Cora Gold Limited - Metallurgical Testwork Results
Metallurgical Testwork Results
· Metallurgical testwork undertaken by
· Samples were selected to represent different host lithologies and depth profiles
· Approximately 207 kg of oxide material was tested, from the Selin, Zone A and Zone B prospects
· The more competent sap rock, derived from deeper in the weathering profile, was characterised as 'very soft' on the crusher, abrasion and rod mill work indices, and 'medium' on the Bond Ball Mill Index
· In whole ore leach tests, all the lithological groups reported gold ('Au') recoveries ranging from 92.4% - 96.7% without oxygen addition, using an optimised 106-micron grind size and a relatively low concentration of cyanide (0.5 g/l)
· Combined Au recovery into a gravity concentrate with leached tailings ranged from 95.4% to 98.3%. Test work on the extraction of gold from the concentrate itself was inconclusive with further work recommended
· Testing for environmental characteristics confirmed that the leach residues are neither acid generating nor carry toxic elements at hazardous levels
· A column leach test over 95 days was continuing to leach Au when the programme was suspended with 56% Au recovery recorded at that time
Although gold recoveries from the column leach don't match that of the cyanide leach approach, it is reassuring that the result obtained is considered a minimum with further upside potential if a programme is pursued over a greater length of time. This work shows that heap leach, with expected lower capex, may still be considered as an option."
The preliminary testwork focused on oxidised volcanic and carbonaceous phyllite from the Selin Prospect.
This work focused on the potential for variability of the oxides and included 187 individual reverse circulation ("RC") samples weighing about 161.9 kg and 37 core samples weighing 45.8 kg. The samples represented complete mineralised drill intercepts at depths that ranged from 18m through to 120m. The samples were from the three current primary target zones, being Selin, Zone A and Zone B. The samples were combined into lithological and zonal groupings as follows:
i) A group called "Sandstones" derived from mineralised oxide intervals at Selin hosted primarily in sandstones with subordinate volcanics.
ii) A group called "Carbonaceous phyllite" derived from mineralised oxide intervals of carbonaceous phyllite at Selin.
iii) A group called "Zone A/B" derived from mineralised oxide intervals from both zones hosted in sandstones, siltstones and minor carbonaceous phyllite.
The spread of lithologies, geographic distribution and depths has provided for a reasonable evaluation of the variability of metallurgical responses that might be expected across Sanankoro, including the range of oxide types from the very soft, near surface saprolite through to the harder sap-rock which lies closer to the fresh rock boundary. In particular, the crushing and grinding test work ("comminution") relied on the harder sap rock samples.
Head Grade; screen fire assay was undertaken on each sample group to determine the distribution of coarse gold (+ 75 micron) to fine gold and to determine the head assay of the samples. The coarse gold represented 18-35% of the gold content, with head grades assayed as: Sandstone 1.93 g/t Au, Carbonaceous phyllite 1.89 g/t Au and Zone A/B 1.47 g/t Au.
Comminution; a composite core sample derived from Zones A and B representing sap rock material. The results showed the oxide to be very soft with low crusher, abrasion and rod mill work index values. The Bond Ball Mill Work Index value was classified as "Medium" with a value of 12.01 kWh/t.
Whole Ore Leach Testwork; each lithological group was subjected to a series of whole ore cyanide leach tests to investigate the effect of a range of variables on the amount of gold and silver that could be recovered. These variables included grind size, cyanide ("CN") strength and the use of oxygen both for pre-treatment and during leaching of the oxide material.
The three groups of lithologies all performed very similarly, with the Zone A/B samples typically responding marginally better.
At a relatively coarse grind size of 80% passing 150 microns and a CN strength of 1 g/l in air, the recovery range was 93.7-94.3%.
When leached at a grind size of 80% passing 106 microns, which was determined to be the optimum size for the remainder of the tests, the following was reported:
1.00 g/l CN strength in air, Au recovery of 94.9-96.6 %
1.00 g/l CN strength in O2, Au recovery of 95.7-97.1% (3L/Min O2)
0.50 g/l CN strength in air, Au recovery of 92.4-96.7%
0.25 g/l CN strength in air, Au recovery of 91.6-96.5%
Gravity Testwork; bulk two-stage sequential gravity testing at 212 micron and 75 microns was conducted to investigate the amount of gold that could be recovered from each sample using a gravity process. The resulting gravity concentrates were then subjected to intensive cyanide leaching using 20g/l cyanide whilst the gravity tailings were leached using 0.5g/l and 1.0g/l cyanide solutions.
The recovery of gold by leaching of the gravity tailings was essentially independent of the strength of CN used with a very similar result for each test. The recovery of gold into a gravity concentrate was as follows:
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Au Recovered into Concentrate
Gravity Tailings 0.5 g/l CN Leached Au Recovered
Total Au Recovered Gravity concentrate
A final step in the gravity testwork looked at the recovery of the gold from the gravity concentrate. As might be expected, a recovery of 99.3% of Au in the gravity concentrate from the sandstone sample was achieved, bringing overall recovery to 95.4%. The gold recoveries in the carbonaceous phyllite and Zone A/B gravity concentrate samples were 75.6% and 76.5% respectively, an unusual result; this lowers the overall gravity testwork gold recoveries to 83.5% and 82.2% respectively. An explanation for the lower recovery of the gold from the concentrate will be checked in a future programme, but there is suspicion that very coarse gold (perhaps millimetric, which is not uncommon in the oxides) failed to fully dissolve and / or gold was not fully liberated when collected in the + 212 micron gravity stage.
Both acid-base accounting ("ABA") and net acid generation ("NAG") testing was conducted on a composite sample of oxide leach residue to investigate the potential for the material to become acid generating. The results of both the ABA and NAG tests showed the oxide leach residue to be non-acid generating.
A separate toxicity characterisation leaching procedure testing did not indicate the presence of any potentially hazardous elements at elevated levels.
Subsequent to the preliminary metallurgical testwork programme, a single column leach test was conducted on an oxide composite sample derived from drill core material sourced from Selin and Zone A in order to simulate potential heap leach kinetics.
The 40 kg composite sample was agglomerated with 22.5 kg of cement and together placed in a 2m x 150mm column which was irrigated for 95 days with a CN solution to maintain a 1 g/l concentration.
The result of the column leach test showed a gold recovery of 56.3%. When compared with the results of the previous coarse ore bottle roll tests, which had achieved gold recoveries of 78.4% (SD0005) and 66.8% (SD0006), the calculated gold recovery from the Master Composite was seen to be lower by approximately 16.6%, based on the average of the coarse ore bottle roll test results, despite the significantly longer leach residence time.
Analysis of the column leach kinetics indicates that the recovery of gold was continuing to increase at the time testing was suspended and that higher metal recoveries would have been achieved if testing had been extended beyond 95 days. Further column tests with optimised cement addition, irrigation rates and leach cycle times are recommended.
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For further information, please visit http://www.coragold.com or contact:
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+44 (0) 20 3239 0010
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SP Angel (Nomad & Broker)
+44 (0) 20 3470 0470
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Turner Pope Investments (Joint Broker)
+44 (0) 20 3657 0050
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+44 (0) 20 7236 1177
Notes to the Editors
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