Corvus Gold Receives Sulfide Gold Recovery At The North Bullfrog Project
VANCOUVER, BC - Corvus Gold Inc. reported initial results from testing the Atmospheric Alkaline Oxidation (AAO) method of oxidizing sulfide mineralization for low cost cyanide gold extraction. Initial results of the AAO method for oxidizing sulfide material resulted in overall gold recoveries of approximately 93% from milled material in the Sierra Blanca tuff (largest host unit), which is comparable to oxidized milled material recoveries in the current production plan. Results for the remaining three units are pending. Estimated additional operational cost to the mill material for the AAO process would raise the mill cutoff grade to approximately 0.65 g/t.
The current North Bullfrog model has several areas below the current designed pit bottom that could be mined at above the projected sulfide mill cutoff grade. Further test work is on going for both mill material as well as heap leach testing of lower grade material using the AAO process. The AAO process has been extensively tested at the Hycroft project in northern Nevada where a pilot scale demonstration plant has been operated as well as test heap leach pads confirming the effectiveness of the processing technique. The AAO process has been included in the recent feasibility study done for the Hycroft project for commercial application.
Jeff Pontius, President and CEO of Corvus said, "The encouraging results from the application of the AAO process to North Bullfrog sulfide material could be a bonus to the project which already hosts a large oxide resource. In addition to the higher-grade, millable sulfide material, Corvus is conducting heap leach applications of AAO similar to the test work conducted at the Hycroft Mine which if positive, could amplify its impact on the project. The current oxide deposit in our PEA study is underlain by an extensive sulfide zone that if amenable to this new low-cost method of extraction could change the way we approach exploration and expansion of the North Bullfrog project and district."
The current test results are from the Sierra Blanca unit, which hosts about 50% of the currently known sulfide mineralization on the North Bullfrog property. The test work conducted standard flotation concentration of the sulfide material which produced a high-quality concentrate that averaged 92% gold in just 12% of the original materials (9:1 concentration ratio) for the Sierra Blanca rock unit. That concentrate was then mixed with a sodium carbonate reagent (Soda Ash) in a reaction tank with oxygen introduced for a period of 72 hours at which point there was no sulfide sulfur remaining in the concentrate. That material was then leached using cyanide in a process similar to that which is proposed for North Bullfrog mill facility. As a control, an unoxidized (no AAO treatment) con centrate sample was cyanide leached which recovered only 12% of the gold. Results for the remaining three units are pending.
The key additional cost elements in adding the AAO process to the existing North Bullfrog mill flow sheet are: a flotation circuit, a fine grinding circuit, AAO reaction tankage; and an oxygen system. These added costs and reagents are currently projected to increase the mill cutoff grade for sulfide material to approximately 0.65 g/t Au. Corvus has identified a number of potential cost saving opportunities for the system that will also be explored in the coming months. The current North Bullfrog project has a number of areas that host sulfide mineralization of sufficient grade to be processed using the AAO system, as well as a local source for soda ash, which is a key reagent. Corvus will report additional results for the other host units and heap leach tests as data becomes available.
Corvus Gold is also conducting column leach tests patterned after those used at the Hycroft project that were scaled up to two, successful 5,000 t test pads. These column tests use the highly reactive sodium carbonate material as an additive to the leach material in place of the standard lime addition. If the heap leach test work is successful, it would lower the cut-off grade required and expand the potential material that could be economically processed.