The orogenic deposit model for gold mineralization in the Guerrero Gold Belt (GGB) is considered to be associated with a Pacific Rim style of mineralization as described by Corbett (1998, 2009). GGB mineralization is related to a late Cretaceous to Early Tertiary age skarn porphyry continuum emplaced during a 62 to 66 million year old intrusive event associated with Laramide Compressional Orogeny. Early stage, essentially barren calc-silicate skarn alteration associated with one or more intrusive phases is thought to have developed as a contact metamorphic aureole surrounding hydrated intrusive bodies.

Gold deposition at Ana Paula tends to occur both contemporaneous with and post intrusion, exhibiting at least two mineralizing events. The earliest consists of Au-As-(Bi-Te) disseminated mineralization characterized by progressive mineralization over time through deposition of gold in breccias, stockworks, contact skarn (both endoskarn and exoskarn) and other replacement bodies.

The second mineralization event (Au-Ag-Pb-Zn Hg-Sb) perhaps related to the epithermal style of alteration may be a later hydrothermal phase of the earliest intrusive event (a retrograde overprint?) or may be younger.

The exact timing of gold deposition and the mechanism of deposition within the GGB and at Ana Paula are not yet fully understood and appears to vary among the known deposits, where each deposit shares important characteristics and differences. Intrusions at Ana Paula have been dated at 66.0 – 66.7Ma ± 0.7- 1.8Ma, Valencia, V.et al, (2008), which may also date the earliest onset on mineralization.

Results from Ana Paula suggest that the bulk of the gold deposition occurs with the dominant (earlier?) AuAs-(Bi-Te) mineralization, and is largely hosted in a northerly trending and westerly dipping corridor of intrusive rocks, at the contacts with sedimentary rocks and hornfels, and within important breccia bodies. Gold deposition within the high grade core of the deposit is structurally controlled, located at the intersection of at least two fault structures and the host stratigraphy. Both skarn type massive and disseminated sulphide (arsenopyrite + pyrite) replacements and some epithermal overprinting have occurred but the extent and relationship to the oldest intrusive rocks have not been studied in detail.

Economically significant gold deposits in the GGB are hosted within a variety of structural, lithological and/or geochemical traps and frequently occur in clusters about a northwesterly trend of intrusions of similar age and provenance which are defined by a co incident northwest trend of magnetic anomalies. The trend is known to exceed 55 km along strike and has become known the Guerrero Gold Belt.

Ana Paula is located along the northwesterly trend of the GGB where it straddles a boundary between two older tectonic sub-terranes; the Teloloapan volcanic-volcaniclastic arc assemblage to the west and the Platform sequence of thick carbonates (limestone and dolomite) overlain by younger marine deposits to the east. The stratigraphy of both sub terranes was deformed during the compressive Laramide orogeny and subsequently intruded by an early Tertiary, ±62-66 million years, calc alkali porphyritic magmatic event associated with an essentially barren skarn and hornfels contact metamorphism. This intrusive event is currently thought to be associated with the timing of mineralization responsible for the gold deposits and showings of the GGB. At least two and possibly three mineralizing events are observed at the scale of the property, but the relationships and timing of these events is not currently known.

Mineralization at Ana Paula occurred during at least two different events or stages. The first event is characterised by gold-arsenic (bismuth-tellurium), (or Au-As-(Bi-Te)), where mineralization is associated with intermediate intrusions emplaced into limestone. The second event locally cuts the first, and is characterized by gold-silver-lead-zinc-mercury antimony (or “Au-Ag-Pb-Zn-Hg-Sb”), containing locally zoned coarse sphalerite. There are also various quartz-calcite veins with epithermal textures, but the relative timing of these veins remains unclear.

Mineralization is hosted in both the sediment intrusive domain and the intrusive domain.

Open pit mining was selected as the method to examine the development of the Ana Paula Project at this time. This is based on the size of the resource, tenor of the grade, grade distribution and topography.

The deposit will be a conventional, open pit, truck-and-shovel operation. A mill feed of approximately 5,000 t/d is planned over an approximate 8-year mine life. There will be pre-strip material in Year -1, with a full production rampup in year 1.

Pit phase designs were developed for Ana Paula using the three pit optimization shells selected earlier.

Equipment sizing for ramps and working benches is based on the use of 63 t rigid frame trucks. The sizing of the ramp is actually sized for the smaller capacity 56 t rigid frame units, as they are slightly wider than the 63 t rigid frame versions. The operating width used for the truck is 5.7 m. This means that single lane access is 17.8 m (2x operating width plus berm and ditch) and double lane widths are 23.5 m (3x operating width plus berm and ditch). Ramp gradients are 10% in the pit for uphill gradients and 8% downhill on the dump access roads. Working benches were designed for 35 to 40 m minimum on pushbacks.

Ana Paula is designed with three phases. The first phase is a starter phase designed to provide early higher grade material for the plant and minimize strip ratio. The second phase expands on the first targeting the larger portion of the ore body. The final phase requires a significant push back from the upper elevations due to local topography.

The Ana Paula processing facility will recover gold and silver by gravity concentration, flotation, oxidation of flotation concentrate and cyanidation of the oxidized concentrate by the carbon-in-leach process. The mill is designed at a nominal capacity of 5,000 t/d at 92% availability. Gold and silver adsorbed on activated carbon are desorbed into solution and then recovered by electrowinning. The recovered metals are smelted into doré bullions, which are the final product of the operations.

ROM ore will be transported by 56-tonne haul trucks from the mine to the primary crusher and dumped into the primary crusher dump hopper with an approximate 120-tonne live capacity (2 truckloads) through a stationary grizzly (opening: 800 mm).

The crushed ore is discharged to the transfer conveyor, which is also the stacking conveyor feeding the coarse-ore stockpile. A self cleaning magnet followed by a metal detector are provided to remove any tramp steel before stockpiling.

The coarse ore stockpile has a live capacity of 10,500 tonnes and a total capacity of 50,650 tonnes. The live capacity is equivalent to 2 days of SAG mill feed at the nominal capacity.

The grinding circuit for the Ana Paula Project consists of a conventional SAG-mill, ball-mill, pebble-crushing system, commonly referred to by the acronym SABC. The grinding line comprises one SAG mill, a pebble wash screen, one ball mill, one cyclone cluster, and a pebble crusher. The SAG mill is in a closed circuit with the screen and pebble crushing. The ball mill is in a closed circuit with the hydrocyclone cluster.

Based on test work, approximately 40% of the gold is gravity recoverable. In the current design, 36% of the circulating load gets treated in the gravity concentration and intensive cyanidation section of the plant. This corresponds to approximately 20% of the gold being recovered by the gravity circuit.

The gravity concentrate is fed to an intensive cyanidation package, a Consep Acacia CS200 or equivalent package, through a reactor feed tank. The pregnant solution is pumped to the electrowinning cells that mainly process pregnant solution from the carbon-in-leach (CIL) process included in this plant. Tailing from the intensive cyanidation reactor are pumped to the CIL process.

Sulfides in the ore will be floated at the ore’s natural pH using potassium amyl-xanthate (PAX) as collector, AERO 3418A as promoter, copper sulfate as activator, and F131A as frother.

Flotation of sulfides will be accomplished by single-stage rougher flotation. Cyclone overflow is first sent to a 41.2 m3 conditioning tank, then to a bank of six 70 m3 tank flotation cells. Each flotation cell mechanism is driven by a 93 kW (125 hp) motor through a gear reducer. Flotation air will be supplied by a 70 kW (94 hp) blower, which can deliver 95 Nm3/min of air.

Flotation concentrates will advance to the concentrate thickener and then to the regrind mill.

The tailing will be pumped to one flotation tailing thickener (28 m diameter high-rate thickener) to be thickened to 55% solids, in preparation for pumping to the tailing storage facility.

Concentrate from the rougher flotation circuit will be dewatered in the 10.5 m diameter high rate thickener to a pulp density of 55% solids. Flocculant will be added to the thickener feed to aid in settling. The withdrawal rate of settled solids will be controlled by one of two underflow pump to maintain either thickener underflow density or thickener solids loading. Each pump will be driven by a 30 kW (40 hp) motor on variable frequency controller to deliver a nominal maximum of 65 m3/h. Underflow from the concentrate thickener will be pumped using variable speed horizontal centrifugal slurry pumps to the regrind mill feed box.

Atmospheric oxidation (AOX) of the sulfide concentrate will be conducted in five agitated tanks. Each tank is 9 meters in diameter and 10 meters high (operating volume of 608 m3), made of 2205 duplex stainless steel. Slurry is fed to each tank through a downcomer and overflows to the next tank or a pump box at the end of the series. Each agitator is powered by a 56 kW (75 hp) motor through a gear reducer. Oxygen will be injected into each tank through finebubble spargers.

The oxidized slurry flows to two neutralization tanks (6 m diameter, 7 m high, 185 m3 operating capacity) where lime is added to increase the pH to 10 to 10.5. The neutralized slurry is then pumped to a pre-leach thickener (10.5 m diameter) to increase the pulp density to 55% solids. Once thickened, slurry is pumped to the carbon-in-leach feed tank where it combines dilution water, sodium cyanide reagent feed, and other process streams, into the first CIL tank.

Cyanide leaching is achieved in six CIL tanks (9.8 m diameter, 9.8 m high, 696 m3 operating capacity) each equipped with 30 kW (40 hp) agitators with two narrow-blade hydrofoil impellers. The tanks are built with epoxy coated mild steel. Air is delivered by a pipe under an inverted cone located directly below the agitator.

Based on leaching test results, a residence time of 48 hours is sufficient to achieve the target recovery for both gold and silver. After leaching, loaded activated carbon is sent to the carbon plant for stripping and electrowinning.

Once the loaded carbon is screened from the leached slurry, it is transported to the carbon handling plant. Loaded carbon is first acid washed with a dilute solution of hydrochloric acid to remove scale from the carbon, rinsed, and then pumped to the carbon stripping vessel. The carbon strip vessel is a pressure vessel, with a capacity to strip 5 tonnes of carbon per batch. The stripping process follows the pressure Zadra procedure developed by the US Bureau of Mines. It involves contacting a hot solution of cyanide and caustic (0.15 % cyanide, 1.25 % caustic) at a rate of 2 bed volumes per hour. The solution is introduced at the bottom of the carbon bed and overflows at the top of the vessel through one of more cylindrical Johnson screens. The solution is preheated to 135oC by heat exchangers. Because of the elevated temperature, the strip vessel is kept at about 550 kPa to prevent boiling.

During stripping, gold and silver adsorbed on activated carbon desorbs into the strip solution. This loaded strip solution is then sent to electrowinning cells through a heat exchanger. In the electrowinning cells, gold and silver are deposited by electrolysis to a stainless-steel cathode. The anode is typically a stainless-steel wire mesh.

When enough is deposited on the cathodes, gold and silver will be pressure washed off the cathodes and collected as sludge at the bottom of the electrowinning cells. The sludge is discharged to a tank and filtered through a plateand-frame filter press.

The filtered residue is finally dried in retorts to remove and collect any mercury, and smelted in a tilting furnace. Metallic gold and silver melt is then poured into bullion molds to produce the final product of the operations – doré bullions.