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Vista Gold Corp. announces improved gold recovery for the Mt. Todd gold project
November 28, 2017

Vista Gold Corp. 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 redesigned, two-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.

Vista Gold’s automated sorting program 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 16 mm (0.63 in.) screened high pressure grinding roll (HPGR) crushed material (approximately 18 percent of the run-of-mine feed) enables the company to reject approximately 10 percent of the run-of-mine feed as below cutoff grade material. This results in an approximate 8 percent improvement in estimated mill feed grade (life-of-mine average 0.91 g/t compared to the 0.84 g/t life-of-mine reserve grade) with an estimated gold loss of only 1.3 percent. With the installation of an automated sorting circuit, Vista Gold expects to reduce total grinding, leaching and tailings handling costs by approximately 10 percent.

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, Vista Gold 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 two-stage grinding, lowers the total estimated project power requirements by approximately 13 percent and enables the company to cost-effectively achieve a nominal P80 grind size of 60μm. Leaching tests on material ground to 60μm in the two-stage grinding tests show improved estimated gold recoveries of 86.2 percent (weighted average, net of solution losses) compared to previous gold recovery estimates of 81.7 percent calculated on the same basis. 

“Mt Todd is one of the largest, advanced-stage gold development projects in Australia,” said Vista Gold’s president and chief executive officer, Frederick H. Earnest. “Last year we initiated a comprehensive program to optimize the Mt. Todd project and demonstrate its viability at a US$1,250/oz gold price. We have now completed test work confirming our ability to achieve a significant improvement in feed grade to the grinding circuit while reducing our grinding, leaching and tailings handling costs. Our newly designed, two-stage grinding circuit is expected to generate a finer product (with lower power consumption), which we have confirmed results in higher gold recovery and a 4.1-percent increase in gold production. We intend to include these process area improvements in an updated Mt. Todd preliminary feasibility study, targeted for completion in Q1 2018. We expect these improvements, along with current foreign exchange rates, and updated capital, labor and operating costs estimates to demonstrate that Mt. Todd will be a long-life, low-cost, significant gold producer, with robust project economics at today’s gold prices.”

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