Late Cretaceous porphyry Cu and epithermal Cu–Au association in the Southern Panagyurishte District, Bulgaria: the paired Vlaykov Vruh and Elshitsa deposits

摘要

Vlaykov Vruh–Elshitsa represents the best ex- ample of paired porphyry Cu and epithermal Cu–Au deposits within the Late Cretaceous Apuseni–Banat– Timok–Srednogorie magmatic and metallogenic belt of Eastern Europe. The two deposits are part of the NW trending Panagyurishte magmato-tectonic corridor of cen- tral Bulgaria. The deposits were formed along the SW flankof the Elshitsa volcano-intrusive complex and are spatially associated with N110-120-trending hypabyssal and sub- volcanic bodies of granodioritic composition. At Elshitsa, more than ten lenticular to columnar massive ore bodies are discordant with respect to the host rock and are structurally controlled. A particular feature of the mineralization is the overprinting of an early stage high-sulfidation mineral assemblage (pyrite ± enargite ± covellite ± goldfieldite) by an intermediate-sulfidation paragenesis with a charac- teristic Cu–Bi–Te–Pb–Zn signature forming the main economic parts of the ore bodies. The two stages of mineralization produced two compositionally different types of ores—massive pyrite and copper–pyrite bodies. Vlaykov Vruh shares features with typical porphyry Cu systems. Their common geological and structural setting, ore-forming processes, and paragenesis, as well as the observed alteration and geochemical lateral and vertical zonation, allow us to interpret the Elshitsa and Vlaykov Vruh deposits as the deep part of a high-sulfidation epithermal system and its spatially and genetically related porphyry Cu counterpart, respectively. The magmatic– hydrothermal system at Vlaykov Vruh–Elshitsa produced much smaller deposits than similar complexes in the northern part of the Panagyurishte district (Chelopech, Elatsite, Assarel). Magma chemistry and isotopic signature are some of the main differences between the northern and southern parts of the district. Major and trace element geochemistry of the Elshitsa magmatic complex are indicative for the medium- to high-K calc-alkaline character of the magmas. 87Sr/86Sr(i) ratios of igneous rocks in the range of 0.70464 to 0.70612 and 143Nd/144Nd(i) ratios in the range of 0.51241 to 0.51255 indicate mixed crustal–mantlecomponents of the magmas dominated by mantellic signatures. The epsilon Hf composition of magmatic zircons (+6.2 to +9.6) also suggests mixed mantellic– crustal sources of the magmas. However, Pb isotopic signatures of whole rocks (206Pb/204Pb=18.13–18.64, 207Pb/204Pb=15.58–15.64, and 208Pb/204Pb=37.69–38.56) along with common inheritance component detected in magmatic zircons also imply assimilation processes of pre- Variscan and Variscan basement at various scales. U–Pb zircon and rutile dating allowed determination of the timing of porphyry ore formation at Vlaykov Vruh (85.6 ± 0.9 Ma), which immediately followed the crystallization of the subvolcanic dacitic bodies at Elshitsa (86.11 ± 0.23 Ma) and the Elshitsa granite (86.62 ± 0.02 Ma). Strontium isotope analyses of hydrothermal sulfates and carbonates (87Sr/86Sr= 0.70581–0.70729) suggest large-scale interaction between mineralizing fluids and basement lithologies at Elshitsa– Vlaykov Vruh. Lead isotope compositions of hydrothermal sulfides (206Pb/204Pb=18.432–18.534, 207Pb/204Pb=15.608– 15.647, and 208Pb/204Pb=37.497–38.630) allow attribution of ore-formation in the porphyry and epithermal deposits in the Southern Panagyurishte district to a single metallogenic event with a common source of metals.