In-situ Micro-scale Pb Isotope Identification Characteristics of Metallogenic and Non-metallogenic Pyrites in Sandstone-type Uranium Deposits
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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204Pb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206Pb/204Pb比正常克拉克值大十几倍甚至数十倍，207Pb/204Pb稍有异常，而208Pb/204Pb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206Pb/204Pb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。
Abstract: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 204Pb 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 206Pb/204Pb ratio is more than ten times or even dozens of times larger than the normal Clark value. 207Pb/204Pb ratio is slightly different, and 208Pb/204Pb 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 206Pb/204Pb 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.
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图 11 区别成矿期黄铁矿与非成矿期黄铁矿的主要标志：(a) 206Pb/204Pb比值差异较大；(b) 207Pb/204Pb比值稍有差异；(c)208Pb/204Pb比值几乎无差异
Figure 11.
表 1 样品采集位置与岩性描述
Table 1. Location of sample collection and lithology description
序号 钻孔号 样品号 采样位置 深度(m) 岩性描述 1 ZKN8-29 ZKN8-29-1 纳岭沟矿床 375.59 浅灰绿色中砂岩，次疏松，含大量碳质碎屑，高矿化，矿样品位100μg/g 2 ZKD112-47 ZKD112-47 大营矿床 581.34 灰白色粗砂岩、钙质胶结，含碳屑黄铁矿结核，遇酸起泡，矿样品位200μg/g 3 ZKD96-31 ZKD96-31 大营矿床 669.46 浅灰绿色粗砂岩，含碳屑及黄铁矿结核，矿样品位140μg/g 4 ZKD112-96 ZKD112-96-1 大营矿床 727.91 灰白色粉砂岩，含黄铁矿结核，碳质碎屑，致密，矿样品位40μg/g 5 ZKN16-56 ZKN16-56-1 纳岭沟矿床 410.31 灰白色粗砂岩，含黄铁矿结核 6 ZKD176-47 ZKD176-47 大营矿床 628.15 浅灰绿色细砂岩，含黄铁矿结核，遇酸起泡，矿化样 
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表 2 黄铁矿微区原位Pb同位素测试数据
Table 2. Pb isotope test data in in situ micro-area of pyrites
序号 样品号 黄铁矿产状 208Pb/204Pb 误差
(SE)207Pb/204Pb 误差
(SE)206Pb/204Pb 误差
(SE)208Pb/206Pb 误差
(SE)207Pb/206Pb 误差
(SE)1 ZKN
112-96-13.1节中的
第1种情况37.751 0.004 15.446 0.001 19.474 0.010 1.9396 0.0010 0.7935 0.0004 2 XINZKD
96-31-13.1节中的
第2种情况37.808 0.005 18.342 0.027 49.948 0.297 0.7569 0.0045 0.3673 0.0016 3 ZKD
96-31-53.1节中的
第3种情况37.956 0.033 16.174 0.018 26.071 0.123 1.4563 0.0064 0.6213 0.0024 4 ZKD
96-31-93.1节中的
第3种情况37.551 0.111 18.343 0.088 46.393 0.770 0.8180 0.0123 0.3990 0.0046 5 ZKD
176-47-53.1节中的
第3种情况37.870 0.008 20.966 0.031 91.288 0.400 0.4162 0.0018 0.2301 0.0007 平均 - 37.766 - 17.076 - 46.6348 - 1.2427 - 0.5452 - 序号 样品号 黄铁矿产状 208Pb/204Pb 误差
(SE)207Pb/204Pb 误差
(SE)206Pb/204Pb 误差
(SE)208Pb/206Pb 误差
(SE)207Pb/206Pb 误差
(SE)6 ZKN
8-29-1-43.1节中的
第1种情况37.805 0.182 67.338 0.391 340.192 2.040 0.1096 0.0001 0.1978 0.0001 7 ZKB
112-47-13.1节中的
第4种情况38.097 0.022 81.762 0.765 651.463 7.400 0.0593 0.0009 0.1260 0.0003 8 ZKB
112-47-23.1节中的
第4种情况37.925 0.051 86.698 0.497 666.826 3.720 0.0569 0.0003 0.1307 0.0003 9 ZKD
176-47-33.1节中的
第5种情况37.704 0.021 23.880 0.096 131.677 1.480 0.2884 0.0033 0.1828 0.0014 10 ZKN
16-56-13.1节中的
第5种情况37.885 0.022 86.323 0.083 461.575 0.403 0.0821 0.0001 0.1869 0.0000 平均 - 37.881 - 61.1612 - 450.3466 - 0.1687 - 0.1757 -
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