Determination of Gold Mobile Fraction in Deep-penetrating Geochemical Samples by ICP-MS with Pre-extraction

BACKGROUND The geochemical exploration for gold deposits is based on the accurate determination of gold. The content of gold in geochemical samples is usually at the level of ng/g, which needs to be preconcentrated before determination by inductively coupled plasma-mass spectrometry (ICP-MS) or graphite furnace absorption spectroscopy (GFAAS). At present, the exploration of buried deposits is the frontier of geochemical exploration technology and the extraction of gold mobile fraction is one of the most effective approaches to find concealed gold deposits. Compared with whole rock gold analysis, the gold mobile fraction content is much lower, which requires specific leaching, efficient preconcentration and accurate determination. OBJECTIVES To selectively extract gold mobile fraction and find surface secondary anomaly information for exploring concealed ore bodies. METHODS Experiments were carried out on the extraction agent and determination of gold mobile fraction by ICP-MS. Ammonium citrate was used to promote the disassociation of the adsorption of exchangeable form gold from the surface of clay minerals and secondary minerals in the soil. The complexation of thiourea and sodium thiosulfate was applied to diffuse the extracted gold into the solution to achieve the purpose of selective extraction. The procedure for gold preconcentration from extraction solution and the ICP-MS determination method were established. RESULTS The experiments determined that the extractant was composed of 5g/L ammonium citrate, 2g/L thiourea, and 5g/L sodium thiosulfate. The extraction time was 24h, and the active carbon was used to preconcentrate gold in the acidic thiourea medium. The gold adsorption rates were 89.6%-109.2%, and the ashing temperature of the concentration materials were 650-700℃. The detection limit of the method was 0.05ng/g, the relative standard deviations (RSDs) ranged from 9.4% to 10.2%, and the recoveries were from 91.2% to 93.4%. CONCLUSIONS Compared with the published method, extraction with ferric sulfate-thiourea-sodium thiosulfate solution and determination by GFAAS, this method has the advantages of low detection limit, wide linear range and fast analysis. This method has been successfully applied in the geochemical exploration of gold deposits in the forest swamp landscape area of Dongan, Heilongjiang Province. Gold anomaly delineated is consistent with the location of the concealed gold deposit.