The geology in the Mara Rosa District is principally delineated by three northeast striking, moderately to steeply northwest dipping belts of metamorphosed volcano-sedimentary and associated intrusive rocks. These belts, referred to as the Western, Central, and Eastern Belts, are separated by broad zones of tonalitic orthogneiss.

The Eastern Belt is bounded to the southeast by the Rio dos Bois fault, which also defines the southeastern limit of the GMA.

Amarillo’s land position within the Mara Rosa District primarily covers the Eastern Belt greenstone assemblage with some coverage of the Western and Central belts as well. The Eastern Belt, has a maximum thickness of 6 km, generally strikes to the northeast and dips moderately to steeply to the northwest. Surface topography is characterised by moderate relief and locally dissected drainages that follow lithologic or structural weaknesses. Depth to fresh bedrock is generally shallow, ranging from 0 to 15 m. The upper portion of the weathered profile consists of clay-rich latosol and saprolite derived from the underlying bedrock.

Rocks of the Eastern Belt are locally intruded by quartz-feldspar-muscovite and biotite granitic rocks and associated aplite and pegmatite dykes, small stocks and dykes of hornblende, biotite and magnetite diorite, and, in its north-central portion, a large body of hornblende-plagioclase gabbro. All units exhibit varying degrees of foliation that typically range from weak to moderate, and generally intensify along sheared contacts. The tonalitic orthogneiss that separates the Eastern and Central Belts is composed of coarse-grained plagioclase, hornblende, and biotite with localised patches of biotite schist near its contact with the Eastern Belt.

Structurally the Eastern Belt is dominated by well-developed, penetrative foliation that strikes 30° to 50° and dips 40° to 70° north-west – an orientation subparallel to stratigraphy. Major structural systems include 50° to 65° striking shears and thrusts and associated drag folds. Shears are most commonly developed along zones of elastic disparity such as lithologic contacts. Shear sense is typically reverse-dextral oblique although a sinistral sense is locally observed. A second set of structures consist of late cross cutting north-west to east-northeast striking brittle faults and fractures that locally offset stratigraphy in apparent dextral strike-slip sense.

The geophysical, geological and geochemical data available demonstrate that the Posse Deposit occurs within a 50 km long shear zone with potassium alteration and lower order gold-copper- molybdenum mineralization. The Posse Deposit has a metamorphosed granodiorite traditionally called a grey gneiss or “Biotite gneiss” in the hanging wall of the fault and amphibolite, “greenstone” in the footwall. Shearing and hydrothermal alteration, of the meta granodiorite has resulted in the formation of mylonitic zones that form a distinct lithologic unit, a quartz- feldspar-mica schist, known as the Posse Schist that is characteristic of the Posse ore zone. This unit has been identified in several other areas including the Posse footwall and on strike extensions of the Posse Ore Zone to the northeast. Shearing is most intense in the footwall. It is speculated that the rheological contrast between the hanging wall and footwall rock types captured the regional thrust (movement west to east) for a 2 km segment of the shear. It is also possible that the chemical contrast between the hanging wall and footwall rocks may have aided in focusing mineralizing fluids. Observations from drill core suggest that an earlier potassic event with quartz veining, chalcopyrite, molybdenum, biotite and K-feldspar was followed by a later phyllic (sericite) event with pyrite, iron-telluride, and gold. Gold occurs as native gold and also with telluride and pyrite.

In general, mineralization at Posse is developed along a 050° to 065° striking fault zone. Mineralization tends to be strongest within mylonitic zones that follow more northerly striking (approximately 030° to 050°) shear strands and dilatant jogs that obliquely transect the contact between the hanging wall and footwall rocks.

The mineralization envelope at Posse is about 30 m thick and over 1 km long. It has a mylonitic appearance that is most noticeable adjacent to the footwall where shearing is the most intense. Higher intensity of shearing is associated with increased sulphide mineralization (up to about 4%), and a slight increase in metamorphic grade from greenschist to high greenschist facies in the hanging wall through to high greenschist/low amphibolite facies in the footwall (biotite flakes and garnet alteration). Higher gold values are associated with increasing intensity of shearing and higher levels of silicification and sulphide mineralization.

The Posse Gold Project is based on a mining concept that uses conventional drill, blast, load and haul techniques for all mining areas and rock types. One hundred per cent of the fresh rock and 30% of the saprolite will be blasted and loaded with excavators into 8x4 on-road trucks, and hauled to final destinations, i.e., primary crusher, low grade stockpiles or waste dumps. Direct mining will be applied to soft material such as soil and fill materials.

Specifically, primary mining will be undertaken by 74-t hydraulic excavators coupled with 45-t heavy tipper trucks.

A 15-month pre-stripping phase, between April 2021 and June 2022, was planned to ensure an initial supply of ore. During this stage the pit will require dewatering of the existing pit lakes. Water will be removed using a pumping system and directed to the planned water reservoir.

The ore and ore/waste contact materials will be mined in 5-m high benches for selectivity purposes, while double benches of 10-m high will be adopted for waste where there is no risk of dilution or ore loss.

The mining method will generate variable quantities of low grade that will require the use of stockpiles. Front-end loaders (FELs) will provide RoM feed and stockpile re-handling.

The mined waste will be distributed into six waste dumps.

The mined materials will be transported along roads cut through the mining area to give a suitable gradient. Double-lane haul roads were designed with 13-m width. Exceptionally, 10-m wide roads were designed to reach the pit bottom. The maximum gradient used is 10%.

A loss of trafficability on haulage routes is anticipated to occur on a seasonal basis during the rainy season. Management plans and risk reduction strategies should be developed which recognize this potential and detail procedures to recover haul roads and mining areas as soon as practicable.

Predominantly, the primary crusher will be directly fed by trucks and, occasionally, by a FEL. The ore stockpiled will feed the plant through a combination of FELs and trucks.

Grade control will be performed via drilling, sampling and assaying potential ore material within the pit boundaries.

The mine will operate 365 days, 24 hours in 3 shifts. The base case for the Project is a contractor operation.

Crushing and Stockpile
ROM ore is discharged to the ROM ore hopper by 45 t dump trucks. The ROM ore hopper incorporates a horizontal static grizzly with 800 mm x 800 mm apertures to screen out lump oversize. The static grizzly oversize is reclaimed by a front-end loader and stockpiled for rock breaking. The static grizzly undersize discharges into the ROM ore hopper. Ore is reclaimed by a vibrating grizzly feeder.

The oversize from the vibrating grizzly enters the 540 t/h (design) Metso C140 1400 x 1070 mm primary jaw crusher powered by a 200 kW motor. The jaw crusher is operated with a closed size setting (CSS) of 120 mm. The vibrating grizzly undersize and jaw crusher product combine and discharge onto the primary crusher transfer conveyor. A tramp magnet removes tramp steel from the primary crushed ore as it transfers by conveyor to the primary classification screen.

The primary classification screen is a double-deck screen, with 70 mm and 13 mm apertures and it operates in open circuit with the secondary crusher. Screen oversize from both decks feeds the secondary crusher, while the undersize is transferred to the stockpile via conveyor, along with the undersize of the secondary screens.

Screen oversize from the primary classification screen feeds the secondary crusher directly. The secondary crusher is Metso HP5 cone crusher with a design throughput of 377 t/h at a CSS of 40 mm with a 370 kW motor. This crusher is in the secondary / tertiary crushing building, in the crushing area. The fine ore product with an 80% passing size (P80) of 47 mm is conveyed to secondary screening for classification, combined with the product from the tertiary crushers. A weightometer measures this recirculating load.

This product is fed to two double screen secondary classification screens, with 22 mm and 12 mm apertures. These screens operate in closed circuit with the tertiary crushers, with one dedicated to each crusher. Screen oversize from both decks feeds the tertiary crusher silos, while the undersize is transferred to the stockpile via conveyor, along with the undersize of the primary screens.

The tertiary crushers are two 200 t/h (design) Metso HP5 cone crushers with CSS of 14 mm and 370 kW motors. They are in the secondary / tertiary crushing building. A surge bin with ten minutes’ retention time, located above each crusher, ensures choke feed conditions. A belt feeder extracts ore from each surge bin to feed each tertiary crusher. The fine ore product of 80% passing size (P80) of 13 mm is conveyed to secondary screening for classification, combined with the product from the secondary crushers, as described previously.

The fine ore stockpile has a live capacity of 1,208 t, providing 4 hours of mill feed. Three variable speed reclaim pan feeders provide three live draw-down pockets. The feeders provide duty/duty mill feed service whereby any single one can handle nominal mill feed demand if the other units are offline.

Grinding and classification
The grinding circuit consists of one single pinion 5.5 m x 8.5 m ball mill followed by one single pinion 5.5 m x 8.5 m ball mill. Each ball mill operates in closed circuit with a hydrocyclone cluster, the first being a direct circuit, i.e. the new material feeds the mill directly and then goes to classification, and the second, reverse, i.e. the new material feeds the classification hydrocyclone cluster first and only the underflow feeds the mill. The final product from the grinding circuit (cyclone overflow) has a target P80 of 53 µm. The first ball mill is equipped with a single 5 MW low speed synchronous motor with liquid rheostat and will operate at 78% of critical speed. The second ball mill is equipped with a single 5 MW low speed synchronous motor with liquid rheostat and a variable speed drive (VSD) to operate between 60 and 80% of critical speed.

The reclaimed fine ore is conveyed by a ball mill feed conveyor from the three fine ore stockpile reclaim feeders under the stockpile to the primary ball mill feed chute. Ball mill slurry discharge overflows onto a rubber-lined trommel screen with trommel oversize discharging to a dumpster for regular collection and disposal by a loader. A weightometer, after the stockpile reclaim, measures the amount of fresh mill feed ore entering the primary ball mill. Quick lime is metered to the ball mill feed conveyor using a rotatory valve or screw conveyor. For both mills, process water is added at a controlled rate into the feed chutes to achieve a nominal pulp density of 68-70% solids at the mill discharge.

The primary ball mill discharge product is screened on the primary ball mill trommel with 10 x 45 mm aperture. Primary ball mill trommel oversize is collected in a bin while the undersize gravitates to the primary classification cyclone feed pumpbox where the slurry is diluted with process water and pumped with a duty/standby cyclone feed pump to the primary classification cyclone cluster. A density meter monitors and controls the amount of process water required to produce a target density to the cyclones. The cluster consists of two cyclones: one operating and one stand-by. The primary classification cyclones produce a ground overflow product with a of P80 of 210 µm.

Secondary ball mill slurry discharge overflows onto a rubber-lined trommel screen with trommel oversize discharging to a dumpster for regular collection and disposal by a loader. The trommel oversize is collected in a bin while the undersize gravitates to the secondary classification cyclone feed hopper where the slurry is diluted with process water and pumped with a duty/standby cyclone feed pump to the secondary classification cyclone cluster. A density meter monitors and is used to control the amount of process water required to produce a target density to the cyclones. The cluster consists of eight cyclones: five operating and three stand-by.

Both mill grinding media (steel balls) supplied in bags, which are unloaded into ball kibbles, one dedicated kibble for each mill, through bag breakers. Each ball kibble is lifted by a 3 t davit crane (also used for cyclone maintenance), discharging the balls into the mills.

Recovery Methods
The unit operations used to achieve plant throughput and metallurgical performance are well proven in the gold/silver processing industry. The flowsheet incorporates the following major process operations:
• three-stage crushing and stockpile
• two stage ball mill grinding and classification
• pre-leach thickening
• pre-oxidation and carbon-in-leach (CIL) adsorption
• desorption, regeneration and gold room
• tailings detoxification, filtration and disposal
• fresh and reclaim water supply
• reagent preparation and distribution.

The solids throughputs for the following parts of the plant are:
• crushing plant is 6,930 t/d or 439 t/h at 65% availability
• process plant is 6,930 t/d or 317 t/h at 90% availability
• tailings filtration plant is 6,930 t/d or 402 t/h at 90% availability

Trash screen undersize is thickened from 28% to 51% solids in a 29 m diameter high-rate pre-leach thickener i ........