Thermochemical modelling of the Kongsberg silver Ore deposit, Norway
|Location||International Geological Congress,oslo 2008|
|Author||Segalstad, Tom V.|
|Holding Date||23 September 2008|
Native silver (Ag) in Permian hydrothermal calcite veins in the Kongsberg District (60°N), Norway, occurs where the veins cross sulfur-rich fahlbands. Ag is formed both from transformation of argentite (Ag2S) and precipitated directly from the mineralizing fluid. Earlier hydrothermal activity precipitated quartz and sulfides in a separate vein system, and barite in a separate vein system. Local extensive wall rock alteration to K-feldspar and chlorite occurs with some of the veins. Coal blende occurs as a characteristic substance with Ag in the veins. Fluid inclusion data by Johansen (1985) showed that Ag formed by heating the hydrothermal system from ~250 to ~300°C and contemporaneous decrease in salinity from ~25 to less than 20 wt.% NaCl-eq. The Ag veins saw at least 3 stages of heating and cooling from ~200 to 300 and back to ~200°C. δ18O of quartz in the quartz-sulfide veins show extremely constant values in the Fiskum Area E of Kongsberg, indicating an open system, with flu.incl. t of formation ~250°C. Computed δ18O(SMOW) for H2O is ca. -1; crustal water in equilibrium with wall rocks. Considering metal solubilities from thermochemical modelling, metal transport may have occurred at ~300°C, pH ~4, log oxygen activity ca. -34. δ13C in coal blende and calcite show that both must have C originating from alum shale. δ18O in calcite shows that the mineralizing systems closed towards the end, when fractionated crystallization occurred from the fluids at pH 4, increasing to pH 5.
Thermochemical modelling of the Ag-mineralizing system at Kongsberg confirm that the characteristic mineralers like barite, argentite, Ag and pyrrhotite could not form in an open system at pH 4 to 5. The veins must have been filled several times by hot fluids separated from their sources, and reacted with host rocks. By reaction with pyrrhotite in the fahlbands protons were consumed and pH increased. After precipitation of sulfides, S in the fluids decreased, and argentite and later Ag reached their stabilities and precipitated. At increasing pH and/or decreasing S in the fluids, pyrrhotite reached stability. Now the fluids had reached equilibrium with the wall rocks, and the Ag precipitation stopped.
Modelling of solubilities for different Ag complexes in equilibrium with Ag under the observed conditions show that the fluids, by heating from 200 to 250°C, can dissolve 1000 times more Ag (~0,005 to 5 ppm). By further heating from 250 to 300°C the solubility decreases, up to 90% of dissolved Ag can precipitate, and up to 9.9% more by further cooling to 200°C. The prosess is concentrating available Ag in the area, having a potential total Ag precipitating efficency of 99.9%. Thermochemical modelling makes the Alum shale probable as the source for dissolution and mobilization of C, Ag, Zn, Pb and Fe; the fahlband as the source for S and Cu; supplied to the mineralizations in the silver-bearing calcite veins at Kongsberg. The heat source may have been Permian granitic magmas.