Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 75

Attempts to Precipitate Gold from Sea-Water

Attempts to Precipitate Gold from Sea-Water.

These experiments were confined to such natural reducing-agents as might naturally occur along the coast at the present day, namely, sulphides of iron (chiefly formed by the reduction of the sulphates of sea-water), and carbonaceous matter of various kinds.

To make a suitable filter, an earthenware pipe 4 inches in diameter and 1 foot long was closed at the ends with strong doth. Next the cloth was placed a loose plug of asbestos, about 2 inches in thickness, wrapped in linen, and the middle part of the pipe was filled with coarsely broken earthenware, with which the reducing agent was mixed. The reducing-agents used were animal charcoal, wood-charcoal, soot, and sulphides of iron, copper and lead—the latter being prepared by precipitating sulphates of iron and copper and the nitrate of lead, in order to make sure that they should contain no gold.

Pelichet Bay is separated from the upper reaches of Otago Harbor by an embankment, in which an opening about 20 feet wide has been left. Through this passage the sea runs with considerable force at most states of the tide. The apparatus above described was fixed beneath the bridge which spans this opening, so that both the ebbing and the flowing current might pass through it. The filter was kept thus immersed for periods varying from one to two months. The reducing-agents were then taken out, roasted and assayed.

The results are given on page 56.

A very large quantity of sea-water must have passed through the apparatus in each of the above cases, but there was no way of estimating it with precision. In view of the negative results of all the experiments, an attempt was made to precipitate gold and silver from a measured quantity of sea-water. Ten thousand grains of artificial sulphides of iron, copper and lead, with animal charcoal and wood-charcoal—all in fine powder—were mixed in a barrel with 60 gallons of sea-water taken from the Pacific Ocean at Tomahawk Head, near Dunedin. The re- page 56
Experiments in Reducing Gold from Sea-Water.
Experiment. Reducing-Agent. Remarks. Gold Found.
a. Animal charcoal, in fine powder. The greater part of the animal charcoal had been washed through the filter.* The remainder was assayed. Nil.
b. Mixture of pounded charcoal and soot. The smaller particles washed away Charcoal remained. Nil.
c. Animal charcoal in lumps. Nil.
d. Sulphide of iron. The sulphide was much oxidized; the whole apparatus being coated with ferric oxide. Nil.
e. Sulphide of lead in lumps. Very little altered. Nil.
f. Sulphide of copper in lumps. Carbonate of copper found in the asbestos filter and other parts of the apparatus. Nil.
g. Mixture of animal charcoal, soot, sulphide of lead, sulphide of copper and sulphide of iron. Very little oxidation, even of the artificial sulphide of iron, was noticed. Nil.
ducing-agents were stirred in the water for half an hour, and the sediment was allowed to settle for some hours. The clear water above was then decanted off, and the barrel was again filled. This operation was repeated fourteen times in the same barrel, or until over 4 tons of sear-water had been treated. The sediment was then collected and roasted at a dull red heat, to incinerate the charcoal and get rid of the sulphur. On assaying the residue, no gold was obtained; but the result was a bead of pure silver weighing 0.0014 grain. I cannot say certainly whether this silver (which contained no gold) came from the sea-water or from the litharge used. I do not think the latter. If it came from the sea-water, however, it is note worthy that the amount from 4 tons represents only 0.00035

* After the first two attempts the reducing-agents were put in, not as powder, but in lumps from ¼ to ½-inch in diameter, it having been found that the strong current carried the finer stuff through the asbestos filter.

page 57 grain per ton, or one five-hundredth part of the quantity found in sea-water by Malaguti and Durocher in 1851.

All my experiments have thus signally failed to show any precipitation of gold (and have practically failed as to silver) from sea-water by natural reagents. So far as they go, they lend no support to the theory that the deposition of gold and silver by such reagents in marine sediments is now going on.

If such deposition had been the rule in former periods, and if this be the origin of the gold in stratified formations, why should only a comparatively small proportion of such formations be traversed by auriferous veins? This point has not escaped the attention of Posepny.* It seems to me that important evidence may be drawn from the examination of stratified rocks known to be consolidated marine sediments, but the lodes in which have not proved auriferous. Table XXIII. gives an examination of nine samples from Skipper's Road, east of Lake Wakatipu, Otago, an area in which the rocks are known to belong to the middle division of the foliated mica-schists (the favorable country-rock for gold in Otago), but in which no auriferous reefs had hitherto been discovered. Only those parts (e.g. fillings of veins and seams, etc.) particularly favorable to the deposition of gold were examined, but no gold was found in any of the samples.

A second series of examinations was made on samples from the west shore of Wakatipu lake, which is largely occupied by rocks of the maitai (Carboniferous) and the Te Anan (Upper Devonian) series. These rocks (mostly sandstones and slates) are undoubted marine sediments, but no gold has been found in this area.

If the gold of the lodes in the foliated mica-schists east of; this lake was originally deposited from sea-water and has since been collected by lateral segregation, it is difficult to understand why gold should not have been deposited in the marine sediments west of the lake also.

I therefore examined 16 samples of quartz, ferric oxide, etc., from the two series above named, where they are exposed north and south of the Greenstone river. There were 4 samples of fine-grained blue slate; 6 of ferric oxide from joints and fis-

* Genesis of Ore-Deposits, p. 307.

page 58 sures; 4 of small quartz-veins, none of winch contained sulphides; 2 of slate containing large crystals of pyrite, amounting in weight to 254 grains for the two samples.

No trace of gold or silver was found in any of these samples.