Bulgaria: gold and copper deposits Tchelopech (Bulharsko: Ložisko zlata a mědi Tchelopech)

Geology

The Chelopech deposit represents one of Europe’s largest producing gold and copper mines. The deposit comprises a number of discrete ore bodies within an andesitic to dacitic volcanic complex of hostrocks on the northern side of a north-easterly trending jog in the regional, east-west trending Balkan fault. The hostrocks in turn comprise a portion of an Upper Cretaceous magmatic and sedimentary assemblage preserved within a north to east trending graben. Basement rocks exposed to the south and east of Chelopech include Precabrian granitoid gneisses, two-mica schists, quartzites and amphipolites.

The northern and north-eastern part of the volcanic complex has been eroded off whilst the southern part has been deeply down faulted by part of the Balkan fault complex. During the first volcanic stage the copper-gold-pyrite mineralisation was deposited. The second stage of volcanic activity was centred to the east of Chelopech - and is illustrated in the figure below.

 

Geological Cross-Section through Chelopech Deposit

In the following stages, sub-volcanic porphyritic andesites, porphyritic syeno-granodiorites and intrusive rhyodacites where injected into the basement. These bodies host and localise several large copper porphyry deposits which run northwesterly across the deposit, including the Elatsite porphyry copper stockwork deposit (5 km north west), the Medetski Reka gold copper deposit (5 km south east) and the Medet porphyry copper molybdenum deposit (10 km north). This is illustrated schematically in the figure below.

 

Mineralisation
The Chelopech ores are developed largely in altered andesitic and dacitic lavas, with the balance being hosted in interbedded fine grained tuffs and agglomerates. The original wallrocks were brecciated and altered during mineralization largely destroying their original fabric. In the mine area the mineralisation is spatially associated with, and restricted to, the andesitic volcaniclastic zones, and is sulfide-dominated and constrained within areas of intense silica and to a lesser extent advanced argillic alteration. The orebodies comprise lenses, discrete pipes and branched and converging geometries. They have been grouped into two mining areas termed the Central and Western Zones, comprising eleven orebodies and nine orebodies respectively. Of these, six are considered significant to the defined resources; these are 18, 19E, 19W in the Central zone and 103, 150 and 151 in the Western zone. Lenses comprising the orebodies vary from 150 – 300m in length, 30 – 120m in diameter and extend for up to 350m down dip.

 

3 D view showing relative locations of the Chelopech Ore Blocks

Silica-sulphide mineralisation is tectonically controlled and predominantly located along, and proximal to, fault structures and, in the case of the larger ore bodies, at their intersections. The ores have between 50% to 75% silica content. The ore is brittle and fractures well with blasting but is very abrasive. The sulphide content of the ores is generally low, with a ubiquitous background of 5% to 10% pyrite, with sulphide content as high as 30% in well mineralised stockworks, and up to almost 100% in the massive orebodies.

The principal ore minerals in the Chelopech deposit are pyrite, tennantite, enargite-luzonite, chalcopyrite, gold and bornite together with subordinate famatinite, sphalerite and galena. Quartz, barite, dickite and kaolinite are the dominant gangue minerals with chlorite, ankerite and gypsum being minor contributors. The overall mineralogy of the Chelopech deposit is complex and to date 71 mineral species have been identified.

Ore Resource
Surface exploration commenced in 1956 and has been carried out extensively throughout the deposit area with approximately 550 diamond drill holes producing 76mm (approximately HQ) core. Many hole were wedged, giving multiple ore body intersections. Most surface holes are vertical or steeply inclined and average 600-700m in depth with some holes extending down to 1000m below surface.
The holes were drilled 50m apart along section lines 50m and occasionally 25m apart. Due to bore hole deviations in depth, some intersections were up to 100m-130m apart and required infill drilling for resource status. The current mine underground drill spacing is nominally 20m square
In 2004, an extensive delineation drilling program totalling, 33,000m of underground diamond drilling, was completed with the objective to upgrade as much of the inferred resource as possible to the indicated category.

The Chelopech Mine Mineral Resources currently stand at:

Lower Cutoff 4.0 (AuEq)	Tonnes (Million)	Gold		Copper
		Grade (g/t)	Au Oz (000)	Grade (%)	Cu (t) (000)
Measured	3.13	4.7	473	1.8	56
Indicated	21.80	3.9	2,733	1.4	305
M&I	24.93	4.0	3,206	1.5	361
Inferred	6.50	3.2	669	1.2	78

MINING

Access and Haulage
The mine is access by a 5.5m diameter concrete lined circular steel shaft (Kapitalna Shaft) which is 345m deep (Collar elevation is at 731m ASL). The operations access is on the 405m ASL elevation which handles wagon haulage and hoisting, and also men and materials on rail track. The shaft bottom is 28m below the 405m level.
Hoisting from the Kapitalna Shaft rated at up to 3000 tons per day is via a double deck twin cage system which hoists two 4.2 tons capacity rail wagons per lift to a semi-automated mechanical wagon exchange and weighing arrangement in the headframe which direct feeds ore into primary crushers.
In addition, there is possible hoisting capacity of up to 1000 tons per day from the Zapad Shaft, which is a 3.3m diameter concrete lined skip/counterweight shaft connecting the 405m ASL rail haulage elevation which is normally used to hoist waste.
The mining operation is accessed via two trackless haulage ramps to the eastern and western zones, accessing the orebodies. The truck haulage (using 40 and 50 tonne capacity trucks) ramps connect at the 450m ASL elevation where a double ore pass truck tipping arrangement feeds rock passes down to the 405m ASL rail haulage level. Chutes then load ore or waste into the 4.2 tons capacity rail wagons for haulage to Kapitalna or Zapad shafts.
During 2004, a new decline from surface was commenced, which will link the mine surface with the 450m ASL elevation, and facilitate the movement of men and materials, along with the instant ability to raise to surface higher tonnages of ore than is possible with the existing shaft capacities.

Ore Production
The mining method used at Chelopech was sub-level caving (SLC). Sub-level intervals, at 15m vertical distance, were developed with interconnecting longitudinal drifts and drawpoint crosscuts set at 12m centres using electro-hydraulic two boom drill jumbos, and mucked using loaders with between 10 and 17 tons bucket capacities.
Production drilling for blasting with ammonium nitrate/fuel oil (ANFO) is carried out using automated, electro hydraulic drill rigs.
Control of the SLC is critical in order to minimise dilution. Grade control geologists monitor all production mucking from the SLC drawpoints on a continuous shift basis.

Sub Level Caving
Studies of the geotechnical characteristics of the geological setting have shown that longhole stoping with subsequent fill can be undertaken at Chelopech. Longhole stoping has the potential to extract reserves at low ore losses and provide the high production rates required by the Chelopech operation. In conjunction with longhole stoping, cemented fill made from process tailings will be introduced to ensure stability and allow the mining of adjacent stopes. The other advantage of this mining method is that it allows the operation to arrest further caving to surface – an effect of sub-level caving methods, and thereby stabilising the environment.
The introduction of longhole stoping with fill will allow the operation to increase production to its current target of 1.5 million tons per annum.
The same equipment that is currently used in the operations will be used for open stoping, with the introduction of additional 50 ton capacity trucks, and 17 ton bucket capacity loaders.

 

Open Stopping

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