The principal mineral occurrence in the Ada Tepe ores is as free gold and electrum, and while there are some veins containing visible gold outcropping at surface, the majority occurs as particles of <15 microns for the Upper oxide and <10 microns for the Wall. It is principally associated with quartz, feldspar and chlorite, and sometimes associated with minor iron oxides, with very minor pyrite, chalcopyrite and arsenopyrite.  There is no evidence of any occurrence in solid solution. The concentration of sulphides in the ore is very low.

In the 2005 program, the two ore major ore zones (Upper and Wall) were further subdivided into oxidized or fresh indicating the degree of weathering that had occurred giving four ore zone classifications. The metallurgical and comminution testwork programs focused on these main ore zones.

The bulk of the detailed comminution testwork program was conducted on the Fresh Wall Zone and Oxidized Upper Zone composites as these were shown to represent the hardest and softest ore zones respectively in the deposit.  The detailed metallurgical program was conducted on the Fresh Wall, Oxidized Wall’ and Oxidized Upper Zone composites representing the bulk of the ore from the deposit.  The metallurgical and comminution variability testwork programs were conducted on samples representing all four ore classifications, various sections of the ore body, various secondary alterations and variations in ore grades.

The work showed that the degree of weathering has minimal impact on the comminution and metallurgical performance of the ores.  The results showed that the Upper Zone ore is moderately competent and a low to medium abrasive index indicating moderate energy demand and media consumption, while the Wall Zone ore is significantly harder and more abrasive, indicating higher energy demand and media consumption.

All of the extensive recovery work confirmed that consistent performance with gold extractions  ranging between 93 and 95% would be achieved across all ore zones at moderate grind sizes (P80 of 75 microns) with a conventional cyanide extraction technology.  The absence of cyanicides throughout the ores also enabled the proposed cyanide destruction circuit to perform with very high efficiencies, and the production of solutions with CN concentrations significantly below those required by current EU and Bulgarian legislation.

The 2005 plant design consequently focused on the range of the characteristics between the two zones for the comminution circuit design, with a fairly standard “Carbon-in-Leach” circuit, with CN destruction on the solid tailings stream. 

The 2009/10 recovery test program concentrated on the development of:

• Determining whether recovery by physical methods could produce acceptable recoveries of the precious metals into conventional concentrates suitable for sale to smelters;
• Confirmation of whether a high-density thickened tails product could be produced; and
• The development of a ”Co-disposal” process of the mined waste rock, and the hi-density thickened tails.

Once the “Proof of Concept” of each of the key parameters was confirmed, DPM actively revisited all aspects of the original project to optimize the process and designs throughout, with the result of recommencing a “DFS” that will reflect these improvements, and the benefits which are described in Recent Developments Section.  The DFS is expected to be released early in the first quarter of 2011.