PROFESSIONAL EDUCTOR DREDGING; PAYSTREAKS.

[RKC]

PROFESSIONAL EDUCTOR DREDGING; PAYSTREAKS.

By RKC

The river miners of earlier times had to learn by experience that gold values are always erratically distributed in auriferous rivers. These days however, the nature of gold deposition in rivers is well understood, with the richest concentrations of river gold referred to by modern day miners as the pay-streak (the zone which carries the profitable alluvium).

The width of a pay-streak can be affected by the method of mining. At one extreme the pay-streak can actually be regarded as wide as the river itself and bulk mining plants such as excavator-fed floating trommel plants and giant bucket-wheel suction dredges, can profitably work the lower reaches of rivers from bank to bank. As these plants can process hundreds of cubic meters of river gravel each day, even river wash with gold values of as low as a few grains per cubic meter can be profitable to mine.



Ten ounces of fine river gold dredged from an Australian waterway.

At the other extreme is the individual miner operating an eductor dredge, who, out of necessity, must mine selectively, and with a great deal of skill. Eductor dredging is never just a simple matter of vacuuming gravel from a river bed, as commonly believed. To be viable a dredger uses his dredge to process only the very richest gravel, and while doing so, making sure he leaves untouched the ground he considers contains marginal or poor values. A suction dredge miner refers to the richness of the river wash in terms of ounces produced each day; whereas, in contrast, the large scale bulk miners refer to an average weight of gold to each cubic meter of the total wash in a river.

Geological laws can be said to govern the formation of pay streaks, and generally gold deposition takes place wherever river velocity drops below its critical value for the size and distribution of the alluvium in transport. Nevertheless, in each individual river the actual mechanism of deposition can differ significantly, as gold deposition is complicated by a multitude of variables such as the form and texture of bedrock; bends in the river; the location of gorges; the position of islands, boulders and pot holes; the river gradient; depressions in bedrock; the severity of floods; and so on. However, while gold deposition is governed by certain geological laws and complicated by numerous variables, the most significant complicating factor in most rivers ‘these days’ that needs to be considered above all others, is the effect of any previous mining.



A 7-inch production dredge.

In a practical sense, what this means to modern day miners, is that a pay-streak can only be located by actually mining. And in this respect eductor dredging is unequalled in its efficiency; making eductor dredges the preferred method to mine rivers in their upper and middle reaches.

A skilled and experienced operator is absolutely essential for any eductor dredging operation to be a success and a skilled dredger is continually aware of the richness of the gravel being processed. Therefore by diligently using an eductor dredge to sink prospecting holes, a pay-streak can be located and its outer limits determined quiet accurately. The critical skill involved in knowing what to look for from a prospecting hole, comes, like any skill, only with experience.

Following the trend from North America, many professional New Zealand dredgers are now concentrating more on dredging pay-streaks comprised of flood gold in preference to hunting around for the single deposit, such as a crevice, or a patch of ground missed by the miners of earlier times. This change in emphasis has occurred primarily as a result of improvements in the design of the larger Production Dredges which combine swift and efficient processing, along with excellent recovery of the fine and flaky gold. The increasing use of eductor dredges by companies on mining claims, particularly in Papua New Guinea, Indonesia and New Zealand, has also contributed to this change in emphasis.



A 7-inch production dredge being used in a Queensland river.

Mining and exploration companies have not always given eductor dredging plants the consideration they deserve when selecting equipment for the prospecting and mining of rivers. And it has only been in recent times that the advantages of using modern eductor dredges have become more widely acknowledged.

During the 1980s, which could be called the eductor dredging industries development phase, the lack of interest in eductor dredges for production mining may well have been due to the poor performance of many dredges, and when recreational dredges were mistakenly claimed to be suitable for use in commercial applications, and were often sold with the erroneous claim made that they were suitable for commercial scale dredging.

These days there is an increasing use of commercial eductor dredges (Production Dredges), which can, apart from actually mining a river deposit, be used to accurately determine if a river has sufficient and payable values of a mineral (gold, platinum, tin, gemstones) to permit mining on a larger scale. During such prospecting, a dredger can also ascertain the ‘make-up’ of the alluvium in a river. And such information can be very useful when it comes to selecting the most suitable plant for larger scale mining.

Victoria’s Goulburn River for example, is a river that would be suited for mining by trommel plant … even though this river has already been subjected to intense mining by eductor dredging for a period of more than more than 10 years! The eductor dredging that was carried out in the Goulburn River during the 1980s left a great deal of ground with gold values payable for larger scale bulk mining. The eductor dredging that was carried out did not result in a river which has been “mined out” but actually had the result of establishing the presence of gold values payable for larger scale mining.

Mining companies can also utilize eductor dredges to thoroughly clean up crevices in hard bedrock unable to be mined by a mechanical excavator, or bucket dredge. There is considerable scope for the use of eductor dredges to exploit such bedrock deposits to achieve a maximum recovery of minerals.



A 7-inch production dredge.

For example, the trommel plants that mined New Zealand’s Shotover River during the 1980s and early 1990s did not excavate crevices, except where the bedrock was soft enough to be broken up by the excavator. The Shotover River plant operators experimented with bedrock clean-up in the early 1980s by using a 6-inch triple sluice eductor dredge. However, that experiment was not a success, most probably due to the selection of an unsuitable dredge and operator inexperience. And now, in recent years, a new suction dredging claim has been granted over much of the Shotover River so the river can be again mined … this time by eductor dredging.

The massive sized bucket dredges that mined many Australian rivers from the turn of the century up until the 1950s were also often incapable of mining irregular bedrock. In hard bedrock, and even occasionally in soft bedrock, the dredge buckets were unable to get down into narrow fissures or crevices; and to this day such crevices remain packed with rich virgin wash.



Alluvial gold from New Zealands famous Arrow river.

There days rivers are, as they have always been, an extremely rich source of minerals. Rivers such as the Buckland and Mitchell in Victoria, the Turon and Macquarie in New South Wales, the Palmer and Russell in North Queensland, the Buller and Arrow in New Zealand, and numerous other rivers in many different countries, await mining. The advantages of using modern eductor dredges include low capital investment and operating costs; selective mining is easily carried out; there is no other river mining method more thorough; indigenous miners can be trained as eductor dredge operators; there is a short lead time to commencement of production; there is no detrimental environmental impact arising from their use; no chemicals are used in the process; and these dredges have an ease of portability unmatched by any other commercial mining equipment.

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Copyright; all rights reserved.
This is an updated (March,2006) article that was first published in Australian Gold Gem and Treasure magazine


Regards,
Rob (RKC)