The 100% owned Las Lagunas project involves the reprocessing of high grade gold/silver refractory tailings from the Pueblo Viejo mine located approximately 105km to the north of Santo Domingo, the capital of the Dominican Republic in the Caribbean.
The tailings were derived from open pit operations at the mine between 1992 and 1999, and are impounded in a valley-catchment dam. The Pueblo Viejo mine has recently been redeveloped by Barrick Gold Corp at a cost in excess of US$3.5 billion.
The tailings were originally generated through the processing of refractory ores by Rosario Dominicana S.A, a State owned mining corporation. The refractory nature and metallurgical complexity of the ore resulted in poor recoveries (<30%) of gold and silver when treated by the conventional carbon-in-leach/cyanidation process plant in place for oxide ore that had been mined earlier. This resulted in significant tonnages of refractory tailings with +3.5g/t gold being deposited in the Las Lagunas dam.
PanTerra Gold's subsidiary, EnviroGold (Las Lagunas) Limited, was successful in an international tender and signed a Contract with the Dominican State in 2004 granting it the right to reprocess the tailings under a profit sharing arrangement with the Government.
Under this arrangement, EnviroGold (Las Lagunas) Limited is exempted from income tax in the Dominican Republic, but will share 25% of its operating profit with the Government from 2017, after the Company has recovered approximately US$75 million of direct investment for process plant construction.
The project involves the reclamation of the existing tailings by dredging, ultrafine grinding, concentration of gold bearing sulphides through flotation followed by sulphide oxidation using the Albion process, prior to extraction of gold and silver utilising standard carbon-in-leach cyanidation.
Dredging of Stored Tailings
Extensive feasibility studies were carried out in 2005-2007 followed by detailed engineering of the Albion/CIL plant which was designed to process 800,000tpa of tailings for 6.5 years. The project had a JORC Indicated Resource of 5.137mt of ore grading 3.8g/t gold and 38.6g/t silver prior to commencement of mining in mid-2012.
Since signing the agreement with the Dominican State, the PanTerra Gold Group has spent approximately US$100 million on the project, including resource definition, metallurgical test work, pilot plant studies, feasibility studies, engineering, site works, procurement of mechanical and electrical equipment, plant construction, project management, administration, holding costs, and acquisition of a minority interest in the project.
The project was funded by a US$37.5 million loan from Macquarie Bank Limited, and facilities totalling US$7.5 million from BanReservas (Dominican Republic Government-owned Bank). Macquarie Bank also paid the PanTerra Gold Group US$7.5 million to purchase a 3% gold royalty for the life of the project, which was advanced to the development.
The Las Lagunas tailings are metallurgical complex because, in addition to being highly refractory, they contain significant levels of talc minerals that hamper floatation recovery of gold, and also contain both organic carbon from the original orebody, and discarded activated carbon from the former Rosario process plant, that act as preg robbers in the CIL circuit. Also, approximately 10% of the contained gold within the tailings is silicate occluded and therefore does not report to the concentrate.
The Las Lagunas plant is now running at its optimum performance, and achieves approximately 81% gold recovery to flotation, and 63% recovery in CIL, for an overall gold recovery of 51%, which equates to 45,000oz of annualised gold production. This is significantly lower than the 71% overall recovery predicted by the feasibility study metallurgical test work.
The Las Lagunas tailings reprocessing is scheduled to be completed in mid-2019 based on current processing rates.
The Company has developed an onsite Albion testing laboratory where concentrates from around the world have been tested as part of investigations into collaboration agreements or possible concentrate supply arrangements. The testwork has demonstrated that when presented with a clean concentrate from a mining operation, the Albion Process provides the oxidation required to achieve +90% gold recovery from concentrates through cyanide leaching.