Great Boulder Resources (ASX: GBR) has completed new metallurgical test results show 99.7% recovery of gold in a sample from the Mulga Bill prospect at the Side Well Gold Project in Western Australia.
In early March 2022, Great Boulder announced a series of three leach tests examined the effect of various grind sizes over a 48-hour period. All three tests achieved similar recoveries in the range of 87.2% to 88.0%, indicating that gold recovery is not affected by grind size.
A fourth leach test completed this week increased overall gold recovery to 99.7% by increasing the cyanide concentration, resulting in a residue or tails grade of 0.1g/t Au. Gravity recovery in this test was consistent with the previous three tests at 62.1%.
“This is a fantastic result. Achieving an overall gold recovery of 99.7% on a high-grade sample is excellent, but only leaving 0.1g/t in the tail is sensational,” Managing Director, Andrew Paterson, said.
“For this test IMO increased the maintained cyanide level from 300ppm to 400ppm and used a coarse grind size of 150 μm, which was enough to improve gold recovery from 87.7% to 99.7%.”
“This is another step in our understanding of Mulga Bill, and we will be doing more comprehensive metallurgical studies on other mineralised zones and on drill core over the next few months.”
The three leach tests completed previously by Independent Metallurgical Operations Pty Ltd (IMO) were conducted at grind sizes of 150 μm, 106 μm and 75 μm.
All three tests had an initial cyanide concentration of 500ppm NaCN, maintained at 300ppm NaCN over 48 hours. Overall gold recovery was 87.7%, 88.0% and 87.2% respectively, which resulted in a residue grade of over 4.3g/t Au in each case.
The sample composite was selected from two high-grade RC intersections. Both were in the transitional zone just above the fresh rock interface, from 91m and 101m downhole respectively.
For the fourth test IMO used a grind size of 150 μm and an initial cyanide concentration of 750ppm NaCN, maintained at 400ppm NaCN over 48 hours. This resulted in an overall recovery of 99.7% and a residue grade of 0.1g/t Au.
Future testing will examine the metallurgical characteristics of other styles of mineralisation at Mulga Bill, including lower-grade zones and areas with significant chalcopyrite (copper) and associated gold mineralisation in order to build up a more comprehensive understanding of the overall metallurgy.
Mineralised samples from drill core will also enable physical testing of the crushing and grinding parameters of different mineralisation styles.
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