Apr 28 2017
Corvus Gold Inc. announces the initial results from testing the Atmospheric Alkaline Oxidation 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."
AAO Sulfide Oxidation Process Test work
The test work conducted by Hazen Research, Inc. on North Bullfrog sulfide material is testing the four rock types which host nearly all of the known sulfide mineralization on the project (Table 1). 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) concentrate 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.
North Bullfrog Mill Concentrate AAO Testing
||Gold Recovery to Concentrate
||Concentration Ratio (ore to concentrate)
||Post AAO CN Gold Recovery (from con)
||Overall Gold Recovery
(from ore tonne)
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.
About the North Bullfrog Project, Nevada
Corvus controls 100% of its North Bullfrog Project, which covers approximately 72 km² in southern Nevada. The property package is made up of a number of private mineral leases of patented federal mining claims and 865 federal unpatented mining claims. The project has excellent infrastructure, being adjacent to a major highway and power corridor as well as a large water right.
The North Bullfrog project includes numerous prospective gold targets at various stages of exploration with four having NI 43-101 mineral resources (Sierra Blanca, Jolly Jane, Mayflower and YellowJacket). The project contains a measured mineral resource of 3.86 Mt at an average grade of 2.55 g/t gold and 19.70 g/t silver, containing 316.5k ounces of gold and 2,445k ounces of silver, an indicated mineral resource of 1.81 Mt at an average grade of 1.53 g/t gold, and 10.20 g/t silver, containing 89.1k ounces of gold and 593.6k ounces of silver and an inferred resource of 1.48 Mt at an average grade of 0.83 g/t gold and 4.26 g/t silver, containing 39.5k ounces of gold and 202.7k ounces of silver for oxide mill processing. The mineral resource for the mill process was defined by Whittle™ optimization using all cost and recovery data and a breakeven cut-off grade of 0.52 g/t gold. In addition, the project contains a measured mineral resource of 0.3 Mt at an average grade of 0.25 g/t gold and 2.76 g/t silver, containing 2.4k ounces of gold and 26.6k ounces of silver, an indicated mineral resource of 22.86 Mt at an average grade of 0.30 g/t gold and 0.43 g/t silver, containing 220.5k ounces of gold and 316.1k ounces of silver and an inferred mineral resource of 176.3 Mt at an average grade of 0.19 g/t gold and 0.67 g/t silver, containing 1,077.4k ounces of gold and 3,799.2k ounces of silver for oxide, heap leach processing. The mineral resource for heap leach processing was defined by Whittle™ optimization using all cost and recovery data and a breakeven cut-off grade of 0.15 g/t.
Qualified Person and Quality Control/Quality Assurance
Jeffrey A. Pontius (CPG 11044), a qualified person as defined by National Instrument 43-101 - Standards of Disclosure for Mineral Projects ("NI 43-101"), has supervised the preparation of the scientific and technical information that forms the basis for this news release and has approved the disclosure herein. Mr. Pontius is not independent of Corvus, as he is the CEO & President and holds common shares and incentive stock options.
Carl E. Brechtel, (Nevada PE 008744 and Registered Member 353000 of SME), a qualified person as defined by NI 43-101, has coordinated execution of the work outlined in this news release and has approved the disclosure herein. Mr. Brechtel is not independent of Corvus, as he is the COO and holds common shares and incentive stock options.
The work program at North Bullfrog was designed and supervised by Mark Reischman, Corvus Gold's Nevada Exploration Manager, who is responsible for all aspects of the work, including the quality control/quality assurance program. On-site personnel at the project log and track all samples prior to sealing and shipping. Quality control is monitored by the insertion of blind certified standard reference materials and blanks into each sample shipment. All resource sample shipments are sealed and shipped to ALS Chemex in Reno, Nevada, for preparation and then on to ALS Chemex in Reno, Nevada, or Vancouver, B.C., for assaying. ALS Chemex's quality system complies with the requirements for the International Standards ISO 9001:2000 and ISO 17025:1999. Analytical accuracy and precision are monitored by the analysis of reagent blanks, reference material and replicate samples. Finally, representative blind duplicate samples are forwarded to ALS Chemex and an ISO compliant third party laboratory for additional quality control.