Abstract

The ‘Metals in Soil Gas’ (MSG) survey has proven to be a useful tool for mineral exploration under exotic overburden. Tracing the source of these metals with Pb isotopes is helpful to understand the formation of MSG. Lead isotope ratios in MSG samples were determined by ICP-MS (Model HP4500); the Pb isotope ratios in loess, red soil, wall rocks and ores were measured following decomposition and separation using a VG-354 thermal ionization mass spectrometer (TIMS) for comparison. The results of the study of samples collected over the Jiaolongzhang base metal deposit show that the Pb isotope ratios of MSG background samples are markedly distinct from those ratios of any medium (loess, red soil layer, wall-rocks and ores) in the vicinity of the deposit. The Pb isotope ratios in the MSG anomalous samples (206Pb/204Pb = 18.34–18.56, 207Pb/204Pb = 15.622–15.809 and 208Pb/204Pb = 38.184–38.691) are totally different from those in the samples of background areas (206Pb/204Pb = 16.46–17.68, 207Pb/204Pb = 13.985–14.945 and 208Pb/204Pb = 34.199–36.884). The Pb isotope ratios of MSG anomalous samples scatter near the ratios of the mineralized wall-rocks (206Pb/204Pb = 18.554–18.874, 207Pb/204Pb = 15.618–15.755 and 208Pb/204Pb = 38.629–39.126) and sulphides (206Pb/204Pb = 18.130–18.251, 207Pb/204Pb = 15.671–15.767 and 208Pb/204Pb = 38.350–38.582). It can be concluded that some of the Pb in MSG anomalous samples originates from deep sulphide mineralization and Pb isotope ratios of MSG anomalous samples indicate that an MSG survey can detect the deeply concealed mineral deposits under exotic cover.

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