The Sepon project area is situated near the eastern margin of the intra-continental Khorat Basin and on the western flank of the Anamite Range fold belt. It lies within the Troungson geological region covering a broad spectrum of rocks ranging in age from Upper Proterozoic to Jurassic. The regional geology is dominated by an Upper Palaeozoic sedimentary belt of arkosic and feldspathic sandstone, variably calcareous and carbonaceous siltstone, shale and limestone which is variably dolomitised and locally marble. There are lesser volcanic rocks, typically comprised of agglomerate, conglomerate, tuffaceous sandstone, and rare coherent volcanics. The belt is cut by plutonic to sub-volcanic bodies of granite, monzodiorite, granodiorite, quartz porphyry, rhyodacite porphyry (RDP) and andesite porphyry. The intrusive rocks are preferentially emplaced along either east or north-west trending well-developed structures.

Several styles of mineralisation have been recognised within the Sepon Mineral District: porphyry-like Cu-Mo-Au mineralisation, skarnoid Cu-Mo-Au mineralisation adjacent to porphyry intrusive, distal skarn related Cu-Au-Ag+/- Pb+/-Zn massive sulphide veins, Carlin type carbonate hosted gold mineralisation and carbonate hosted Mississippi Valley type Pb-Zn-Ag mineralisation. In addition weathering and supergene re-mobilisation has created supergene copper, exotic supergene copper, oxide gold and alluvial gold in karst fill deposits.

All primary deposits are hydrothermal and, at least spatially, related to the RDP intrusive. Supergene copper mineralisation results from the oxidation, dissolution and transport of primary sulphide hosted copper mineralisation to sites where chemical conditions result in copper precipitation (reduced groundwater, replacement of sulphide, reaction with alkali carbonate). Supergene copper mineralisation occurs above and down slope of primary mineralisation. Chalcocite mineralisation replaces massive pyrite immediately above the skarns. Copper carbonate mineralisation occurs where copper rich groundwater reacts with carbonate rocks. The best supergene copper zones occur above higher grade zones of primary mineralisation and have a vertical profile with the best grades immediately above the base of weathering.

Primary gold mineralisation occurs as Carlin style gold forming distally to the copper skarn systems. Mineralisation occurs in association with decalcification and partial silica replacement of calcareous mudstones (‘jasperoid’) along steep faults, and is typically best developed at the contact of the Nalou Formation (dolomite) and the overlying Discovery Formation (nodular calc-shale).

Sepon mine is an open pit operation that is mining and processing supergene copper ore. The operation uses a fleet of excavators and articulated trucks along with a fleet of auxiliary equipment. This mining method is appropriate for the style and size of the mineralisation.

Pit optimisations and designs adhered to recommended geotechnical parameters. The inter-ramp slopes used range between 18° – 40° for the clay material and 35° – 50° for the more competent material.

The Sepon mine is an operating entity. The metallurgical process is a hydrometallurgical process involving grinding, tank leaching, autoclave leaching, solvent extraction and electrowinning.

The Sepon copper operation produces approximately 80,000 tonnes of copper cathode (copper sheets) per year from the nearby Khanong and Thengkham ore bodies.

Crushing and milling
Run-of-mine (ROM) ore is crushed and then milled, which reduces the diameter of the pieces of rock from about 500mm to about 0.106mm (106 microns). The milling process at Sepon is unusual as it uses acidic solution instead of water to help grind and dissolve some of the copper. The mixture of ore and acidic solution resembles a thick liquid or paste, and is called slurry. 90% of the copper in the slurry can be extracted by Sepon’s copper processing plant.

Leaching and thickening
The slurry is heated and leached at 80°C for five hours. It is then cooled and thickened, before being washed in a counter decantation circuit to remove soluble copper. The pregnant leach solution (a copper rich solution) from the counter decantation circuit is then clarified to remove the finer solids, before being pumped to a solvent extraction and electrowinning circuit.

Neutralisation circuit
After the copper has been extracted from the slurry, the slurry is mixed with limestone and lime to neutralise the acidic solution. Contaminants in the slurry are also made safe. Slurry from both the gold and copper plants are mixed together before being pumped to a tailings storage facility.

Autoclave circuit
A number of reagents (substances used to bring about a chemical reaction) are used in the leaching circuit. The reagents are expensive and required in large quantities, so instead of buying them and transporting them to Sepon, they are produced on site in an autoclave.