Sedimentary-rock-hosted epithermal systems of the Tertiary Eastern Rhodopes, Bulgaria: new constraints from the Stremtsi gold prospect

Abstract

AbstractPrecious 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) and87Sr/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 by40Ar/39Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with40Ar/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.