Summary:
The Chelopech deposit is located within the Panagyurishte metallogenic district. It formed during Late Cretaceous magmatic-hydrothermal events, defined by a north-northwest alignment of porphyry copper-gold (Elatsite, Assarel and Medet) and epithermal copper-gold deposits that is oblique to the east-west orientation of the Srednogorie belt. The geology of the Panagyurishte metallogenic district comprises a basement of Precambrian granitoid gneisses intruded by Palaeozoic granites and overlain by Late Cretaceous magmatic and sedimentary sequences.
The Chelopech area stratigraphy consists of pre-mineral and post-mineral sequences separated by a Late Turonian erosional surface and controlled by an inherited and intermittently reactivated regional Variscan basement relay structure. The pre-mineral and syn-mineral formations consist of the following units (from oldest to youngest): (i) high and low-grade metamorphic complexes that form the Palaeozoic Basement unit; (ii) the Basal Turonian unit of quartz-rich sandstones and conglomerates deposited in a shallow-marine setting; (iii) the Late Turonian Mixed Unit that consist ofshales, dark grey wake sandstones and weakly-sorted epiclastic poly-mictic debris-flows deposits and hydro-magmatic surge deposits, including exhalative sulphide zones; and (iv) the Turonian Magmatic Chelopech mine Formation, a shallow porphyritic diorite/microdiorite intrusive system with phreatomagmatic breccia pipes. The post-mineral sequence consists of an older Monolithic Rock-Avalanche Breccia unit made up of angular to sub-angular polymictic debris-flows deposits and younger sedimentary rocks accumulated as a Gosau-type subbasin formation with characteristic rapid facies changes, post-mineral thrusting and subsequent normal faulting, all contributing to the preservation and distribution of the mineralization.
The Chelopech hydrothermal system is genetically related to a multi-phase 91.9±0.2 Ma old dioritic shallow intrusive system which extends at least over an area of 5 by 4 kilometres and hosts various types of mineralization, including (1) the economically most important high-sulfidation style gold-copper mineralization in the Chelopech mine, West Shaft and the Krasta prospects (2) a sub-economic porphyry copper-molybdenum-gold stockwork mineralization in the Petrovden prospect, (3) distal gold-rich base metal sulfide veins in the Vozdol and Wedge prospects, and 4) epiclastic-hosted re-worked copper-gold mineralization in the Sharlo Dere prospect.
The economically significant high-sulfidation style gold-copper mineralization is controlled by phreatomagmatic breccia pipes and syn-mineral hydromagmatic surge- and epiclastic debris-flow deposits. Ore shoots are associated with the high-porosity breccia–diorite contacts, breccia pipe cupola zones, surge flows with VMS-like exhalative ore zones and WNW- and ENE-striking steep structural feeders, which follow regional and local trends. Mineralization is represented by sulphide- and sulphosalt-rich replacement zones associated with a well-zoned advanced argillic alteration footprint. The complex branched pipe-like individual ore bodies vary from 40 to 200 metres in length, are 20 to 130 metres thick and can extend up to 480 metres down plunge.
The main ore bodies are spatially grouped into two major mining areas, with semi-circular distribution that are thought to be controlled by favorable breccia and host rock contact zones and structure intersections within the breccias. The Central zone consists of ten mineralized ore blocks (16, 17, 18, 19, 5, 25, 10, 7, 8 and 700), whilst the Western zone comprises a further 12 ore blocks (103, 144, 145, 146, 147, 148, 149, 149 South, 150, 151, 152 and 153). Advanced argillic alteration related to Chelopech ore system extends toward the southeast, beneath the Chelopech thrust fault, and is associated with a zone of blind breccia pipes known as the Southeast Breccia Pipe Zone.
Mineralisation is hosted within the Lower Chelopech Formation and is characterised by typical epithermal, high-sulphidation (HS) alteration. Alteration and mineralisation are typically zonal with central, high-grade units associated with well-developed stockworks and massive sulphide mineralisation. These units are surrounded by lower-grade haloes dominated by disseminated sulphides and pervasive silica overprinting. These two zones are respectively referred to as “Stockwork” and “Silica Envelopes” and are used as hard boundaries during the estimation of Mineral Resources.
The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. In gross terms, about 45% of the copper is in the form of arsenides and sulfosalts, 50% as chalcopyrite, and 5% as oxides. Gold occurs in a variety of forms but is dominated by refractory species and is typically fine-grained averaging 5–20 microns in diameter.
Three successive mineralisation stages have been recognised at Chelopech, including an early iron- sulphur stage consisting mainly of disseminated and massive pyrite, a second copper-arsenic-sulphur stage which is the economic copper and gold stage, and a late lead-zinc stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.
The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. Sub-vertical vein mineralisation is volumetrically the most important mineralisation style at Chelopech (Chambefort, 2005).
Sulphide mineralogy is dominated by pyrite, marcasite, melnikovite, tennantite, enargite-luzonite, and chalcopyrite, together with subordinate famatinite, sphalerite and galena. In gross terms, about 45% of the copper is in the form of arsenides and sulfosalts, 50% as chalcopyrite and 5% as oxides.
Quartz, barite and kaolinite are the dominant gangue minerals with chlorite, ankerite and gypsum subordinate. Quartz barite-sulphides mineralization with high gold grades and low copper is typical for peripheral zone near the covering sediments (Block 700).
Gold occurs in a variety of forms, both as native metal with admixed silver in a stoichiometric form approximating to Au3Ag and in auriferous tellurides. The gold is fine grained (5–300 microns, with 5–20 microns the norm). Metallurgical studies have shown a significant proportion of the gold is refractory, typically:
• 45% intergrown within pyrite, chalcopyrite and sphalerite;
• 25% intergrown with enargite, luzonite, tennantite, tetrahedrite and bornite;
• 20% finely intergrown with chalcedonic silica;
• 10% as free gold.
Silver-bearing rock and native silver are usually spatially associated or finely intergrown with pyrite and galena (62%) with enargite, tennantite and tetrahedrite (15%) and as electrum (23%).
Other major sulphides and arsenides exhibit simple crystalline and intergrown forms with the pyrite and occur in intra-crystal spaces as replacements, as replacements of pyrite, as cross-cutting veinlets and as overgrowths. Intergrowths of the cupriferous minerals are commonplace, both as aggregates and as complex textures with several intergrown minerals.