Blind and buried mineral deposits in dry climates

In the CSIRO project, four deposits classified as blind by company geologists and located in areas with dry climates were studied. Significant Hg and base-metal soil anomalies were detected at three of these deposits, indicating that the deposits or their primary halos outcrop. 

The failure to detect anomalous Hg above these buried deposits can at least in part be related to features of the weathering environment. In some of the case histories it was determined that the present water table was above the depth of oxidation of sulphides with the result that active sulphide oxidation had ceased. In others it was apparent that the low levels of Hg restricted the development of a halo. 

In contrast to the very restricted size of the soil-gas anomaly at Jabiluka, Zonghua and Yangfen (1981) describe broad anomalies associated with a buried skam Cu deposit near Shanghai, China. The ore body is confined to the contact zone of granodiorite and Palaeozoic and Mesozoic sedimentary rocks. The major sulphide minerals in the ore body are chalcopyrite, pyrite and molybdenite. Part of the deposit has been oxidised to limonite. The area is covered by 140-180 m of alluvium. A soil-gas survey of the area was complemented by a multielement study of soils including the determination of Hg. Significant anomalies of Hg in soil and soil gas occurred above the buried sulphide 

Similar anomalies over buried ore deposits have been reported by other Chinese workers (see Chapter 13). Using a gold-film detector, workers at the Chinese National Non-ferrous Metals Corporation (CNNC) analysed Hg in soil gas and near-surface atmosphere at the Naking Pb-Zn mine, where stratabound ores occur in fractures in Permo-Carboniferous strata. The region is covered by about 30m of allochthonous overburden. Mercury anomalies of up to three times background occur in both soil gas and near-surface atmosphere above a fault that intersects the ore (Fig. 12-23). These are coincident with higher-contrast Hg soil anomalies, but corresponding Pb and Zn data are not available. 

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