The Variscan KaÅ¡perské Hory orogenic gold deposit, Bohemian Massif, Czech Republic

KaÅ¡perské Hory, one of the largest gold deposits in the Bohemian Massif, is characterised by shear zone-related gold-bearing quartz veins at the tectonic boundary of the two metamorphic terranes of the Moldanubian Unit. Studies of the geology, mineral paragenesis, fluid inclusions and oxygen geothermometry have been carried out to determine the mineralisation events and alternative P-T-t model of this deposit.The pre-mineralisation stage is characterised by geochemical evidence of pegmatites (~ 700 °C and 1.1-0.45 GPa) and metamorphic structures resulting from deformation Dx + 1 at the regional metamorphic peak (650 °C and 0.5 GPa).The mineralisation in Dx + 2 began to develop from the formation of the Q1 and Q2 quartz veins (Stage I) under brittle-ductile conditions with the N-S extension; these quartz veins were filled mainly by apatite, plagioclase-1, muscovite and arsenopyrite-1. Revised and reinterpreted gechronological data indicate that the earliest quartz veins can be dated at 344 Ma. The Q3 veins were formed later under brittle to brittle-ductile conditions with crystallisation of plagioclase-2, arsenopyrite-2, scheelite, chlorite, pyrite and calcite (Stage II). The corresponding fluids of Stages I and II belong to the C-H-O-N system. The P-T conditions correspond to 590-520 °C and to 0.44-0.25 GPa for Stage I and 480-390 °C and ~ 0.2 GPa for Stage II. Stage III is characterised by the precipitation of younger quartz, chlorite, molybdenite, calcite, pyrrhotite-1, galena-1, native gold, bismuth, and other Bi-Te minerals; the associated aqueous fluids (salinity 1.5, 8 wt.% NaCl equiv.) follow fluid inclusion trails and were trapped at relatively low pressures and temperatures (290-180 °C and < 0.1 GPa). Stage IV is mainly dominated by carbonate and fluorite fillings of the open fractures and cracks.Arsenopyrite-1 and ‐2 and molybdenite are the main gold-bearing and gold-carrying ore phases at this deposit. The means of microanalysis indicate the presence of both micron-sized and probably submicron-sized particles of gold (~ 920/1000) in the fractures and fissures of the arsenopyrite-1 and structurally bound invisible gold in the arsenopyrite-2 crystal lattice (e.g., gold as Au(I)). Economic gold mineralisation (native-free gold flakes; 930/1000) is linked to Stage III.

► KaÅ¡perské Hory is a shear zone-controlled gold deposit. ► Mineralization has been formed in four stages of fluid movement and cooling. ► Gold is mainly present as a native‐free gold flakes (930/1000). ► Pegmatites indicate the pre-existence of an earlier higher pressure metamorphic stage.