The primary processing option for the Helen and Gap resources is toll milling and treating through a third-party roasting or pressure oxidation operation.
Report context
This Preliminary Economic Assessment for the Cove Project, Lander County, Nevada, was prepared for i-80 Gold Corp and dated January 25, 2021. The report addresses processing via toll milling at existing Nevada facilities including Jerritt Canyon Gold and operations under Nevada Gold Mines.
Processing route
Resource processing concept
The proposed design for the Cove Project is to transport Helen and Gap resource material to a third-party toll processor for treatment. Premier Gold solicited terms and conditions from a prospective toll operator with both roasting and pressure oxidation (POX) operations. The prospective toll operator provided terms and conditions for processing Helen Zone and Gap resource material through existing facilities.
Historical testwork
Laboratory bench-scale batch roasting and pressure oxidation tests were completed using plant conditions from the prospective toll operator. The conditions provided approximate the expected operating conditions in the prospective toll operator's roasting and POX facilities. Premier Gold provided a package of Helen Zone metallurgical data from the 2017 test program to the prospective toll process operator as the basis for toll processing.
Feed specifications
Table 17-1 in the report shows proposed feed specifications applicable to both roaster and pressure oxidation feed. Previous testing identified that mercury and arsenic concentrations would likely exceed the specified limits for both Helen and Gap resources, apart from Gap which did not exceed mercury limits in the samples tested. Other elements of concern include barium and chromium (Gap only). Selenium and cadmium were not assayed to a sufficiently low detection limit to determine if they are problematic.
Blending at either the Cove mine site or at the toll processor would be required to meet the maximum concentration limits for the elements of concern.
Roaster feed considerations
Roaster feed specifications show the following areas that are out of range:
- CO3 – both Helen and Gap samples were below minimum specification
- TCM – both Helen and CSD samples exceeded maximum specification
- Sulfide sulfur – both Helen and Gap samples were below the minimum specification
POX feed considerations
For POX feed specifications:
- Acid autoclave – Generally the Helen Zone samples did not meet these specifications while most of the Gap samples met this specification.
- Alkaline autoclave – Some Gap samples met this specification and would be directed to the alkaline system.
Metallurgical amenability
The test data indicates that the Helen Zone composites were generally more amenable to roasting than POX. The assay data for the Helen composites indicates that there may be problems from some areas to meet roaster feed specifications. Onsite blending of Helen resource material to meet specifications prior to shipping to the toll processor, provided that resource material is available for blending, will likely be required. Alternately, blending with the toll processor's own feed material may also be possible.
Conversely, the Gap test data shows more amenability to POX. Helen resource may generally be more amenable to roasting, but it is likely that there will be production that can be directed to POX. The reverse would be likely for the Gap production.
Projected gold and silver recoveries
Roaster and pressure oxidation recoveries assuming CIL processing were projected based on 2017 SGS test programs. These are initial projections and further sampling, assaying, and testing will be needed to confirm the projections and increase understanding of recoveries by roasting or pressure oxidation within the metallurgical zones. Some projections were extrapolated for samples where CIL was not performed. The testing showed that CIL significantly increased gold and silver recoveries over direct leaching.
Payable content terms
The proposed toll processing terms contained terms for determining recoverable metals by roasting and POX processes. Payable metal content was generally based on feed head grades of gold and silver. The proposed terms state that at month's end for each period covered by a potential contract, the recoverable gold will be adjusted based on the toll processor's actual plant recoveries and proportions of toll resource to the processor's own materials processed. This could be a positive or negative adjustment.
The same end-of-month recovery adjustment applies to alkaline POX recovery. The proposed terms indicate that recoverable silver will be 10 to 20% and will be adjusted at month's end in a similar manner as for gold. Silver recovery in roasting and POX operations is typically low. The 2017 test work indicates that the Helen and Gap resource material may yield higher silver extractions; however, the proposed terms will likely pay for lower amounts unless the toll processor's silver recovery is higher when processing the Helen or Gap resource materials.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Processing method | Toll milling via roasting or pressure oxidation | Proposed design |
| Feed specifications | Per Table 17-1 (roaster and POX feed) | Proposed by toll operator |
| Mercury and arsenic in feed | Exceed specified limits for Helen and Gap (except Gap mercury) | Historical testwork |
| Roaster feed: CO3 | Below minimum specification | Historical testwork |
| Roaster feed: TCM | Exceeded maximum specification | Historical testwork |
| Roaster feed: Sulfide sulfur | Below minimum specification | Historical testwork |
| Helen Zone amenability | Generally more amenable to roasting | Historical testwork |
| Gap amenability | More amenable to POX | Historical testwork |
| Gold recovery method | CIL after roasting or POX | Projected from testwork |
| Silver payable | 10 to 20% | Proposed terms from toll operator |
| Recovery adjustment | Monthly based on actual plant recoveries | Proposed terms |
Project website: https://www.i80gold.com/cove-2/
Project website: https://www.newswire.ca/news-releases/premier-enters-into-definitive-agreement-to-acquire-the-high-grade-cove-gold-project-in-nevada-510332661.html
Technical qualifications
The feed specifications appear to be somewhat rigid and could preclude some material being sent to the toll processor. Blending may allow shipment of some off-specification material provided appropriate material is available for onsite blending prior to shipping.
The recovery terms appear to be the result of analyzing the metallurgical data provided by Premier Gold. The terms appear to be consistent and typical with those encountered in the industry.
The report recommends that the resource model should be advanced to include arsenic, TCM, TOC, mercury, lead, zinc, total copper, selenium, barium, cobalt, nickel, and cadmium, as these will be important for predicting grades if toll processing offsite is used and potentially for estimating extractions within the resources.
Additional metallurgical testing should be conducted to confirm the proposed payable recoveries are appropriate for the resources. Development of a preliminary or conceptual onsite blending program is recommended. The next phase metallurgical program should examine blending of out-of-specification resource materials to produce on-specification material based on material projected to be mined in a given period.
The report also recommends confirming the availability of toll processing under current conditions in Nevada.
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*Source: Practical Mining LLC, Preliminary Economic Assessment for the Cove Project, Lander County, Nevada, January 25, 2021, Sections 17 "Recovery Methods"*
