As for the leaching component, gold recoveries from the two low-grade composites of Mt. Todd ore that were tested ranged from 84.77% to 91.3%, depending on grind size. The finer the grind was, the higher the gold recovery. "Some indications point to even higher recoveries on average and higher-grade material," said Frederick H. Earnest, the president and CEO.
Average head grades on the two composites were 0.65–0.66% grams per ton gold (0.65–0.66% g/t Au). Individual head grades for Composite 1 were between 0.64 and 0.69% g/t Au. Those for Composite 4 were even higher, from 0.60–0.79 g/t Au.
Regarding the grinding tests, they were done to better quantify required power usage and determine final particle size, both for the second stage of the Mt. Todd grinding circuit, in which Vista plans to use an IsaMill mill.
Core Metallurgy Pty Ltd., which specializes in IsaMill testing and simulations, showed that grinding of the Mt. Todd ore consumed less energy, grinding media and time than anticipated, and therefore produced a finer product.
"The implementation of stirred media grinding technology in the design of the second stage grinding circuit has allowed us to achieve significant energy efficiency improvements," Earnest said. "We believe that we may be able to achieve another significant reduction in final particle size without any increase in power consumption."
In light of these findings, Vista plans to conduct additional grinding tests. Its goal is to determine how much power and grinding media would be consumed when Mt. Todd ore of various head grades is processed through the IsaMill. The product resulting from those efforts will then be leached to establish leach recovery versus head grade curves.
Vista will use those results, anticipated later this year, to reassess the economics of the Mt. Todd project and decide whether or not those in the updated feasibility study need revising. "This is a continuation of our strategy to add value at a relatively low cost," Earnest said.
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