A 2012 NI 43-101 technical report snapshot of the Briggs Mine processing operation in Inyo County, California, describing a three-stage crushing and heap leach circuit with carbon adsorption, stripping, electrowinning, and doré refining.
Report context
This profile is drawn from the March 26, 2012 effective-date technical report prepared for Atna Resources Ltd. by independent qualified persons . The report covers the Briggs Mine, which originally operated from 1996 to 2004, then recommenced operations in early 2009. The processing discussion addresses both the historical operating period and the current (as of early 2012) operation, emphasizing that this is a snapshot of an active mine with ongoing leaching and future production scheduling through 2017.
Processing route
Ore processing at Briggs uses a conventional crush-agglomerate-heap leach flowsheet with carbon-in-pulp adsorption. All ore is crushed to minus ¼ inch in a three-stage closed-circuit crushing plant before stacking on a permanent, lined leach pad.
Crushing and stacking
The crushing plant operates at a nominal rate of 650 tons per hour, with an operating capacity of 640 tons per hour. Annual crusher capacity exceeds 3,500,000 tons per annum; as of the report date, crusher production was at approximately 3.2 million tons per annum. The plant runs seven days per week, 24 hours per day, with 24 hours of scheduled maintenance downtime spread across the week.
Crushed ore is transported by a system of fixed and mobile conveyors and stacked in 30-foot lifts to a total heap height of 190 feet. The leach pad comprises 13 constructed cells; remaining capacity was sufficient for approximately 12 million tons of ore at the time of the report. Additional pad capacity had been allocated at the Federal level, but State and County permits were still required.
Leaching
A dilute cyanide solution is distributed over the heap by drip irrigation. The leach solution pumping system is designed to deliver 3,000 gallons per minute. As leach area increased over time, pumping rates were raised to near capacity levels. Solution percolates through the heap, dissolving gold, and flows to storage ponds or tanks via a leachate collection and recovery system installed over the impermeable pad liner.
The leaching cycle extends 12 to 18 months, depending on stacker locations that require sections of the pad to dry before additional stacked lifts are placed. This cycle includes three rest periods of two months each. Leaching continues beyond the crushing and stacking period, with gold production planned for an additional one to two years. The operating permit requires a fresh water rinse of the pad during final closure.
Gold recovery from solution
Pregnant solution, typically containing approximately 0.010 oz/ton Au, is processed through a five-stage carbon adsorption circuit at a rate of 1,500 gallons per minute. Gold adsorbed on carbon is desorbed in a pressure stripping circuit. An electrowinning operation recovers gold from the stripping solution; the resulting sludge from cathodes is smelted to produce doré bullion. The Briggs refinery typically produces a doré of 70 percent gold and 20 percent silver.
Laboratory and staffing
A laboratory adjacent to the process plant performs fire assays, solution assays, and water analysis for environmental monitoring. The process department, including crushing, conveying, leaching, gold recovery, and lab, comprises 55 employees.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Total ore processed (historical, 1996–2004) | 733,850 (contained Au placed on heap) | ounces | Actual |
| Total gold recovered (1996–2004) | 554,607 | ounces | Actual |
| Total ore processed (1996–2004) | ~26 million (20M crushed + 6M ROM) | short tons | Actual |
| Overall leach pad recovery (1996–2004) | 75.4 % | , | Actual |
| Predicted recovery (1996–2004) | 75.1 % | , | Design estimate |
| Crushed material average recovery (1996–2004) | ~80 % | , | Actual/testwork |
| Crushing plant nominal rate | 650 | tons per hour | Design |
| Crushing plant operating capacity | 640 | tons per hour | Actual |
| Annual crusher capacity | >3,500,000 | tons per annum | Design |
| Current crusher production rate | ~3.2 million | tons per annum | Actual (as of 2012) |
| Crushed product size | 80–85 % passing minus ¼ inch | , | Design specification |
| Leach solution pumping capacity | 3,000 | gallons per minute | Design |
| Carbon adsorption circuit flow rate | 1,500 | gallons per minute | Design |
| Leach cycle duration | 12–18 | months | Design |
| Pregnant solution grade | ~0.010 | oz/ton Au | Typical operating |
| Doré composition | 70 % Au, 20 % Ag | , | Typical |
| Current recovery model target | 80 % | , | Design estimate |
| Actual cumulative recovery since restart (2009) | 78.6 % | , | Actual (as of early 2012) |
| Process department staffing | 55 | employees | Operating |
Project website: https://www.briggsandstratton.com/en-us/
Technical qualifications
The report describes an operating mine with two distinct historical phases: original operation (1996–2004) and restart (2009 onward). All recovery metrics presented are actual operating data or testwork results unless otherwise noted. The 78.6 percent actual cumulative recovery since restart is compared against the 80 percent target; the report states that the difference may be partly attributable to reduced leaching time on some pad sections due to conveyor placement, and anticipates achieving or exceeding the target once full leach cycles are completed.
Future production schedules (Table 17-1) are estimates for 2013–2017, not actual results. The report explicitly notes that additional State and County permits are required for leach pad extension beyond the existing 13 cells. Metallurgical testwork (column leach and bottle roll) on monthly crusher composites from January 2011 through February 2012 indicates extracted recoveries ranging from 79.0 to 93.8 percent, with the report suggesting that the 80 percent target "may understate final recovery potential."
Source: *2012 NI 43-101 Technical Report on the Briggs Mine, Inyo County, California*, effective March 26, 2012, report date May 29, 2012. Sections 13 (Mineral Processing and Metallurgical Testing) and 17 (Recovery Methods).
