Bias in the analysis of geological materials for gold using current methods

A consistent pattern has been observed in the results obtained for Au in three soil and two porphyry copper ore samples serving as control reference standards in geochemical analyses. The mean reported by wet chemical methods (regardless of the measurement technique) was less than the mean by fire assay-based methods which, in turn, was less than the mean reported by laboratories using direct instrumental neutron activation analysis (INAA). These data have been obtained from 16 laboratories, some employing more than one method. Compared to INAA, values obtained for Au (at the 30–300 ppb level) using aqua regia (AR) dissolution were low by 24–42%, while those reported by fire assay-based methods were low by 14–26%.Studies of these samples and 32 rocks of widely varying composition revealed that the amount of Au remaining in the residue after AR attack ranged from 4% to 59% of the total. Gold dissolved by AR was determined by graphite furnace atomic absorption spectrometry (GFAAS) after extraction into MIBK (methyl isobutyl ketone), whereas Au remaining in the residue was determined by INAA. The slope of the line obtained by plotting Au by INAA vs. the sum of Au by AR/GFAAS and Au in the residue was not significantly different from 1.0.Modifications to the AR procedure such as prior attack by HCl or HF did not improve the recovery of Au in the three reference soils. However, reduction of sample size from 10 g to 1 g while maintaining the volume of AR at about 30 ml did increase results for Au. Furthermore, addition of HF in the attack on 1-g samples yielded results virtually identical to those obtained by INAA. It is thought that the poor extraction efficiency by AR is due to non-wetting of the larger sized sample, a lack of intimate, prolonged contact between the grains of the sample and the acid mixture and the presence of insoluble gangue surrounding Au particles. Constant agitation of the sample during evaporation with AR is desirable.The mean values obtained for Au in the soils by fire assay methods were not significantly different from the results by INAA (low by 14–19%); this was not the case for the two copper ore samples (low by 26%). This probably reflects the difficulties encountered in fire assay by high concentrations of Cu which hinder effective collection and separation of Au into the Pb button. However, the accuracy of the INAA method has not been established and is dependent upon measurement procedures and the degree of certainty associated with the Au values assumed for the reference materials employed for calibration. While estimation of precision does not present a problem, accuracy is difficult to assess in the absence of certified reference materials for Au at concentrations in the 10–300 ppb range. However, it is concluded that methods based upon AR dissolution can lead to low results, the magnitude being dependent upon the sample matrix and the mineralogical association of the Au present.     

Instrumental neutron activation in geoanalysis

Theoretical aspects of instrumental neutron activation analysis (INAA) are discussed. Various applications of INAA to exploration geochemistry are described demonstrating its capabilities and its limitations.Cost, turnaround time, high sensitivity for certain elements, the non-destructive nature of analysis and its precision and accuracy have combined to make INAA an indispensible method for multielement determination on virtually all geological matrices. Humus, vegetation, heavy minerals, lake bottom sediments as well as rocks and soils comprise the major sample types analyzed by INAA. As many as 50 elements can be determined routinely and easily by INAA.     

中国火成岩化学元素的丰度与分布

在中国各构造单元构选采了１０３６４件火成岩样品合成１１３１个分析样，采用仪器中子活化法，Ｘ射线荧光光谱法，原子吸收法，原子荧光光谱法，催化波极谱法和分光光度法等１５种可靠分析方法进行测试，对一些难测痕量元素采用了最新的分析研究成果，以同类国家一级标准物质监控分析质量。     

GGR Biennial Review: Atomic Absorption,Inductively Coupled Plasma-Atomic Emission Spectrometry,Neutron Activation Analysis and X-Ray Fluorescence Spectrometry Review for 2008–2009

This review describes developments major and in trace element determination using atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), instrumental neutron activation analysis (INAA), and X-ray fluorescence (XRF) spectrometry that were reported in 2008 and 2009. Publication levels were comparable to those of previous years, except for XRF which showed an increase in the number of articles published in the past 2 years. In terms of number of publications and impact, the most active field was AAS, while INAA was the least active. As expected for well-established and mature methods, novel developments for all four analytical techniques were relatively sparse. For AAS, the most notable publications concerned developments in sample introduction, particularly focussing on solid samples, increasing analyte sensitivity, and higher sample throughput. For ICP-AES, publication of developments in sample introduction, remediation of matrix effects and calibration continues. Compared with past years, there was a clear focus on sample preconcentration methods, but very few publications reporting new hyphenated speciation methods. For INAA, there were several publications exploring the accuracy and robustness of the method, as well as the requirements for INAA to meet criteria for a primary method of measurement. Two other related techniques, delayed neutron activation analysis, and prompt gamma activation analysis, were also described. Hyphenated XRF techniques showed interesting developments in enabling XRF and XRD analysis on the same spot, and further work characterising and calibrating three-dimensional micro-XRF shows promising results for investigating sample heterogeneity.     

地气测量研究及在东季金矿的试验

用中子活化分析技术及新研制的地气采集装置,在山东省东季金矿上进行了地气探测及异常研究。文中讨论了对于找隐伏金矿,INAA优于PIXE技术。在构造含矿带上方有明显的La、K、Na、Fe、Sc、Au、As、Cr、Sb及Zn等元素的地气异常。在隐伏矿上方有Au、As、Cr、Sb及Zn等元素异常存在。认为地气中这些金属元素的异常是一种良好的地下金矿化的地表直接指示。     

Comparison of Major and Trace Element Analyses by ICP, XRF, INAA and ID Methods

Element concentration data for a set of silicate igneous rocks have been determined by four independent analytical methods: inductivelycoupled plasma emission spectroscopy (ICP), instrumental neutron activation analysis (INAA), x-ray fluorescence (XRF), and isotope dilution mass spectrometry (ID). The results show excellent agreement among elements determined by more than one technique, and demonstrate the suitability of commercially available ICP instrumentation for the routine analysis of geologic material compared to other standard methods.     

Geochemical Anomaly at the Devonian / Carboniferous Boundary,Huangmao, Guangxi,China

Abstract The abundances of nearly 40 elements, Ir included, have been measured using radio-chemical and instrumental neutron activation analysis (RNAA and INAA) across a Devonian / Carboniferous (D / C) boundary section at Huangmao, Guangxi, China. The Ir anomaly has been found in the D / C boundary bed. Its peak value is 156 ppt, richer by a factor of 12 than that in the underlying strata. Besides, as with Ir, other siderophile and chalcophile elements such as Au, Ni, Co, As and Sb are also enriched. The cause for the abundance anomalies of Ir and other elemets is discussed. Neither volcanic eruption nor extraterrestrial impact can explain it satisfactorily. The real mechanism for the anomalies awaits further study.     

Determination of Rare Earth Elements in Geological Reference Materials: A Comparative Study by INAA and ICP-MS

Data was obtained for the rare earth elements (REE) by instrumental neutron activation analysis (INAA) and inductively coupled plasma-mass spectrometry (ICP-MS) in twenty geological reference materials. In general, the precision obtained by ICP-MS is better for the light REE, decreasing with increasing atomic number. This is partly a result of the occurrence of the heavy REE at low concentrations. The precision of the data obtained by INAA is good (5% RSD). The data obtained also showed that for the elements determined by both methods, the accuracy is similar for the light REE and better for the middle and heavy REEs by INAA. Higher uncertainty is achieved by ICP-MS mainly for elements at very low concentrations, occurring at about ten times the chondritic values.     

Comparison of three analytical methods in the determination of gold in six Finnish gold ores, including a study on sample preparation and sampling

We used samples from six Finnish ore deposits to evaluate the efficiency of sample pretreatment procedures — crushing, splitting and grinding — and to compare three analytical methods based on the atomic absorption determination of gold following: (1) classical lead fire assay (FA); (2) the aqua regia leach (AR) followed by Hg coprecipitation of Au; and (3) the sodium cyanide (NaCN) leach. Sample size used for the method comparison is 20 g. The Au deposits and ore types were: Suurikuusikko and Osikonmäki, refractory ores in which Au is associated with arsenopyrite and pyrite; Pampalo and Kutemajärvi ores with metallic Au and Au tellurides; and Jokisivu and Pahtavaara ores containing coarse-grained metallic Au. After crushing, the samples were split into three parts, one of which was put aside into storage. Two splits were further divided into two subsamples which were ground to two grades of fineness (<0.03 and <0.06 mm). The four subsamples thus obtained were analysed for Au using the three analytical methods. Each determination was performed five times on each of the four subsamples. According to t-tests on the FA results of the two splits, crushing and splitting produced samples of equal Au content in all six cases. Grinding to a finer grain size gave a significant difference in Au results only for the Pahtavaara ore sample. If the FA results are assumed to represent 100% recovery of Au, we obtained greater than 95% recoveries for all but the Suurikuusikko sample (87% recovery) by the AR leach method. We also obtained recoveries of over 95% by the NaCN leach method for the Pampalo, Kutemajärvi and Pahtavaara samples, whereas recoveries for the other three samples varied between 73 to 92%. The AR leach was also performed on 1-g samples and the NaCN leach on 250-g samples. For three of the ore samples, decreasing sample size from 20 g to 1 g did not cause a significant difference in the variance of the Au results. Increasing the sample size from 20 g to 250 g significantly improves the representativity of only the Pahtavaara sample. For the Kutemajärvi, Pahtavaara and Jokisivu ores, a sample larger than 250 g is needed in order to obtain a precision equivalent to that for reference samples.     

Bias in the Determination of Zr,Y and Rare Earth Element Concentrations in Selected Silicate Rocks by ICP‐MS when Using Some Routine Acid Dissolution Procedures: Evidence from the GeoPT Proficiency Testing Programme

The analytical results for the modified river sediment, SdAR‐1 circulated in Round 31 of the GeoPT proficiency testing programme, revealed unexpected discrepancies for Zr, Y and most higher atomic number rare earth elements, in determinations made by ICP‐MS using acid attack digestion methods. This investigation compares these ICP‐MS results with those obtained for SdAR‐1 by three other methods: (a) ICP‐MS using fusion/sintering for sample digestion, (b) XRF analysis and (c) INAA. The distribution of results for the elements Rb, Sr, Ce, Y, Yb and Zr is examined and compared with those of the test material for Round 25 of GeoPT, Paraná basalt, HTB‐1. A substantial proportion (though not all) of Y, Yb and Zr determinations in SdAR‐1 by ICP‐MS/acid attack was variably low (sometimes very low) compared with other methods. A detailed evaluation of the procedures used to determine these results indicated that successful determinations by ICP‐MS/acid attack could be made if digestions were made at 180 °C for 48 hr using at least 2 ml HF per 100 mg of sample. We suggest that the more benign conditions (used by many laboratories) resulted in incomplete digestion of resistant minerals, such as zircon.     

The content of gold in Lower Paleozoic phosphatized biogenic remains in Baltoscandia

According to instrumental neutron activation analysis (INAA) data, the average content of Au in conodont elements and phosphate brachiopod shells from Lower Paleozoic sections in Baltoscandia is 4.9 and 0.79 μg/g, respectively. Concentration of Au in phosphatized biogenic remains suggests that they served as an important intermediate concentrator of this metal in the sedimentary cover of the East European Platform.     

Geochemical dispersion of gold related to copper-gold mineralization in northeastern Thailand

The nature of gold dispersion in soils and stream sediments associated with a copper-gold-mineralized system in northeastern Thailand has been investigated as a basis for identifying appropriate geochemical exploration techniques for the search for comparable deposits in similar environments.Soils were collected with varying relationships to mineralization as a basis for determining sample representativity, size distribution of gold, variation with soil horizon and possible pathfinder elements. Similarly, stream sediments were collected to estimate sample representativity, size distribution of gold, variation of gold with depth in the stream sediment profile and to compare the relative recoveries of gold in field-panned and laboratory-prepared heavy-mineral concentrates. Samples were analyzed for Au and potential indicator elements by a variety of methods but mostly by instrumental neutron activation analysis.Results indicate the consistent distribution of fine-grained gold in soils which allows Au analysis of relatively small samples from B-horizon soils to be used effectively and reliably to identify the surficial patterns of gold mineralization in the study area. Anomalous patterns of other indicator elements, Co, As, Cu, Sb, W, Pb, Zn, Ag, Fe and Mn, may contribute additional information regarding type of mineralization. This finding indicates the effectiveness of soil surveys in gold exploration, particularly in areas of deep weathering where fresh bedrock exposures are infrequent.Unlike soils, size distributions of gold in stream sediments, as a result of the local flow regime, vary both between sampling sites and at depths within a sampling site. Exploration requires Au analysis of the fine fraction (minus 63 μm) of active stream sediments to reduce the problem of sampling representativity. The presence of coarse-grained gold in the stream channel has drawn attention to the possible benefit of using the conventional field-panning method as a semiquantitative technique for providing immediate results. However, highly erratic distribution of pannable gold on a very local scale together with variable proportions of the total gold recovered in field-panned or heavy-mineral concentrates highlights a potentially serious drawback of the method. Combination of analysis of the minus 63 μm fraction and field panning appears warranted to cover the possible existence of gold of a wide size range in stream sediments.The overall results indicate the utility of geochemical exploration techniques in the search for gold mineralization. However, particular care is necessary in the design and implementation of geochemical techniques to ensure maximum reliability of exploration.     

Iron meteorites with low Ga and Ge concentrations—composition,structure and genetic relationships

Twenty-one iron meteorites with Ge contents below 1 μg/g, including nine belonging to groups IIIF and IVB, have been analyzed by instrumental neutron activation analysis (INAA) for the elements Co, Cr, As, Au, Re, Ir and W. Groups IIIF and IVB show positive correlations of Au, As and Co (IIIF only) with published Ni analyses, and negative correlations of Ir, Re, Cr (IVB only) and W (IIIF only) with Ni. On element-Ni plots, the gradients of the least squares lines are similar to those of many other groups, excluding IAB and IIICD. With the inclusion of a new member, Klamath Falls, group IIIF has the widest range of Au, As and Co contents of any group and the steepest gradients on plots of these elements against Ni. It is likely that these trends in groups IIIF and IVB were produced by fractionation of elements between solid and liquid metal, probably during fractional crystallization.It has been suggested that some of the 15 irons with <l μg/g Ge which lie outside the groups might be related. However, the INAA data indicate that no two are as strongly related as two group members. These low-Ge irons and the members of groups IIIF, IVA and IVB tend to have low concentrations of As, Au and P, low $\text{Co}\text{Ni}$ ratios and high Cr contents. The depletion of the more volatile elements probably results from incomplete condensation into the metal from the solar nebula.The structures of low-Ge irons generally reflect fast cooling rates (20–2000 K Myr^?1). When data for all iron meteorites are plotted on a logarithmic graph of cooling rate against Ge concentration and results for related irons are averaged, there is a significant negative correlation. This suggests that metal grains which inefficiently condensed Ge and other volatile elements tended to accrete into small parent bodies.     

生物成分系列标准物质的研制

GSB系列生物标准物质包括大米、小麦、玉米、黄豆、圆白菜、菠菜、茶叶、奶粉、鸡肉和苹果等10种生物样品，用冷冻干燥等技术制备。由14个分析水平较高的实验室协作，采用等离子体质谱、等离子体光谱和仪器中子活化为主的10余种分析方法测试，共进行了22477次测定，取得了5136组平均值数据，定值元素59种，定标准值元素54种。     

Role of the biogenic factor in platinum accumulation by oceanic ferromanganese nodules

A complex study of oceanic ferromanganese nodules (FMN) from the abyssal Clarion-Clipperton Fracture Zone and Lamont Guyot (Pacific Ocean) is performed. The concentrations of noble and trace elements are determined by AAS, INAA, and RFA-SR. Furthermore, SEM, and activation autoradiography are applied. Experiments are carried out using Pt, Au, and Ir radionuclides (radioisotope indicator method).     

Trace elements in native gold by solution ICP-MS and their use in mineral exploration: A British Columbia example

An elegantly simple, aqua regia-based, ICP-MS analytical procedure is used to compare the trace element composition of density-separated alluvial native Au from seven stream silt samples with three samples of geographically-associated Au from a prospective ore deposit in central British Columbia. Not all of the alluvial Au could have come from the ore deposit based on present drainage. The silt sample Au, averaging four alluvial grains and totaling 12–250 μg per sample, generally yielded measurable concentrations for V, Fe, Cu, As, Pd, Ag, Sb, Pt and Bi. The bedrock Au samples represent the three dominant rock types in the showing. Their Au trace element compositions largely bracket the alluvial Au. Multidimensional scaling (exploratory statistics) shows that trace elements in the native Au form lithophile, chalcophile and siderophile groupings. This indicates that a small set of geochemical processes formed all the Au in one geologic environment. Previous work shows that Au from individual deposits has distinct assemblages of detectable elements. Given these observations and that detectable elements are the same in both the deposit and alluvial Au, and that concentrations in the former bracket those of the latter, it is concluded that the source of the alluvial Au has probably been identified. Apparently neither mineral inclusions nor weathering impaired fingerprinting of the Au. The simplicity of the approach indicates that this is a useful exploration tool for determining the bedrock source of alluvial Au. The study also shows that silt sample exploration in glaciated terrains must recognize that paleo-ice movement and paleo-stream directions can yield geographic distributions of alluvial Au that cannot be explained by present-day drainage patterns. Thus this simple analytical/exploration technique is potentially very useful to the exploration industry.     

A SELF-CONSISTENT COMPILATION OF ELEMENTAL CONCENTRATION DATA FOR 93 GEOCHEMICAL REFERENCE SAMPLES

Concentration data obtained by instrumental neutron activation analysis (INAA) are presented for up to 36 chemical elements in 93 geochemical reference samples, including some for which there are little previous data. Because all data are based on at least three independent analyses, and for many of the data the uncertainty associated with counting is an insignificant source of error, the values presented here are considered of higher precision than generally reported by INAA. Information on subsampling error (sample heterogeneity) is also presented.     

Multielemental Neutron Activation Analysis of Chinese Geochemical Reference Samples

The concentrations of 36 elements in geochemical reference samples issued by the Ministry of Geology and Mineral Resources of China were determined by neutron activation analysis. Three main variants of the technique, instrumental, epithermal, and nreirradiation separation neutron activation analysis (INAA, ENAA, PNAA), were employed in a systematic study of the samples by three laboratories: the Institute of Atomic Energy of the Academia Sinica (INAA, ENAA), the Institute of High Energy Physics of the Academia Sinica (INAA), and the Institute of Geophysical and Geochemical Prosnecting of the Ministry of Geology and Mineral resources (INAA,PNAA). Both long and short irradiations and both Ge(Li) and HPGe detectors were used. A supplementary software nackage for data processing was developed. About 81% of the data determined by neutron activation agreed with recomended values with in 15%.     

植物样品中无机元素分析的样品前处理方法和测定技术

植物样品中无机元素的分析测定在环境地球化学和生物地球化学的研究中起着重要作用。植物样品中元素含量一般较低,须选用科学合理的前处理技术和灵敏度高、精密度好、检出限低的测定方法。本文针对植物样品前处理方法和无机元素分析测定技术的研究进展、优势与不足进行评述。前处理方法主要根据样品和待测元素的性质进行选择:干法灰化所用试剂少、空白值低,但组织致密型的样品不易灰化完全、高温下易造成元素挥发损失;湿法消解样品消解较为完全,但试剂消耗大、空白值高、操作繁琐;微波消解可以防止部分易挥发元素损失,用酸量少、消解速度快,但称样量相对较小,不适于需要大称样量的样品分析。几乎所有针对元素分析的仪器分析技术都可以用于植物样品分析,主要根据仪器适用的元素、必要的干扰校正以及基体改进等方面进行选择:电感耦合等离子体质谱法可同时测定植物样品中40种以上的元素,高分辨质谱的检出限可达fg/mL;电感耦合等离子体发射光谱法适用于某些植物样品中含量较高的P、K、Na等元素的测定;原子吸收光谱法可分析元素达70余种,是普及程度最高的仪器分析技术之一;原子荧光光谱法与氢化物发生技术的联用,在元素含量较低的植物样品分析中技术优势更加明显;新兴的激光诱导击穿光谱技术已被应用于植物样品分析,无需复杂的样品前处理,操作简单快速,可实现原位、在线、实时、多元素同时检测;其他选择性强、灵敏度高的分析技术,满足了一些特定元素不能用常规分析技术测定的需求。当前主流分析技术的样品前处理方法都存在着缺陷,固体进样技术将成为植物样品分析领域的发展方向之一。