Application of portable XRF to the direct analysis of till samples from various deposit types in Canada

In this study, results by direct portable XRF (‘pXRF’) on unsieved till samples were compared with those by established laboratory methods (aqua regia or fusion ICP-MS and ICP-ES) on the <0.063-mm fraction to determine if the application of direct pXRF in the field would serve as an acceptable guide for immediate follow-up work. Four test sites in Canada were chosen: the Halfmile Lake Cu-Pb-Zn VMS deposit; the intrusion-hosted W-Mo Sisson deposit; a Pb-Zn Mississippi Valley–type (MVT) deposit in the Pine Point district; and the Triple B kimberlite. Unsieved till samples from the GSC archive collection were used for this study and included samples from background areas, immediately overlying, and at various distances down-ice of each deposit. Ziploc® and Whirl-Pak® bags that were used to contain the samples in the field were tested for their properties of X-ray attenuation and contamination. In general, the performance of pXRF in the four test areas was very good where concentrations of elements of interest (indicator or pathfinder elements) were substantially above detection limits by this technique (in the low ppm range for many elements). The following elements, shown to be useful indicator elements (important constituents of the ore/commodity) or pathfinder elements (those associated with the commodity elements) by the established methodology, showed similar patterns by pXRF on the unsieved material: Zn, Cu, Pb, and As at Halfmile Lake; W, Mo, Cu, Zn, Pb, and As at the Sisson deposit; Zn, Pb, and Fe at Pine Point; and Ca, Sr, Cr, and Ni at Triple B. Pathfinder elements whose concentrations were too low for determination by pXRF include: Ag and Sb at Halfmile Lake; Ag and Cd at Sisson; Cd, S, and Se at Pine Point; and Co, Mg, P, U, and Th at Triple B. The high background for Bi by pXRF, equivalent to c. 50 ppm, and its noisy signal precluded its use at Halfmile Lake and Sisson. Elements which tended to show poor precision (three analyses each sample) by pXRF in some samples due to sample heterogeneity include Sn, V, and W. Mercury was erroneously reported for the majority of samples in the low ppm range by pXRF whereas its concentration in fact was in the low ppb range. Several Pb-, Zn- (c. 1% Pb, Zn) and Fe-rich (up to 16% Fe) samples demonstrated spectral interferences by: Pb on As, Th and Se; Zn on Cu; and Fe on Co. Results for six till samples analysed in Ziploc® and Whirl-Pak® bags showed that Ziploc® absorbs fewer low-energy photons and hence is preferable for determining light elements such as Si, K and Ca.

本研究针对未筛分冰碛物样品，采用便携式X射线荧光光谱仪（portable XRF，pXRF）直接开展现场测试，并将测试结果与针对<0.063 mm粒级组分的标准实验室方法（王水（aqua regia）或熔融法结合电感耦合等离子体质谱法（ICP-MS）与电感耦合等离子体发射光谱法（ICP-ES））的测试结果进行对比，旨在探究现场直接使用pXRF能否作为即时后续工作的可靠指导方案。研究选取加拿大境内4处测试靶区：Halfmile湖铜-铅-锌火山成因块状硫化物（Volcanogenic Massive Sulfide, VMS）矿床、赋存于侵入体中的钨-钼锡森矿床、派因波因特矿区内的铅-锌密西西比河谷型（Mississippi Valley–type, MVT）矿床，以及Triple B金伯利岩筒。本次研究使用的未筛分冰碛物样品取自加拿大地质调查局（Geological Survey of Canada, GSC）档案库，涵盖背景区域样品、直接覆盖于各矿床上方的样品，以及位于各矿床冰流下游不同距离处的样品。对野外封装样品所用的Ziploc®与Whirl-Pak®包装袋开展了X射线衰减与污染特性测试。总体而言，在目标元素（指示元素或找矿指示元素）浓度显著高于该技术检出限（多数元素可达低百万分之一（ppm）级别）的4处靶区中，pXRF的测试表现优异。经标准方法验证为有效的指示元素（indicator elements，即矿石/矿种的重要组成成分）与找矿指示元素（pathfinder elements，即与矿种元素伴生的元素），在未筛分样品的pXRF测试中呈现出一致的分布模式：Halfmile湖矿区的锌、铜、铅与砷；锡森矿床的钨、钼、铜、锌、铅与砷；派因波因特矿区的锌、铅与铁；Triple B金伯利岩筒的钙、锶、铬与镍。部分找矿指示元素因浓度过低，无法通过pXRF准确定量：Halfmile湖矿区的银与锑；锡森矿床的银与镉；派因波因特矿区的镉、硫与硒；Triple B金伯利岩筒的钴、镁、磷、铀与钍。pXRF对铋的本底值较高（约50 ppm）且信号噪声较大，因此无法在Halfmile湖与锡森矿区使用该元素作为指示指标。部分样品因非均质性，导致pXRF测试精度欠佳（每个样品开展三次平行分析）的元素包括锡、钒与钨。多数样品的pXRF测试结果错误地显示汞浓度处于低百万分之一级别，但实际汞浓度仅为低十亿分之一（ppb）级别。部分富铅、锌（铅、锌含量约1%）与富铁（铁含量最高可达16%）的样品存在光谱干扰：铅对砷、钍与硒的干扰，锌对铜的干扰，以及铁对钴的干扰。针对封装于Ziploc®与Whirl-Pak®包装袋内的6件冰碛物样品的测试结果显示，Ziploc®包装袋对低能光子的吸收更少，因此更适用于硅、钾、钙等轻元素的定量分析。