Piaba is a structurally-controlled, tabular, orogenic gold deposit with a strike length of ~3.3 km, width of 10 – 50 m, and down-dip extent of at least 600 m, beyond which the deposit is sparsely explored. The deposit is hosted in the brittle-ductile Aurizona shear zone (ASZ) that is ENE-WSW striking and steeply north dipping to the NNW.

Within the Piaba deposit, the ASZ system is divided into ten shear zones and/or brittle faults, partially segmenting the deposit with limited offsets. The maximum offset observed is 100 m, whereas most offsets are in the order of 10 m. The most significant of these structures is the Pirocaua fault zone at the east-northeast end of the deposit. This brittle fault zone is up to 350 m wide and locally disrupts the Piaba gold zone (maximum offset 75 m). At the western end of Piaba on section 1900W there is a northwest striking fault, that truncates the gold zone at depth.

Mineralization is primarily hosted in the quartz diorite that is also described as a dike-like c. 2.0 Ga granophyric granodiorite (e.g. Freitas and Klein, 2013; Klein et al., 2015) but has also been interpreted as a cataclastic unit. Ore-related alteration includesstrong to intense sericite-carbonate-silica-sulphide alteration in the central part of the structure (i.e. in the quartz diorite) flanked by chlorite-dominant alteration in the foot and hanging walls. Gold is mostly hosted in thin, millimetre to centimetre-scale, and quartz -carbonate ± sulphide± tourmaline bearing shear veins. Native gold is rarely observed at wall rock-vein contacts. Sub-horizontal quartz-carbonate extensional veins commonly cut shear veins and can contain gold. Increased vein density and sulphide abundance are the best indicators of gold mineralization.

Fluid inclusion, stable isotope, and radiometric isotope data (Klein et al., 2015) indicate the Piabadeposit was formed from a reduced, low-salinity, aqueous-carbonic metamorphic fluid, with oredeposition occurring at 250–330 °C, 1.25 to 2.8 kbar (c. 4 – 9 km depth), and between 1.98 – 2.01 Ga.

- Piaba is hosted in quartz-diorite rocks with diorite in the hanging wall and chemical-carbonaceous metavolcano-sedimentary rocks in the footwall.

- The deposit displays intense weathering alteration with an average depth of oxidation at 60m.

- The contact of the footwall metavolcano-sedimentary rocks with the overlying quartz diorite is often marked by fractured zone filled by quartz,sulphide, and carbonate.

- The quartz diorite hosts the bulk of the auriferous zone at Piaba. The quartz diorite is limited in the lower portion by thin, ductile shear zones (shearing bands with graphite and quartz veins with thicknesses from 1 to 10m) or by faults with fracturing and intense brecciation.

- The hanging wall contact of the quartz diorite unit with the overlying diorite unit is gradational, but locally exhibits thin, ductile shear zones(1 to 10m), or even fault zones with intense fracturing and infill by carbonaceous-graphitic material. The diorite unit has mixed characteristics between volcanic and plutonic features.

- The entire package is cut by north or north-northwest trending andesitic dikes. The deposit trends east-northeast and dips steeply (80°) north-northwest.

- Shearzones are present in the hanging wall and footwall of the Piaba deposit. The shearzones are represented by decimetric to metric zones of increased foliation development.

- In the hanging wall of the Piaba deposit, this increase in foliation development often occurs in proximity with the hanging wall of both the quartz-diorite and the gold mineralization. A similar increase in foliation development can be observed through the gold zone and into the feldspar quartz diorite in the footwall.

- In the gold zone, brittle-ductile deformation evidenced by excessive fragmentation of the rocks, brecciation, intrusion of quartz-carbonate-sulphide veins, shear bands, infilling of the matrix with carbon material, chlorite, carbonate and sericite and mineral stretching, can be recognized in the core samples.

- Two ages of shearzones are present at Piaba, earlier auriferous chloritic shear zones that are reactivated, and shear zones cross cut by graphitic shear zones. Where reactivated by subsequent graphitic shear zones, these shear zones can be associated with additional auriferous quartz veining.

- The contact of the feldspar quartz diorite with the metavolcano-sedimentary rocks of the footwall generally appears as fractured and at times sheared, with coarse graphite and quartz-carbonate veining. In the metavolcano-sedimentary rocks in the footwall, another shear zone occurs, marked by a zone of increased foliation (1 to 12m wide). Millimetric and centimetric quartz-carbonate-sulphide veins parallel to the main shearzone, are displayed along the strike and dip of the deposit.

- Discrete northwest-trending graphitic shear zones and brittle faults appear to modify the geometry of the gold zone. Nine shear zones and/or brittle faults have been defined in the Piaba deposit. These shear zones and/or brittle faults partially segment the deposit, although offsets of the gold zone along them are limited. The maximum offset observed is 100 m, whereas most offsets are approximately 10m.

- The Piaba deposit is crosscut by the Pirocaua faultzone at the east-northeast end of the deposit. This brittle faultzone is up to 350m wide and locally disrupts the Piaba gold zone (maximum offset 75m).

- From a hydrogeological perspective, the mine has excess water during the wet season therefore, sedimentation control is required for all mine runoff and process water prior to discharge in the surrounding estuaries. The climate is tropical and often humid, with annual rainfalls of up to 3,000 mm. The rainy season occurs from mid-December to mid-July with the heaviest rains from February through May.

Longhole stoping was selected as the preferred mining method due to the fair/good orebody and country rock ground conditions, and the dip and thickness of the deposit.

The regular, steep dipping geometry of the vein indicates a long hole stoping method to be appropriate. The width of the economic veins typically varies from 1.6m to 15m and averages 7.2 m width with some pinching and swelling. The vertical sub level interval was limited to 23 m measured floor to floor. AGP assumed a minimum planned stoping width of 1.7 m, adjusting the planned development and stope grades to include the non-vein (planned dilution) material. Although the Piaba underground orebody is a relatively low-grade deposit, there are higher grade areas contained within that would warrant 100% extraction through the use of cemented backfill. For the purposes of this study, low-grade stopes were defined as stopes with a mill feed grade less than 2.6 g/t Au and high-grade stopes with a mill feed grade above 2.6 g/t Au. With this in mind, three variants of longhole mining techniques were ultimately selected:

- Longhole stoping with cemented backfill: (i.e. high-grade stopes with no rib pillars): Blast holes will be drilled downward from the drilling level to the mucking level, retreating from the vein extents of each particular mining level towards the central stope access of that mining level. Each stope would be 35m in length. After extraction, the empty stope will be filled with cemented rockfill and allowed to cure. The next stope to be extracted on the level ‘in retreat’ would be mined immediately adjacent to the previously backfill stope. Stoping would follow a ‘bottom-up’ sequence beginning on the lowest level of a mining area and progressing upwards to the top of the mining area, using the previous drill level of the stope below as the mucking level for the next level of stopes above. Approximately 60% of the deposit will be extracted using this method.

- Longhole stoping with uncemented backfill: (i.e. low-grade stopes, with rib pillars): Stopes would be drill and blasted in the same fashion as described above. Each stope would be 35m in length. After extraction, the empty stope will be filled with unconsolidated waste rock, and an unrecoverable rib pillar would be left to separate the backfilled stope from the next stope to be extracted on the level ‘in retreat’. Again, stoping would also follow a ‘bottom-up’ sequence. Approximately 38% of the deposit will be extracted using this method.

- Longhole blind up-hole retreat stoping: Blast holes will be drilled upwards from the mucking level to an economic cut off or to the limits of the open pit crown pillar, retreating from the vein extents towards the central access ramp. Each stope would be 35m in length, with the stope height being less than the normal level interval. The stopes would be left open (i.e. not backfilled) with an unrecoverable rib pillar used to separate adjacent stopes. The method will be employed only in those areas where an upper stope drilling level is not practical or economic to develop. Approximately 2% of the deposit will be extracted using this method.

Subdivision of the underground mine into separate mining areas provides the opportunity to optimise level spacing and location for each of the separate mining areas in future studies.

A stope development drift height of 4.5m was selected to suit medium-sized mobile production equipment. Thus, all production stopes were designed to be 18.5 vertical metres in height (23m less 4.5m). A minimum stope design width of 1.7m true width was applied.

Longhole stope outlines were designed in on 12m-spaced sections using the appropriate grade shell and vein wireframe as a guide. There are several areas where two or more (up to four) potentially economic veins are located in close proximity to each other on the same level. A minimum 5m waste interval was applied between adjacentstopes. Where the vein interval waslessthan 5m, multiple veins were included in the stope outlines if the resulting grade exceeded cut-off grade.

The process of wire framing stope outlines resulted in contiguous stope wireframes as long as 1,400 m in length on some levels. Overall, the underground mine had a total strike length in the order of 3.3 km in length. The long strike length required that the underground mine to be logically split into smaller strike length ‘mining areas’ that can be accessed and mined independent from each other, thereby increasing the available working places and allow for a relatively high overall production rate from the underground mine. Ultimately the underground mine was broken into eight separate mining areas.

Geotechnical guidance with respect to required pillars was incorporated into the stope design process in the following manner:
- Crown Pillar: The crown pillarseparatesthe open pit from the underground stoping operation. A 20m crown pillar thickness was designed below the open pit floor. The crown pillar is assumed to be unrecoverable.
- Rib Pillars: 5m wide rib pillars were designed to separate adjacent low-grade stopes. These are backfilled with unconsolidated waste rock and are therefore unrecoverable.
- Sill Pillars: In order to increase the number of working places within the underground mine, the more extensive mining areas were further divided into separate upper and lower mining sections. This allowsstoping activitiesin the upper and lower mining sectionsto occur concurrent to each other. The stopesimmediately below the sill pillar were extracted under the previously backfilled upperstopes. If extracted in this manner, this approach to sill pillar recovery will require the use of high strength backfill in the first level of upperstopesin order to allow safe extraction of the stopes below. Higherstope dilution and lowerstope recovery factors were assigned to sill pillar recovery.

In the Base Case, mill feed from the underground mine is first achieved in Q2 of Year3 from the start of underground mine development. The underground mine reaches a nominal steady-state production rate of 2,800 tonnes/day in Q4 of Year 4 and continues at that rate until near the end of life.

The Aurizona Mine was placed on care and maintenance status in late 2015 and had operated successfully for over 4 years prior to that. The process plant was subsequently upgraded during 2018-2019, mining resumed in 2019 and Aurizona achieved commercial production July 1, 2019. The process plant now consists of the following main processing facilities with a nominal processing rate of 8,000 t/d:

- primary crushing and associated material handling equipment;
- crushed ore surge bin, emergency stockpile, associated feed and reclaim systems;
- grinding circuit, including a SAG mill, ball mill, and associated pumping and material handling systems;
- a gravity circuit with intensive leach reactor, an electrowinning cell and associated equipment;
- cyanide Leach/CIP circuit and associated gold recovery and carbon handling circuits, including pre-leach thickening, leach and CIP tanks, acid wash and elution, carbon reactivation, gold electrowinning and smelting;< ........