In addition, as a result of detailed delineation drilling completed in the underground Stockwork Zone in 2012, 121,000 ounces of contained gold were upgraded from the inferred category to high-grade underground Stockwork Zone indicated resources.
Kumtor's inferred resources decreased by 437,000 contained ounces of gold from 2011 year-end, to total 2.7 million contained ounces of gold at December 31, 2012, which includes 1.9 million contained ounces of high-grade underground inferred resources in the SB and Stockwork Zones. The inferred resources in the high-grade underground SB Zone decreased by 0.5 million contained ounces to 1.2 million contained ounces of gold with an average grade of 11.2 g/t. This reduction is the result of the expansion of the Central Pit, which captured 1.2 million ounces of contained gold that were previously classified as part of the high-grade underground SB Zone inferred resources.
These inferred resources are now within the new open pit design and therefore, are included in the Central Pit as probable reserves. This reduction in inferred resources has been partially offset by the positive exploration results in 2012 including 17 additional diamond drill holes completed in the fourth quarter of 2012 after the KS-13 resource estimate was announced in November 2012. These holes have extended the SB Zone to the southwest and northeast adding 651,000 contained ounces of gold to the high-grade underground SB Zone inferred resource, which remains open at depth.
In addition, the high-grade underground Stockwork Zone inferred resource increased by 76,000 ounces of contained gold to 705,000 ounces of contained gold with an average grade of 11.0 g/t. Underground drilling in the Stockwork Zone added 168,000 contained ounces of gold to the inferred resources which was partially offset by the conversion of 121,000 contained ounces of gold to high grade Stockwork Zone indicated resources with an average grade of 10.7 g/t.
At the 100% owned Oksut project in Turkey, Centerra calculated its initial resource estimate based on the successful 2012 drilling program. As of December 31, 2012, the Oksut project has an indicated resource of 682,000 ounces of contained gold and an inferred resource of 477,000 ounces of contained gold. The resource estimation is contained in two zones; the Ortacam North Zone which has an indicated resource of 682,000 ounces of contained gold and an inferred resource of 353,000 ounces of contained gold and the Ortacam Zone which has an inferred resource of 124,000 ounces of contained gold.
The initial Oksut indicated and inferred resource estimate includes only oxide mineralization and is based upon 52 drill holes in the Ortacam North Zone and 21 drill holes in the Ortacam Zone. The resource estimate is contained within a preliminary whittle shell using a cut-off grade of 0.2 g/t and raw assays were top-cut to 15 g/t before compositing.
At the ATO project in Mongolia, measured and indicated contained gold, resources have decreased by 53,000 contained ounces of gold from 2011 year-end, to a total of 771,000 contained ounces of gold at December 31, 2012. Extensive metallurgical testwork completed in 2012 has resulted in revised lower process recovery and net smelter return assumptions for the sulphide mineralization which has lowered the contained gold resources. This has also decreased the corresponding contained silver, lead and zinc resources. Some of the decrease has been offset by increased recovery assumptions for the oxide mineralization and the addition of 50 new exploration drill holes completed in 2012.
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