Pilot study of partial extraction geochemistry for base metal exploration in a thick loess-covered region

• First successful trial to use partial extraction in loess covered region in China
• 2% HCl leaching procedure can provide meaningful responses related to concealed mineralization.
• The pH values indicated the existence of a reduced chimney in an alkaline environment.

The Jiaolongzhang polymetallic deposit in Eastern Gansu is located under loess terrains with thicknesses ranging from < 50 m to > 200 m. The mineralization zones are completely covered by loess and by a red soil layer. A soil survey along Line 48 by using the total digestion technique at a depth of 30 cm showed no responses for deposit-related elements Pb, Zn, Cu, and Ag. Different top soil leachants and leaching procedures were tested, and several elements were analyzed by using ICP–MS to decipher the leaching geochemistry in a loess terrain. Deionized water, NH4Cl, and HOAc did not detect any signals for the buried deposits, whereas 2% HCl leaching effectively detected buried deposits. Strong multi-point anomalies were found in many elements above the concealed ore bodies. The highest Cu, Pb, and Zn concentrations reached 800, 2000, and 8000 ng/g in the anomalous samples, respectively, and reached 200, 70, and 400 ng/g in the background samples, respectively. The elemental distribution patterns corresponded with the results of the metals-in-soil-gas survey. Most pH values of soil in background and anomalous samples were higher and lower than 8.45, respectively; thus, the pH values corresponded to a pattern of reduced chimneys.