History of alluvial gold mining in PNG started in 1873 when Captain Moresby reported minor amounts of gold in the hills surrounding what is now Port Moresby Harbour. The Port Moresby discovery was not worked until 1877 when first gold rush started involving 100 European miners, who mined the Laloki and other rivers inland from Port Moresby.
Mining effectively commenced in Papua New Guinea in 1888 with the discovery of payable gold in Sudest Island (near Misima Island).Morobe Goldfield was discovered in 1922, which continued to operate till the Second World War. It was a world class goldfield by the standards of the time, reaching a peak production of 8.5 tonnes/year of gold in 1942. The Bulolo Gold Dredging Company which operated up to eight dredges in the Bulolo River valley between 1932 and 1965, accounted for most of the production in an operation that was particularly remarkable for pioneering large scale air transport. With considerable ingenuity, the components of the 2 000 tonne dredges were airlifted in and assembled on site and the operation was la rgely supported by air transport as there was no road network in the country at that time.
Alluvial Mining Techniques
Present day alluvial mining techniques can be categorised into the following two main types:
- Small Scale Artisanal Mining
- Small to Medium Scale Mechanised Alluvial Mining.
Small Scale Artisanal Mining
Very small family orientated miners, which are also called Artisan Miners, are analogous to subsistence farmers in the agriculture sector. The main livelihood of these miners depend on the amount of gold they collect per year, sometimes only 0 to 10 grams which feeds and supports themselves and their families.
In 1996 the United Nations calculated that this small scale mining sector provided employment for in excess of six million people worldwide, equivalent to 20% of global mining industry employment. Assuming an average of four additional family members per worker, the total number of people dependent on small scale mining could exceed 30 million.
However, the most recent new ILO report calculates that the world’s small scale mining operations employ a total workforce of up to 13 million people and estimates that as many as 100 million depend on small scale mining for their livelihoods. This is broadly similar to the number dependent on the mechanised mining sector. The report says that small scale mining activity in 35African, Asian and Latin American countries has grown by an average of 20% over the past five years, and that growth seems set to continue at a similar rate in most countries surveyed. The small scale artisan miners in many developing countries employ very basic tools and equipment to conduct mining operations. In PNG, small operations consist of less capital intensive manual excavations of alluvial gold deposits found in streams/rivers, puddles and ancient river terraces.
Panning dish, pick and shovel are the main tools used. Wooden boxes, in most instances without riffles and recovery mats are used to conduct sluicing operations. The capital investment required in such operations would be US$50-100 or even less.
This method of mining has been inefficient. It leads to loss of very fine – micron size gold particles due to poor design and construction of sluice boxes. The coarse fraction of gold is recovered near the top of the box using either cloth or sometimes dry grass.
The major drawbacks of sluice box operating practices in PNG are: the use of gravel with wide size distribution as feed material and the use of insufficient or excessive wash water flow rates.
To mitigate these problems it is recommended that:
- the feed should be pre-screened to remove excessively large gravel. In situations where coarse gold is present, two sluice boxes should be used in parallel to treat the oversize and undersize material separately.
- optimal wash water flow rate be used so that the recovery of gold particles is maximised and gangue recovery is minimised.
Recently, new steel or aluminium metal sluices are being marketed utilising expanded metal riffles and are gaining wider acceptance.These rely on higher wash water velocities to function and result in significantly higher throughputs for the miner and consequently higher returns. Our Department in Lae fabricates these improved sluice boxes designed by Hancock and supplies to small miners at a very minimal cost on non-profit basis.
The total throughput achieved by these operations varies considerably from about 1 m3 per day for hand picked sluice boxes to 5 m3 /day for the more efficient true sluicing operations.
Many times these improved sluice boxes, which also require motor driven pumps for higher wash water velocities, are beyond the reach of small artisanal miners with the result that these improved techniques are used more frequently by the relatively well-off mechanised small miners.
Mechanised Small to Medium Mining
Mechanised mining techniques commonly involve the use of hydraulic excavators or wheeled loaders feeding gravel wash through a trommel with gold recovery being achieved by a range of techniques from simple sluices to sluices with hydraulic riffles through to complex jig and Knudsen bowl or Knelson concentrator systems. These plant configurations may be either fixed or mobile plants with skids or floating on pontoons.
These systems have throughputs of between 25 and 200 m3 per hour (typically in the range of 40 to 120 m3 per hour).
These methods are both capital intensive and mechanically more complex and therefore are used only by Nationally owned companies, which have access to large capital, ie Eddie Creek Mine near Wau.
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