Getting Gold: A Practical Treatise for Prospectors, Miners and Students

by J. C. F. Johnson

"An American scientist has just concluded a very interesting and suggestive experiment. He took a crushed sample of rich ore from Cripple Creek, which carried 1100 ozs. of gold per ton, and digested it in a very weak solution of sodium chloride and sulphate of iron, making the solution correspond as near as practicable to the waters found in Nature. The ore was kept in a place having a temperature little less than boiling water for six weeks, when all the gold, except one ounce per ton, was found to have gone into solution. A few small crystals of pyrite were then placed in the bottle of solution, and the gold began immediately to precipitate on them. It was noticeable, however, that the pyrite crystals which were free from zinc, galena, or other extraneous matter received no gold precipitate. Those which had such foreign associations were beautifully covered with fine gold crystals."

Experimenting in a somewhat similar direction abut twelve months since, I found that the West Australian mine water, with the addition of an acid, was a solvent of gold. The idea of boiling it did not occur to me, as the action was rapid in cold water.

Assuming, then, that gold originally existed as a mineral salt, when and how did it take metallic form? Doubtless, just in the same manner as we now (by means of well-known reagents which are common in nature) precipitate it in the laboratory. With regard to that found in quartz lodes finely disseminated through the gangue, the change was brought about by the same agency which caused the silicic acid to solidify and take the form in which we now see it in the quartz veins. Silica is soluble in solutions of alkaline carbonates, as shown in New Zealand geysers; the solvent action being increased by heat and pressure, so also would be the silicate or sulphide of gold. When, however, the waters with their contents were released from internal pressure and began to lose their heat the gold would be precipitated together with the salts of some other metals, and would, where the waters could percolate, begin to accretionise, thus forming the heavy or specimen gold of some reefs. On this class of deposition I shall have more to say when treating of the origin of alluvial gold in the form of nuggets.

Mr. G. F. Becker, of the United States Geological Survey, writing of the geology of the Comstock lode, says:—"Baron Von Richthofen was of opinion that fluorine and chlorine had played a large part of the ore deposition in the Comstock, and this the writer is not disposed to deny; but, on the other hand, it is plain that most of the phenomena are sufficiently accounted for on the supposition that the agents have been merely solutions of carbonic and hydro-sulphuric acids. These reagents will attack the bisilicates and felspars. The result would be carbonates and sulphides of metals, earth, alkalies, and free quartz, but quartz and sulphides of the metals are soluble in solutions of carbonates and sulphides of the earths and alkalies, and the essential constituents of the ore might, therefore, readily be conveyed to openings in the vein where they would have been deposited on relief of pressure and diminution of temperature. An advance boring on the 3000 ft. level of the Yellow Jacket struck a powerful stream of water at 3065 ft. (in the west country), which was heavily charged with hydrogen sulphide, and had a temperature of 170 degrees F., and there is equal evidence of the presence of carbonic acid in the water of the lower levels. A spring on the 2700 ft. level of the Yellow Jacket which showed a temperature of about 150 degrees F., was found to be depositing a sinter largely composed of carbonates."

It may be worth while here to speak of the probable reason why gold, and indeed almost all the metals generally occur in shutes in the lodes; and why, as is often the case, these shutes are found to be more or less in a line with each other in parallel lodes, and why also the junction of two lodes is frequently specially productive. The theory with respect to these phenomena which appears most feasible is, that at these points certain chemical action has taken place, by which the deposition of the metals has been specially induced. Generally a careful examination of the enclosing rocks where the shute is found will reveal some points of difference from the enclosing rocks at other parts of the course of the lode, and when ore shutes are found parallel in reefs running on the same course, bands or belts of similar country rock will be found at the productive points. From this we may fairly reason that at these points the slow stream filling the lode cavity met with a reagent percolating from this particular band of rock, which caused the deposition of its metals; and, indeed, I am strongly disposed to believe that the deposition of metals, particularly in some loose lodes, may even now be proceeding. But as in Nature's laboratory the processes, if certain, are slow, this theory may be difficult to prove.

Why the junction of lodes is often found to be more richly metalliferous than neighbouring parts is probably because there the depositing reagents met. This theory is well put by Mr. S. Herbert Cox, late of Sydney, in his useful book, "Mines and Minerals." He says:—"It is a well-known fact in all mining districts that the junctions of lodes are generally the richest points, always supposing that this junction takes place in 'kindly country,' and the explanation of this we think is simple on the aqueous theory of filling of lodes. The water which is traversing two different channels of necessity passes through different belts of country, and will thus have different minerals in solution. As a case in point, let us suppose that the water in one lode contained in solution carbonates of lime, and the alkalies and silica derived form a decomposition of felspars; and that the other, charged with hydro-sulphuric acid, brought with it sulphide of gold dissolved in sulphide of lime. The result of these two waters meeting would be that carbonate of lime would be formed, hydro-sulphuric acid would be set free, and sulphide of gold would be deposited, as well as silica, which was formerly held in solution by the carbonic acid."

Most practical men who have given the subject attention will, I think, be disposed to coincide with this view, though there are some who hold that the occurrence of these parallel ore shutes and rich deposits at the junctions of lodes is due to extraneous electrical agency. Of this, however I have failed to find any satisfactory evidence.

There is, however, proof that lodes are actually re-forming and the action observed is very interesting as showing how the stratification in some lodes has come about. Instances are not wanting of the growth of silica on the sides of the drives in mines. This was so in some of the mines on the Thames, New Zealand, previously mentioned, where in some cases the deposition was so rapid as to be noticeable from day to day, whilst the big pump was actually choked by siliceous deposits. In old auriferous workings which have been under water for years, in many parts of the world, formations of iron and silica have been found on the walls and roof, while in mining tunnels which have been long unused stalactites composed of silica and calcite have formed. Then, again, experiments made by the late Professor Cosmo Newbery, in Victoria, showed that a distinctly appreciable amount of gold, iron, and silica (the latter in granular form) could be extracted from solid mine timber; which had been submerged for a considerable time.

This reaction then must be in progress at the present time, and doubtless under certain conditions pyrites would eventually take the place of the timber, as is the case with some of the long buried driftwood found in Victorian deep leads. Again, we know that the water from some copper mines is so charged with copper sulphate that if scrap iron be thrown into it, the iron will be taken up by the sulphuric acid, and metallic copper deposited in its place. All this tends to prove that the deposition of metals from their salts, though probably not now as rapid as formerly, is still ceaselessly going on in some place or another where the necessary conditions are favourable.

With regard to auriferous pyritic lodes, it does not appear even now to be clear, as some scientists assert, that their gold is never found in chemical combination with the sulphides of the base metals. On the contrary, I think much of the evidence points in the other direction.