The sulphide ore comprises both undeformed massive sulphides and deformed sulphide ore with the latter characterised by brecciation, mylonitisation, folding and shearing. The most abundant ore minerals are pyrite, sphalerite, galena, arsenopyrite and chalcopyrite. Similar chemistry and fine zoning of sphalerite in both deformed and undeformed ore suggest that they come from the same system and were deposited after the onset of regional metamorphism. In addition copper-lead- zinc– silver bulk ore chemistry and together with lead silver relations, irrespective of deformation, indicate similarity to recognised skarn – replacement type deposits (Kalogeropoulos, Kilias and Bitzios, 1989).

The Olympias mineralisation is a massive stratabound polymetallic replacement deposit hosted in the marble-gneiss contact of the Kerdilia Formation. The deposit comprises pyrite, arsenopyrite, sphalerite, galena, tetrahedrite-tenantite, boulangerite and chalcopyrite with secondary cerussite, chalcocite and covellite. Manganese oxides represent alteration of rhodochrosite.  Gangue minerals include quartz, calcite, rhodochrosite, feldspar, kaolinite, chlorite, ankerite and graphite. The ore minerals are mainly massive in form. Kaolinisation is the main type of alteration associated with the deposit. Hydrothermal brecciation is common and chloritisation and silicification of host rocks immediately around the mineralised section is frequently observed.   

The mineralisation occurs within two orebodies.  The West orebody is approximately 250 m wide and plunges 1,500 m to the southwest.  It has been intersected from surface to a depth of 500 m and is open down plunge.  Its width varies between 5 m to 15 m, with dips averaging 30° to 35° east.  The East orebody lies some 150 m east of the West orebody, has an anticlinal structure, exhibiting axial thickening, with steeper dips toward the peripheries.  It dips an average 25° to 30° to the southeast. Its width averages 75 m and its average thickness is 7 m.  The mineralisation has been traced for 600 m down plunge.

The ore zones at the Olympias Project are variable in ore geometry and ground conditions. The ore contacts are well defined and are often irregular on the footwall.  The high grades associated with these types of ore dictate that a selective mining method will be used. The mining method will be conventional transverse and longitudinal Drift & Fill. A double lift Drift & Fill method (Mini Bench & Fill) in wide orebody areas has been considered but is only applicable to approximately 15% of the reserve and only in very good, stable geotechnical hangingwall conditions. As a result, the mineral reserves have been computed based solely on the conventional Drift & Fill method.  A conventional drill and blast and load-haul-dump (LHD) method of mining will be employed in both production and development. The method is flexible allowing headings to change direction with changing ore geometry.

The overall geometry of the Olympias East and West orebodies is shown in Figure 16-1. The gold shaded area is mined out with Sub-level Cave or drift & fill or undercut & fill. The brown and green areas are planned areas of drift & fill.

An existing ramp has been driven in the footwall from surface at +50 m down to -230 m level. This ramp will be enlarged and resupported to allow access and ventilation. This existing ramp will form the West Ramp from the -110 m junction (with the new Olympias Decline) and will eventually be extended to the base of the orebody at -630 m level. A new Olympias Decline of 1,300 m will be developed from surface in the area north west of the concentrator to intersect the existing ramp at -110 m where a new East Ramp will commence to access the East ore body. This decline is necessary primarily for ventilation purposes but also provides an ore/waste truck haulage route directly to the process plant on surface in the initial years. Access crosscuts to intersect the orebody will be driven from the main West and East ramps.

The mining method is expected to be longitudinal or transverse Drift & Fill depending on orebody geometry.  If the distance between the hanging wall and the footwall is very variable from narrow to wide, then a mixed method of longitudinal and transverse would be adapted to suit the conditions.
   
Generally, for footwall to hanging wall thickness of 10 m or less, the stope will be developed for longitudinal drift & fill mining. This involves developing on the footwall contact avoiding exposing poor hanging wall ground conditions.    After completion of the footwall drift, depending on the orebody width and ground conditions, the ore on the hanging wall side will be either stripped on the retreat, or the drift will be filled and a second drift on the hanging wall side started.

For a thickness greater than 10 m, the transverse drift & fill method is preferable driven along the centre of the orebody from which transverse drifts are mined to the hanging wall and footwall on the retreat from the ends of the orebody. This method of mining from footwall to hanging wall has the advantage of offering increased hanging wall stability by reducing the area of exposure at any one time.  Where primary and secondary mining is scheduled, productivity is also increased as a single stoping level will have many more available faces than with the longitudinal method.

Ore is removed from the stopes after blasting using load haul dump machines.  It is tipped into the orepass system alongside the main ramp or in the lower areas directly into the dump trucks. A loading bay is developed if logistically necessary.  The stoped area is then filled.

As part of the phased approach, Hellas Gold initially rehabilitated the existing concentrator plant to treat the historical tailings. The Phase I plant was commissioned in 2012 and continued operating until the first quarter 2016 when Phase II Plant refurbishment commenced. The underground workings have been refurbished and developed to allow production of ROM ore to be processed in the Olympias Phase II concentrator at a rate of up to 430 ktpa. The Phase II plant was commissioned in May 2017 and commenced commercial production in Q4 2017, producing three concentrates bearing lead-silver, zinc and gold, respectively. The Phase III project would see mine production increased to approximately 800 ktpa on completion of an 8km decline (the Stratoni-Olympias Decline) linking the underground workings at Olympias to a new concentrator to be built in the Stratoni Valley.

In Phase II, the processing circuit comprises 3-stage crushing, transfer to a fine ore bin ahead of a single ball mill. After comminution the overflow from the hydro-cyclones enters the flotation circuit which sequentially separates three products, firstly lead/silver, secondly zinc, and thirdly the gold bearing pyrite/arsenopyrite concentrate. Selectivity of these three concentrates is achieved through a combination of variable pH control and conventional reagents for the depression and activation of the various mineral species. The facilities at Olympias also include concentrate filtration and bagging equipment, a tailings dewatering circuit and a water treatment plant.

Tailings material will either be placed back underground as paste fill for backfilling underground voids created by production activities or transported over to the Kokkinolakkas valley. It is expected that under normal conditions, almost all of the tailings material will be placed back underground