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Orthogneisses derived from granitoids with Variscan protolith ages dominate the lower unit of high-grade metamorphic basement of the Eastern Rhodope Massif in south Bulgaria. We present whole-rock geochemistry and Sr–Pb isotopic composition of these orthogneisses, which are compared with Pb isotopes of parametamorphic rocks, and hydrothermal ore deposits and associated rocks, to better constrain their composition, origin, and contribution to late Alpine hydrothermal processes. The igneous mineral assemblage is partly preserved, and the field textures and microstructures of the orthogneisses are consistent with a ductile, amphibolite-grade tectono-metamorphic overprint during Alpine time, when they were involved in the metamorphic nappe stack. Whole-rock geochemistry revealed compositions of the orthogneisses largely unaffected by the amphibolite-grade metamorphism, displaying a magmatic differentiation trend of the igneous protoliths. The protoliths are peraluminous medium-K calcalkaline S-type granitoids, whose tectono-magmatic setting discrimination consistently indicates a continental volcanic arc origin. The orthogneisses present trace element and rare-earth elements (REE) patterns based on which a group of high-field strength elements-depleted and REE fractionated orthogneisses and a group of LREE-enriched orthogneisses can be distinguished. Both geochemical groups show compositions similar to the bulk and upper continental crust and its sedimentary counterparts. Crustal Pb isotope ratios (206Pb/204Pbi518.24–18.66) of the orthogneisses are comparable to the paragneisses (206Pb/204Pbi518.31–18.93) and uniform in both (207Pb/204Pbi515.64–15.72) and 208Pb/204Pbi ratios in the paragneisses (38.23–38.60) and the orthogneisses (38.32–38.56). The trace element data and 87Sr/86Sri isotopes of the orthogneisses (0.7050–0.7117) overlap those of the parametamorphic rocks (0.7039–0.7144), and confirm the supracrustal origin of the igneous precursors. A heterogeneous crustal source region is suggested in which melting and crustal contamination during magma genesis with subsequent fractional crystallization was involved in the petrogenesis. Comparative Pb isotope systematics suggests that a significant crustal Pb input to ore-forming hydrothermal fluids was derived primarily from the metamorphic basement, implying that the brittlly deformed basement during crustal extension acted as an immediate environment for fluid leaching during late Alpine hydrothermal ore-forming processes

Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins.Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ34S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ34S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction.Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ18O (+12.7‰ to +19.7‰ V-SMOW) and δ13C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ18O values (+13.4‰ to +23.3‰ V-SMOW) with δ13C values (−0.6‰ to −3.9‰ V-PDB) and 87Sr/86Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87Sr-enriched fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing.A plateau age of 37.57±0.31 Ma obtained by 40Ar/39Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40Ar/39Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

The Iran Tepe volcanic complex occurs in the south-eastern part of the Eastern Rhodope massif. The rocks are represented by calc-alkaline and high-K calc-alkaline basaltic andesite to dacite epiclastics, lava flows and dikes, which are crosscut by andesitic and latitic dikes and rhyolitic dykes from the Planinets dyke swarm. Stratigraphic data and existing K/Ar ages suggest that the Iran Tepe volcanic complex is Upper Eocene (35-39 Ma), and is one of the oldest volcanic structures in the Eastern Rhodopes. However, new 40Ar/39Ar laser fusion and incremental step-heating experiments on biotites and isotope dilution – thermal ionization mass spectrometry (ID-TIMS) U-Pb age data on single zircons from the bottom and top lava flows and dykes more precisely constrain the ages and time span of volcanic activity, and show that the volcanism is younger. Volcanic activity started with calc-alkaline andesites and dacites at the beginning of the Oligocene (~33.9 Ma) and culminated with the intrusion of latitic dykes at ~33.0 Ma. Rhyolites from the Planinets dyke swarm yield a similar age (32.8 Ma), but their genetic relationship with the more mafic Iran Tepe lavas remains unclear.

The Rhodope-Serbo-Macedonian massif in northern Greece and southern Bulgaria represents a world-class case to study exhumation processes in an extensional back-arc setting, looking at supra-detachment basin formation, contemporaneous magmatism, and gold mineralization, analogous to the Basin and Range Province of the western United States and elsewhere. This field trip examined four different magmatic-hydrothermal systems in the Rhodope metallogenetic province. The guidebook contains five papers that describe the characteristics of the selected systems, including detailed guides for each day of the four-day field trip.