Regional Geology

Bulgaria is located on the south east part of the Balkan Peninsula which lies within the Alpine geosynclinal belt. The Chelopech deposit is located within the Panagyurishte metallogenic district in the central part of the Srednogorie zone. The geology of the Panagyurishte district comprises a basement of pre-Cambrian granitoid gneisses intruded by Palaeozoic age granites and overlain by Upper Cretaceous magmatic and sedimentary sequences. The basement rocks form a series of uplifted blocks between which a series of three north to east trending sub-parallel basins contain the Cretaceous sequences.

Regional geology of the Panagyurishte metallogenic district

Geology of the surrounding area of the Chelopech Deposit

Regional Mineralization

Mineralization within the Panagyurishte district is dominated by porphyry-copper type deposits such as at Elatsite, Assarel and Medet and by cupriferous massive pyrite deposits as at Chelopech. Alluvial and vein-hosted gold mineralization is also known in the area and has been previously exploited to a small extent.  Mineralization in the Panagyurishte district is intimately related to Cretaceous magmatic activity. In general terms, the cupriferous massive pyrite deposits are associated with Upper Cretaceous volcanics of andesitic and dacitic affinity whilst the porphyry deposits are generally associated with deep level intrusive rocks.

Chelopech Mine Geology

The basement in the vicinity of Chelopech is comprised of pre-Cambrian granite gneisses, schists, quartzites and amphibolites.  This basement is unconformably overlain by Permian volcanics and in turn by a marine sequence of Triassic, Jurassic and Cretaceous ages. The Cretaceous Chelopech Formation reaches a thickness up to 2,000 metres and hosts the mineralization.

The Chelopech formation can be subdivided into a lower (pre-mineralization) sequence of sediments and an upper (post-mineralization) volcanic sequence located within the Chelopech volcanic centre, on the northern side of a north-easterly trending jog in the regional, east-west trending, Balkan fault. The northern and north-eastern part of the volcano has been eroded off whilst the southern part has been thrust down by part of the Balkan Fault complex. The Chelopech volcanic centre was formed in two stages, corresponding to:

• Stage 1: The Lower Chelopech Formation was deposited and hosts the copper-gold-pyrite mineralization; and
• Stage 2: Volcanic activity was centred on Petrovden. During this event shallow level intrusive packages were intruded into the basement rocks and into the overlying Cretaceous age sediments and volcanics.

The period of mineralization related to Stage 2 produced several large copper porphyry deposits such as the Elatsite Mine, located 10 kilometres northwest of Chelopech.

Within the mine environment, ore is associated with the Lower Chelopech Formation, comprising propylitically altered andesitic and dacitic lavas, tuffs and agglomerates. Key features of the mineralization are summarized below:

• the 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 11 ore bodies whilst the western zone comprises a further nine ore bodies. Of these, seven are considered significant to the defined mineral resources;
• 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
• there is good correlation between copper, gold and sulphur, thus indicating that the main phase of mineralization is, as observed, dominantly copper-gold-sulphur associated with pyrite (and other sulphides and arsenides). Negative correlation between copper, gold and lead, zinc is to be expected as the latter elements dominantly occur in peripheral zones to the core mineralized zones characterized by copper-gold-sulphur mineralization.

Schematic litho-stratigraphic cross section through the Chelopech region