DENVER, Nov. 27, 2017 /PRNewswire/ — Vista Gold Corp. (the “Company,” “we” or “our”) (TSX and NYSE American: VGZ) today issued the results of its previously announced metallurgical testing program for the Mt Todd gold project located in the Northern Territory, Australia. The test work confirms that the inclusion of automated sorting and a re-designed, 2-stage grinding circuit will enable the project to achieve a finer grind size, higher gold recoveries/higher gold production, and lower processing costs with no material increase in project capital.
The Company’s automated sorting program (see the Company’s July 24, 2017 press release for full details) is now complete, with assays for all size fractions. The test work confirmed that the combination of x-ray transmission (“XRT”) and laser sorting on the +5/8″ (16mm) screened HPGR crushed material (approximately 18% of the run-of-mine feed) enables the Company to reject approximately 10% of the run-of-mine feed as below cut-off grade material (this uneconomic material is hereafter referred to as “waste”). This results in an approximate 8% improvement in estimated mill feed grade (life-of-mine average 0.91 grams gold per tonne “g Au/t” compared to the 0.84 g Au/t life-of-mine reserve grade) with an estimated gold loss of only 1.3%. With the installation of an automated sorting circuit, the Company expects to reduce total grinding, leaching and tailings handling costs by approximately 10%.
Newly Designed 2-Stage Grinding Circuit
Past metallurgical studies at Mt Todd demonstrate a strong correlation between finer grind size and higher gold recovery. To efficiently achieve a finer grind size and higher recovery, the Company has optimized the crushing and grinding circuits. Excess capacity in the HPGR crusher will now be used to produce finer feed for the grinding circuit. Each grinding module is now comprised of a primary ball mill and three, small fine-grinding mills. The combination of the reduced material volume (as a result of automated sorting), smaller mill feed size, and 2-stage grinding, lowers the total estimated project power requirements by approximately 13% and enables the Company to cost-effectively achieve a nominal P80 grind size of 60µm. Leaching tests on material ground to 60µm in the 2-stage grinding tests show improved estimated gold recoveries of 86.2% (weighted average, net of solution losses) compared to previous gold recovery estimates of 81.7% calculated on the same basis.
Vista’s President & CEO, Frederick H. Earnest, commented, “Mt Todd is one