砂岩型铀矿 

分 类 号：P618.11[天文地球—矿床学] P595[天文地球—地质学]