Method for Ultra-Low-Level Analysis of Gold in Rocks

Abstract

A new method for analyzing gold at ultralow concentrations (<10 pg/g) in geological samples has been developed involving HF-aqua regia acid digestion, chromatographic separation of Au from matrix elements using DIBK supported on an inert resin, and analysis by inductively coupled plasma-mass spectrometry (ICPMS). This method has an analytical detection limit of 2 parts per trillion (pg/g), significantly lower than most routinely used methods developed for analysis of ore samples with Au concentrations considerably higher than average crustal abundance ( approximately 2 ng/g). Such methods commonly have detection limits in the low nanogram per gram range. Many areas of geological research including ore genesis, crustal mobility and redistribution, planetary differentiation, and plume volcanism require quantitative analysis of geological materials with much lower Au concentrations. We present a rapid, easy to use method where Au is separated from matrix elements onto extractant primed chromatographic resin and analyzed by quadrupole ICPMS. The method is suitable for the relatively rapid analysis of a large number of samples and is reliable over a wide range of concentrations from picogram to microgram per gram level. Analysis of four different geostandards, GXR1, GXR4, CH-3, and SARM 7, yields concentrations within error of the published concentrations with accuracies of >95%.

Mobility of gold during metamorphism of the Dalradian in Scotland

“…Gold analysis was carried out using a Thermo XSeries 2 ICP-MS following the ultra-low detection limit method (b10 ppt) described in Pitcairn et al (2006b). Analytical precision for Au analysis at Stockholm was controlled through analysis of CANMET reference material TDB1 and Rocklabs reference materials OxA71 and SE44.…”

“…The instrumental error is extremely small compared to the variability in the distribution of Au within each sample. For example analyses of different aliquots of the same sample shows a variability of 20% (Pitcairn et al, 2006b) whereas repeat analysis of the sample digest on the ICP shows a variability of less than 1%.…”

“…We focus specifically on Au, As and Sb as these elements have given the strongest signals of metamorphic mobility in previous studies (Pitcairn et al, 2006a(Pitcairn et al, , 2010a(Pitcairn et al, , 2014a. Specifically developed analytical methods with ultra low detection limits have been used for this study (Pitcairn et al, 2006b) as these methods provide the analytical power to observe depletions in low abundance elements. Our results provide further understanding of the controls on development of abundant orogenic Au mineralisation during metamorphism.…”

Selective detection of gold using genetically engineered bacterial reporters

“…Thus, efforts have been directed to improve both the discovery of economically suitable gold ores and the extraction procedures . Several analytical methods, such as flame atomic absorption spectrometry (FAAS), inductively coupled plasma optical emission spectrometry (ICP-OES), ICP mass spectrometry (ICP-MS), and the expensive neutron activation analysis (NAA), have been used to evaluate Au content in deposits (Caporali et al, 2010;Pitcairn et al, 2006). All these methods are sensitive enough to detect total Au content in soils and alluvial deposits, but require sophisticated instrumentation and trained personnel.…”

Sources of Metals and Fluids in Orogenic Gold Deposits: Insights from the Otago and Alpine Schists, New Zealand

“…In order to observe depletions in these elements relative to these already low crustal abundances, analytical methods with ultralow detection limits were used. In the case of gold, a new method with an ultralow detection limit of <10 ppt was developed (Pitcairn et al, 2006).…”