Nano-metals in soil gas (geogas) have been tested as a medium for mineral exploration for over 15 years in China. Significant progress has been made since the development and adoption of liquid collectors with an active sampling procedure and the ICP-MS analytical method. The liquid collectors are very low in trace metals, can be controlled with blanks and can be accurately and directly analyzed with ICP-MS.
The pilot study, conducted at several known sulphide deposits covered by loess, Tertiary sandstone and quaternary alluvium, shows that:
1. The distribution patterns of elements in soil gas correlate well with those in crust.
2. Gaseous metal anomalies (mainly of Cu, Pb, Zn, Cd, Ni, Sb, Bi, Tl, Au and Ag) were observed over almost all sulphide mineralizations covered by transported overburden.
3. The highest Cu and Zn in soil gas above sulphide orebodies can reach over 300ppm and 200ppm per litre gas volume, although these metals in barren area are often lower than 1 ppb for Cu and several ppb for Zn.
4. Analysis of Pb isotopes in a known polymetallic deposit covered by loess and Tertiary sandstone shows that the Pb isotope ratios of nano-metals in background areas are different from those in any single surrounding medium, which includes loess, Tertiary red earth and sandstone, host rocks and ores. Great differences were found between the Pb isotope ratios in anomalous samples above the orebodies and those in background samples, which indicates that some anomalous Pb may be turning up in the soil gas. The ratios of Pb isotopes in soil gas above the mineralizations, which show the smallest variation and a distribution similar to that in sulphides, suggest that some metals in anomalous samples originate from deep orebodies.
Since 2001 nano-metal surveying has been used as a tool for exploring mineral gold, polymetallic and nickel deposits. It has proved successful in prospecting for polymetallic mineralizations in several blind areas. One may conclude that the phenomenon of nano-metals in soil gas has several puzzling aspects that require further study, but that nano-metal surveying is indeed a practical tool of great potential for mineral exploration in areas with exotic overburdens.