| Autor: |
Cameron, Eion M., Hamilton, Stewart M., Leybourne, Matthew I., Hall, Gwendy E.M., McClenaghan, M. Beth |
| Zdroj: |
Geochemistry: Exploration, Environment, Analysis; 2003, Vol. 4 Issue: 1 p7-32, 26p |
| Abstrakt: |
Geochemical methods are widely applied during exploration for deeply-buried mineral deposits. This paper describes the project Deep-Penetrating Geochemistry, supported by the Canadian Mining Industry Research Organization and 26 companies. Phase I considered the theoretical framework for the movement of material from buried deposits to the surface. In arid or semi-arid terrain, with a thick vadose zone, advective transport is the only viable means of moving elements to the surface; diffusion of ions in water or gases in air is orders of magnitude slower. Examples of advective transport are pumping of mineralized groundwater during seismic activity and extraction of air plus gas by barometric pumping. In wetter climates several mechanisms have been proposed for movement of elements through water-saturated cover; diffusion-based models again suffer from slow rates of migration. Phase II involved studies at ten sites, including porphyry deposits in northern Chile, a gold-copper deposit in the Carlin district of Nevada, and volcanogenic massive sulphide bodies in the Abitibi belt of Ontario. In all cases anomalies were found in soils above buried mineralization. Anomaly formation is episodic and cyclic: batches of metal in water-soluble form are introduced and the metal is progressively incorporated into the secondary soil minerals. Selective leaches that dissolve specific soil phases can detect these anomalies. We compare the results for five selective leaches plus aqua regia. Efficiency of leaches depends on the ratio of introduced (exogenic) metal in the soil secondary minerals to that of endogenic origin derived from primary soil minerals. For the Chile and Nevada sites, anomaly formation may have extended over a million years or more, which permitted metals to be incorporated into many secondary minerals. For these sites, some anomalies can be detected by aqua regia, although the anomaly/background contrast is less than for selective leaches. At Ontario sites only a few thousand years have elapsed since deposition of the glacial sediment cover sufficient time for metals to have been incorporated into just the most labile secondary minerals, only leaches that can dissolve these minerals that can successfully identify anomalies. The optimum sampling depth in soils is critical to detecting anomalies. |
| Databáze: |
Supplemental Index |
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