GOLD DREDGES.

Ashburton Guardian, Volume 60, Issue 196, 28 May 1940, Page 3

GOLD DREDGES.

EVOLUTION IN NEW ZEALAND. HUGE MODERN MACHINES. One of the .Centennial postage stamps depicts an old-time miner mining gold with his shovel and pan, and contrasting with his method is a modern gold dredge shown on the other half ol the stamp. The intervening history was dealt with by Mr W. J. Synie in a paper discussed by the conference of the New Zealand Institute of Engineers. ,

He said that although it was a. fact that the first gold dredges in the world were developed and used on the Central Otago rivers only a few years after the discovery of alluvial gold at Gabriel’s Gully in 1861, the subject had' hardly, received the attention from the technical Press which one might have expected. The annual reports of the Mines Department, however, gave an indication of the development of the gold-dredging industry through the years. The Otago miners of the sixties were soon faced with the problem of lifting rich gold-bear-ing gravels from river beds, and their first efforts produced the “spoon” dredge. The spoon consisting of a rawhide bag with iron cutting ring, was attached to the end of a pole by means of which it could be raised to discharge into the rockers on deck after dragging along the riyer bed. The next important development was the “current wheel” dredge, equipped with a bucket.line driven bv gearing from current-actuated paddl©rwheels on each side of the pontoon. These paddle-wheels were also provided with buckets to raise water for sluicing purposes. Dredges of this type could not work a all in slack water, and since the accumulation of “tailings” frequently diverted the current, their usefulness was limited.

The Boom Period. The boom period of 1900 found the great majority of dredges—there were 50 working on the West Coast and 151 . working in Otago and Southland in 1903—0 f a type which was the forerunner of designs of to-day. These small dredges were built of timber and were provided with iron digging ladels, open connected fabricated bucket lines, and perforated revolving screens. They were suitable for working “inland” claims, whomever the water supply was sufficient, and the lattice girder tailing stacker with bucket conveyer was in general use. The common source of power was a steam boiler and engines for the main drive, with reciprocating pumps for lifting water for breaking up and gold saving. Many dredges in Otago were driven by high pressure water, brought on to the dredge by a pipeline with flexible couplings and driving a Pelton wheel, the exhaust water supplying the goldsaving tables.

In the last forty years there had been tremendous development in materials and methods of construction, and, thanks to the pioneering work of the early designers, .both here and abroad, the designer of to-day can find much material to 1 aid bis judgement. Further, the development of hydroelectricity had placed at the disposal of the industry, a most economical source of power. It was well known that the first dredges worked only the richest spots and CTiat, judged by modern standards, efficiencies were appalling low. To-day, a large machine will cost jip to a quarter of a miilon pounds, and will dredge profitably suitable ground containing only two grains of gold to the cubic yard.