Boroo Gold LLC (“BGC”, or the “Company”) holds 100% ownership of two active gold surface mining projects, namely Boroo (or Boroo Gold) and Ulaanbulag and both are located in the Selenge province of Mongolia.
On August 1, 2024, Steppe Gold Ltd. announced the successful completion of the previously announced agreement between Steppe Gold and Boroo Gold LLC (Boroo Gold), whereby Steppe Gold has acquired all of the issued and outstanding common shares of Boroo Gold from an indirect, wholly-owned subsidiary of Boroo Pte Ltd. (“Boroo Singapore“).
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Summary:
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 an 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.
Geology of Boroo deposit
The geology of the Boroo area is dominated by the folded Haraa sediments a fairly monotonous sequence of flysch sediments consisting of siltstone, sandstone and greywacke. These rocks are of regional extent and are interpreted to be of Lower Palaeozoic age. Intrusive rocks of the Boroo Complex, of early Palaeozoic age (~520 to 450 Ma), have intruded the sediments.
Detailed drilling around the Boroo gold deposits shows that the contact between the intrusive and the sedimentary rocks is highly irregular, with sedimentary xenoliths floating in the intrusive rocks in the border zone. A significantly younger igneous event of probably late Palaeozoic age is restricted to narrow vertical and shallow dipping dykes and fissures of granitic to dioritic composition.
Much of the general area around the mine is covered by overburden that can reach several tens of metres in thickness and that consists of colluvium and loess, and minor alluvium deposited in head water drainages. The alluvial deposits can contain significant gold placer deposits. In addition, the colluvium deriving from Zone 3 also contains placer resources.
Oxidation has affected the rocks in the area to a depth of 40 m to 60 m. Oxidation is accompanied by kaolinization of the feldspar crystals in the granitic rocks, but, not having taken place under tropical conditions, has not progressed to the formation of a saprolite profile, with the rocks retaining most of their original strength even near surface.
Mineralization
The bulk mineable gold mineralisation at Boroo is hosted in a strongly quartz-sericite altered and sulphidised 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 northwest and trending northeast. 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 mineralisation decrease. The Boroo fault is variously altered and mineralised, and where these features are strongest, individual deposits are formed. These are termed, from north to south, Zone 2, 3, 4, 5 and 6. All of the deposits are elongated in a north-easterly direction, with a length to width ratio of about two to one. In Zone 2, 4, 5 and 6, mineralisation is controlled by Boroo fault and is in the footwall. But in Zone 3, there is low grade mineralisation in both hanging wall and footwall. Grade thickness contours show the same overall elongation probably caused more by the width than by the gold grade, with the multiple superimposed zones of alteration and mineralisation 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.
Two main types of mineralisation have been noted:
• Gold-sulphide zones host the largest proportion of gold mineralisation at Boroo. This type of mineralisation is strongly altered quartz-sericite sulphidised 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, arsenopyrite and rarely chalcopyrite, tetrahedrite and galena occur. It appears that the gold in this mineralisation is relatively fine grained.
• 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 mineralisation 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 mineralisation described above have different gold grade distribution patterns. Gold content is high in quartz. Gold values are also higher where there is quartz stockwork mineralisation 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 Zone 5 and Zone 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 mineralisation is relatively low, typically a few percent. Arsenic is highly anomalous (up to 21,500 g/t) but highly variable in the different zones; 103–112 g/t in Zone 2, 3,158–3,843 g/t in Zone 3 and more than 1% in the metasediments of Zone 5. A positive correlation with gold is restricted to gold values up to about 2 g/t.
It has long been recognised 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 oxidised 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 oxidised grains. In the fresh zone, there is no discernible oxidation in the sulphide minerals.