The ATO (Altan Tsagaan Ovoo) is an epithermal gold and polymetallic deposit of transitional sulfides in breccia pipes in a Mesozoic continental rift zone.

Three mineralized pipes at ATO (Pipe 1, 2, and 4) have been emplaced into stratified rocks as young as presumably Early to Middle Jurassic. Pipe 4 is mainly concealed. Pipe 3 contains abundant pyrite, but no significant amounts of Au, Ag, Pb, and Zn. There is a strong Au anomaly in soil at TO, as well as other accompanying metals, particularly Pb. In the aeromagnetic field, only post-mineral young dikes have a prominent positive response; much weaker responses outline some ring-shaped features. ATO resides near the center of the latter. Sinter and silicified rocks are reflected as shallow resistivity anomalies, but broad clay and chlorite-altered rocks are characterized by low resistivity. The pipes coincide with chargeability anomalies that overall really are quite weak.

An upper zone of Au-Pb-ZnAg mineralized rock at Pipe 1 is approximately oval in shape and is about 320 m wide. Pipe 2 is elongate to the NE and is approximately 320 m by 160 m in maximum dimension. Pipe 4, which is completely concealed, also is elongate to the NE and is about 400 m by 200 m. The pipes taper slightly with depth and have a carrot-shaped configuration, narrowing gradually to depths of approximately 200 m.

The deepest hole into Pipe 1, inclined 60 degrees, is about 700 m. Silica cap rock has a variable thickness in Pipe 1, generally tapering from a maximum thickness of about 40 m under the topographic high point of the pipe to less than 1 m near its margins. However, bottom surface of the cap rock is highly irregular, showing sharp undulations with underlying quartz-veined Middle-Late Jurassic gravel and coarse pebbly sandstone, some blocks of which are totally engulfed by massive silica.

Despite the three mineralized pipes being so geographically close to one another, there are distinct differences in their metal geochemistry. Pipe 2 is notably base-metal enriched; Pipe 1 contains less base metals, and Pipe 4 contains further decreases in base metals, particularly near its margins where extremely high Ag contents (locally 100s of ppm Ag across narrow intercepts) are present in association with base metal concentrations of a few hundred ppm in all.

ATO is characterized by an absence of adularia that is relatively abundant elsewhere in epithermal Au-Ag deposits. At ATO is a reflection of high Mg/2H+ ratios and a correspondingly low K/H+ ratio in mineralizing fluids associated with an underlying largely dioritic magmatic complex. At ATO, siliceous sinter forms a cap rock at the top of Pipe 1 and includes barite (in narrow micro veins as well as tabular crystals in open cavity fillings), clinochlore (Mg chlorite), and less abundant Mg-Fe chlorite. With increasing depth, silica is increasingly present in the pipes as relatively late paragenetic stage vein quartz filling central parts of sulfide mineral-rich veins. Though clays (mostly kaolinite) occur with sinter at the top of the mineralized column at Pipe 1, clinochlore (Mg chl) is the dominant alteration hyrosilicate mineral at depth throughout mineralized breccia. At depths near 200 m, however, clinochlore begins to give way to phlogopitic white mica. White mica also increases near margins of Pipe 4. Gypsum (after anhydrite) is concentrated near margins of the pipes.

Though quartz is the dominant alteration mineral at the surface, magnesium minerals also are widespread in the pipes. They, in essence, replace rock flour during pipe development and eventually comprise a fluidized matrix together with iron sulfide and base- metal sulfide minerals. Their importance is well indicated by mineralized core contents of many 10s of thousands ppm Mg to greater than 100,000 ppm Mg throughout the mineralized pipes. The dominant Mg alteration mineral in the pipes is clinochlore (hydrous Mg Al silicate member of the chlorite group). With increasing depth in the pipes, clinochlore becomes progressively enriched in Fe and assumes petrographic characteristics of typical chlorite and, in turn, becomes associated with increased abundances of phlogopitic white mica.

Typical appearance in core at ATO of the various altered rocks recognized thus include oxidized siliceous sinter; steeply dipping quartz veinlets with associated pyrite and base metals; matrix supported mineralized breccia where magnesian chlorite is the dominant hydrosilicate in the matrix; and lastly gypsum after anhydrite especially concentrated near pipe margins.

The ATO project has been planned as an open-pit truck and shovel operation. The truck and shovel method provides reasonable cost benefits and selectivity for this type of deposit. Only open- pit mining methods are considered for mining at ATO.

Pit optimization was done to define pit limits with input for economic and slope parameters. Optimization used only Measured and Indicated Resources for oxide ore processing. Pit designs were created to use 2.5 meters benches for mining. Slope parameters were applied as up to 50 degree inter-ramp slope angles; up to 70 degree bench face angles. Ramps were designed to have a maximum centerline gradient of 10%. In areas where the ramps may curvealong the outside of the pit, the inside gradient may be up to 11% or 12% for short distances. Design criteria accounts for 3.5 times the width of the truck for running room in areas using two- way traffic. For roads designed outside of the pit, and additional safety berm is accounted for in the road widths. The ultimate pit is an integrated design using interior pit phases and it separates 3 small mines. Based on a resource block size of 5 m by 5 m by 2.5 blocks, ore loss and dilution was assumed not more than 3% and 2%.

The gold recovery process was designed on the basis of leaching 1.2Mt of ore per year with an average gold grade of 1.25 g/t, average silver grade of 10.0 g/t at an overall gold recovery of 70%, silver recovery 40%.

The process plant, located near to and down- gradient from the heap leach facility HLF to minimize the pumping and pipeline requirements for pregnant and barren solutions, will operate 365 days per year. The pregnant solution will flow to the plant at a nominal rate of 200 m3/h and a design flowrate of 250 m3/h. The plant is designed to process 5 ton of carbon per day using an adsorption, desorption and refining (ADR) process to extract gold from the pregnant solution to produce the gold doré. The gold ore processing facilities will include the following unit operations:

Crushing and Ore Handling
- Primary crusher: a vibrating grizzly screen and jaw crusher in open circuit producing a final product P80 of approximately 190 mm;
- Secondary and tertiary crusher: a vibrating screen and cone crushers operating in reverse closed circuit producing a final product P80 of 25 mm, and;
- Heap placement: crushed ore stacked to a 3,000 ton - capacity stockpile, reclaimed by a radial stacker.

Heap Leach Pad
- Crushed ore stacking
- Ore leaching; and
- Barren and pregnant solution delivery and recovery piping systems.

ADR Plant
- Carbon-in-Column (CIC) Adsorption: adsorption of solution gold onto carbon particles.
- Desorption: acid wash of carbon to remove inorganic foulants, elution of carbon to produce a goldsilver rich solution, and thermal regeneration of carbon to remove organic foulants, carbon stripping to recover gold into solution.
- Gold refining: gold electrowinning (sludge production), filtration, drying, mercury retorting, and smelting to produce gold dore.

A single crushing circuit will be utilized for preparation of the heap leach feed comprised of a conventional closed-circuit jaw / cone / cone crusher flow sheet.

The HLF comprises the following:
- Conventional, three stage lift (nominally 8 m per lift), free-draining heap over a gently sloping heap leach pad (HLP) along the axis of the ridgeline west of the ADR plant;
- The leach pad will be graded and constructed in a nominally balanced cut-and-fill manner using locally borrowed (within the heap boundary) rock for structural fill, supplemented as needed by mine waste including waste rock and, if available, thaw-stable soil for lining the pad subgrade before placement of the liner system;
- Permanent and interim perimeter diversion channels and berms to manage surface water flows;
- Perimeter access and ore haulage roads.

The carbon adsorption circuit consists of a train of five cascading carbon columns. The pregnant or goldenriched solution will be pumped to the carbon adsorption circuit across a stationary trash screen for removal of any debris from the heap leach pad. The solution will flow counter- current to the movement of carbon from column 1 to column 5. The solution overflow from the final column will discharge onto a screen in order to recover any carbon. The barren solution, which at this stage has had most of the gold in solution adsorbed, will discharge from the final carbon column and will be pumped to the barren tank.

The stripped carbon from the strip vessel will be pumped to the vibrating carbon-sizing screen. The kiln-feed screen doubles as a dewatering screen and a carbon-sizing screen, where fine carbon particles will be removed. Oversize carbon from the screen will discharge by gravity to the 7.5 ton carbon-regeneration kilnfeed hopper. Screen undersize carbon will drain into the carbon-fines tank and then be filtered and bagged for disposal. A 250 kg/h diesel-fired horizontal kiln will treat 5.0 ton of carbon per day at 650°C, equivalent to 100% regeneration of carbon. The regeneration-kiln discharge will be transferred to the carbon quench tank by gravity, cooled by fresh water or with carbon-fines water, prior to being pumped back into the CIC circuit.

Pregnant solution will flow by gravity to a secure gold room. The solution will flow through one of two 3.54 m3 electrowinning cells. Gold will be plated onto knitted-mesh steel wool cathodes in the electrowinning cell. Loaded cathodes will be power washed to remove the gold-bearing sludge and any remaining steel wool. The gold-bearing sludge and steel wool will be filtered to remove excess moisture and then retorted to remove any mercury. The retort residue will be mixed with fluxes consisting of borax, silica and soda ash before being smelted in an induction furnace to produce gold doré and slag. The doré will be transported to an off-site refiner for further purification. Slag will be processed to remove prills for re- melting in the furnace. The gold bars will be stored in a vault located in the gold room prior to secure off-site transportation by aircraft.