THE GENESIOLOGY OF GOLD--AURIFEROUS DRIFTS
Having considered the origin of auriferous lodes, and the mode by which
in all probability the gold was conveyed to them and deposited as a
metal, it is necessary also to inquire into the derivation of the gold
of our auriferous drifts, and the reasons for its occurrence therein.
When quite a lad on the Victorian alluvial fields, I frequently heard
old diggers assert that gold grew in the drifts where found. At the time
we understood this to mean that it grew like potatoes; and, although not
prepared with a scientific argument to prove that such was not so, the
idea was generally laughed at. I have lived to learn that these old
hard-heads were nearer the truth than possibly they clearly realised,
and that gold does actually grow or agglomerate; and, indeed, is
probably even now thus growing, though it is likely that the chemical
and electric action in the mineral waters flowing through the drifts is
not in this age nearly so active as formerly.
Most boys have tried the experiment of dipping a clean-bladed knife into
sulphate of copper, and so depositing on the steel a film of copper,
which adheres closely until worn away. This is a simple demonstration of
a hydro-metallurgical process, though probably young hopeful is not
aware of the fact; and it is really by an enlargement of this process
that our beautiful and artistic gold-and silver-plated ware is
produced.
In the great laboratory of Nature similar chemical depositions have
taken place in the past, and may still be in progress; indeed, there is
sound scientific reason to suppose that in certain localities this is
even now the case, and that in this way much of our so-called alluvial
gold has been formed, that is, by the deposition on metallic bases of
the gold held in solution.
We will, however, take, to begin with, the generally accepted theory as
to the occurrence of alluvial gold. First, let it be said, that certain
alluvial gold is unquestionably derived from the denudation of quartz
lodes. Such is the gold dust found in many Asiatic and African rivers,
in the great placer mines of California, as also the gold dust gained
from the beach sand on the west coast of New Zealand, or in the enormous
alluvial drifts of the Shoalhaven Valley, New South Wales. Of the first,
many fabulous tales are told to account for its being found in
particular spots each summer after the winter floods, and miraculous
agency was asserted, while the early beachcombers of the Hokitika
district found an equally ridiculous derivation for their gold, which
was always more plentiful after heavy weather. They imagined that the
breakers were disintegrating some abnormally rich auriferous reefs out
at sea, and that the resultant gold was washed up on the beach.
The facts are simply, with regard to the rivers, that the winter floods
break down the drifts in the banks and agitate the auriferous detritus,
thus acting as natural sluices, and cause the metal to accumulate in
favourable spots; whilst on the New Zealand coast the heavy seas
breaking on the shingly beach, carry off the lighter particles, leaving
behind the gold, which is so much heavier. These beaches are composed,
as also are the “terraces” behind, of enormous glacial and fluvial
deposits, all containing more or less gold, and extend inland to the
foot of the mountains.
It is almost certain that the usually fine gold got by hydraulicing in
Californian cañons, in the gullies of the New Zealand Alps, and the
great New South Wales drifts, is largely the result of the attrition of
the boulders and gravel of moraines, which has thus freed, to a certain
extent, the auriferous particles. But when we find large nuggety masses
of high carat gold in the beds of dead rivers, another origin has to be
sought.
As previously stated, there is fair reason to assume that at least three
salts of gold have existed, and, possibly, may still be found, in
Nature--silicate, sulphide, and chloride. All of these are soluble and
in the presence of certain reagents, also existing naturally, can be
deposited in metallic form. Therefore, if, as is contended, reef gold
was formed with the reefs from solutions in mineral waters, by
inferential reasoning it can be shown that much of our alluvial gold was
similarly derived.
The commonly accepted theory, however, is that the alluvial gold of our
drifts has been ground out of the solid siliceous lodes by glacial and
fluvial action, and that the auriferous leads have been formed by the
natural sluicing operations of former streams. To this, however, there
are several insuperable objections.
First, how comes it that alluvial gold is usually superior in purity to
the “reef” gold immediately adjacent? Second, why are masses of gold,
such as the huge nuggets found in Victoria and New South Wales, never
been discovered in lodes? Third, why are these heavy masses which, from
their specific gravity, should be found only at the very bottom of the
drifts, if placed there by water action, sometimes found in all
positions from the surface to the bottom of the wash? And, lastly, why
is it that when an alluvial lead is traced up to, or down from, an
auriferous reef, that the light, angular gold lies close to the reef,
while the heavy masses are often bedded much farther away? Any one who
has worked a ground sluice knows how extremely difficult it is with a
strong head of water to shift from its position an ounce of solid gold.
What, then, would be the fluvial force required to move the Welcome
Nugget? Under certain circumstances, Niagara itself would not be equal
to the task.
The generally smooth appearance of alleged alluvial gold is adduced as
an argument in favour of its having been carried by water from its
original place of deposit, and thus in transit become waterworn; while
some go so far as to say that it was shot out of the reefs in a molten
state. The latter idea has been already disposed of, but if not, it may
be dismissed with the statement that the heat which would melt silica in
the masses met with in lodes would sublimate any gold contained, and
dissipate it, not in nuggets but in fumes. With regard to the assumed
waterworn appearance of alluvial gold, I have examined with the
microscope the smooth surface of more than one apparently waterworn
nugget, and found that it was not scratched and abraded, as would have
been the case had it been really waterworn, but that it presented the
same appearance, though infinitely finer in grain, as the surface of a
piece of metal fresh from the electrical plating-bath.
Let us then try if we cannot suggest, if not a complete answer to the
many perplexing problems which trouble the alluvial miner, at least a
theory that will bear investigation, and by the application of which
many apparent paradoxes may be explained. While admitting that certain
alluvial gold has been ground out of its siliceous matrix, and
distributed and re-collected by water action, we may inquire what would
be the result if we extend the experiment of the boy and knife, or the
electro-metallurgical bath to alluvial drifts, and show that it is not
only possible, but almost certain, that similar action has obtained, and
may now be occurring.
Mr. Daintree, of the Victorian Geological Survey, many years ago
discovered accidentally that gold chloride would deposit its metal on a
metallic base in the presence of any organic substance. Mr. Daintree
found that a piece of undissolved gold in a bottle containing chloride
of gold in solution had, owing to a portion of the cork having fallen
into the liquid, grown or accretionised so much that it could not be
extracted through the neck. This led Mr. Charles Wilkinson, who has
contributed much to our scientific knowledge of metallurgy, to
experiment further in the same direction. He says: “Using the most
convenient salt of gold, the terchloride, and employing wood as the
decomposing agent, in order to imitate as closely as possible the
organic matter supposed to decompose the solution circulating through
the drifts, I first immersed a piece of cubic iron pyrites taken from
the coal formation of Cape Otway, far distant from any of our gold
rocks, and therefore less likely to contain gold than other pyrites. The
specimen (No. 1) was kept in dilute solution for about three weeks, and
is completely covered with a bright film of gold. I afterwards filed off
the gold from one side of a cube crystal to show the pyrites itself and
the thickness of the surrounding coating, which is thicker than ordinary
notepaper. If the conditions had continued favourable for a very
lengthened period, this specimen would doubtless have formed the nucleus
of a large nugget. Iron, copper, and arsenical pyrites, antimony,
galena, molybdenite, zinc blende, and wolfram were treated in the above
manner with similar results. In the above experiments a small chip of
wood was employed as the decomposing agent. In one instance I used a
piece of leather. All through the wood and leather gold was disseminated
in fine particles, and when cut through the characteristic metallic
lustre was brightly reflected. The first six of these sulphides were
also operated upon simply in the solution without organic matter, but
they remained unaltered.”
Wilkinson found that when the solution of gold chloride was as strong
as, say, four grains to the ounce of water, that the pyrites or other
base began to decompose, and the iron sulphide changed to yellow oxide,
the “gossan” of our lodes, and that though the gold was deposited, this
occurred in an irregular way, and it was coated with a dark brown
powdery film something like the “black gold,” often found in drifts
containing much ferruginous matter. Such were the curious Victorian
nuggets Spondulix and Lothair.
Professor Newbery also made a number of similar experiments, and arrived
at like results. He states as follows: “I placed a cube of galena in a
solution of chloride of gold, with free access of air, and put in
organic matter; gold was deposited as usual, in a bright metallic film,
apparently completely coating the cube. After a few months the film
burst along the edges of the cube, and remained in that state with the
cracks open without any further alteration in size or form being
apparent. Upon removing it a few days ago and breaking it open, I found
that a large portion of the galena had been decomposed, forming chloride
and sulphate of lead and free sulphur, which were mixed together,
encasing a small nucleus of undecomposed sulphate of lead. The formation
of these salts had exerted sufficient force to burst open the gold
coating, which upon the outside had the mammillary form noticed by
Wilkinson, while the inside was rough and irregular with crystals
forcing their way into the lead salts. Had this action continued
undisturbed, the result would have been a nugget with a nucleus of lead
salts, or if there had been a current to remove the results of the
decomposition, a nugget without a nucleus of foreign matter.”
But Newbery also made another discovery which still further establishes
the probability of the accretionary growth of gold in drifts. In the
first experiments both investigators used organic substances as the
reagent to cause the deposit of gold on its base, and in each case these
substances whether woodchips, leather, or even dead flies, were found to
be so absolutely impregnated with gold as to leave a golden skeleton
when afterwards burned. Timber found in the Ballarat deep leads has been
proved to be similarly impregnated.
Newbery found that gold could also be deposited on sulphurets without
any other reagent. He says: “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. Thus the aggregation of the nuggety 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 or 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 such matters disseminated or even stopping 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 are to a
certain extent polarised, 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 polarisation 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 oxidised to sulphuric acid and its
iron to sesquioxide 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 oxidising
substances other than gold, the strength of the gold solution, length of
exposure to it, the rate of supply and 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 (bisulphide of iron) weighing only 12 lbs. is
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. Thus we find that no straining of the imagination is
required to conceive of this mode of formation for the huge masses of
gold found in Australia in particular, such as the Welcome Nugget, 184
lbs. 9 oz.; the Welcome Stranger, a surface nugget, 190 lbs. after
smelting; the Braidwood specimen nugget, 350 lbs., two-thirds gold;
besides many other large masses of almost virgin gold which have been
obtained from time to time in the alluvial diggings.”
The author has made a number of experiments in the same direction, but
more with the idea of demonstrating how possibly gold may in certain
cases have been deposited in siliceous formations after such formations
had solidified. Some of the results were remarkable and indeed
unexpected. I found that I could produce artificial specimens of
auriferous quartz from stone which had previously contained no gold
whatever, also that it was not absolutely necessary that the stone so
treated should contain any metallic sulphides.
The following was contributed by the author and is from the
“Transactions” of the Australasian Institute of Mining Engineers for
1893:--
“THE DEPOSITION OF GOLD.
“The question as to how gold was originally deposited in our
auriferous lodes is one to which a large amount of attention has
been given, both by mineralogists and practical miners, and
which has been hotly argued by those who held the igneous theory
and those who pronounced for the aqueous theory. It was held by
the former that as gold was not probably existent in nature in
any but its metallic form, therefore it had been deposited in
its siliceous matrix while in a molten state, and many ingenious
arguments were adduced in support of this contention. Of late,
however, most scientific men, and indeed many purely empirical
inquirers (using the word empirical in its strict sense) have
come to the conclusion that though the mode in which they were
composed was not always identical, all lodes, including
auriferous formations, were primarily derived from
mineral-impregnated waters which deposited their contents in
fissures caused either by the cooling of the earth’s crust or by
volcanic agency.
“The subject is one which has long had a special attraction for
the writer, who has published several articles thereon, wherein
it was contended that not only was gold deposited in the lodes
from aqueous solution, but that some gold found in form of
nuggets had not been derived from lodes but was nascent in its
alluvial bed; and for this proof was afforded by the fact that
certain nuggets have been unearthed having the shape of an
adjacent pebble or angular fragment of stone indented in them.
Moreover, no true nugget of any great size has ever been found
in a lode such as the Welcome, 2159 oz., or the Welcome
Stranger, 2280 oz.; while it was accidentally discovered some
years ago that gold could be induced to deposit itself from its
mineral salt to the metallic state on any suitable base, such as
iron sulphide.
“Following out this fact, I have experimented with various salts
of gold, and have obtained some very remarkable results. I have
found it practicable to produce most natural looking specimens
of auriferous quartz from stone which previously, as proved by
assay, contained no gold whatever. Moreover, the gold, which
penetrates the stone in a thorough manner, assumes some of the
more natural forms. It is always more or less mammillary, but at
times, owing to causes which I have not yet quite satisfied
myself upon, is decidedly dendroidal, as may be seen in one of
the specimens which I have submitted to members. Moreover, I
find it possible to moderate the colour and to produce a
specimen in which the gold shall be as ruddy yellow as in the
ferro-oxide gangue of Mount Morgan, or to tone it to the pale
primrose hue of the product of the Croydon mines.
“I note that the action of the bath in which the stone is
treated has a particularly disintegrating effect on many of the
specimens. Some, which before immersion were of a particularly
flinty texture, became in a few weeks so friable that they could
be broken up by the fingers. So far as my experiments have
extended they have proved this, that it was not essential that
the silica and gold should have been deposited at the one time
in auriferous lodes. A non-auriferous siliceous solution may
have filled a fissure, and, after solidifying, some volcanic
disturbance may have forced water impregnated with a gold salt
through the interstices of the lode formation, when, if the
conditions were favourable, the gold would be deposited in
metallic forms. I prefer, for reasons which will probably be
understood, not to say exactly by what process my results are
obtained, but submit specimens for examination.
“(1) Piece of previously non-gold bearing stone. Locality near
Adelaide, now showing gold freely in mammillary and dendroidal
form.
“(2) Stone from New South Wales, showing gold artificially
introduced in interstices and on face.
“(3) Stone from West Australia, very glassy looking, now
thoroughly impregnated with gold; the mammillary formation
being particularly noticeable.
“(4) Somewhat laminated quartz from Victoria, containing a
little antimony sulphide. In this specimen the gold not only
shows on the surface but penetrates each of the laminations, as
is proved by breaking.
“(5) Consists of fragments of crystallised carbonate of lime
from Tarrawingee, in which the gold is deposited in spots, in
appearance like ferrous oxide, until submitted to the magnifying
glass.
“The whole subject is worthy of much more time than I can
possibly give it. The importance lies in this: That having found
how the much desired metal may have been deposited in its
matrix, the knowledge should help to suggest how it may be
economically extracted therefrom.”
A very remarkable nugget weighing 16¾ oz. was sluiced from near the
surface in one of my own mining properties at Woodside, South Australia,
some years ago, which illustrated the nuclear theory very beautifully.
This nugget is very irregular in shape, fretted and chased as though
with a jeweller’s graving tool, showing plainly the shape of the
pyritous crystals on which it was formed while the interstices were
filled with red hematite iron just as found in artificially formed
nuggets on a sulphide of iron base. The author has a nugget from the
same locality weighing about 1½ oz. which exhibits in a marked degree
the same characteristics, as indeed does most of the alluvial gold found
in the Mount Lofty Ranges; also a nugget from near the centre of
Australia weighing four ounces, in which the original crystals of
pyrites are reproduced in gold just as an iron horse-shoe, placed in a
launder through which cupriferously impregnated water flows, will in
time be changed to nearly pure copper and yet retain its shape.
Now with regard to the points I have put (p. 36) as to the apparent
anomalies of occurrence of alluvial gold. The explanation for alluvial
gold being of finer quality as a rule than reef is probably that while
gold and silver, which have a considerable affinity for each other, were
presumably dissolved from their salts and held in solution in the same
mineral water, they would in many cases not be deposited together, for
the reason that silver is most readily deposited in the presence of
alkalies, which would be found in excess in mineral waters coming direct
from the basic rocks, while gold is induced to precipitate more quickly
in acid solutions, which would be the character of the waters after they
had been exposed to atmospheric action and to contact with organic
matters.
This, then, may explain not only the comparatively greater purity of the
alluvial gold, but also why big nuggets are found so far from auriferous
reefs, and also why heavy masses of gold have been frequently unearthed
from among the roots even of living trees, but more particularly in
drifts containing organic matter, such as ancient timber.
All, then, that has been adduced goes to establish the belief that the
birthplace of our gold is in certain of the earlier rocks comprising the
earth’s crust, and that its appearance as the metal that is valued so
highly is the result of electro-chemical action, such as we can
demonstrate in the laboratory.