The Stratoni polymetallic ore consists of stratabound replacement mineralisation hosted within marble horizons and may be termed a distal skarn. The mineralisation was emplaced by hydrothermal fluids driven by a granodioritic intrusive which was intruded some 30 million years ago. The mineralised zone is often referred to as the Stratoni-Piavitsa belt, and the total sulphide content of this belt prior to mining has been estimated at 25Mt. The deposits consist of massive sulphide carbonate and aplite replacement, restricted to two Marble horizons, interlayed with biotite gneiss and schist. The model therefore requires two components; marble horizons that have been thickened and structurally prepared by thrusting and folding prior to the introduction of mineralising fluids and; a structural architecture which has allowed the channelling of fluids from a magmatic source to the chemically reactive marbles. Exploration uses these two elements as a guide to identifying potential new ore zones.

The massive sulphide mineralisation has a relatively simple mineralogy, and the most abundant ore minerals are pyrite, sphalerite, galena, arsenopyrite and chalcopyrite. The dominant exploited minerals historically have been sphalerite, pyrite and galena; the ore is often abbreviated to BPG from the old notation blende, pitch, galena. The ore is also gold bearing, mostly associated with the arsenical pyrite and arsenopyrite. Quartz, calcite and minor rhodochrosite form the gangue minerals.

The mineralisation is hosted either in the marble or replacing aplite as irregular and sub-parallel layers of sphalerite, pyrite and galena. These lenses occur from footwall to hanging-wall, varying from few centimetres to tens of metres. Galena is seen as the most deformed mineral, commonly re-crystallised into coarse grains. Galena also exhibits curved cleavage and elongated and foliated grains. Zonation of the sulphides is not clear.

The Mavres Petres orebody is some 500 metres in strike length, between 100 and 340 metres in dip extent. The true width is on average 25 metres. The strike of the orebody is close to east west and the dip is 30° to the south.

The Mavres Petres orebody shows an average gold grade of around 4 to 6 g/t Au. Its presence is strongly associated with the arsenical pyrite and arsenopyrite, which appears to be a different stage of mineralisation within the matrix of hydrothermal breccias or in veins cutting the Pb-Zn sulphides. The gold mineralisation is not coincident with the massive sulphide mineralisation. The gold is completely refractory and not considered to be of any economic value.

Mining is a combination of transverse and longitudinal drift-and fill methods with rock breaking by conventional drill and blast. The stoped area is then tight filled with cemented tailings from surface. Previous mining has generated considerable experience with the ground conditions and the rock mass’ physical and geotechnical characteristics. Support varies based on size and ground conditions. Some areas are supported with backpacked steel sets while others are supported with rock bolts and shotcrete.

The Stratoni concentrator plant is currently operating on a campaign basis, five days a week at a rate of approximately 50 tonnes/hour of ROM ore from the Mavres Petres mine. Lead, silver and zinc recoveries of 91.5%, 80% and 91.5%, respectively, are achieved. The grade of the bulk lead/silver concentrate is typically 70.5% Pb with approximately 1,600 g/t Ag; the zinc concentrate contains 50% Zn. The crushing circuit is capable of crushing up to 750,000 dmt per annum and as a consequence is currently operating for approximately one-third of the available time. Fine ore which has been crushed to minus 12mm is then ground to 80% minus 200 microns in a conventional rod mill/ball mill circuit, then sent to the differential flotation circuit. Lead concentrate is recovered first, then zinc minerals are subsequently recovered from the lead circuit tailing. Thickeners and disc filters are used to dewater lead and zinc concentrates, which are weighed and conveyed to storage sheds for shipment to the smelter. Shipment occurs either through the loading facility at Stratoni port or via the Thessaloniki port. The Stratoni port can be used for materials being sold into the European Mediterranean market with the larger vessels using the alternate port.