Cosmo Gold Project — 2013 Technical Report

This 2013 technical report describes the Union Reefs carbon-in-leach processing facility used for Cosmo deposits ore, located approximately 62 kilometres from the project.

Report context

The Technical Report for the Crocodile Gold Corp. Cosmo Gold Project, dated July 2013, describes the processing methods for gold production from the Cosmo deposits. All Cosmo ore is processed at the Union Reefs processing facility, commissioned in 1994 by Acacia Resources Ltd and acquired by Crocodile Gold in 2010. The report presents the plant design, historical operating data to June 2013, and processing circuit details.

Processing route

Crushing circuit

Run of mine ore is broken to minus one metre by blasting in open pit operations. Larger rocks from blasting are broken to suitable size by rock breaker. Ore is transported by truck directly to the ROM feed bin or ROM stockpile. A three-stage crushing circuit processes ore at a rate of up to 2.5 million tonnes per annum. The circuit includes a primary jaw crusher operating in open circuit and secondary and tertiary cone crushers operating in closed circuit with a double deck banana screen. Crushing circuit product at a nominal size of 12 millimetres is conveyed to the grinding circuit via the Fine Ore Bin.

The Fine Ore Bin has a live capacity of 2,500 tonnes, providing a buffer of approximately seven to eight hours between crushing and grinding circuits. Ore is reclaimed via a slot feeder at a variable rate and conveyed to the grinding circuit. Because crushing circuit capacity exceeds milling circuit capacity, crushed ore is stockpiled and fed back into the grinding circuit using a front end loader into an emergency feed hopper and feeder arrangement.

Grinding and gravity circuit

Crushed ore is ground in a circuit consisting of two ANI single-stage rubber-lined ball mills operating in closed circuit with a nest of Warman cyclone classifiers. A proportion of ball mill discharge is directed to the gravity circuit incorporating four Knelson concentrators, two per mill. Knelson concentrator tailings report back to the mill discharge stream, while concentrated coarse gold is sent to the gold room for further processing.

The remainder of the mill discharge and the Knelson concentrator tailings are pumped to cyclone classifiers. Coarse underflow fraction reports back to the ball mill for further grinding. Fine overflow fraction (P80 75-106 micrometres) gravitates to a single high-rate thickener for density control before being pumped to the first of two leach tanks.

Leaching and carbon-in-leach circuit

Cyanide is added into two leach tanks to leach gold into solution before the material gravitates into the CIL circuit. High purity oxygen is added into the leach tanks from the Pressure Swing Absorption plant on site. The CIL circuit comprises seven leach/adsorption contactors gravity fed through open launders. All tanks are agitated and aerated and fitted with hollow shaft mechanical agitators. Barren slurry exits from the last CIL tank and gravitates to the residue treatment circuit. Activated carbon is pumped counter current to the process slurry to recover gold from solution, achieving the highest gold on carbon loading in CIL tank 1. Carbon from tank 1 is pumped to the elution circuit. Tailings slurry is pumped to the Crosscourse pit tailings facility, with process water recycled back from the Crosscourse pit.

Elution and gold recovery

Loaded carbon recovered from the CIL circuit is screened to remove pulp and subjected to a desorption stage (split AARL, 4 tonne capacity) to remove gold as an auriferous caustic-cyanide solution from which gold is recovered by electrowinning. Stripped carbon is reactivated in a vertical kiln and returned to the CIL circuit for reuse.

Gravity gold recovered in the Knelson concentrators is periodically discharged to a settling cone located in the gold room. The gold is then intensively leached in an Acacia reactor and the pregnant solution electrowon onto steel wool.

Electrowon gold and gravity-recovered gold are calcined in an electric oven and smelted separately in a gas-fired furnace into doré bullion. Bars are stamped for identification and dispatched via security service to AGR at Perth International Airport.

Plant control and infrastructure

The whole plant is controlled by a CITECT process control system over an Allen Bradley PLC. Water is supplied from various dams strategically located to maximise catchment of run-off drainage. A dam constructed on the nearby McKinlay River provides make-up water if required. Power is supplied under contract from the Northern Territory Government owned Power and Water Corporation. All processing consumables are provided by local suppliers based in Darwin or surrounding areas.

The laboratory is used for sample preparation and analysis of gold in solution only. All solids and carbon samples are assayed at an external laboratory. The laboratory is not set up for grade control samples or metallurgical testwork; these services are provided by external contract laboratories.

Proposed design configuration

The Union Reefs CIL treatment plant was designed by Kinhill and commissioned in December 1994 with a throughput capacity of 1.25 million tonnes per annum, including a gravity circuit to extract coarse gold. JR Engineering carried out an upgrade in 1998 involving installation of a tertiary crusher, second ball mill, and two additional leach tanks. The plant currently has a maximum capacity for 2.5 million tonnes per annum, depending on ore types, and is configured with three-stage crushing and two single-stage milling circuits.

Historical operating data

Prior to the plant being placed on care and maintenance in 2003, the milling rate at Union Reefs was typically 335 tonnes per hour at a P80 of 75 micrometres. Plant availability was typically 96-98 percent.

In August 2004, the Burnside JV partners purchased the Union Reefs Gold project for A$4 million on a walk-in, walk-out basis before being acquired by GBS Australia. In August 2006, GBS Australia re-commissioned the Union Reefs plant on the larger of the two mills while leaving the other smaller ball mill in a care and maintenance state. The first source of feed material was low grade stockpiles from Cosmo and an alluvial tailings deposit from the Union Reefs site. Following commissioning, ore was sourced from a blended mix of oxidised and fresh underground and open pit mines.

Crocodile Gold announced commercial production for the Union Reefs plant in June 2010. The period from December 2009 until June 2010 was commissioning. Production at Union Reefs has continued since commercial production was declared in 2010.

Total gold production from the commencement of operations at Union Reefs in 1994 to June 2003 was estimated by AngloGold to be 953,294 ounces from the treatment of approximately 21 million tonnes of ore grading an average 1.5 grams per tonne gold. Total production from November 2009 when Crocodile Gold commenced operations totalled 207,700 ounces from 4.7 million tonnes at an average grade of 1.4 grams per tonne gold as of December 2012.

Testwork basis

The laboratory at Union Reefs is not set up for metallurgical testwork. These services are provided by external contract laboratories. This process was under review for suitability to current ore types, with potential work required using fire assay, which could be completed in Pine Creek at NAL.

Key reported parameters

Technical qualifications

The report states that Union Reefs plant performance figures were achieved on a variety of ore types including oxide, sulphide and low-grade ores from Union Reefs, Brocks Creek, various open pits and historic stockpiles from places such as Moline and Glencoe. The plant demonstrated capability to process in excess of 2.8 million tonnes per annum of free milling ore from a blend of oxide, transition and fresh ore types. The laboratory on site is not set up for metallurgical testwork; these services are provided by external contract laboratories, and this process was under review for suitability to current ore types at the time of reporting.

Source: Technical Report, Crocodile Gold Corp., July 2013, Cosmo Gold Project, Sections 17 Recovery Methods.