Paste fill mix and cost-optimisation efforts at Olympias mine

Olympias is an underground mine located in Chalkidiki, Northern Greece, and it is owned and operated by Hellas Gold Single Member S.A., a wholly owned subsidiary of Eldorado Gold Corporation. The mine extracts a high-grade polymetallic deposit using the drift and fill (DAF) mining method. Cemented paste fill has been implemented as the predominant backfilling method at Olympias mine. Cemented paste backfill is an integral part of the mining cycle and constitutes a major component of underground mining costs. During paste system commissioning and initial operations, a high-density paste backfill mix, meeting the high target strengths (1 MPa before vertical exposure and 4 MPa at 28 days curing age) and required flow properties, was developed by incorporating superplasticiser admixtures. Following a revised environmental impact assessment in 2022, the current minimum required backfill strength for DAF mining method is 0.5 MPa for sidewall exposures and working floors, and 1.5 MPa for sill pillars, respectively. The current production schedule and DAF cycle require paste fill achieving strength of 0.5 MPa within three days curing age. This paper presents results of the recent extensive laboratory test work and mix optimisation efforts for developing cost-effective paste fill mix designs that improve the economic viability of the mine while delivering operational efficiency and sustainability. These efforts have contributed to approximately 16% cost reduction per cubic metre of paste fill and there is still room for further improvement as mining and backfill optimisation efforts are ongoing.