Comparison of three analytical methods in the determination of gold in sixFinnish 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, splining and grinding - and to compare three analytical methods based on the atomic absorption determinati on of gold following: (1) classical lead fire assay (FA); (2) the aqua regi a leach (AR) followed by Hg coprecipitation of Au; and (3) the sodium cyani de (NaCN) leach. Sample size used for the method comparison is 20 g. The Au deposits and ore types were: Suurikuusikko and Osikonmaki, refractory ores in which An is associated with arsenopyrite and pyrite; Pampalo and Kutema jarvi ores with metallic Au and Au tellurides; and Jokisivu and Pahtavaara ores containing coarse-grained metallic Au. After crushing, the samples wer e split into three parts, one of which was put aside into storage. Two spli ts 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 a nalysed for Au using the three analytical methods. Each determination was p erformed 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 An content in all six cases. Grinding to a finer grain size gave a significant difference in Au results only for the Pahtavaara ore sample. I f the FA results are assumed to represent 100% recovery of Au, we obtained greater than 95% recoveries for all but the Suurikuusikko sample (87% recov ery) by the AR leach method. We also obtained recoveries of over 95% by the NaCN leach method for the Pampalo, Kutemajarvi and Pahtavaara samples, whe reas recoveries for the other three samples varied between 73 to 92%. The A R leach was also performed on l-g samples and the NaCN leach on 250-g sampl es. For three of the ore samples, decreasing sample size from 20 g to 1 g d id not cause a significant difference in the variance of the Au results. In creasing the sample size from 20 g to 250 g significantly improves the repr esentativity of only the Pahtavaara sample. For the Kutemajarvi, Pahtavaara and Jokisivu ores, a sample larger than 250 g is needed in order to obtain a precision equivalent to that for reference samples. (C) 1999 Elsevier Sc ience B.V. All rights reserved.