Art. LV.—On the Formation of Gold Nuggets in Drift.
[Read before the Wellington Philosophical Society, 23rd October, 1872.]
The manner in which those gold nuggets have been formed which are found in our drift or fluviatile deposits has long been a subject of profound interest. Our Victorian friends in particular have been greatly exercised with this matter, no doubt from having it so frequently and forcibly presented to them by the almost regular announcement from time to time of the discovery of nuggets so large as to be entitled to description in the annals of their gold fields, and to names to identify them by.
From the circumstance of their attention being thus given to this subject many valuable observations have been recorded by them and published in the periodicals or other works emanating from their colony.
The first theory broached to account for the presence of these nuggets in drifts was that they had been broken off some rich reef and transported by water bodily to the positions in which they are now found by us. At first sight this appears very plausible, but there are several considerations which, when allowed to have their due weight, rather tend to shake our belief in its competency to explain the case. These considerations have been discussed pretty freely in the works alluded to so I need not detail them here, but will only state that, briefly put, the chief of them are as follows:—The large size of many of these nuggets as compared with any of the masses of gold yet found in our reefs; their position in the drifts, lying sometimes as they do in the
upper layers; and their superior fineness of quality as compared with that of any of the reef gold found in their vicinity.
Impressed by these facts Mr. A. C. Selwyn proposed another theory for explaining the origin of these nuggets, and one which certainly appears to meet the question upon the particular points just cited. This theory is “that nuggets may be formed and that particles of gold may increase in size through the deposition of gold from the meteoric waters percolating the drifts, which water, during the time of our extensive basaltic eruptions, must have been of a thermal, and probably of a highly saline character, favourable to their carrying gold in solution.”*
At the time this idea was broached nothing systematic or thorough had been undertaken towards investigating this matter as to the probable presence of gold in those meteoric or saline waters referred to, and nothing whatever had been accomplished towards showing any likely means by which gold, depositing from such solutions, would be determined upon itself as a continuous coating, and in such quantity as occasionally to form nuggets of the enormous size we find them in such drifts, nor did Mr. Selwyn indeed make any suggestion on this matter; perhaps considering the initiation of such an idea sufficient for his part, he left the support of it to the ingenuity of chemists, to whom in fact such a labour rightfully belonged; in reality, so little was known in support of this theory at the time of its evolution that it seemed in the highest degree chimerical. Since then, however, chemical investigations have given us results greatly in favour of this idea. Thus in the first place as regards the presence of gold in a soluble state in the waters percolating our drifts, it appears that Mr. Daintree found gold in pyrites which had obviously replaced the organic structure of a tree occurring in a drift-bed, and Mr. Newbery, Analyst to the Geological Survey of Victoria, afterwards obtained the same results upon other pyrites occurring in a similar manner, both results showing that gold must have been “presented to the pyrites in a soluble form.”
Since that time gold has been by no means unfrequently discovered to be present in certain mineral and mine waters, and indeed Mr. Daintree has recently found gold while testing the water of a mine in Victoria.
Perhaps, though, the most important communication we have relative to this subject is that of E. Sonstadt, “On the Presence of Gold in Sea-water,” (“Chemical News,” 4th October, 1872). This metal has indeed before this been alleged to exist in sea-water, bnt these allegations have not been sustained with such evidence and accompanied with such detailed description of processes employed as entitled them to an unreserved belief on our part. Son-
[Footnote] * “Trans. & Proc. Roy. Soc. of Victoria,” Vol. IX., p. 53.
stadt's experiments on the other hand are detailed minutely, and his statements are supported by the results of different processes.
The amount of gold present in the water taken from Ramsey Bay, he states to be very minute, “less than one grain in the ton,” still the fact of its presence at all in such water is exceedingly interesting as showing an escape of gold from the land seaward, and so confirms the correctness of the various allegations I have referred to respecting the auriferous character of certain of our springs and mine waters.
Thus in different ways the first question involved in this theory of Mr. Selwyn is answered in a most satisfactory manner.
As to the means by which the gold present in these waters has been reduced therefrom and aggregated in masses, solid, homogeneous, and occasionally of considerable size, we have no lack of substances certain to be present in these drifts, and capable of effecting the reduction of gold and silver from the kind of solutions likely to be present there. In various kinds of organic matter and in sulphate of iron we have substances which will effect this with facility, but we have no sure evidence as yet to show that either of these substances will aggregate the gold, which they reduce or locate in a marked manner, or preferentially, upon the gold already reduced.
That gold will be reduced by these substances is certain, but all our present experience in regard to the deposition of gold by them shows that gold so reduced will be dispersed rather than aggregated, so that it would appear that nuggets of gold could not well be formed in this manner.
In our mineral sulphurets, however, we have agents which are not only capable of reducing gold and silver from solution, but besides are capable of locating them when so reduced in coherent and bulky masses.
I may state that their nuclear action upon gold depositing from solution by aid of organic matter was suggested by Mr. Charles Wilkinson,* while their competency to reduce the gold from solution without addition of organic matter was shown by me in Vol. III. of our Transactions, pp. 227–230; thus the aggregation of the nuggetty forms of gold from solution becomes a still more simple matter, only one reagent being necessary, so that there is a greater probability of such depositions obtaining than were a double process necessary.
Knowing the action of sulphides, the manner of the mode of formation of a portion at least of these nuggets seems apparent. Conceive a stream or river fed by springs rising in a country intersected by auriferous reefs, and consequently in this case carrying gold in solution; the drift of such a country must be to a greater or lesser extent pyritous, so that the débris forming the beds of these streams or rivers will certainly contain nodules of
[Footnote] * “Trans. Roy. Soc. Vict.,” Vol. VIII., Art. II.
such matters disseminated, or even topping them in actual contact with the flow of water.
It follows then from what has been previously affirmed, that there will be a reduction of gold by these nodules, and that the metal thus reduced will be firmly attached to them, at first in minute spangles isolated from each other, but afterwards accumulating and connecting in a gradual manner at that point of the pyritous mass most subject to the current, until a continuous film of some size appears; this being formed, the pyrites and gold is to a certain extent polarized, the film or irregular but connected mass of gold forming the negative, and the pyrites the positive end of a voltaic pair; and so according as the polarization is advanced to completion, the further deposition of gold is changed in its manner from an indiscriminate to an orderly and selective deposition concentrated upon the negative or gold plate.
The deposition of gold being thus controlled, its loss by dispersion or from the crumbling away of the sustaining pyrites is nearly or quite prevented—a conservative effect, which we could scarcely expect to obtain if organic matter were the reducing agent.
Meanwhile there is a gradual wasting away of the pyrites or positive pole, its sulphur being oxidized to sulphuric acid, and its iron to sesqui-oxide of iron or hematite, a substance very generally associated with gold nuggets. According to the original size of the pyritous mass, the protection it receives from the action of oxidizing substances other than gold, the strength of the gold solution, length of exposure to it, and rate of supply (velocity of stream), will be the size of the gold nugget.
As to the size of a pyritous mass necessary to produce in this manner a large nugget, it is by no means considerable. A mass of common pyrites (bi-sulphide of iron) weighing only about twelve pounds being competent for the construction of the famous “Welcome nugget,” an Australian find, having weight equal to 152 lbs. avoirdupois.
Such masses of pyrites are by no means uncommon in our drifts or the beds of our mountain streams. The general velocity of the current flowing over such pyritous matters would in all probability be such as would prevent the development of any crystalline form in the gold thus deposited, as we know very well that for such development motion is unfavourable. The form most likely to be assumed by these deposits then would be the mammillary, precisely that in which our nuggets as a rule occur.
Upon this mode of accounting for the presence of large nuggets in our drifts, their occasional great superiority in point of size to any auriferous mass as yet found in our reefs, and their superior fineness to such reef gold, admits of easy explanation.
Firstly, as regards their comparative size, if we only admit that reef gold
is also deposited by pyrites, as I attempted to show in the paper just alluded to, and if we assume that the strength of the gold solutions forming these varieties of gold respectively was not greatly different, it is only reasonable to suppose that the gold masses formed in this manner in drift would attain the greatest dimensions, for in the first place this gold in depositing would certainly aggregate more as the pyrites in the drifts or river beds would be less continuous and more sparsely distributed than that in reefs.
Secondly, the supply of gold to pyrites lying in these drifts or river beds (and so exposed to rapidly changing waters) would be far more copious than to pyrites cooped up in a rocky fissure, and so in contact only with water stagnant or nearly so.
And, thirdly, as regards the generally superior quality of these nuggets to gold found in the reef, it will, I think, appear from the following considerations that such a difference in favour of drift gold is to be expected.
I have previously shown* that silver is deposited with greatest rapidity and certainty upon pyrites from solutions which are alkaline from presence of the fixed alkalies or alkaline earths, and that as such solutions are passed from this condition to an acid one the silver present in them is retained in solution; any gold, however, that may be mixed with such silver is deposited upon this reducing agent, no matter which of these conditions the solvent is in.
Now this alkaline condition is precisely that in which, as far as we can ascertain, our lodes or rocks must have been at the time of the deposition of the gold and silver now found in them, and this alkalinity would especially manifest itself in those reefs which traverse rocks of a basic nature, such as diorites or serpentines: hence, by the way, the large proportion of silver alloying the gold found in these reefs, as compared with that alloying the gold found in the lodes of our schists or older formations.
But though the waters percolating our reefs must be to a more or less extent of an alkaline nature the drainage waters issuing from them will lose a portion of this alkalinity as they are exposed to the air, or to the products of decomposing organic matters, from having absorbed a quantity of carbonic or other acids (sulphuric, humic, etc.), thus in some measure, according to the distance such waters have travelled from their springs, will their condition be changed until their alkalinity may give way to neutrality, or even acidity, either of which conditions are, as I have stated, unfavourable to the liberal deposition of silver along with gold from such waters. Hence it is apparent that from the instant the waters percolating rocks or lodes leave them to form springs, etc., they are continually passing from a favourable condition to one eminently unfavourable for the deposition upon pyrites of what silver they
[Footnote] * Trans. N.Z. Inst., Vol. III., Art. XL.
may contain. Consequently the deposition of gold from solution being as we know unaffected, or but slightly so (comparatively), by the condition of the solvent, the great purity of gold deposited from these surface waters is explained.
The above explanation of the greater purity of our alluvial or drift gold over gold found in the reef is, I think, much more plausible than that which attributes this difference to the interaction of solutions of gold upon the auriferous masses transported from the reef, whereby the silver of these masses is replaced by gold and so removed, leaving the mass correspondingly richer in gold. That this process can be continued until our largest auriferous masses can be thus affected throughout appears to me impossible when we consider the imperviousness of such metallic masses to liquids, and how nearly the atomic volumes of gold and silver approximate. That a superficial change, however, in this direction may occur is by no means improbable, but such would escape detection unless it were especially sought for. Thus the hypothesis advanced by Mr. Selwyn as to the manner in which the nuggets of our drifts may have been formed receives support upon all those points which appear of any importance.
That nuggets of some size may, however, be in a few instances transported bodily from these matrices into the drifts or water-courses is by no means improbable, but in this case they would, I think, partake of the usual quality of the reef gold of the country about, and so would be inferior in this respect to gold formed in the manner above described.
Whatever may be the origin, however, of any particular nugget, or of nuggets generally, when we consider the auriferous nature of many mine waters, also that of sea-water, together with the decomposing and aggregating action of metallic sulphurets upon the gold of these waters, we cannot avoid the conclusion that gold is now being deposited and aggregated in many of our drifts, and that such depositions have been going on from remotest times.
In conclusion, the questions as to the source of the gold of our nuggets, the nature of the agencies by which it is dissolved, and the precise chemical state in which it exists in our auriferous waters, are subjects which it is not incumbent upon me to discuss here. I will, however, take leave to make a few observations upon them now.
As regards their source I think this is rather in gold as disseminated in certain of our slate, sandstone, or schist rocks, than in that of our reefs.
In reference to the nature of the solvent I have shown* that sulphuretted hydrogen attacks gold at ordinary temperatures, forming a sulphide of the metal, and we know that all the sulphides of this metal we have to this time formed are soluble in alkaline sulphides; therefore, as both these agents are
[Footnote] * Trans. N.Z. Inst., Vol. III., Art. XXX.
generally present in waters situated at some depth in our rocks, we may very reasonably suppose that a portion, if not all, of our gold has been brought into solution by these agents.
The state to which such auriferous solutions might pass when exposed to air and carbonic acid is not easy to determine, but of this we may be certain, that it could not well be one unfavourable to the exercise of the reducing properties of metallic sulphurets upon the gold compound present in them.