豆荚状铬铁矿以及其中铂族元素矿物的成因问题——进展与展望

铂族元素矿物(Platinum Group Mineral:简称PGM)资料的不断积累,丰富了人们对蛇绿岩中豆荚状铬铁矿成因的认识。文章总结近年来有关PGM的新资料和取得的新认识,探讨豆荚状铬铁矿以及其中PGM的成因问题。幔源岩浆结晶过程中,铬铁矿周边熔体减少将诱发那些易氧化的铂族元素(Os、Ir、Ru)在熔体中达到饱和状态,并结晶形成纳米级PGM。在地幔熔体中,随着硫逸度升高,PGM微粒与熔体中的硫反应并逐渐长大。多期次的熔体抽提和熔体-岩石反应事件,可以在地幔源区通过逐步降低硫逸度、促进含铂族元素的贱金属硫化物分解,形成PGM以及铂族元素合金。低硫逸度环境更有利于PGM的形成和保存。在变质环境或流体环境中,这些PGM往往会与流体反应,造就了PGM矿物的多样性。原生PGM与变质流体反应并发生原地去硫化作用,可以形成次生的PGM环边或者纳米级PGM包体。铬铁矿的多阶段蚀变/再平衡过程可以导致PGM溶解—沉淀—均一化,并扰动Os同位素体系。不同类型矿石在有限空间伴生的现象以及它们所具有显著差异的地球化学特征,说明蛇绿岩是不同地幔组分的机械混杂。随着俯冲板片,铬铁矿团块被拖曳到地幔深部,并通过地幔对流重新出现在扩张中心附近,最终混杂在蛇绿岩中。发生循环的铬铁矿团块因此可以与新生铬铁矿及其围岩伴生在同一蛇绿混杂岩中。     

Geochemistry of Some Marine Fe–Mn Nodules and Crusts with Respect to Pt Contents

Bulk chemical analyses for Pt and Pd in marine Fe–Mn nodules and crusts from different provenances are presented, together with a wide range of elements. Platinum contents vary from 70–328 ppb, whereas Pd contents extend from 0.6–4.7 ppb only. Bromine and Pb show strong positive correlations with Pt. Lead is remarkably enriched in Fe–Mn precipitates over seawater, but Br is a conservative‐type element in seawater and shows no enrichment in Fe–Mn precipitates. Hence, the Pt–Br–Pb element association combines two elements, Br and Pb, of extremely contrasting enrichment factors in Fe–Mn precipitates.     

内标校正-电感耦合等离子体发射光谱法测定磷矿石中的磷

采用电感耦合等离子体发射光谱法(ICP-OES)可以同时测定磷矿石中的主、次量元素,但在实际测量过程中,仪器的漂移以及基体效应引起的分析误差不容忽视。本文以碲为内标,对高含量磷的测定进行校正,可以有效降低由仪器漂移以及基体效应引起的分析误差,12次平行测定磷的相对标准偏差(RSD)为0.5%,与未经校正的RSD值(4.1%)相比,显著提高了分析精度,极大地降低了仪器波动对测定结果的影响,因此获得了更低的方法检出限(0.0044%)。实验还对比了王水溶样与盐酸-硝酸-氢氟酸-高氯酸(四酸)溶样两种样品前处理方法对磷矿石分析的影响,结果表明:以王水对磷矿石标准物质进行快速溶解,磷、钙、铁、镁、锰等元素的测定结果与标准值间的相对误差范围为-6.7%~1.6%,满足日常测定要求;而钾、钠、铝是否能够完全溶出与样品的矿物组成及氟含量存在联系,若需要分析钾、钠、铝等元素,如果样品中的硅酸盐矿物组成不多,氟含量较高时可以考虑采用王水溶解;四酸可以完全溶解样品,适合于磷、钙、铁、镁、铝、钾、钠、锰、锶等元素的同时测定。     

桂西北微细浸染型金矿中铂族元素的发现及其找矿意义

本文基于辽宁第九地质大队2015—2017年间在桂西北尾腊沟金矿勘查过程中的新发现而撰写。桂西北尾腊沟金矿体主要赋存于中三叠统兰木组二段第3亚段的压碎泥岩、粉砂岩、细砂岩中;而铂族元素矿则是在兰木组二段第1、2、3亚段地层中,均存在不同程度的铂(族)矿化。区内的铂族元素不单纯局限在金矿体内,在其围岩中也同样存在,找矿潜力乐观。本次在桂西北中三叠统砂岩、泥岩中发现高品位的铂族元素,突破了我国在川黔滇围绕寒武系黑色页岩中寻找铂族元素矿的束缚,无疑对今后铂族元素的找矿方向拓宽了新的思路,也为研究铂族元素的成矿机制提供了新的线索。     

High‐precision Determination of Gold Mass Fractions in Geological Reference Materials by Internal Standardisation

Determination of gold abundances in natural rock is critical for applications, but very challenging. Here, we report a method for determining gold with a very low mass fraction (> 0.01 ng g^?1) in rocks. The method involves Carius tube digestion with reverse aqua regia, chromatographic separation to remove most of the sample matrix and measurement by high‐sensitivity ICP‐MS. The mono‐isotopic element gold was quantified by external calibration using an internal standardisation of gold to platinum that was precisely determined by isotope dilution. The method is robust and the obtained results are indistinguishable (< 5–10%, 2s) from those independently obtained by a standard addition technique on the same solution. The results from reference materials TDB‐1 and GPt‐2 are consistent with the certified values and those determined by HF‐aqua regia digestion, confirming the validity of the method. TDB‐1 (n = 20), GPt‐2 (n = 6), BHVO‐2 (n = 9) and other mafic RMs are homogenous for gold (10–20%, 2s) at the 2 g test portion level; however, sample heterogeneity affects some RMs. Gold and platinum‐group elements also display different extents of sample heterogeneity for different RMs. Given the homogeneity observed for TDB‐1, GPt‐2 and BHOV‐2, they are recommended as well‐suited RMs for inter‐laboratory comparison studies of gold.     

甘肃省贵金属矿和铂族元素地球化学勘查的发展及新案例

由于Ａｕ、Ｐｔ、Ｐｄ等元素分析测试技术的突破，使中国的贵金属地球化学勘查得到迅速发展。甘肃省自８０年代以来，金矿地球化学勘查取得了丰硕的成果。９０年代初，在研究改进Ｐｔ、Ｐｄ、Ａｕ化学－光谱法快速联测分析方法的基础上，在祁连山地区开展了针对金和铂族元素的区域地球化学勘查。工作结果，Ｐｔ、Ｐｄ、Ａｕ的报出率都达到了１００％，从而可靠地圈定了Ｐｔ、Ｐｄ、Ａｕ的地球化学异常。这一案例证明，地球化学勘查在寻找金和铂族元素矿产方面都是有效的     

A stream sediment geochemical re-investigation of the discovery of the platiniferous Merensky Reef, Bushveld Complex

The platiniferous Merensky Reef was discovered on the farm Maandagshoek in the Bushveld Complex in August 1924. A historical review of these events is presented, based partly on an unpublished report and a map by Dr Hans Merensky. It has been supposed that Merensky first traced the platinum to discordant ultramafic pipes. However, a re-examination of the topography and river systems in the area shows that this was not possible. A stream sediment study from this area has been undertaken to determine the source of the platinum originally panned at the discovery site. Determination of the Pt:Pd:Au ratio in these samples indicates a strong similarity with ratios in the Merensky Reef, and is completely different from the ratio found in the pipes, which are deficient in Pd and Au. These ratios also suggest that weathering and transportation has occurred predominantly due to physical rather than chemical processes. Two platinum-group element anomalies in the stream sediments are found upstream from the Merensky Reef outcrops. This results from a change in the fluvial system due to Pliocene to recent up warping with a reversal in flow direction.     

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6-15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.     

交换树脂—活性炭动态吸附极谱法测定矿石中的微量铂

矿石中微量铂经 71 7阴离子交换树脂 -活性炭动态吸附富集后 ,Pt的回收率在 95 %～ 1 0 3 %。以H2 SO4-NH4Cl-六次甲基四胺 -硫酸联氨为底液 ,极谱法测定。方法经GBW系列国家标准物质分析检验 ,结果与标准值相符 ,对w(Pt) =6.4× 1 0 -9的GBW 0 72 90国家标准物质测定 7次 ,RSD =7.1 %。方法适用于矿石中Pt含量在 1 0 -6～ 1 0 -9级的测定。     

密闭溶样-活性炭吸附-化学光谱法测定化探样品中的痕量铂

通过对介质酸度、缓冲剂、活性炭用量进行试验，建立针对化探样品中痕量铂的测定方法。经过化探标样考核，表明该方法快速准确、劳动强度低。