Distribution of trace Hg and its mode of occurrence over Fankou Pb---Zn deposit, Guangdong, China

Blind ore bodies more than 200 m below the surface and ore-bearing faults are clearly indicated by soil-air Hg anomalies through an overburden 20–40 m thick over the Fankou Pb---Zn deposit. The background of soil-air mercury in this area is 50 ng m⁻³ and the highest value encountered is 2000 ng m⁻³. The maximum anomaly to background contrast is 40. The width of anomalies higher than 500 ng m⁻³ can be greater than 600 m. The background Hg in soils of the area is 0.06 ppm and increases to 0.15 ppm where there are soil-air anomalies. The contrast of Hg in soils is only 2.5. Mercury in the unconsolidated overburden is in the form of free gas and is absorbed onto soil colloids such as clay minerals and Fe-Mn oxides. The primary sulphides contain 14–320 ppm Hg where Hg is incorporated isomorphically in sulphide minerals, which is confirmed by microprobe analysis.Based on a systematic analysis of the materials from surface and underground workings for the mode of occurrence of Hg, the authors suggest that the Hg-vapour anomalies in soil-air are the oxidation products of the ore.