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 &lt;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 <em>c</em>. 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- (<em>c</em>. 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.

本研究将未筛分冰碛物样品的便携式X射线荧光光谱仪（portable XRF, pXRF）直接检测结果，与采用标准实验室方法（王水消解或熔融法结合电感耦合等离子体质谱法（ICP-MS）及电感耦合等离子体发射光谱法（ICP-ES）对<0.063毫米粒级组分的分析结果）进行对比，旨在探究野外直接使用pXRF能否作为即时后续工作的可靠指导依据。研究选取了加拿大的四个试验矿区：Halfmile湖铜-铅-锌块状硫化物（Volcanogenic Massive Sulfide, VMS）矿床、赋存于侵入岩体中的钨-钼Sisson矿床、Pine Point矿区的密西西比河谷型（Mississippi Valley–type, MVT）铅-锌矿床，以及Triple B金伯利岩岩体。本研究使用了加拿大地质调查局（Geological Survey of Canada, GSC）档案库中的未筛分冰碛物样品，涵盖背景区域样品、直接覆盖于各矿床上方的样品，以及顺冰流方向距各矿床不同距离处的样品。对野外盛装样品所用的Ziploc®袋与Whirl-Pak®袋进行了X射线衰减特性与污染情况的测试。总体而言，在目标元素（指示元素或探途元素）浓度显著高于该技术检测限（多数元素的检测限处于低ppm量级）的四个试验矿区中，pXRF的检测表现极佳。通过标准实验室方法确定的若干有用指示元素（矿石/目标矿种的重要组成元素）及探途元素（与目标矿种元素伴生的元素），在未筛分样品的pXRF检测中呈现出相似的分布规律：Halfmile湖矿区的锌（Zn）、铜（Cu）、铅（Pb）和砷（As）；Sisson矿床的钨（W）、钼（Mo）、铜（Cu）、锌（Zn）、铅（Pb）和砷（As）；Pine Point矿区的锌（Zn）、铅（Pb）和铁（Fe）；以及Triple B矿区的钙（Ca）、锶（Sr）、铬（Cr）和镍（Ni）。部分探途元素的浓度过低，无法通过pXRF准确测定，包括Halfmile湖矿区的银（Ag）和锑（Sb）；Sisson矿床的银（Ag）和镉（Cd）；Pine Point矿区的镉（Cd）、硫（S）和硒（Se）；以及Triple B矿区的钴（Co）、镁（Mg）、磷（P）、铀（U）和钍（Th）。pXRF对铋（Bi）的本底值较高（约50ppm）且信号噪声较大，因此无法在Halfmile湖和Sisson矿区使用该元素。部分样品因样品非均质性，导致pXRF检测的精密度较差（每个样品进行三次分析），涉及元素包括锡（Sn）、钒（V）和钨（W）。多数样品的pXRF检测结果错误地报告了汞（Hg）处于低ppm量级，而实际汞浓度仅处于低ppb量级。部分富铅、锌（铅、锌含量约1%）及富铁（铁含量最高达16%）的样品存在光谱干扰：铅（Pb）对砷（As）、钍（Th）和硒（Se）的干扰；锌（Zn）对铜（Cu）的干扰；以及铁（Fe）对钴（Co）的干扰。对分别装入Ziploc®袋与Whirl-Pak®袋的6件冰碛物样品的检测结果显示，Ziploc®袋吸收的低能光子更少，因此更适合用于测定硅（Si）、钾（K）和钙（Ca）等轻元素。