Midas Gold Corp. today reported assay results from a 45-hole, 1,106m hollow stem auger and sonic drilling program designed to explore the potential for the reprocessing of historic tailings located on its Golden Meadows Project at Stibnite in Valley County, Idaho.
Reprocessing of the historic tailings could provide Midas Gold with an opportunity to remediate legacy disturbance on the Golden Meadows site, while contemporaneously providing economic value to shareholders since the drill results indicate potential for the definition of a mineral resource in the tailings with significant gold, silver and antimony values. Reprocessing of these tailings were not previously considered in any prior evaluations completed by Midas Gold.
The weighted average grade of all the tailings intercepts in the 41 holes that intercepted tailings is 1.63g/t gold equivalent (1), comprised of 1.17g/t gold, 3.0g/t silver and 0.17% antimony, plus 0.02% tungsten (not part of the gold equivalent grade) over an average 8.4m thickness. The tailings were tested over an area extending approximately 800m by 400m, although the outer edges of the drill pattern had minimal to no tailings as illustrated in the attached map. Results from individual holes are summarized in Table 1 attached and are illustrated in the map and section that can be found by clicking here.
Midas Gold's review of historical data indicated that sub-optimal recoveries (common with older flotation methods) resulted in some gold, silver, antimony and tungsten remaining in the historic tailings, with metal grades which may warrant reprocessing of this material if environmental, technical and economic factors prove supportive. Flotation methods for antimony-bearing stibnite and gold-bearing pyritic sulphide ores were developed in the Stibnite Mining District from the 1920s to the mid-1950s, with lower recoveries in the early years and improving in later years, after reagents and flotation methods were optimized through experimentation and practical experience. The historic tailings represent a significant legacy disturbance at the site, and reprocessing of this material could result in significantly lower metal content exposed to the environment. The historic tailings could be removed from their current unlined facility in the Meadow Creek valley, reprocessed to recover sulphide minerals that host the metals of economic interest, and the resultant "cleaned" tailings placed within a modern lined tailings facility, designed with strict environmental protections, that may be constructed to contain tailings generated by hard rock mining and milling operations that may be proposed by Midas Gold in the future, subject to confirmation of economic viability and the appropriate regulatory approvals.
In the 1980s and 1990s, the historic tailings were capped with several million tonnes of spent ore remaining from seasonal "on-off" heap leach operations conducted by prior operators of the site. This spent ore may represent suitable construction material for potential future facilities constructed on site, should such a project be supported by environmental, technical and economic factors currently being evaluated, in addition to providing further remediation opportunities.
The drill program was designed to: (1) quantify the characteristics, metal content, volumes, thicknesses and relative spatial distribution of spent ore above the tailings; (2) determine the characteristics, metal content, volumes, thicknesses and relative spatial distribution of the tailings; (3) collect samples for metallurgical and environmental baseline characterization; and (4) determine the nature and position of the natural materials underlying the tailings - such as overburden and gravels.
The drilling program was primarily completed with a conventional hollow-stem auger system with a 7.6cm inner tube driven in advance of the auger flights for continuous recovery of materials and to eliminate cross contamination. A total of 42 auger holes, totalling approximately 978m were completed in 2013 in addition to three sonic holes completed in 2011. Drill holes were completed on an approximately 75m x 75m grid across the main tailings area, excluding areas known to be underlain by waste repository materials from past reclamation actions, areas known be underlain by wet conditions or near suspected subsurface springs or near the adjacent stream diversion constructed in 1998-2000. A rigorous set of protocols were used to ensure drilling did not produce cross-contamination between various subsurface layers, including the underlying native materials, the tailings or the overlying spent ore pile. Hole conditions, as well as the composition and character of subsurface materials intersected in the auger holes, were carefully logged and monitored by the drillers, an on-site geologist and environmental staff. Holes were plugged with appropriate hole-plugging material as recommended by state and federal agencies and were abandoned per applicable requirements upon completion of drilling and the drill sites reclaimed. Drill collars were surveyed with a survey-grade instrument to provide accurate information for later volumetric calculations.
Sample lengths were variable and dependent upon the ability of the drive stem to penetrate the material. Sample lengths through the tailings were, on average, approximately 0.6m. Samples were photographed and logged on site and then split in half, with one half sent to the laboratory and the other half of the split placed in plastic bags and archived for future reference. Typical sample weights for sample splits shipped to the laboratory were 1.4kg/sample, providing a reasonable and appropriate sample weight (given the fine-grained nature of the tailings materials), approximately midway between that that would be provided by split HQ and PQ core samples. Sample densities were determined by Strata Labs and averaged 1.50 t/m3.
Results from the program indicate that the historic tailings beneath the spent heap leach material have an average overall thickness of approximately 8.4m, with the thickest tailings situated near the northern (lower) end of the former tailings impoundment, and tapering out to a feather-edge at the limits of the spent heap leach material. Figure 1 is an isopach (contoured thickness) map of the tailings in the areas sampled. Additional tailings are known to underlie several other areas in the immediate vicinity, but were not sampled or evaluated as part of this program. The length weighted average grade of all intervals of tailings drilled, with no cut-off grade applied, was 1.17 g/t Au, which is very close to the estimated value of 1.12 g/t Au derived from back-calculating expected tailings grades from past mining, milling and production records.
Midas Gold sent composite samples from the 2011 sonic holes to SGS Labs for metallurgical testing and results indicate that recoveries similar to those achieved for hard rock mineralization utilized in the 2012 Preliminary Economic Assessment (see news release dated September 4, 2012) are achievable. Additional metallurgical testing is planned.
Mineral Resource Estimation
The drilling data gathered during the 2011 sonic drilling program and the 2013 auger drilling program will be used, along with historic information, to complete a mineral resource estimate within the next month.