Midas Gold Corp. today provided an update on its extensive independent metallurgical test program completed for the Golden Meadows Project, which is located in the historic Stibnite mining district of Idaho.
Since the completion of the Preliminary Economic Assessment published in September 2012, Midas Gold's process and metallurgical consultants have focused on defining the metallurgical parameters for the Project for incorporation into a Preliminary Feasibility Study scheduled for completion later in 2014.
"During the past 24 months, Midas Gold and its independent metallurgical consultants have completed an extensive and rigorous metallurgical program designed to support completion of a Preliminary Feasibility Study," said Stephen Quin, President & CEO of Midas Gold Corp. "Successful completion of this program marks a major milestone on the road towards completion of the Preliminary Feasibility Study," he said. "Overall results of the metallurgical test program support excellent overall gold and antimony recoveries from all three deposits."
Following the successful completion of an extensive independent metallurgical test program, Midas Gold is able to demonstrate, to a level of confidence required to support its planned PFS, metallurgical parameters that are generally comparable with those set out in the Preliminary Economic Assessment published in September 2012. Taking into consideration the different deposits, types of mineralization and processing methods contemplated, it is anticipated that recovery parameters to be utilized in the PFS will range between 80-90% for antimony, while overall gold recovery to doré after POX and/or leach are forecast to average 88-91% for Hangar Flats, 81-84% for West End and 90-92% for Yellow Pine. Additional details on the metallurgical program are set out below, while more detailed information will be included in the Technical Report summarizing the results of the PFS later, which will be filed later in 2014.
Metallurgical Testing Details
Independent metallurgical consultant, Blue Coast Metallurgy Ltd. ("Blue Coast"), supervised an extensive metallurgical test program on behalf of Midas Gold and in support of the upcoming PFS. The test work was carried out at a variety of independent labs under Blue Coast's direction, including SGS, JK Tech, Process Mineralogy Consultants, Surface Science Western, ACTLabs, Kingston Process Metallurgy and Pocock Industrial, Inc. In total, approximately 800 samples were utilized to generate over 100 variability composites and six global master composites covering mineralization from each of the deposits with reported mineral resources, as well as additional composites for historic tailings and the Scout prospect, and representing the variable styles of mineralization within deposits, including sulfide, oxide and transition materials, as well as high and low antimony grades.
Mineralogical studies, including QEMSCAN, confirmed that antimony occurs as stibnite and that, when present in economic quantities, is coarse enough for good flotation recoveries at the chosen grind, while gold primarily occurs in solid solution in pyrite and, to a much lesser extent, arsenopyrite. Except where the mineralization is oxidized, free gold is relatively rare. Overall, the sulfide content of the deposits is relative low, with West End having relatively less sulfides than Hangar Flats and Yellow Pine.
Comminution (crushing and grinding)
A wide spectrum of comminution testing was completed on the different composites to determine appropriate crushing parameters, as well as SAG and ball milling conditions. Overall results confirmed a conventional crushing-SAG-ball mill combination as the optimal comminution circuit. The Yellow Pine and Hangar Flats mineralization are broadly similar and have similar grindability characteristics, whereas the West End mineralization is somewhat more resistant to SAG milling and may require additional crushing. The PFS design parameters (excluding any additional crushing that may be required for West End) to be used in the PFS are anticipated to be single stage crushing with a 2m x 1.5m opening 373kW (79"x59", 500HP) jaw crusher, primary grinding in one 9.1m x 4.9m 7,500kW (30' x 16', 10,000HP) SAG mill, followed by a single 7.3m x 12.2m 13,500kW (24' x 40', 18,100HP) ball mill for secondary grinding to 80% minus 75 microns prior to flotation.
Per the currently conceptualized mine plan, the substantial majority of the mineralization is sulfide and transitional material (~86%), with the balance of the mineralization being oxide ores from the West End deposit. Extensive flotation test work, including multiple locked cycle tests, was undertaken on a variety of global composites from each deposit representing the PEA established average deposit grades of gold, sulfur and antimony. Test work confirmed that the optimal recovery circuit for sulfides involves a conventional sequential flotation circuit of stibnite (antimony) flotation (where antimony grades warrant, in approximately 14% of the sulfide feed at grades generally >0.1% Sb) followed by flotation of other sulfide minerals into a gold-bearing sulfide concentrate. Where warranted, tailings from flotation will undergo agitated leaching to recover additional gold, while oxide material will bypass flotation and go straight to agitated leaching. Due to the high carbonate content of West End materials, a cleaning flotation stage will be undertaken to reject carbonates from the concentrates. While further value to the project may be attainable through the use of additional metallurgical processing, the flow sheet selected is designed to constitute the minimum technical risk and, as such, uses only technologies that are in wide use industry-wide.
Stibnite flotation was successful in producing a marketable, high quality antimony concentrate and, due to the good concentration ratio achieved in the gold-bearing sulfide flotation, it is anticipated that rougher concentrates produced from Yellow Pine and most of the Hangar Flats ores can be sent straight to sulfide oxidation treatment without a cleaning stage of flotation. The West End mineralization and some from Hangar Flats would require cleaning of those concentrates to increase sulfur grades to achieve target levels and reject carbonates to keep levels low. Leaching of gold in transitional and oxide materials has shown fast leaching kinetics and low reagent consumption. Historic tailings were also shown to be successfully co-processed with Yellow Pine ores and, when mixed, behaved somewhat similar to transitional material.
Antimony recoveries are forecasted to be in the 83-89% range from Yellow Pine, and approximately 70-84% at Hangar Flats, while 1-3% of the gold is expected to report to the antimony concentrate. Gold flotation recoveries are expected, on an annual basis, to range from close to 90% for Yellow Pine and the Historic Tailings-Yellow Pine blend, to 81-88% for Hangar Flats and West End sulfides, and down to 64-69% in West End transitional material. Subsequent leaching of the flotation tailings are expected to increase recoveries by ~1% in Yellow Pine, 1-7% in Hangar Flats and up to 17-23% from the West End transitional mineralization. West End oxide leach recoveries are forecast to range from 78-84%.
Pressure Oxidation and Gold Recovery
As identified in the 2012 PEA, pressure oxidation is required to prepare the sulfide gold concentrates for leach recovery of gold. One 4.6m ID x 32.3m (T/T)(15.1' ID x 106') autoclave would operate at 220°C and 2,930 kPag pressure (428°F, 425 psig) for 60 minutes to oxidize the refractory gold-bearing concentrates. To date, two concentrates from each deposit have been tested at these conditions, the results of which confirmed fast oxidation kinetics and high gold liberation, with 95-99% oxidation of sulfides achieved in the studies. Subsequent neutralization and leaching of the oxidized material demonstrated high gold recoveries of 97-99% and low to modest reagent consumption.