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

Abstract

Vlaykov Vruh–Elshitsa represents the best example 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 central Bulgaria. The deposits were formed along the SW flank of the Elshitsa volcano-intrusive complex and are spatially associated with N110-120-trending hypabyssal and subvolcanic 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 characteristic 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–mantle components 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.

Appendices

Appendix 1—Analytical techniques

Whole-rock major oxides were analyzed by X-ray fluorescence spectrometry on powdered samples fused with Li₂B₄O₇ on a Philips PW2400 spectrometer with Rh anode at the “Centre d’Analyses Minérales,” University of Lausanne, Switzerland. Trace elements and Rare Earth elements (REE) were analyzed by laser-ablation inductively coupled plasma-mass spectrometry (ICP-MS) on the same pellets at ETH—Zurich, Switzerland, applying the methodology described in Günther et al. (2001). Analytical results for major, REE and trace elements are presented in the Appendix 2.

Lead isotope analyses on whole-rocks were carried out at ETH—Zurich, Switzerland. Samples of about 50 mg of finely ground whole rock were washed in 3 N HNO₃ at 80 °C for 30 min. The residues were digested in 10-ml PFA-Teflon beakers with a mixture of 2 ml HF (21 N) and 4 ml HNO₃ (7.5 N) at 160 °C. After 5 days, the samples were dried and transferred to a nitrate form. Lead was separated by Sr Resin® (Eichrom, 50–100 µm) in 100-μl columns. The total procedure blank for lead was better than 10 pg. Pb was loaded on outgassed Re-filaments using the silica gel technique and measured on a MAT Finnigan 262 mass spectrometer in static mode. The fractionation for lead was based on repeated measurements of the SRM982 standard. The fractionation factor was 0.095% per atomic mass unit. The analytical errors (2σ) were 0.03% for ²⁰⁶Pb/²⁰⁴Pb, 0.04 % for ²⁰⁷Pb/²⁰⁴Pb, and 0.08 % for ²⁰⁸Pb/²⁰⁴Pb.

Lead isotope analyses on separates of chalcopyrite, pyrite, galena, bornite, enargite, and sphalerite–galena mixtures were carried out at the Department of Mineralogy, University of Geneva, Switzerland. Samples of 50 mg of ore minerals (except for galena—10 µg) were digested in sealed 20-ml Teflon beakers with a mixture of 3 ml 7 M HCl and 1 ml concentrated HNO₃ at 180°C. After conversion of the samples to bromide form, lead was separated by ion exchange chromatography. Procedural blanks were less than 120 pg Pb. Fractions of the purified lead were loaded on Re-filaments using the silica gel technique, and lead isotope ratios were measured on a MAT Finnigan 262 thermal ionization mass spectrometer in static mode. Lead isotope ratios were corrected for fractionation by a +0.10% atomic mass unit correction factor based on repeated analyses of the SRM981 international standard. The analytical errors (2σ) were 0.07% for ²⁰⁶Pb/²⁰⁴Pb, 0.10% for the ²⁰⁷Pb/²⁰⁴Pb, and 0.14% for the ²⁰⁸Pb/²⁰⁴Pb.

Strontium and neodymium isotope analyses on whole-rock samples were conducted at the Department of Mineralogy, University of Geneva, Switzerland. About 200 mg of finely ground rock were digested with concentrated HF/HNO₃ in stainless steel bomb with Teflon liners. All solutions were evaporated, and the dry residues were dissolved in 2.5 N HCl, followed by Sr and Nd separation in ion exchange columns. Strontium and neodymium isotope analyses were performed on a seven-collector MAT Finnigan 262 thermal ionization mass spectrometer with extended geometry and stigmatic focusing using double Re-filaments. Strontium isotopic analysis was conducted in a semidynamic mode (triple collectors, measurement in jumping mode). The Sr isotope ratios were mass fractionation corrected to ⁸⁸Sr/⁸⁶Sr = 8.375209 and normalized to the Eimer and Amend® SrCO₃ standard, with ⁸⁷Sr/⁸⁶Sr = 0.708000 using an average of 0.708028 ± 5 × 10⁻⁶ (2SE; n = 52) measured during the period of analyses. ¹⁴³Nd/¹⁴⁴Nd was measured in a semidynamic mode (quadruple collectors, measurement in jumping mode), mass fraction corrected to ¹⁴⁶Nd/¹⁴⁴Nd = 0.721903, and normalized to the La Jolla standard = 0.511835. An average of 0.511838 ± 6 × 10⁻⁶ (2SE; n = 28) was measured during the analyses.

For strontium isotope analyses of hydrothermal minerals, approximately 100 mg of finely ground anhydrite, barite, and calcite have been used. Barite and anhydrite were leached overnight in 6 N HCl at 110°C, and calcite samples were dissolved in 1.5 N HCl for 20 min. After evaporation of the solutions, the dry residues were dissolved in 2.5 N HCl, followed by Sr and Nd separation in cation exchange columns. Analytical conditions during the analyses on the mass spectrometer were similar to those used for the whole-rock samples. The Sr content of anhydrite, barite, and calcite samples was determined by atomic absorption with an analytical precision of ±5%, using a GBS 905AA spectrophotometer at the Atomic Absorption Laboratory of ISTO-CNRS, Orléans, France.

U–Pb dating of magmatic zircons and hydrothermal rutiles was performed at ETH—Zurich, Switzerland. High-precision “conventional” U–Pb zircon analyses were carried out on single zircon grains (except for three analyses, for which a multigrain technique was used). Selected zircons were air-abraded to remove marginal zones with lead loss, washed in warm 4 N HNO₃ and rinsed several times with distilled water and acetone in an ultrasonic bath. Dissolution and chemical extraction of U and Pb was performed using miniaturized bombs and anion exchange columns. Blanks for the entire procedure were <2 pg Pb and 0.5 pg U. A mixed ²⁰⁵Pb/²³⁵U tracer solution was used for all analyses. Both Pb and U were loaded with 1 µl of silica gel–phosphoric acid mixture on outgassed single Re-filaments and measured on a MAT Finnigan 262 thermal ionization mass spectrometer using an ion counter system. The performance of the ion counter system was checked by repeated measurements of the NBS 982 standard solution. The reproducibility of the ²⁰⁷Pb/²⁰⁶Pb ratio (0.467070) was better than 0.05%. The calculations of the U/Pb ratios include uncertainties of the spike calibration; Pb blank measurements, common Pb correction, U and Pb fractionation, and U decay constant errors. All uncertainties are included in the error propagation for each individual analysis. Mean age values are given at the 2σ level.

Hf isotope ratios of zircons were measured on a Nu Instruments multiple collector ICP-MS at ETH—Zurich. During analysis, the ¹⁷⁶Hf/¹⁷⁷Hf ratio of the JMC 475 standard was measured at 0.282141 ± 5 (1σ) using the ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 ratio for normalization (exponential law for mass correction). For the calculation of the ε-Hf values, the following present-day ratios (¹⁷⁶Hf/¹⁷⁷Hf)_CH = 0.28286 and (¹⁷⁶Lu/¹⁷⁷Hf)_CH = 0.0334 were used, and for 85 Ma, an average ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.005 for all zircons was taken into account.

Appendix 2

Table 8 Major- and trace-element analyses of whole rock samples from the southern part of the Panagyurishte district