Abstract
The Au value of stream sediments in swamp-forest areas in Northeast China is lower than in other areas, according to results from the Regional Geochemistry–National Reconnaissance Program. Two dissolution methods for measurement of Au levels were compared in this study. The Au values in samples dissolved by using four acids (HClO4 + HF + HNO3 + HCl) are larger than those dissolved by using aqua regia, especially when the Au content is greater than 1.25 ng/g. We found that a large amount of Au may be absorbed to or/and included in SiO2 colloids, apart from that occurring in organically bound forms in organic-rich sediments of swamp-forest areas. The Au absorbed to or/and included in SiO2 colloids is related to plant metabolism. This Au can migrate for long distances protected by high-concentration organic acids in water solution, but cannot be dissolved by aqua regia. The results of a pilot survey with an ultra-low sampling density in the Jiayin area, Heilongjiang Province, show that the Au anomalies obtained by using four-acid dissolution are not only larger in scale but also more coincident with corresponding metallogenic belts than those using aqua regia dissolution.
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