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.

Styles of Sulfide-mineralized Rock at ATO
A number of styles of sulfide-mineralized rock are present below the oxide zone in the pipes at ATO, ranging from disseminated flooding by sulfide minerals in matrix of breccia to multiply banded veins. The latter may be either flat lying or steeply dipping. Generally, sphalerite becomes more Fe rich with depth in the system; compare honey brown sphalerite at 82.4 m with brown sphalerite at 97.80 m associated with weakly amethystine quartz, or honey brown sphalerite at 85.9 m versus dark brown sphalerite at 142.7 m in DDH ATO-20 in Pipe #2. The latter also is associated with amethystine quartz. Further, flat-lying galena-sphalerite flooding at 266.5 m in DDH ATO-111 can be followed down hole within a few meters by steep veins at 275.1 m. The latter veins also cut paragenetically earlier disseminated sulfide minerals that form a matrix support to mineralized breccia. In addition, late paragenetic stage manganiferous calcite, in places actual rhodocrocite, rarely cuts across locally layered breccia. In addition, the multiple bands of sulfide minerals and quartz in many veins suggest a protracted period of vein emplacement after initial onset of brecciation associated with pipe emplacement.

Mineralization at ATO
The ATO base (Pb-Zn- (Cu)) metal and Au-Ag mineralized pipes are intermediate sulfidation (IS) epithermal deposits characterized by an absence of adularia. They are apparently magma affiliated on the basis of their sulfur isotopic data, and must thus be associated with emplacement of Jurassic igneous rock at depth. The predominance of low FeS sphalerite, galena, Ag-bearing tetrahedrite-miargyrite, and chalcopyrite at ATO unquestionably are all compatible with an IS state. ATO appears instead to be associated with more mafic, dioritic Jurassic arc terrane magmas. These magmas thereby must have contributed to a correspondingly high Mg/2H+ component in fluids associated with mineralization that in turn inevitably led to widespread presence of the Mg chlorite clinochlore as a gangue mineral in the pipes as opposed to adularia. In addition, Jurassic volcanic rocks are not present in the region of ATO even though the pipes appear to have been formed near surface (banded silica, siliceous reedbearing sinter at Pipe 1). This in turn suggests rapid emplacement of the pipes occurred as a temporally transient or fleeting event. Regardless, well-formed sulfide-silicate banded veins in the pipes suggest protracted veining continued after formation of the brecciated columns of rock. However, the question still remains as to whether or not the mineralized pipes at ATO represent distal parts of a deep-seated porphyry environment.

The ATO mine is 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 designs were created to use 5.0 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%. Design criteria accounts for 3.5 times the width of the truck for running room in areas using twoway traffic. For roads designed outside of the pit, and additional safety berm is accounted for in the road widths. The ultimate pit is 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 waste dump is designed as a flat surface extending through the valley to the north. The dump height is not expected to exceed 10 m between the dumping crest to the underlying topography. Waste material is dumped against a berm on the dump face, and dozers are used to maintain the dumping face.

The waste dump has a total capacity of more than 20 million tons of the required 2,846 thousand tons of waste, though there is suitable space to expand the dump to the north or upward.

The final production schedule uses trucks and shovels as required to produce the ore to be fed into the process plant.

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RECOVERY METHODS
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:

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 t ........