Art. LI.—On the Mode of Producing Auriferous Alloys by Wet Processes.
[Read before the Wellington Philosophical Society, 23rd October, 1872.]
In former papers read before this Society* I showed that metallic sulphides generally reduced gold from both acid and alkaline solutions; that silver as nitrate was reduced by galena and sulphide of copper, but not by iron pyrites, while its ammoniated solutions were unaffected by any of the sulphides experimented with; and from a consideration of these results I suggested that most of our native deposits of noble metals have been formed by the agency of metallic sulphides, and not by that of organic matter as has hitherto been generally supposed.
The question which naturally presented itself to me at the time as to the capability of processes of this nature producing alloys of such metals (as found in nature) was tacitly left over for consideration until the behaviour of these sulphides with metallic solutions should be more fully examined.
In pursuit of this question as to the possibility of obtaining mixed metallic deposits or alloys by the agency of metallic sulphides I have from time to time, as opportunity offered, studied the behaviour of different sulphides when in contact with various salts of gold and silver, and the principal results thus obtained I now beg to state.
1. That solutions of chloride of silver in alkaline chlorides, rendered alkaline by addition of potash, soda, or lime, are readily decomposed by common iron pyrites.
2. That this effect is not produced if such solution of silver is either acid or neutral.
3. That when chloride of gold is added to an alkaline argentiferous solution of this nature, such mixed solution is capable of depositing the metals contained in it in the form of coherent alloys upon metallic sulphides generally when presented to them.
4. That these alloys can also be formed from such solutions by voltaic action.
As will be seen these results show that, by allowing an alkaline solution of gold and silver contact with iron pyrites (a mineral of most common
[Footnote] * See Trans. N.Z. Inst., Vol. III., Arts. XL. and XLI.
occurrence in our reefs), we obtain that mixed deposit or alloy we are seeking to produce.
The only condition which appears to me from the results of numerous experiments necessary in regard to such metallic solutions is that they should be not only alkaline, but alkaline from presence of a fixed alkali or alkaline earth, and it will be remembered, perhaps, in connection with this circumstance, that this alkaline condition is one which I have recently shown* to be that of our silicates generally, whether simple or compound, with the exception of silicate of alumina and other corresponding silicates of the sesqui-oxides, while quartz itself, whether free or in combination, is either quite neutral or of such very feeble acidic powers as regards intensity that when united, even in very greatly disproportionate quantity, with alkalies or alkaline earth the resulting compound gives an alkaline reaction.
If such then are the conditions (alkalinity or neutrality) of our surface rocks generally, it is clear that the condition of the water permeating them at some distance from the surface would be alkaline, the intensity of which would be largely increased were the retaining rock subjected to those hydro-chemical influences popularly supposed to have operated for the deposition of their older vein matters and their metallic contents.
The fact that many mine waters are acid does not affect the truth of the conclusion thus drawn as to the general alkaline condition of the waters charging our rocks, since this acidity is, as is well known, brought about by the contact of air with pyritous matters, metallic sulphates thus being formed which communicate an acid reaction to water dissolving them. This is a mere surface affair as it were, and is besides in its manner of production entirely distinct from, if not antagonistic to that by which gold has been deposited in the pyritous portions, for a slight examination of these shows that the gold present in them has been deposited there before they were oxidized by atmospheric agency.
Alkalinity being then certainly the general condition of our waters permeating rocks, closed, or partially so, against the atmosphere, and this condition of liquids appearing essential for the production of alloys by humid methods, as shown here, it does seem highly probable that our native alloys of gold and silver have been deposited from alkaline solutions, and by such agents as I have suggested, viz., the metallic sulphides.
I will only add, in reference to the mode in which our deposits of native gold have been formed, that while the number of substances capable of precipitating this metal from solution is many, the number of those at all likely to have been actually concerned in the production of these deposits is very small indeed. As far as at present appears, the substances capable of reducing
[Footnote] * Trans. N.Z. Inst., Vol. IV., Art. LVI.
gold, and likely to occur in the interior parts of our rocks, are ferrous sulphate of iron, organic matter, and metallic sulphides. These also reduce silver from certain of its solutions, but, as already noted, the difficulty is in finding a substance which will reduce these two metals simultaneously in coherent forms, and from such kinds of solutions as generally permeate our rocks. With this double duty to perform, and limited in this manner as regards nature of solvent, I cannot avoid thinking that but one of these reducing agents, the metallic sulphides, will be found equal to the occasion. The ferrous sulphate is thrown out at once from this service on account of its insolubility in such a menstruum, while organic matter appears to have a decided tendency to scatter the gold it reduces (see Art. LII), nor have we, as far as I am aware, produced any true alloy of gold and silver by their use.
I would not intend to convey the idea that such a mixed deposition is impossible, but only that, from what we at present know of this subject, the production of such an alloy by these means appears a very difficult undertaking. However, this particular question is, I understand, now being dealt with by Mr. Daintree, late assistant geologist to the Victorian Geological Survey,* so that the propriety or otherwise of retaining this theory of the origin of our auriferous deposits in their lodes by the interaction of organic matter may be left in abeyance until Mr. Daintree publishes the results of his inquiry, as promised.
I will therefore leave the question in this state, merely observing that should Mr. Daintree be unable to obtain the results he is in search of, I shall then claim for our metallic sulphides the sole duty of depositing at least that portion of our native gold which occurs in the reefs or fissures of our metamorphic rocks.
[Footnote] * “Athenæum,” 22nd July, 1871.