A significant amount of metallurgical testing has been completed and the Deposit's metallurgy is considered excellent. In 1995 and 1996, bottle roll and column leach tests were performed by Hazen Research, Inc. (Golden, Colorado) on Echo Bay drill core and RC cuttings. All bottle roll tests resulted in greater than 90% gold extractability and showed that the mineralization is not refractory. Column leach tests indicate 94.3% recovery on oxidized material (-1/2" crush size) and 86.9% on mixed/partially oxidized material (-1" crush size), material types which together comprise over 85% of the deposit. As noted in a 2002 Rayner and Associates and Van Brunt NI 43-101 compliant report on the property, the extraction of 86.9% gold on mixed material after 75 days of leach time is excellent and suggests that even coarser crush sizes may also lend themselves to favorable extractability, thus enhancing deposit economics.

In 2010, Otis reported the receipt of additional positive gold recoveries from column leach tests performed on bulk samples. These results, in conjunction with historic column leach testing from the deposit, provided continuing support that the deposit is readily amenable to standard heap-leach process technology.

Otis Gold Column Leach Data

The column leach tests were run by McClelland Laboratories, Inc. of Sparks, Nevada, on composites of the three main host rock types collected throughout the deposit area. Column percolation tests demonstrate that three composites are amenable to simulated heap-leach cyanidation treatment at the 80% -1/2 inch feed size. The bulk of the Mine Ridge mineralization occurs in two rock types, felsic dikes and lithic tuff, which together comprise an estimated 93% of the deposit and exhibit column leach recoveries of 85.3% and 81.0% respectively. The tests performed indicate that up to 77% of the gold was recovered within 30 days.


Table 1. Results from Otis Gold's 2010 Column Leach Tests on ½ inch crushed material.

% Gold
Rock Type as
Percentage of Mine
Ridge Deposit
(lbs/ton ore)
(lbs./ton ore)
Felsic Dike
Lithic Tuff
Aspen Fm.


Note 1: Commercial CN consumption in heaps is generally 25%-33% of NaCN consumption achieved from laboratory tests.

Tests were conducted on between 93 and 123 kg composite samples from a variety of drill holes scattered throughout the deposit from split HQ-3 drill core. Each composite was stage crushed in its entirety to 80% -1/2" in size. Crushed composites were each thoroughly blended and split to obtain approximately 68 kg for a column leach test and each sample was blended and split to obtain 1 kg for triplicate head assays. Column tests were not optimized for sodium cyanide ("NaCN") consumption and used 3.1 to 3.9 lbs NaCN; however, projected NaCN consumption in production heaps is typically 25 to 33% of the NaCN consumption achieved in laboratory testing. McClelland reports, "Column test cyanide consumptions encountered during commercial production would probably not exceed 1.3 lbs NaCN/ton of ore." Lime additions of 2.0 to 4.5 lbs/ton were sufficient for maintaining protective alkalinity during leaching.

In 2011, the Company announced the receipt of additional positive column leach gold recoveries from coarse-crush bulk samples. These results provided continuing evidence that the Kilgore Mine Ridge deposit is readily amenable to standard heap-leach process technology and suggest that the gold ore is not particularly sensitive to crush size.

Otis Gold Column Leach Profiles

The column leach tests were performed by McClelland Labs of Reno, Nevada on three samples comprising oxidized and non-oxidized dike (both thought to be responsible for the mineralizing hydrothermal fluids supplying gold to the host rock) and oxidized lithic tuff (the dominant host rock at Kilgore). Both dike samples and the lithic tuff samples were composited from drilled PQ-diameter core (85 mm) that was stage crushed to a nominal 80% passing a 38 mm (1.5 inch) screen. Each sample was divided and one split was further crushed to 80% -12.5 mm (0.5 inch) to directly compare the difference in gold recovery between the two crush sizes.

Column leach tests show that the three samples are amenable to simulated heap leach cyanidation treatment. Further, there was not a substantial decrease of gold recovery in the coarser crush material suggesting that the ore may not require processing by a tertiary crusher. The tests further imply that yet coarser crush material should be tested to determine if part of the deposit might be processed by run-of-mine material that would result in a great cost savings for Otis.


Table 2: Results from Otis Gold’s 2011 Column Leach Tests on 38 mm (1.5”) and 12.5 mm (0.5”) crushed material

Rock Type
Crush Size
Leach Time
% Au Recovery
CN Consumption (lbs/ton)
Lime Consumption (lbs/ton)
Oxidized Felsic Dike
12.5 mm/0.5”
78 days
83.3 %
38.0 mm/1.5”
78 days
Unoxidized Felsic Dike
12.5 mm/0.5”
91 days
38.0 mm/1.5”
91 days
Oxidized Lithic Tuff
12.5 mm/0.5”
91 days
38.0 mm/1.5”
91 days


Overall, the tests revealed that the gold came out quickly in all samples from both size fractions, with 85 to 90% of the recovered gold leaching in just 30 days. Both cyanide and lime consumption were relatively low and the ore charges did not require agglomeration pretreatment.

The results show that the unoxidized felsic dike and the oxidized tuff leach at about the same rate for 80% -12.5 mm and 80% -38 mm. The tuff, which makes up an estimated 65% of the deposit by volume, leaches at the same rate for both crush sizes as the tuff is poorly welded, lightly compacted and has very good porosity and permeability characteristics. Likely the cyanide solutions had excellent percolation and residence rates throughout the sample and the gold was recovered from both sample-sizes with about an 85% recovery rate. The unoxidized dike showed a similar recovery rate of approximately 75% with the coarser crushed material having an even better recovery at 78%. Only the oxidized dike showed feed size sensitivity in regard to gold recovery in column leach tests possibly due to the shorter residence time of 78 days. However, the oxidized dike represents only an estimated 14% of the deposit by volume.

During the tests, the Company conducted tail screen analysis on both crush sizes for the three samples. Residual gold values were generally evenly distributed throughout the various size fractions and it is doubtful, considering the low grade nature of the samples tested, that any improvement in recovery obtained by finer grinding would be sufficient to offset the associated higher processing costs.