The Boroo gold deposit is a low silica Au+As sulphide system associated with a zone of quartz-sericite-pyrite (QSP) alteration in the sub horizontal Boroo fault. Boroo is a intrusion-related gold deposit and hosted by a Cambrian-Ordovician sequence of highly deformed shales, siltstones and fine sandstones of the Haraa turbidite sediments, and the Paleozoic granitoids of the Boroo Complex.

The bulk mineable gold mineralization at Boroo is hosted in a strongly quartz sericite altered and sulfidized nearly flat lying zone controlled by the Boroo fault. The fault has been traced for a distance of 2.4 km and is thought to be a thrust fault that dips at an angle of 10° to the north west and trending north east (see Figure 7). It cuts across the intrusive contact between sediments and granitic rocks in the north, but is entirely contained within the sediments in the south.

In the cross section, the Boroo fault shows a slightly undulated shape with the structure becoming thicker to the northwest, where the alteration and mineralization decrease. The Boroo fault is variously altered and mineralized, and where these features are strongest, individual deposits are formed. These are termed, from north to south, Pit 2, 3, 5 and 6. All of the deposits are elongated in a northeasterly direction, with a length to width ratio of about two to one. In Pit 2, 5 and 6, mineralisation is controlled by Boroo fault and is in the foot wall. But in Pit 3, there is low grade mineralisation in both hanging wall and foot wall. Gradethickness contours show the same overall elongation (Figure 9) probably caused more by the width than by the gold grade, with the multiple, superimposed zones of alteration and mineralization responsible for the thicker parts. The thickness of the individual deposits thus varies from a few metres at the deposit edges to several tens of metres. Figures 10, 11 and 12 provide sectional views of the three main deposits, with Pits 5 and 6 shown together in Figure 12.

Two main types of mineralization have been noted:
* Gold-sulphide zones host the largest proportion of gold mineralization at Boroo. This type of mineralization is strongly altered quartz-sericite sulphidized zones that occur in thin, irregular veinlets, less often in breccia zones, and disseminated within the pervasive alteration. The intensity of sulphide mineralisation depends on primary host rock and intensity of alteration being stronger in the granites than metasediments. The main sulphide minerals are pyrite, arseonpyrite and rarely chalcopyrite, tetrahedrite and galena occur. It appears that the gold in this mineralization is relatively finegrained.
* Gold-quartz vein type. The second major gold bearing facies is massive, white quartz veins in which gold is commonly coarse-grained. The thickness of quartz veins varies from a few centimetres up to 3 m and appear as infill veins and veinlets in fractures within mostly metasediments. Veins contain small amount of sulphides and mostly coarse grained visible gold. This type of mineralization from a volume perspective is subordinate, however, can carry very high gold values of up to several hundred grams per tonne.

The two main types of mineralization described above have different gold grade distribution patterns. Gold content is high in quartz. Gold values are also higher where there is quartz stockwork mineralization associated with pyrite-arsenopyrite ore. Silver values are generally low and are not obviously correlated with gold. Silver values can be higher in the quartz veins in Pit 5 and Pit 6. Silver values, higher than 10 g/t, occur mostly in quartz veins in metasediments and are very variable. The sulphide content in both types of mineralization is relatively low, typically a few percent. Arsenic is highly anomalous (up to 21 500 g/t) but highly variable in the different pits; 103-112 g/t in Pit 2, 3158-3843 g/t in Pit 3 and more than 1% in the metasediments of Pit 5. A positive correlation with gold is restricted to gold values up to about 2 g/t.

It has long been recognized that the degree of oxidation is an important economic parameter at Boroo, as the gold in the fresh ore has a refractory component that limits the metallurgical recovery. Three facies of oxidation have been defined. All sulphides are completely or predominantly oxidized in the oxide zone, and additionally, the feldspars in the granitic rocks have been partly or completely altered to kaolin. In the transition zone, kaolinization of the feldspars is partial and the original sulphides survive in the core of oxidized grains. In the fresh zone, there is no discernable oxidation in the sulphide minerals.

The Boroo deposit was mined using conventionalb open pit mining methods. Mining was done with bench heights of five metres, with ore mined on half-benches for improved grade control in the flat lying ore. Three to four benches were typically under development at any given time. Blast hole drilling was carried out with two rotary-percussion drill rigs. Bulk explosives trucks blended ammonium nitrate with fuel oil or emulsion for wet holes as each hole was loaded. The principal mining equipment included two hydraulic excavators and ten 50-tonne haul trucks, and the waste rock mined was deposited on waste dumps immediately adjacent to the individual pits. Additional mining equipment included two front-end loaders for supplementary loading, ore handling and blending, two tracked dozers and one wheel dozer for the maintenance of waste dumps and benches and two graders for the maintenance of the roads and bench floors. Grade control was achieved by sampling of the blast hole cuttings. The blast hole assay data was determined at a laboratory in Ulaanbaatar and combined into an ore control model. The model was used to determine the boundaries for the various ore, stockpile and waste categories and to estimate the monthly pit production. Boundaries between material types were surveyed and digging supervised by grade control staff to ensure that ore and waste rock were separated correctly. Ore in seven blasthole samples was randomly selected for leach testing.

The Boroo flowsheet for ore processing is a standard layout that consists of crushing, grinding, gravity concentration, cyanide leaching and gold recovery in a CIL circuit. The mill was designed with a capacity to process 1.8 million tonnes of ore per year but the actual mill throughput is currently approximately 2.1 million tonnes per year or approximately 6,840 tonnes per day.

Mill recovery is dependent on the head grade and mineralization of the ore feed material. Recovery has steadily decreased as oxide and transitional ores have been depleted. When processing sulphide or fresh ore, mill recovery is typically in the range of 40% to 70%. A significant portion of the recovery has historically been achieved by gravity separation and it is expected that this would continue for the remaining ore at Boroo and will continue for the Gatsuurt ores.

A jaw crusher reduces the ore to 100% minus 20 centimetres. The crushed ore is fed directly to a semi-autogeneous (“SAG”) ........