The Porgera gold mine is a joint venture operation located at an altitude of 2,200-2,700 metres in the Enga Province in the highlands of Papua New Guinea. The Porgera JV operation is 95% owned and operated by a subsidiary of Barrick where both open-pit and underground mining methods are employed.
In 2009, the company’s share of production totaled approximately 551,000 ounces of gold at cash costs of $515 per ounce1. The Company’s share of proven and probable mineral reserves at December 31, 2009 was 7.7 million ounces of gold.
The property is composed of approximately 2,255 hectares located in Porgera Valley in the Enga Province, about 130 kilometres west of the established town of Mount Hagen, 600 kilometres northwest of Port Moresby, and about 680 kilometres by road from the coastal port of Lae from where all materials are freighted.
Gold mineralization occurs in the Porgera intrusive complex, around and within the margins of intrusive breccia bodies associated with the intrusives. The gold is mostly submicroscopic and associated with disseminated pyrite.
Mineralisation occurs along the margins of the Porgera intrusive bodies. The Porgera Zone VII orebody is an epithermal style orebody hosted within thermally metamorphosed sediments of the cretaceous. The majority of gold occurs as submicroscopic gold associated with pyrite.
Mining & Operations
There are both open pit operation and underground operation at the mine. The ore is milled and then processed through a series of stages. Stage 1 consists of a concentrator and leach/CIP circuit which produces a gravity concentrate and a sulphide flotation concentrate which is leached to recover gold and silver (up to 70% gold recovery). The discharged residue was retained in a storage pond until the Stage 2 pressure oxidation circuit was completed in Oct/91. The sulphide flotation concentrate is now fed into the pressure oxidation circuit with the gold being released and then recovered by a cyanide leach and CIP circuit, followed by refining on site into dore. Stage 3 was placed into production in Sept 1992 and consists of a gyratory crusher, semi-autogenous mill, ball mill and expansion of the present concentrator.
The Porgera deposit is currently being mined using open-pit and underground mining methods. In 2006, mill feed, on a tonnage basis, was sourced 85% from open pit and stockpiled ore, and 15% from underground. Underground ore accounted for 30% of the contained gold in mill feed.
More recently with the discovery of the Eastern Zone, further reserves was added to Porgera and extended the life of mine.
Open pit mining is a typical hard rock operation utilizing 10 metre benches. The current mining fleet of 7 DML blast hole drills, 5 O&K RH200 hydraulic face shovels and 32 Cat 789 haul trucks, gives a nominal mining production capacity in the order of 62 million tonnes per annum. During 2005 the mining fleet will be reduced to 4 DML blast hole drills, 4 O&K RH200 hydraulic face shovels and 25 Cat 789 haul trucks. This will give a nominal mining production capacity in the order of 45 million tonnes per annum.
The underground production fleet consists of 1 Atlas Copco H322 rockbolter, 2 Atlas Copco H352 jumbos, 2 Tamrock 1006 production drills, 2 Elphinstone R2900 loaders, 1 Elphinstone 2800 loader, 2 Elphinstone 1700 loades, 4 Elphinstone AD45 haul trucks and 3 Eimco EJC430 haul trucks.
Process - Run-of-mine ore is crushed and ground, free gold is recovered in a gravity circuit and flotation is used to recover a sulphide concentrate. This is then oxidised using autoclaves, producing feed for conventional carbon-in-pulp cyanide leaching to recover the contained gold.
Low-grade stockpile and open pit ore are crushed using a gyratory crusher. Coarse ore is conveyed from the stockpile to two parallel SAG mills. The discharge is pumped to three clusters of cyclones in closed circuit with three ball mills. Coarse SAG mill discharge is crushed in two cone crushers and returned to the SAG feed. The slurry is then pumped to the flotation circuit. A Knelson concentrator gravity separation circuit has improved recovery of free gold prior to flotation.
Flotation concentrates, thickened to 50% solids, form the feed for the oxidation process. Autoclave feed is pumped through three closed stainless steel carbonate reaction tanks in series, the concentrate being mixed with recycled oxidised slurry. The autoclaves operate at 1,725kPa pressure and 197°C, producing feed for the leaching and CIP circuits. Combined leach and CIP recovery is 90-95% of the contained gold.