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Geological setting and mineralization

Geological Setting and Mineralization Regional Geology

The Chelopech deposit is located within the Panagyurishte metallogenic district in the central part of the Srednogorie zone, a volcano-intrusive zone that formed during the Late Cretaceous magmatic evolution in Bulgaria The Panagyurishte mineral district is defined by a NNW alignment of porphyry-copper (Elatsite, Assarel and Medet) and epithermal Cu-Au (Chelopech) deposits, which is oblique to the east-west orientation of the Srednogorie belt. Alluvial (Topolnitza and Luda Yana) and vein-hosted (Svishti Plas) gold mineralization is also known in the area and has been previously exploited on a small scale.

The geology of the Panagyurishte mining district comprises a basement of Precambrian granitoid gneisses intruded by Palaeozoic granites and overlain by Late Cretaceous magmatic and sedimentary sequences. In some parts of the district, these rocks are overlain by Late Cretaceous flysch and by Palaeogene and Neogene molasse.

The basement rocks form a series of uplifted horst-anticlinal structures between which a series of three north to east trending sub-parallel grabens contain the Cretaceous sequences. To the north and towards Chelopech, the Srednogorie massif forms the basement.

Mine Geology

The Chelopech area consists of pre-mineral and post-mineral sequences, separated by a Late Turonian erosional surface. The pre-mineral formation consist the following units (from older to younger):

  • The Paleozoic basement unit is composed of high and low-grade metamorphic complexes with the tectonic boundary between them being part of the Variscan ductile shear zone Sturgel-Boluvanya.
  • The Basal Turonian unit with quartz-rich sandstones and conglomerates show very changeable lateral characteristics and indicate shallow-marine setting, interfingering with coal-bearing packages or red-colored alluvial conglomerates but also limestones lenses or poorly lithified clasts.
  • Based on recent geological mapping and reinterpretation, the Turonian Magmatic Chelopech Mine Formation belongs to an intrusive system, significantly different from the earlier andesitic strato-volcanic model. The high-sulfidation hydrothermal system in the upper part of the structure developed within a shallow intrusive porphyritic diorite/microdiorite which is pierced by a number of vertically-extended intrusion-related breccias bodies. These breccia bodies include both blind breccia pipes as well as a surface-reaching diatreme (i.e. phreatomagmatic explosive origin).

The post-mineral formation consist the following units (from older to younger):

  • Monolithic Rock-Avalanche Breccia unit is made up of angular to sub-angular debris-flows deposits, which are locally distributed and cannot be correlated laterally. The unit indicates tectonic subsidence and deposition in fault-bounded depocenters.
  • The Two Mica Sandstone unit is represented by thick-bedded to massive sandstones to gravelly, rare pebble sandstones. These Santonian sediments can be interpreted as high-density sandy turbidites indicating redeposition of large amounts of terrigenous material over a short time period. From drill hole data it appears the Two Mica Sandstone unit was deposited mainly in a fault bounded local depocenter with features of a pull-apart sub-basin. This explains the variability of the unit thickness from a few tens of meters to more than 400m.
  • The Polylithic Breccia unit is made up of high-density epiclastic flows deposits. It generally underlies the Limestone unit, but similar polylithic breccia units are found on top of the limestone, as wedges into the limestone, and over the unconformity with the pre-mineral formation at the base of the Two Mica Sandstone unit.
  • The Limestone unit (Mirkovo Limestone Formation) is represented by thin-bedded to massive limestones with various color (mainly reddish) and is interpreted as marking the post-rift stage of the basin system. The thickness varies from 5 metres to 32 metres due in part to large syn-sedimentary slump folds. Interfingering with the other post-mineral units is common. The age of the unit is Late Santonian-Early Campanian.
  • The “Flysch” unit (Chugovitsa Formation) is composed of thin to thick-bedded, low to high-density sandy and muddy turbidites in the topmost part. The unit is characterized by three packages, including: 1) a topmost package with muddy turbidites and marls (background deposits) with rare thin to very thin sandy turbiditic and carbonate turbiditic layers, indicating low-terrigenous input; 2) a middle package represented by medium to thick-bedded high-density sandy turbidites, the entire interval is dominated by synsedimentary slump folds; and 3) a lower package represented by of low- to high-density sandy/epiclastic turbidites and debrites with thin background sediment layers. The thickness of the unit is approximately 450m. The chronostratigraphic range of the unit is assumed as Santonian – Late Maastrichtian.
  • The Quaternary Zlatitsa Graben (basin) unit is developed over the metamorphic allochthon of the main thrust. The graben is strongly faulted and five depocenters can be delineated. Some of them, especially along the northern board indicate extension reactivation of older structures.

The overall structural complexity of the Chelopech area is interpreted as a result of pre-existing fabrics, faults and weak structural zones in the basement that were intermittently reactivated. Recent detailed structural mappings on surface and section reinterpretations suggest that the Chelopech imbricate thrust is a significant post-mineral structure that splits the entire sequence into a structurally less complex autochthon in the footwall of the thrust complex, and a complex allochthon in the hangingwall of the thrust complex. Recent mapping and structural interpretation confirmed syn-sedimentary faults in post-mineral cover sequence and numerous late – N-S and E-W striking – normal faults. The Petrovden fault is one of the main E-W striking normal fault, and juxtapose pre-mineral phyllic-altered rocks of the deeper part of the Chelopech intrusive complex in the north against unaltered post-mineral epiclastic rock units and sedimentary cover in the south. This implies a normal vertical dislocation of several hundred of meters.

Current interpretations show that several mineralization types occur within the Chelopech-Vozdol area, including porphyry stockwork Cu-Au-Mo, base-metal-Au-Ag veins and breccia hosted epithermal Au-Cu mineralization. These mineralization types are potentially genetically related, but their exact relationships are still unclear. As a general criterion it is agreed that the Petrovden fault (and other associated syn-sedimentary normal faults) played a key role in the preservation of the economically most interesting epithermal Au-Cu mineralization (Chelopech and Sharlo Dere deposits).

Key features of the Chelopech high-sulphidation type Au-Cu mineralization are summarized below:

  • Mineralization occurs within sulphide-rich zones of replacement silicification surrounded by haloes of silica-sericite alteration;
  • The ore bodies, which formed as both complex branched bodies and discrete pipes, are grouped into two mining areas. The Central zone consists of five mineralized bodies (16, 17, 18, 19E, 19W and 8) whilst the Western zone comprises a further eight mineralized bodies (5, 25, 103, 145, 147, 149, 150 and 151).  The mineralized zones vary from 150-300 metres in length, are 30-120 metres thick and can extend at least 350 metres down plunge;
  • Mineralized zones are structurally controlled; and
  • Good correlation between copper, gold and sulphur and a strong relationship between copper (and to some extent gold) mineralization and stockwork veining.

Earlier models have interpreted that the Chelopech deposit is located within the Chelopech andesitic strato-volcanic sequence with complex structural and laterally correlatable ‎stratigraphic control. Continuous effort to improve this model and as the result of two year re-logging program a new geological model is being developed. The main implication of new model is that the ore-hosting magmatic environment at Chelopech belongs to an intrusive system. The high-sulfidation hydrothermal system in the upper part of the structure developed within a shallow intrusive porphyritic diorite/microdiorite rock-body pierced by a number vertically-extended intrusion-related breccias bodies of blind breccias-pipe and surface-reaching maar-diatreme (i.e. phreatomagmatic explosive). The diatreme hosting the bulk of Chelopech deposit is one large body hosted by unbrecciated remnants of the single large pre-diatreme diorite. The diatreme is related to a granodiorite phase that also has porphyry Cu-Mo-Au stockwork and potassic-phyllic alteration (in Petrovden area), distal base metal sulfide veins (Petrovden and Vozdol areas), and the high-sulfidation Cu-Au system including the ore is localized in an overprinting hydrothermal breccia related to porphyry dikes (Chelopech and Sharlo Dere areas). The high-sulfidation system is localised within the diatreme by structures that have been intruded by porphyry dikes, so an understanding of the geometry of these structures and the dikes is a predictive tool for near mine exploration.

The shift of paradigm from volcanic to intrusive with respect the host environment of the mineralization processes, in particular the lithological control on ore deposition at Chelopech, radically changes both the perspectives of, and the strategies to be followed by, further exploration endeavors.

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