The Chirano gold deposits can be described as epigenetic, mesothermal gold deposits, demonstrating a strong structural control and a brittle structural style. They are hosted by mafic volcanics and granite, ranging from stacked parallel veinlet systems to vein stockworks, breccias and cataclasites. The veinlets are dominated by quartz, with lesser ankerite, calcite, albite and traces of pyrite and hematite. The deposits show varying degrees of ankerite-albite-muscovite-pyrite alteration superimposed on earlier hematite alteration.

The deposits occur close to a major fault (the Chirano Shear), and is considered likely that any new deposits found will also be closely associated with faulting. In particular, individual deposits are often closely associated with small dextral jogs in the host structure. Although the currently known gold deposits are in granite, there are also strong gold anomalies in Birimian metasediments elsewhere within the mine area, which require concerted follow-up exploration.

The deposits range in strike length from 150 meters to 700 meters, and range in thickness from a few meters over 70 meters. They vary from rather tabular (Obra, Sariehu, Suraw) to more pipe-like (Tano and Akoti North) morphologies. The longer, the more tabular bodies generally comprise at least two shorter lenticular shoots, such as the Obra main and north lenses. These lenses may be separated by a small dextral jog such as those at Obra and Sariehu. Within the Paboase Bulge there are several parallel lodes, whereas elsewhere along the mineralized horizon there is commonly only a single zone is evident.

Most of the deposits dip steeply to the west, however shallow west, vertical and steep east dips occur locally. The mineralization plunges either directly down dip or steeply northwards. The mineralization demonstrates excellent continuity, there being no known gaps due to oblique faults or dykes.

Unusually flat dips have been noted in short sections of the lode horizon at Mamnao Central (39,850 N to 39,975 N), Obra South (36,850 N to 36,950 N), Sariehu (38,400 N), and Akoti South (34,635 N), however these areas do not demonstrate any spatial relationship with thicker or higher grade mineralized intervals.

In some of the deposits, thicker zones of gold mineralization appear to have formed where nearby parallel lodes have coalesced. Such deposits have a single thick zone in the core of the deposit, which splits into two or three thinner zones along strike. Tano is the best example of this type of deposit geometry.

Deformation.
The deposits comprise fractured, veined, altered and slightly pyritic mafic volcanics and granite. Within each deposit there is generally a positive correlation between the intensity of fracturing and brecciation and intensity of gold mineralization, however the degree of fracturing varies greatly between the deposits.

The gold mineralization at Obra is generally hosted in severely deformed and brecciated granite (cataclasite), whereas much of the Tano lode is less fractured and can be considered more of a stockwork or vein swarm.

At Obra there is clear evidence that brecciation, veining and alteration have been prolonged, or the result of repeated episodes of deformation, and diamond drill core shows a complex array of small scale structures that often appear ambiguous or contradictory. For example, some rock fragments in the Obra cataclasite contain veins that predate the brecciation and later veins cut through the breccia. Fragments of altered and unaltered rock are juxtaposed in some parts of the breccia, implying alteration before deformation, however adjacent fragments show alteration that overprints the brecciation. In addition, stylolites have been observed to cut across the breccia and early veins, but are cut by later veining.

Veining.
All the gold deposits at Chirano contain numerous quartz and ankerite veinlets and there is generally a positive correlation between intensity of veining and elevated tenor of gold mineralization. The majority of observed veining is oriented parallel to the dip of the overall mineralized zone horizon, however veins have also been noted to dip more shallowly to the west, and some deposits have a sub-horizontal vein set in addition to the dominant west-dipping vein set.

The shallowly west-dipping veins have been interpreted to result from 'west-block-up' shearing in the mineralized zone. The veinlets are mostly a few millimeters to a few centimeters thick. More massive vein quartz (sometimes meters thick) occurs locally, usually on the eastern side of a deposit close to a footwall shear and usually carries only low gold grades. This feature has been observed at Sariehu and Tano.

The quartz veins vary in style from early veins (which may be recrystallized, folded, boudinaged, corroded by pressure solution, offset by micro faults or truncated at the edges of clasts) to late quartz veins (which may be undeformed and exhibit evidence of internal zonation such as carbonate crystals lining the vein selvedge). Some veins contain pyrite replacing hematite in the adjacent rock (sulphidation). At Obra the ankerite veins tend to comprise irregular networks, and may have formed early in the paragenetic history.

Sulphide Development.
The deposits contain trace amounts of pyrite, typically 1% or 2% by volume, rarely exceeding 5% by volume. It is noted that the surrounding barren rocks contain lower levels of disseminated sulphides than the mineralized horizon. Mineralogical studies indicate that the pyrite has a very high gold content.

The pyrite may be very fine grained and disseminated throughout the rock mass, as at Obra, or occur as cubic euhedra a millimeter or two in diameter (and rarely larger at Tano and Sariehu). Pyrite may also occur as rare aggregates to a centimeter in size, and has also been observed to form concentrations along stylolites. The quartz-carbonate veins can also contain pyrite, and pyrite has also been noted as an alteration selvedge to the quartz-carbonate veins. Pyrite also occurs disseminated through the altered host to veining.

Processing capacity is 3.5 Mt/a using a conventional three stage crushing circuit, followed by primary and secondary ball mills for fine grinding. After grinding and 24 hours of cyanide leaching, a CIL circuit extracts gold in solution to activated carbon. A conventional carbon elution and electro-winning circuit recovers gold which is then smelted to gold doré for shipment to international gold refiners. Gold recovery using the above described process istypically 91 to 92%.

Ore is transported to the plant by truck and deposited onto the Run-Of-Mine pad (ROM). Crushing of the ore takes place in three stages; a primary jaw crusher that reduces ore to less than 150mm; a secondary cone crusher and two tertiary cone crushers. Secondary crushed ore is conveyed to a screening section before two tertiary stage crushers.

Crushed ore is transferred from the fine ore bin at a controlled rate to the primary ball mill (a converted SAG mill) by means of a conveyor belt. Water is added to the mill feed to permit wet grinding and slurry pumping. Ore passes through the mill, is reduced in size and is pumped to hydrocyclones for classification. Ore that is less than 106 microns in size exits the grinding circuit together with some process solution and passes to the leaching circuit.

The leaching circuit is a carbon-in-leach circuit. The slurry gravitates across a trash screen into the pre leach tank and then to the first of nine agitated leaching tanks. Dilute sodium cyanide solution and lime are added to start the chemical dissolution of gold from the ore. Compressed air is pumped into each tank to accelerate the dissolution. Activated carbon granules adsorb the dissolved gold from solution. Carbon that has a high gold content is termed “loaded” and is pumped from time to time to the elution circuit for recovery of the gold.

To recover gold from the carbon, batches of carbon are subject to a high pressure and temperature process called elution. A hot caustic solution is used to remove the gold from the carbon and into solution. The gold is recovered from the caustic solution by electrolysis onto mild steel wire wool cathodes in an electro-winning tank. The loaded wool is removed regularly, mixed with fluxing chemicals and smelted on site to produce bullion bars of approximately 75% purity. The bullion is shipped to a refinery for further refining and sale. After gold recovery the “barren” carbon is heated to reactivate it, and returned to the circuit.