Coeur Mining owns the Rochester silver-gold mine near Lovelock in Pershing County, Nevada, US. The mine consists of the main Rochester deposit and the nearby Nevada Packard deposit, on the south-western side of Rochester.
Coeur Mining operates the open-pit mine through its wholly-owned subsidiary Coeur Rochester.
A re-scoped mine plan and preliminary economic assessment for the project was completed in February 2018, which focused on project expansion by including two high-pressure grinding rolls (HPGR) to Rochester’s crushing circuit. The first HPGR was commissioned in the second quarter of 2019, at a cost of approximately £15m ($20m). A second HPGR unit is anticipated to be brought online in 2020.
The project is anticipated to produce between 4.2 and five million ounces (Moz) of silver and between 40,000oz and 50,000oz of gold in 2019. The production is expected to continue until 2032, while residual leaching will last until 2033, following the expansion.
Rochester silver-gold mine location and geology
The Rochester mine is located on the southern flank of the Humboldt Range of northwestern Nevada, roughly 42km from Lovelock. The property comprises 16,494 net acres, entirely controlled by Coeur Rochester.
Rochester silver-gold mine development history
Coeur acquired the mining property from ASARCO in 1983 and conducted a large-scale development drilling programme.
Open-pit mining at Rochester began in 1986 and 387Mt of material was mined until the 2007 shutdown due to low metal price and exhaustion of known reserves. Coeur processed ore by heap leaching mode between 2007 and 2010.
Coeur conducted completed feasibility studies in 2009 and 2010 and secured a Decision Record (DR) and Finding of No Significant Impact (FONSI) from BLM for the mine in October 2010. Silver and gold mining operations were recommenced at Rochester in 2011.
Coeur submitted a revised plan of operations (PoO) in June 2013, which included the expansion of the stage IV heap leach pad and installation of a new HLP.
Rochester silver-gold mine reserves
As of 2018, the total proven and probable gold reserves in Rochester mine were estimated to be 106.2Moz of silver and 684,000oz of gold.
Mining at Rochester
Mining at Rochester is done by conventional open-pit method, involving drilling and blasting followed by hauling and loading the ore by trucks.
The mined material is either fed directly into the primary crusher dump pocket or crushed at an in-pit crusher system or placed directly onto a heap leach pad for run-of-mine (ROM) processing.
Processing at Rochester Mine
The circuit currently comprisesfour dedicated valley-fill heap leach facilities, referred to as Stage I, II, III, and IV. The plant employs Merrill-Crowe zinc precipitation method to extract silver and gold from the process leach solution.
The ore undergoes three-stage crushing to generate a nominal 3/8-inch passing product. The crushed material or run-of-mine (ROM) ore is then placed on heap leach pads. Cyanide is added to extract silver and gold from the mineralised ore.
Pregnant solution from the heap leach pads is clarified in three clarifiers, followed by de-aeration (oxygen removal) in two deaerator towers. The solution is then mixed with zinc dust to precipitate precious metals, which are then filtered out of the solution using filter presses.
The metal precipitates are separated and placed into a retort oven for moisture removal and for mercury extraction. The precipitate is then dried and mixed with flux comprising variable concentrations of silica, sodium carbonate, borax, and nitre (potassium nitrate).
The fluxed precipitate is smelted by using a propane-fired reverberatory furnace, resulting in the production of final gold and silver doré product.
The Rochester silver-gold mine is accessed by a 4.8km-long arterial branch of the Unionville-Lovelock County road.
NV Energy is supplying electricity to the Rochester mine through a 60kV transmission line running along Rochester Canyon (ROW N-043389). Water required for the processing plant and storage tanks is collected from three production wells.
On-site infrastructure include offices, maintenance facilities, warehouse, and various ancillary facilities.
MMTS prepared a life of mine (LOM) planning project comprising detailed pit phases and mining schedules, for the Rochester resource.