Lithogeochemistry in exploration for Proterozoic porphyry-type molybdenum and copper deposits, southern Finland |
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| Authors: | Pekka A Nurmi |
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| Institution: | University of Helsinki, Department of Geology, Box 115, SF-00171 Helsinki, Finland |
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| Abstract: | Over 450 samples were collected around four Proterozoic porphyry-type occurrences and analyzed for 30 elements. Relatively broad primary halos are associated with all four occurrences in spite of their moderate size and grade. The halos are characteristic of each particular occurrence depending on the geological and geochemical conditions. Copper forms extensive and highly contrasted anomalies, whereas Mo anomalies are of limited size. Gold, Zn, Pb, As, and Sb seem to be useful pathfinders for slightly eroded or blind occurrences. Pyritization and wall-rock alteration are weaker than in Phanerozoic porphyry deposits, consequently, S halos are small and often erratic. K2O, Rb, and SiO2 produce weak anomalies near the core of the occurrences. A density of a few tens of samples per km2 is required for detailed follow-up exploration.The host granitoids have normal chemical compositions outside the mineralized areas with low and constant background contents of ore elements and are thus very suitable for lithogeochemical exploration. On a regional scale a density of one to five samples per km2 is sufficient to identify mineralized intrusive phases and areas, because the porphyrytype occurrences were formed by extensive hydrothermal processes, which produced scattered anomalies over large areas. |
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