Metallurgical testing of the Kilgore Deposit has been ongoing since the 1990’s; the following represents a summary of the metallurgical test results to date. It is important to note that tests conducted before the introduction of NI 43-101 do not meet reporting requirements and are provided for reference only.
Metallurgical Testing Results - 2019
In December 2019, results were received for metallurgical testing from an additional 450 kg of PQ core from drill hole 17OKC-379 from Resource Development Inc. (RDi) of Denver, Colorado under the supervision of Global Resource Engineering Limited (GRE) of Denver, Colorado. The drill samples to be tested came from the interval: 13ft (4.0m) to 133ft (40.5m), an interval of 120ft (35.6m), and returned weighted average assays of 2.05 g/t Au and 2.01 g/t Ag, including 5ft (1.52m) from 119ft to 124ft (36.3-37.8m) of 42.68 g/t Au and 15 g/t Ag. These assays are significantly higher than those from the original hole 17OKC-356 (124ft averaging 0.31 g/t Au) and reflect the variable nature of gold mineralization at Kilgore. The samples, as analyzed in both holes, came from near surface, hydrothermally altered and oxidized rhyolite-composition lithic tuffs.
Metallurgical testing of the samples consisted of:
- Initial head grade assay of each of the drill intervals submitted
- Bench-top bottle roll leach tests
- Column leach tests at ½-inch, 1½-inch and 3-inch crush sizes
The test work was conducted to confirm the suitability of this material to treatment by cyanide heap leaching and to validate the crush size required for efficient gold extraction. Each of the tests was conducted in a typical heap leach configuration.
The column test results indicate that a finer crush size improved gold extraction up to 1.5” with further size reduction beyond this showing additional benefit. Final gold extraction after approximately 200 days (see Figure 1) at a 0.5” crush was 92.6%, and 85.9% at 1.5”, and 56.1% at 3.0” inch crush after 120 days that was terminated when no further gold extraction was recorded. Silver extractions were 41.9%, 37.2%, and 17.4% respectively.
|Table 4: Summary of Column Leach Results
||NaCN Consumption (kg/t)
||Lime Consumption (kg/t)
Finer crush sizes improved the gold extraction rate reaching 60% gold extraction in approximately 24 days (1/2”) versus 56 days at the next coarser crush size (1.5”). The majority of silver leached in the first 25 days. Also notable was that at half inch (1/2”) and one and a half-inch (1.5”) crush gold recoveries had not plateaued after 200 days.
Mineralogical examination of higher-grade gold occurrences within the samples submitted revealed gold grains associated with Jarosite varying in size from <2 microns to 70 microns. The identification of a direct association between gold and jarosite is an important step in understanding the nature and character of mineralization at Kilgore (see Figure 2).
Kilgore Project - Drill Hole OKC17-379 Thin Section Micrographs showing Gold Occurrences
Cluster of bright Au in a matrix of Jarosite – 200X RL
Several grains of bright Au in Jarosite with prismatic Quartz – 200X RL
In 2018, Otis submitted approximately 1,400 kg of whole PQ core from drill hole 17OKC-379 to RDi of Denver, Colorado under the supervision of GRE of Denver, Colorado and received the results in 2019. Drill hole 17OKC-379 was a PQ twin hole of 17OKC-356 that returned 129.4 meters(m) averaging 1.66 g/t Au from 155.4 to 285.0m; including 24.4m averaging 3.45 g/t Au from 157.0 to 181.4m (note: reported widths are estimated to be 80-100% of actual true widths). These drill results come from a package of lithologies including Tertiary volcanic rocks and Cretaceous sedimentary rocks; the lower volcanic stratigraphy rocks included intensely altered volcanic rocks of indeterminate composition.
Four mineralized samples were selected from the material submitted for petrographic analysis. The work was performed by DCM Science Laboratory Inc. of Wheat Ridge, Colorado. Of the four samples, three were sourced from Aspen Formation sediments, and the other from material determined to be part of a sill, known historically as crystal tuff or andesite dike. The results of the study confirmed that the Aspen Formation rocks are siliciclastic arkosic sediments that are silicified with a significant introduction of adularia between round quartz, and sub-angular igneous feldspar grains; the primary quartz and feldspar grains have been corroded by the alteration process. Carbon / graphite is present as discontinuous strings, dust and fragments and grains are small in size (<50 microns). Gold occurs as discrete grains <15 microns in size and associated with secondary silica along crystal boundaries and with minor siderite.
The fourth sample of indeterminate lithology from the Sill Domain was identified as porphyritic igneous rock showing strong to intense quartz-adularia alteration. Optical petrology combined with trace and whole rock geochemistry has interpreted the sill material to be from a series of faulted, porphyritic andesite flows that lie uncomformably over the Aspen Formation sediments; gold occurs within the groundmass of the andesite.
Fourteen bulk samples were submitted to RDi for comprehensive metallurgical testing. Tests included Abrasion and Crusher Work Index testing, QEMSCAN and Acid -Base accounting tests, Bottle Roll Leach Tests, and eight Column Leach tests including 12.5mm (0.5") and 38mm (1.5") feed sizes. The metallurgical tests were carried out on rocks not previously submitted for testing, that is carbonaceous Aspen siltstone and shale, altered Aspen sandstone and intrusive sill material now interpreted as andesite flows.
The gold grades varied significantly from 0.2 g/t Au to 16.9 g/t Au. Cyanide soluble gold values ranged from 0.04 g/t to 1.24 g/t Au which followed the head assay results; silver was detected in all the samples tested, ranging from 0.8 g/t Ag to 6.8 g/t Ag (note: reported widths are estimated to be 80-100% of actual true widths). Gold-silver ratios are highly variable and show no significant correlation.
The bulk density test results are 2.47 for the Sill Domain and ranged from 2.99 - 2.65 for the Aspen sediments.
Crusher work index testing determined the samples to be medium- to very-hard. The Sill Domain was the softest at14.0kWh/mt. The Aspen sediments are harder at 20.3 kWh/mt. Abrasive index testing determined that the samples were abrasive to very abrasive. The Sill Domain is the most abrasive at 0.717 while the Aspen Sediments are less abrasive at 0.458.
|Table 1. Crusher Work and Abrasion Index Results
||CWi (kWh/metric ton)
||20.3 - Very Hard
||15.5 - Medium Hard
||14.0 - Medium
Bottle roll leach tests conducted with individual samples of each of the 14 samples submitted indicate that the Aspen sediments have preg-robbing characteristics, even though test results reveal that organic carbon occurs only in small quantities. Initial gold extractions at a particle size of 10 mesh ranged from 7.5% to 68%. Subsequent leach tests at a particle size of 200 mesh exhibited gold extractions from 3.6% to 85.4% but were on the whole lower. Higher gold extraction was seen from two samples sourced from the upper Aspen stratigraphy; these samples also contained the highest gold grades of 8.13 g/t Au and 16.89 g/t Au respectively. CIL test work on select Aspen sediment samples improved gold extraction from 85.7% to 87.9% versus the original values of 15.0% to 31.4%.
|Table 2. CIL Test Results for Aspen Samples
|Lower Aspen Samples
|Upper Aspen Samples
Sill Domain samples readily leached by bottle roll without the need for CIL. Each sample achieved gold extractions of 87.8% and 84.9% at a particle size of 10 mesh, while the 200 mesh grinds achieved 92.3% and 93.8% gold extraction. Silver extractions during bottle roll leaching were similar for all samples, ranging from 34.5% to 74.8%.
Column leach tests of the Sill Domain samples indicated that the material would readily heap leach. Final gold extraction at 12.5mm (0.5”) crush was 74.8% versus 83.5% at 38mm (1.5”) crush. Silver extractions were 42.8% and 29.4% respectively. The 12.5mm (0.5”) crush material leached faster than the 38mm (1.5”) crush material, reaching 60% gold extraction in approximately 12 days of leaching versus 26 days. The 38mm material leached slower, but achieved a higher overall gold extraction. Leach tests were conducted over a period of 135 to 140 days.
Head assay grades for the Sill Domain samples returned an average of 0.89 g/t Au however the calculated assay grades (extracted gold + residual column gold) returned 1.38 g/t Au and 1.42 g/t Au respectively. The variation between head assay grade (one assay ton with AA finish) and calculated assay grade suggests that gold may occur as coarse grains. The results from column leach tests on the 38mm crushed core show that gold recovery is slow and continues to increase up to a leach time of 107 days, again suggesting coarse grained gold. Optical petrology shows that gold grains are very small (2-15 microns), hard to find, and do not appear to reflect the calculated average grade or presence of coarse gold.
|Table 3. Summary of Column Leach Results
Echo Bay – 1995/1996 Metallurgical Testing
Echo Bay Mines submitted drill core and reverse circulation cuttings to Hazen Research Inc. (Hazen) in Denver, Colorado in 1995, in anticipation of starting up a small-scale open pit heap leach mining operation on the Kilgore deposit. The material submitted was divided into three classes: oxide , sulfide and mixed material, and each was submitted for mineral characterization studies, bottle roll studies on reverse circulation cuttings, and column leach tests on drill core samples. It was noted from mineral characterization studies that the sulfide material did not contain significant sulfides but that it was unoxidized.
The results from 96-hr bottle roll tests on reverse circulation cuttings showed that extractions by cold cyanide solution varied from 82.9% to 94.8% with little variation between the oxide, sulfide, and mixed material types, though cyanide consumption rates were significantly higher in the oxide material. It was concluded at the time that the rocks submitted showed that the Kilgore deposit may be amenable to heap leaching which lead to further metallurgical testing.
The same three categories of mineralization: oxide, sulfide and mixed, were submitted for column leach tests; the samples weighing approximately 45.3kg (110lbs.) each were crushed to 80% passing 12.5mm (0.5”) mesh and placed in 1.8m (6ft.) by 0.15m (6”) diameter columns. Column leaching used cyanide with lime to maintain high pH and the test conducted over a 60-day period. Results returned 94.3% gold recovery for the oxide , 80.8% gold recovery for mixed and 63.8% gold recovery for sulfide mineralization.
In 1996, Echo Bay submitted 80 kg of split drill core from drill hole 95EKC-128 that consisted of mixed oxidized and unoxidized material. Head assay grades returned an average of 0.7 g/t Au for the whole sample. The sample was crushed to 80% passing 25mm (1”) mesh and placed in 3.05m (10’) by 0.15m (6”) columns; testing was conducted over a 75-day period. Results showed an average gold extraction of 85.5% after 75 days which is very similar to the shorter period (60-day) 12.5mm (0.5”) test result suggesting that maybe the Kilgore deposit ore was crush size insensitive.
An independent study in 1996 concluded that the Hazen testing had been conducted over too short a period, 60- and 75-days respectively, and did not provide for sufficient leach time. Ore porosity had not been taken into account; the suggestion was that the difference between oxide and sulfide had no relation to primary mineralogy; no significant sulfide content had been reported and the study concluded that differences are due to porosity and that gold recoveries should be higher than metallurgical testing had reported.
Otis Gold – 2010/2011 Metallurgical Testing
In 2010, Otis Gold submitted four composite samples to McClelland Laboratories Inc. of Reno, Nevada. The composites comprised three principal host rock types identified at that time from the Kilgore deposit: Aspen sandstone, lithic tuff, and felsic dike and were derived from four drill holes: 09OKC-195, -197, -205 and -206.
The calculated head grades for the four composites were: Aspen sandstone averaged 1.44 g/t Au, for lithic tuff _1 averaged 0.55 g/t Au, for lithic tuff _2 averaged 0.69 g/t Au, and the Felsic Dike averaged 1.47 g/t Au. Silver grades averaged 3.77 g/t Ag, 5.82 g/t Ag, 7.54 g/t Ag and 3.77 g/t Ag respectively.
A direct agitated cyanidation (bottle roll) test was conducted on each composite at 80% passing 10 Mesh feed size; this test was conducted to obtain preliminary information concerning heap leach amenability. The results showed that gold recoveries obtained after 96 hours ranged from 50.0% to 78.9%; silver recoveries ranged from 37.5% to 59.1%. Cyanide and lime reagent consumptions were low.
Subsequently, column percolation leach tests were conducted on each composite at an 80% passing 12.5mm (0.5”) feed size to determine gold and silver recovery, recovery rate and reagent requirements, under simulated heap leaching conditions. Results showed that all four composites were amenable to simulated heap leach cyanidation treatment at the 12.5mm (0.5”) feed size. Column test gold recoveries obtained from the four composites were 69.8%, 57.1%, 81.0% and 85.3% respectively after 80 to 109 days of leaching and rinsing, though one composite Lithic Tuff _2 was substantially complete after 30 days. Gold recovery rates generally were fairly slow. Silver recoveries obtained from the four composites were 36.4%, 44.4%, 58.3% and 58.3%, respectively with silver grades varying between 3.77 g/t Ag and 7.54 g/t Ag.
Of note the physical characteristic show that very little “slumping” of the sample charge occurred during leaching. Sample apparent bulk densities were essentially the same before and after leaching. Also, solution requirements were fairly low to typical for finely crushed feeds. No solution percolation, fines migration or solution channeling problems were encountered during leaching. These characteristics suggest that the samples will be physically amenable to typical heap leach.
In conclusion McClelland stated that:
- The Kilgore drill core composites were amenable to simulated heap leach cyanidation treatment, at an 80% 12.5mm (0.5") feed size;
- Gold recovery rates generally were fairly slow;
- Cyanide consumptions were high, but should be substantially lower in commercial production;
- Lime additions of 2.0 to 4.5lb/ton ore were sufficient for maintaining protective alkalinity during leaching; and
- Bottle roll tests (10m feed size) were not particularly useful for predicting column test
gold recoveries, but were effective for estimating column test lime demand.
Otis Gold - 2012 Metallurgical Testing
In 2012, a further three drill core composites were submitted to McClelland Laboratories Inc. for metallurgical testing. The objective of the testing was to determine heap leach amenability of the three composites to comparative feed sizes. Bottle roll tests were conducted for 80% passing 1.7mm feed size. Column leach tests were conducted for 80% passing 38mm (1.5”) feed size and 80% passing 12.5mm (0.5”) feed size. In addition, abrasion and work index tests on the three composites was performed.
The three composites came from whole PQ core from drill holes 11OKC-285 and -287 which were twin holes of 11OKC-258 and 10OKC-228 respectively. The composites comprised 38m (124.5ft) of oxidized lithic tuff with an average calculated head grade of 0.46g/t Au; 38.1m (125.0ft) of oxidized felsic dike, and 32.9m (108ft) of unoxidized felsic dike with an average calculated head grade of 1.15g/t Au (note: reported widths are estimated to be 80-100% of actual true widths).
Bottle roll tests showed that all three composites were amenable to direct agitated cyanidation treatment at the 1.7mm feed size. Gold recoveries ranged from 72.1% to 91.7%. Reagent cyanide and lime consumptions were low, and low to moderate respectively.
The column leach tests showed that all three composites were amenable to simulated heap leach cyanidation treatment at 80% passing 1.5” and 0.5” feed sizes. Gold recoveries for 80% passing 38mm (1.5”) feed size were 78.5% for unoxidized felsic dike, 84.9% for oxidized lithic tuff, and 71.2% for oxidized felsic dike; gold recoveries for 80% passing 12.5mm (0.5”) were 74.5%, 85.5%, and 83.3% respectively.
In conclusion McClelland stated:
- All three core composites were amenable to direct agitated cyanidations treatment at 80% passing 1.7mm feed size;
- All three core composites were amenable to simulated heap leach cyanidation treatment at 80% passing both 38mm (1.5”) and 12.5mm (0.5”) feed sizes;
- The oxidized lithic tuff did not show feed size sensitivity in regard to gold recovery;
- The oxidized felsic dike gold recovery increased with decreasing feed size;
- The unoxidized felsic dike required a longer leach cycle than the other two composites;
- Cyanide consumptions were generally low (≤ 1.0 kg NaCN / mt ore); and
- Lime requirements were generally low but higher for the oxidized lithic tuff.
As with testing in 2011, the physical mineralization characteristics were the same. Importantly, hydraulic conductivities under simulated stack heights of 40m were significantly higher than equivalent solution application rates typically employed during commercial heap leaching. The composites had hydraulic conductivities of 9.79 x 10-2, 5.72 x 10-2 and 5.47 x 10-2 cm/sec respectively.