Art. XLVII.—A Theory on the Formation of Gold into Specks and Nuggets.
[Read before the Nelson Philosophical Society, 16th September, 1889.]
In this paper I shall open the question, How is gold formed into specks and nuggets? and also dispute the theory that alluvial gold was not derived from reefs, but was formed in the drift in which it is found. This, I consider, is a theory as yet quite unsupported by any facts, as the reasons relied on to establish it are quite untenable.
If the gold were deposited from solution as a dark impalpable powder, how did it get into the quartz in the shape of specks and nuggets, in which it is found? This is a question I have often heard asked, but I have never heard or seen it answered.
In accounting for it, we must quite put aside the idea that the reefs have been molten, and that the gold has been melted into the shapes in which it is found. There has never been a reef found with evidence of its having been formed by molten matter; but, even if there had, it would not account for the way the gold is found in the many reefs that evidently have not been affected by heat. The heat necessary to melt quartz is so great that it would have changed the character of the rock forming the walls of the reef, with which the molten matter must have been in contact; and the other minerals found in the quartz in masses, and crystals containing such volatile substances as sulphur, arsenic, antimony, &c., must have been altered, and the volatile portions driven off by the intense heat.
But the walls only show the action of water, and in the hard quartz we find perfect crystals of minerals that less than a red heat would destroy: yet these are perfect; and the hard quartz surrounding the crystal takes the exact form of it, showing that the quartz was soft when the crystal was formed, though not molten.
Beyond doubt the gold was carried into the reefs in a solution by water, and, meeting with a reagent, was precipitated as a powder, the particles of which were as fine as or finer than anything we can conceive, and this was deposited on and with silica in a soft state. There are several reagents that will precipitate gold from solution, such as sulphate of iron, formed by the decomposition of iron-pyrites, vegetable matter, &c., and I think it very probable that in the same reef the gold has been precipitated by different causes at various parts and
periods of its formation, and that, like most things in nature, a reef, with its minerals, is the result of a combination of many causes, of which, although there are a few we do know, there are a great many we do not know as yet.
The atoms of precipitated gold, having a very strong attraction for each other, would gradually draw together, forming minute specks, which would also be drawn towards each other, forming larger and irregular specks as they joined, until the hardening of the quartz prevented any further movement.
The time taken by quartz in hardening would probably vary in different reefs, and also at different times and places in the same reef. Where the hardening has been slow the gold would be coarse, and, if a flat reef, towards the foot-walls, as its great specific gravity would give it a downward tendency. Where the gold-precipitate was evenly distributed with the silica, and the hardening of the quartz was not very slow, the gold would probably be in fine specks through the stone, as there would only be time for the contiguous atoms to join before the quartz became too hard for further union. We can see by the hot springs in the North Island that when silica has been held in solution in hot water it hardens much more quickly than when the water is cold.
At the Thames and Te Aroha, where the hot springs have evidently played a part in the formation of the reefs, we find the gold in very fine particles; and I suspect that a good deal of the gold (not recognizable as such, and) said to be an ore of gold is really the gold as precipitated, the quartz having hardened too quickly for the atoms to join together to form specks. We must bear in mind that gold is only yellow by reflected light, and that in a state of fine division it is simply a dark powder, and would look like an ore.
It is very common in quartz-specimens to get what is called a nest of gold, which generally consists of one large speck surrounded by a number of smaller ones at various distances from it. These, I believe, are a number of atoms attracted together, gathering and joining other particles as they are drawn towards the centre; and if the quartz had remained soft long enough they would have joined the centre one at different points, forming one large irregular speck. If we consider the great ductility and malleableness of gold, we shall not be surprised at the powerful attraction the particles seem to have for each other, as those qualities are due to the cohesive attraction of the atoms of which it is composed; and if this is so great when near together it does not seem unreasonable to expect that when separated by a short distance the same power should exist, although perhaps in lessened degree according to distance. A certain degree of heat seems to increase this. I have often seen with alluvial gold, when heated a good
deal below red heat and shaken, the specks of gold would adhere firmly together—sufficiently so to remove them from the sand—and then fall apart as they cooled. If the gold formed into specks in the quartz by the attraction of its atoms, so did the pyrites, crystals, &c., by a similar process before the quartz hardened. At the Parapara, ironstone pots or bombs can be seen in all stages of formation, from the first rough outline, enclosing a quantity of iron-containing earth, to the highly metallic shell enclosing the earth from which every particle of iron has been attracted to form the pure-iron-looking bomb.
In his “Chemistry of the Goldfields” Professor Black mentions a theory originating with Mr. Selwyn several years ago; and the question is put in this way: “Do nuggets grow?” Of course, the growing must be understood to mean an increase of size by the addition and cohesion of atoms of gold. In this way I think they undoubtedly do grow; and if a flow of water carrying gold in solution—but not silica—were to pass along a lode under favourable conditions for deposition for a sufficient time they might grow to any size. But, in an ordinary way, the water carrying gold in solution does carry silica with it, which, being deposited and hardened, prevents further union of the gold atoms, thus limiting the size of the nugget and forming a reef. The exceptional ones, carrying gold in solution without much silica (or not depositing it), allow the gold to accumulate in the favourable places for its deposition as excrescences on the walls, or vughs, in the watercourse. Although I agree that nuggets do grow, I do not think there is evidence of their having done so in the drifts or washes, the strongest arguments brought forward in support of the theory only tending to show the possibility of it. The following are the chief arguments used in favour of the theory that alluvial gold has grown in the drifts:—
“Nuggets are found in the alluvial drift of larger size than any piece of gold ever got from reefs.,” In answer to this we must consider the very small amount of quartz reefs that have been broken up and worked by man, as compared with the immense quantity broken up by Nature and forming the drifts. The difference is so immeasurable that the comparison is absurd.
“That alluvial gold is of a higher quality than reef-gold from the same neighbourhood.,” This is said to be a very strong argument, but I consider it requires very careful investigation before it is taken as a fact. I do not admit that the statement is correct as a rule, and I will mention two cases to show why: First, in Bedstead Gully there were two distinct qualities of alluvial gold—one a rough honeycomb gold, derived from the reef at the head of the gully, and of the
same quality; the other a more travelled, much water-worn gold. At the Quartz Ranges, again, a similar thing occurs—the rough gold being of the same quality as the gold in the leaders now being prospected; the other sample showing signs of being more water-worn, and having travelled some distance. Now, if, in either of these cases, the sample to be tested was the travelled one, and it was compared with the reef-gold known in the neighbourhood, it would appear of very different quality. The quality of the gold may vary in reefs even close together; so that it is not surprising in drifts—which may have, and probably have, derived their gold from several sources—to find that the analysis does not coincide with the gold from the only one source known.
“There is a nuggety or mammillated appearance about drift-gold which reef-gold does not present.,” This is quite correct; and it is the result of the bruising and wearing action of the gravel, &c., it has come in contact with in the drift. Take Mahakipawa Creek, for instance: examine the gold in the lower part of the creek, and it is a smooth, solid, water-worn gold; further up it becomes more mammillated and less worn; and as it gets nearer to the source from whence it was derived the mammillations become rougher and sharper, until it appears the sharp-pointed rough gold nearly as it came from its matrix. This would point to the mammillated appearance being caused by a certain amount of attrition on the rough, pointed, matrix gold. The same thing can be seen in other places, but I mention Mahakipawa because, from the coarseness of the gold, it is more easily noticed there.
“That nuggets often contain a core or nucleus of oxide of iron.,” This is no evidence that nuggets were formed in the drift, as the same thing could occur equally well if the nuggets were formed in the rock according to argument 10.
“Gold has been repeatedly found in solution in mine-waters by analysis. It has also been found as a golden incrustation in iron boilers fed with mine-waters.,” If this is any evidence at all, it is in favour of nuggets being formed in the rock, as it proves the necessary gold-solution there, but it does not prove the gold-solution in the drift.
“Many fragments of wood lying among the gold-drift debris have been found petrified into iron-pyrites, and such pyrites, when analysed, has always yielded gold, generally at the rate of several ounces to the ton.,” If it were certain that no mine-water had been in contact with them this would seem to be the best and only argument brought forward, as it would tend to show that drift-water may carry gold, and deposit it under favourable conditions.
Professor Egleston, of New York, points out “that good gold is often found buried to the depth of 8in. or 10in. in the
bottom or bed-rock, especially where the bed-rock is soft and porous. The gold so found could not by any conceivable process have got there except in solution.,” In gold-digging this is a very simple every-day experience. If a ground-race for sluicing golden dirt is made across ground that does not contain gold, under the conditions given (a soft porous rock), the fine shotty gold will work down in the rock not only 8in. or 10in., but in many cases more than twice that depth; and the process can be seen. The water penetrates porous places, softening and sometimes removing some of the lighter material they contain, thus allowing the fine round gold to sink; and gold may be obtained as far as this action has gone, but not farther.
“That, if the gold in the deep placers had come ready formed from the vein-matter in the reefs, it would be distributed in layers of unequal richness through the bed—the richness depending on the amount of deposition taking place at any one time—and would not occur in increasing richness from top to bottom.,” Professor Egleston, who points out this argument, has evidently no knowledge of how drifts are formed. In the first place, the layers are of unequal richness, and naturally get poorer and the gold lighter upwards from the bottom. The richness of the layer does not so much depend on the amount of deposition at one time as on the quantity and quality of the débris passing over, and from which the deposition has been made. Take the Maitai River, for instance, and suppose the rock it has cut through and washed away to have been gold-bearing down to its present level; then let an obstruction occur at the lower end, causing a layer of gravel to accumulate all up the valley. At this level the liver might run for ages, depositing whatever gold and heavy mineral might be washed into it. The accumulations and running-levels might go on indefinitely, but in each succeeding level there would be less gold per ton, and of a lighter description, for these reasons: that the bottom layer would have all the gold contained in the rock washed away in forming the whole length, breadth, and depth of the valley up to the time of the obstruction, concentrated into a narrow strip. Each succeeding layer would cover a much wider area, and, even if it contained the same number of ounces, would be much poorer, as the gold would be dispersed through so much more stuff. Another reason why succeeding levels would not be so good is that only very light gold will travel over gravel; so that, with the creeks and feeders, most of the gold would remain on the bed-rock above the junction with the layer. It is a common thing on diggings for the gullies round a flat to be payable and the flat too poor; and in the case of the Australian deep leads the original bottom levels, containing
the gold concentrated out of an immense extent of country into narrow channels, have been found and followed.
“It is on hard bottoms, false or true, over which the drift-water mostly flows, that the washdirt as a rule is richest.,” This is not borne out by experience. The richest gold is found on the bottom that forms and retains the best mechanical catches with regard to the course of the depositing stream; and where the bottom has any advantage for catching and retaining the gold, there the richest places will be; and, as a rule, these are on the higher portions, where the drift-water would not flow.
“The marked quantity of reddish oxide of iron at the level of the richest washdirt is partly accounted for by the oxidizing power of chloride of gold.,” It may be; but I do not see how this would account for the fact that all minerals of high specific gravity associated with the gold in the rock are found deposited with it in all its situations, and, like the gold, more or less water-worn. To any one experienced in ground-sluicing this is clear enough, as precisely all the action of drift-formation is seen on a small scale, and the phenomena mentioned to show that the gold formed in the drift will invariably take place in it. It is the natural result of force of water acting on material of different specific gravities; and the action is just the same, the only difference being the magnitude of the operations. If gold were found in the drift clearly separated from any of its heavy rock-associates, it would be a stronger argument in favour of its having been formed there than anything that has been brought forward yet.
The question is not, Is it possible under certain conditions for gold to form in a drift? but, Is there any evidence to show that it has done so to any appreciable extent? While admitting that it is quite possible under favourable conditions for gold to have formed in the drifts, I do not think that there is the slightest evidence that it has done so to any noticeable extent. A careful examination of drift-gold under a powerful magnifying-glass would be of assistance in determining the question: for if it formed in the drift it certainly should not have an abraded surface; and this, I think, all drift-gold will be found to have, with the possible exception of a very little that has been released by disintegration of the matrix while in the drift. I believe that nuggets have grown as defined above, but think it has been in the hollow vughy water-channels or lodes in the rock, not in the drift-gravel.