Organic and inorganic geochemistry of gold mineralisation at the Zloty Stok, Southwest Poland S. Z. Mikulski* and S. Speczik Inorganic and organic geochemical data obtained from contact-metasomatic and skarn-like mineralisation and associated country rocks from the Zloty Stok Au–As deposit in Southwest Poland have been studied. Magnetite-rich calcite-dolomite marbles have the highest total organic carbon content of the rocks studied. Organic matter consists of solid bitumen, with a minor extractable fraction composed mainly of low molecular weight aromatics. There are positive correlations between organic matter and Fe 2 O 3 and MgO content, suggesting remobilisation of organic matter by the hydrothermal fluids during serpentinisation processes. Results of the organic geochemical and isotope studies suggest that the hydrocarbons were formed by thermal and later hydrothermal alteration of indigenous organic matter. The ranges of d 34 S values of sulphides show that in Au-rich arsenopyrite and pyrrhotite, most of the sulphur may have originated from a magmatic source. However, some of results indicate a contribution from isotopically heavier sulphur, most probably associated with fluids derived from the metamorphic sedimentary environment. Rare earth element patterns of the mineralised samples point to a granitic source of the studied orebodies. Keywords: Gold, Organic carbon, Stable isotopes, Geochemistry, Sudetes Introduction In the Polish part of the Bohemian massif, the most important source of primary gold (from medieval times to present) were deposits are associated with arsenic-rich polymetallic mineralisation. 16 Among the known depos- its the most important is the Zloty Stok deposit where gold extraction has been carried out for over 800 years. The Zloty Stok mine was closed in 1961 due to the poor quality of the gold-bearing arsenic ores (grades ,3gt 21 Au). According to old mining reports almost 20 tonnes of gold were mined within the Zloty Stok district. In the early 1990s, extensive exploration and re-evaluation work started at Zloty Stok, as well as in the surround- ings of the Klodzko-Zloty Stok intrusion (KZSI). These provided new insights into our knowledge of the KZSI evolution and related mineral deposits, as well as highlighting the future economic potential of the whole Zloty Stok gold district. Geological setting The Zloty Stok Au–As deposit is located in the eastern part of the Western Sudetes that constitute the north- eastern part of the Saxothuringian Zone of the European Variscan belt. The basement of the Western Sudetes is built of Neoproterozoic and Paleozoic rocks affected by Cadomian and Variscan metamorphism. Neoproterozoic rocks record magmatism and meta- morphism related to the Cadomian orogeny that occurred between 530–600 Ma. 10 These rocks – grey- wackes and granodiorites – represent a magmatic arc in an active margin of the Gondwana continent. 20 Late Cambrian-Early Ordovician continental rifting in the Sudetes is marked by widespread granitic intrusions, which were deformed into gneisses during Variscan tectonism. 14 The Ordovician to Devonian volcano- sedimentary basinal sequences were deposited in an Ordovician continental rift (characterised by eruption of bimodal volcanics) that evolved during Silurian- Devonian times into an oceanic basin (characterised by MORB-type metabasalts and associated deep marine deposits). 6 During the Late Silurian-Early Devonian (420–400 Ma) 21 ophiolite succession composed of serpentinised ultramafics, massive gabbros and cumu- lates, sheeted dykes, pillow lavas and radiolarian cherts developed and this sequence is preserved in the Go ´ry Sowie Mountains. The metamorphosed and exhumed Sudetic rock complexes formed substratum for auto- chtonus intramontane basins which formed in the Late Devonian and Early Carboniferous and are represented by flysch with olistoliths. 14 During this time, tectonic activity was dominated by strike-slip motions along major faults (e.g. the Intra-Sudetic fault) and shear Polish Geological Institute, 4 Rakowiecka street, 00-975 Warsaw, Poland *Corresponding author, email stanislaw.mikulski@pgi.gov.pl ß 2008 Institute of Materials, Minerals and Mining and The AusIMM Published by Maney on behalf of the Institute and The AusIMM Received 3 July 2007; accepted 10 November 2008 DOI 10.1179/174327508X387474 Applied Earth Science (Trans. Inst. Min. Metall. B) 2008 VOL 117 NO 4 149