LATE CRETACEOUS STRUCTURAL CONTROL AND ALPINE OVERPRINT OF THE HIGH-SULFIDATION CU–AU EPITHERMAL CHELOPECH DEPOSIT, SREDNOGORIE BELT, BULGARIA

Abstract

The Chelopech epithermal high-sulfidation deposit is located in the Panagyurishte ore district in Bulgaria, which is defined by a NNW alignment of Upper Cretaceous porphyry–Cu and Cu–Au epithermal deposits, and forms part of the Eastern European Banat–Srednogorie belt. Detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the Chelopech deposit from the Late Cretaceous to the present. The Chelopech deposit is characterized by three fault populations including ~N55, ~N110, and ~N155-trending faults, which are also recognized in the entire Panagyurishte district. Mapping and 3-D modeling show that hydrothermal alteration and orebody geometry at Chelopech are controlled by the ~N55-trending and ~N110-trending faults. Moreover, the ~N155-trending faults are parallel to the regional ore deposit alignment of the Panagyurishte ore district. It is concluded that the three fault populations are early features and Late Cretaceous in age, and that they were active during high-sulfidation ore formation at Chelopech. However, the relative fault chronology cannot be deduced anymore due to Late Cretaceous and Tertiary tectonic overprint. Structurally controlled ore formation was followed by Senonian sandstone, limestone, and flysch deposition. The entire Late Cretaceous magmatic and sedimentary rock succession underwent folding, which produced WNW-oriented folds throughout the Panagyurishte district. A subsequent tectonic stage resulted in overthrusting of older rock units along ~NE-trending reverse faults on the Upper Cretaceous magmatic and sedimentary host rocks of the high-sulfidation epithermal deposit at Chelopech. The three fault populations contemporaneous with ore formation, i.e., the ~N55-, ~N110- and ~N155-trending faults, were reactivated as thrusts or reverse faults, dextral strike–slip faults, and transfer faults, respectively, during this event. Previous studies indicate that the present-day setting is characterized by dextral transtensional strike–slip tectonics. The ~NE-trending overthrust affecting the Chelopech deposit and the reactivation of the ore-controlling faults are compatible with dextral strike–slip tectonics, but indicate local transpression, thus revealing that the Chelopech deposit might be sited at a transpressive offset within a generally transtensional strike–slip system. The early WNW-trending folds require a roughly NNE–SSW shortening, which is incompatible with the present-day dextral strike–slip tectonic setting and the ~NE-trending thrust formed during the tectonic overprint of the Chelopech deposit. This reveals a rotation of the principal stress axes after Late Cretaceous high-sulfidation ore formation and post-ore deposition of sedimentary rocks. The nature of the sedimentary rocks interlayered and immediately covering the Upper Cretaceous magmatic rocks hosting the Chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation. It is concluded that the Chelopech deposit was formed when the tectonic setting changed from extensional during Late Cretaceous basin sedimentation and magmatism, to compressional producing WNW-trending folds under a roughly NNE–SSW compression, possibly in a sinistral strike–slip system. Thus, like other world-class, high-sulfidation epithermal deposits, the Chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation, which are particularly favorable tectonic settings for the formation of high-sulfidation epithermal deposits. The exceptional preservation of the Upper Cretaceous Chelopech epithermal deposit is explained by the combined deposition of a thick Senonian sedimentary sequence on top of the Upper Cretaceous magmatic host rocks of the deposit, and the later overthrust of older rock units on top of the deposit. Our study at Chelopech supports previous studies stating that post-ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion. The tectonic evolution of the Chelopech deposit is similar to that of the entire Panagyurishte ore district. This coherence of the magmatic, hydrothermal, and tectonic events from north to south suggests that the ore deposits of the entire Panagyurishte ore district were formed in a similar tectonic environment.