RE-OS ISOTOPIC AGE OF THE CU-AG SULPHIDE ORE AND ITS MINERALOGICAL AND GEOCHEMICAL CHARACTERISTIC IN THE LUBIN–POLKOWICE MINING AREA (SW POLAND)

Abstract

In the paper we present the results of Re-Os isotopic studies as well as the mineralogical and geochemical characteristic of bornite veinlets with chalcopyrite ± chalcocite margins and chalcopyrite veinlets that are parallel to sub-parallel to bedding in Kupferschiefer from the underground workings of the Lubin and Polkowice mines in SW Poland. Kupferschiefer samples are of grade from 2.5 to 14.2% Cu and with silver admixtures from 40 to 900 ppm and organic matter contents ca. 6%. The ratios of Co/Ni = 0.1–0.7, V/Cr = 4–17 and Fe2O3/C org = 0.6–1.9 are in the range of values typical for the Kupferschiefer. Besides, copper sulphides, which commonly contain silver admixtures (e.g. chalcocite – 0.44–5.03 wt.%., bornite – 0.33–0.77 wt.%., chalcopyrite 0.09–0.20 wt.%) are associated with minor galena, sphalerite, Ag-minerals and common pyrite framboids. In the isotopically analysed bornite and chalcopyrite samples Re concentrations ranging from 5.7 to 12 ppb, and total Os concentrations ranging from 27 to 52 ppt. Significant common Os is present in all of the analysed sulphides. The isotopic ratios of 187Re/188Os are very high (range: 2269–2942), and of 187Os/188Os from 9.8 to ca. 12.4. Re-Os model ages calculated for these isotopic ratios are in the range from 256 to 268 Ma and for one of the chalcopyrite veinlet was 217 ±2 Ma. Taking into the account the values of the isotopic ratios of 187Re/188Os, it was possible to construct the Re-Os isochrone age for A Model 1 regression based on four different samples. They yields age of 212 ±7 Ma, with an initial 187Os/188Os ratio of 2.13 ±0.31 (MSWD = 1.3). Re-Os isochrone age indicates for bornite and chalcopyrite crystallization event of the Ag-bearing Cu sulphide mineralization within the Kupferschiefer in the Late Triassic (Norian), ca. 212 ±7 Ma.