The Buriticá deposits are classified as intermediate sulphidation epithermal using the terminology of Sillitoe and of Hedenquist (in Simmons et al, 2005). Corbett and Leach (1997) recognized a specific sub-class of low sulphidation epithermal deposits with high base metal contents and carbonatebearing gangue, which they named CBM association. Although such mineralization is perhaps better described as “base metal carbonate sulphide-rich”, and may occur over depth extents much larger than many low sulphidation epithermals, the Buriticá systems are referred to as the CBM association.

Mineralization at Buriticá is a porphyry related, carbonate base metal (CBM) gold vein/breccia system. High-grade precious metal mineralization in CBM systems may occur over substantial vertical intervals, to well in excess of a kilometre, from the porphyry level to below the shallow epithermal range. Compared to low sulphidation epithermal styles CBM mineralization is sulphide-rich, with abundant pyrite ± pyrrhotite + sphalerite + galena along with minor sulfosalts, chalcopyrite and quartz-carbonate gangue mineralogy. Gold in CBM systems may be free-milling or refractory. Mineralization in CBM systems typically comprises sheeted veins, stockworks and mineralogically similar breccias with some fracture-related dissemination in wallrocks.

CBM systems are widespread in circum-Pacific magmatic arcs, and include the supergiant Porgera deposits in Papua New Guinea (25 million ounces of gold, previous production plus remaining reserves and resources) and the Kelian deposits (7 million ounces (Moz) of gold produced + resources) in Indonesia. Gold mineralization at Porgera has been mined over a vertical range of 500 m but potentially economic mineralization extends over more than 1,000 m depth range.

CGI recognized the strong similarities of the Porgera and Buriticá mineralized systems and this gave CGI the confidence to drill extensively and deeply at Yaraguá and Veta Sur.

Precious metal mineralization in Yaraguá and Veta Sur appears to be related to two main depositional stages. Stage I is represented by banded base metal (iron, zinc and lead) sulphide-rich mineralization with variable amounts of quartz-carbonate gangue and bands. As well as in subparallel narrow vein arrays, Stage I mineralization also occurs in veined (dilational) breccias in places occupying substantial areas of both Yaraguá and Veta Sur, but at grades typically lower than those of the high-grade veins. CGI’s experience at the Yaraguá mine indicates that Stage I mineralization is non-refractory and recoverable by simple gravity and flotation circuits, flotation concentrates being cyanided, gold and silver then recovered by the Merrill Crowe process. Wall-rock alteration around Stage I veins comprises narrow phyllic assemblages ± K-feldspar. Stage I mineralization evidently overprints earlier potassic, phyllic and propylitic alteration.

Stage II mineralization is a texturally and chemically distinctive high-grade gold mineralization that locally cross-cuts and overprints Stage I mineralization as veins and breccia veins. Stage II mineralization is characterized by abundant free (and commonly visible) gold in siliceous and carbonate gangue, associated with arsenical pyrite and with low zinc and lead contents, relatively high arsenic and antimony but low bismuth contents. Stage II mineralization is, to date, largely known from the Veta Sur system in which this style of mineralization contributes to some very highgrade precious metal subzones, but has also been encountered in the Yaraguá system.

Three underground mining methods were selected for Buriticá; longhole open stoping (LHOS), cut & fill (C&F), and shrinkage stoping. Mining method selection was driven primarily by geomechanical rock quality, vein geometry, depth, proximity to old workings, stope margin and vein continuity. Unless geomechanical and geometry characteristics required either C&F or shrinkage, longhole mining was the preferred mining method due to higher productivities and lower mining costs compared to either C&F or shrinkage.

There are two vein systems that will be exploited in the Buriticá mine: Yaraguá and the Veta Sur.
The Yaraguá system generally strikes to the east, and has been drill intersected over 1,125 m along strike and 1,540 m vertically. There are several veins within the system that strike to the northwest, intersecting the east-striking veins. Vein strike lengths vary from 50 to 1,100 m and vein dip distances vary from 50 to 1,300 m. The Veta Sur system strikes to the Northeast, and has been drill intersected over 1,140 m along strike and 1,600 m vertically. Vein strike lengths vary from 70 to 1,000 m, and vein dip distances vary from 150 to 1,350 m.

Cemented backfill will be used to fill stope voids to provide confinement on the waste rock pillars in the hanging wall and footwall between parallel stopes spaced close together, and as the fill develops strength, to reduce lateral loads on these pillars. This is an important aspect when mining closely spaced sub-vertical veins so adjacent veins planned for mining are not sterilized due to potentially unstable waste rock pillars between sub-parallel stopes. The use of cemented backfill also enables secondary stopes along strike to be mined directly adjacent to the primary backfilled stopes without requiring a ribdip pillar.

Longhole Mining
LHOS will be used where rock quality and ground conditions allow and where vein thickness is relatively uniform. It is the highest productivity method selected for the production phase, but given many veins are relatively narrow, considerable preparation is required to maintain stope inventory.

LHOS is the least selective of the three methods when applied over long vertical distances due to drillhole deviation and vein geometry variations. To mitigate these effects, longhole drilling distances at Buriticá will be limited to 12 m.

According to final stope design, the longhole mining cycle will begin with blasting the drop raise to provide a free face for the first longhole round and initial empty volume for blasted swell. Production blasting will begin at the stope ends and retreat to the access.

Cut and Fil
C&F mining methods were selected for areas that have lower quality rock. C&F was selected for all stopes within 70 m of surface due to observed weathering profiles, which are associated with weaker ground conditions. In areas of documented historic mining, C&F was also selected because these areas are typically near surface and the selectivity allows safe and high recovery extraction. This method limits vertical stope wall exposure during extraction.

The C&F mining cycle begins with the pivot ramp driven downward from the footwall to access the bottom of the stope block at its midpoint along strike. Once the bottom sill is mined, production uppers (2 m) are drilled and blasted for the first cut, and then the ore is mucked and the stope backfilled with mine waste rock using a remote LHD. The back is then taken down in the pivot ramp to access the next lift. The stope is then supported with the drill jumbo or miners using jacklegs working from the waste fill. Once ground support is installed, mine geologists will map and sample the stope back as part of the grade control protocol. The next cut won’t be drilled until the limits of economic mineralization are marked on the back.

Shrinkage Stopin
Shrinkage stoping accounts for less than 2% of planned mill feed but is nonetheless an important option for mining methods selected for Buriticá and offers additional production flexibility. Shrinkage stopes are designed and planned in isolated areas of the mine where extensive level development is not justified or required. It will be the least productive of the three mining methods and will be used only where economically justified.

Stope development begins with sill cuts driven on vein at the top and bottom of the stope block along with conventional raises driven at both ends. Raises provide personnel and service access, as well as flow through ventilation. Extraction drifts and drawpoints are driven on the bottom level.

Production mining commences with drilling the full stope with uppers using conventional jackleg drills and stopers. After blasting, approximately 30% to 35% of ore blasted within the stope must be drawn down in a controlled fashion so enough broken ore remains in the stope as a work platform. The process of drawing down ore can result in an uneven working surface within the stope so slushers and shovels will be used to level the stope floor as ground support is installed on reentry.

Crushing
ROM mineralized material will be trucked from the underground mines by 30 and 40-tonne rear dump haul trucks to stockpiles to be blended as needed prior to feeding primary crusher. The ROM mineral stockpile will be designed to contain 14,000 t. A front-end loader (FEL) will feed a stationary grizzly over the crusher dump hopper. Alternatively the trucks may dump directly into the dump hopper. A mobile hydraulic rock breaker will be provided to break oversize ROM material at the stockpile, the stationary grizzly, and at the crusher dump pocket.

An apron feeder will discharge the scalped ROM material directly into a jaw crusher, where it will be reduced from a size of F100 = 357 mm to a product size of P80 = 150 mm. The jaw crusher product will discharge onto the crusher discharge conveyor. The crusher discharge conveyor will discharge to the covered coarse ore stockpile. A belt weigh scale mounted on the crusher discharge conveyor will measure the feed to the coarse mineralized material storage.

The primary crushing facility will be equipped with an air compressor for maintenance equipment such as air tools. A selfcleaning electric magnet will be installed over the discharge of the stockpile feed conveyor to remove tramp metal.

Grinding
The grinding circuit will process an average of 3000 t/d, at 91% availability, operating 24 hours per day, 365 days per year. A SAG mill will reduce crushed mineralized material from F80 = 150,000 µm to T80 2000 µm and a ball mill will further reduce the mineral to P80 = 74 µm.

A SAG mill measuring 5.8 m diameter and 4.2 m long (3.8 m effective grinding length), powered by a 2000 kW variable speed motor will perform primary grinding of the material in closed circuit with a vibrating screen. The undersize material will pass through the vibrating screen into a gravity feed box, where it will combine with ball mill discharge.

Oversize material will be transferred to a SAG mill oversize conveyor and will be crushed in pebble crusher and returned to the SAG mill. Ball chips will be removed prior to the crusher using a self cleaning electric magnet. Combined SAG and ball mill discharge slurry will be pumped using variable speed horizontal centrifugal slurry pumps to a splitter box ahead of the gravity circuit. Slurry will be split to feed two bowl concentrators each preceded by a scalping screen to remove oversize from the bowl concentrator feed. Scalping screen oversize will flow by gravity to the cyclone feed box. Gravity tailing will flow by gravity to the cyclone feed box.

Slurry will be pumped from the cyclone feed box to hydro-cyclones. The underflow of the hydro cyclones will flow by gravity to a ball mill measuring 4.6 m diameter and 7.5 m long (7.2 m effective grinding length), powered by a 2600 kW motor. The ball mill will discharge over a trommel screen. The trommel product will be washed by process solution sprays and ball chips will be rejected out the end of the trommel into a tote bin. Trommel undersize will be combined with SAG mill discharge screen undersize. Cyclone overflow (final grinding circuit product) is directed to the leach circuit.

The process design follows these steps: Crushing > Grinding > Gravity Concentration > Cyanide Leach > Counter-Current Decantation (CCD) > Merrill Crowe > On-site refining to doré bars.

Concentrate from the centrifugal gravity concentrators will feed a magnetic separator to remove grinding media and then to a cleaner gravity table. Concentrate from the cleaner table will be recycled and upgraded to produce a gravity concentrate that can be refined directly. Cleaner gravity tailing will be returned to the cyclone feed sump.

The hydro-cyclone overflow will flow by gravity to a trash screen for removal of tramp material. Trash screen oversize will discharge into a tote bin to be periodically removed for disposal. The trash screen undersize will report to the leach feed pump box. The leach circuit will consist of five agitated tanks. Cyanide solution may be added to the tanks as required to maintain desired free cyanide levels. Oxygen from the pressure swing adsorptio ........