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 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. 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 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.
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.
Production
Since acquisition, Boroo restarted operations in 2019 since its care and maintenance mode in 2016. Heap Leach operation restarted in March 2019, followed by Boroo Mill operation restarted with throughput 5,500tpd in July 2020.
| Commodity | Units | 2021 | 2020 |
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Gold
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koz
| ....  | .... |
All production numbers are expressed as metal in doré.