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

Abstract

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.

From obduction to continental collision: new data from Central Greece

“…In this hypothesis, melt production started as soon as the subducting slab reached the partial melting zone and continued throughout Late Cretaceous time fuelled by a continuous subduction until continental collision in latest Cretaceous time (Gallhofer et al 2015). On the other hand, recent research along the Alpine–Himalayan belt has revealed that many magmatic deposits previously assumed to be subduction related are in fact syn- or post-collisional (Shafiei, Haschke & Shahabpour, 2009; Moritz et al 2010; Imer, Richards & Creaser, 2013) and sometimes occur long (30–40 Ma) after the end of oceanic lithosphere subduction (Hou et al 2009, 2011).…”

Long-lived, stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus, Armenia and Nakhitchevan

“…This complex converging system resulted in Jurassic-Cretaceous and Paleogene subduction-related magmatism and ore formation, followed by various collision to post-orogenic magmatic and ore forming events throughout the Cenozoic (e.g. Marchev et al, 2005;Von Quadt et al, 2005;Perelló et al, 2008;Yigit, 2009;Moritz et al, 2014;Hou et al, 2015;Richards, 2015).…”

Unconventional non-magmatic sulfur source for the Mazarrón Zn–Pb–Cu–Ag–Fe epithermal deposit (SE Spain)

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