Origin of the gold deposit in the Polkowice-West Mine, Lubin-Sieroszowice Mining District, Poland

The recently discovered gold deposit in the Lubin-Sieroszowice district lies 0.0 to 1.5 m below the stratabound copper-silver orebody, mainly in the Polkowice-west mine. The deposit extends over 60 km(2), has a thickness of 20-80 cm and a conservative estimate of the average grade of 1.5 g/t gold, 0.3 g/t Pd and 0.2 g/t Pt. It contains gold and electrum in association with minor amounts (up to 0.4 vol%) of hematite, chalcocite, digenite, djurlite, bornite, chalcopyrite, pyrite, gersdorffite, rammelsbergite and clausthalite. The gold-bearing zone lies mainly in the Weissliegendes sandstone, but locally transgresses the stratigraphy into the overlying Kupferschiefer shale and Werra Dolomite of the Zechstein evaporite sequence. It lies within a transition zone between the oxidized, hematite-stained Rote Faule facies and the overlying reduced organic carbon-bearing sediments. At the Polkowice-west mine, but only where gold is present, this zone is characterized by hematite-stained 'patches' within grey to maroon sediment that contains little or no hematite or organic carbon. These textures were caused by downwelling reducing sulfur-rich solutions, flowing through previously oxidized sediments, and by gravitational instability between the reducing and oxidizing fluids. The oxidizing fluid originated within the Rote Faule facies, whereas the reducing fluid originated in the overlying Zechstein sequence from which it was expelled downward during gypsum dehydration accompanying burial. Gold transport as the chloride complex in the oxidizing fluid is unlikely at the low temperature (<100 degrees C) prevailing during mineralization. Some gold was probably transported as the bisulfide and, to a lesser extent, the polysulfide complexes, in the descending reducing fluid. However, the potential source rocks do not contain enough gold to account for the known deposit. Most of the gold was probably transported as the thiosulfate complex during replacement of reduced sediments by the Rote Faule facies. Parts of the district where reduced fluids penetrated deeply into the Weissliegendes, and where the Rote Faule facies transgressed the greatest volume of reduced sediments, should be evaluated for the presence of gold. We propose that the Polkowice-West mine represents a new class of gold deposit.