THE GENESIOLOGY OF GOLD--AURIFEROUS LODES
Up to a comparatively recent time it was considered heretical for any
one to advance the theory that gold had been deposited where found by
any other agency than that of fire. As late as 1860 Mr. Henry Rosales
convinced himself, and apparently the Victorian Government also,
that quartz veins with their enclosed metal had been ejected from
the interior of the earth in a molten state. His essay, which is very
ingenious and cleverly written, obtained a prize which the Government
had offered, but probably Mr. Rosales himself would not adduce the
same arguments in support of the volcanic or igneous theory to-day. His
phraseology is very technical; so much so that the ordinary inquirer
will find it somewhat difficult to follow his reasoning or understand
his arguments, which have apparently been founded only on the occurrence
of gold in some of the earlier discovered quartz lodes, and the
conclusions at which he arrived are not borne out by later experience.
He says:--"While, however, there are not apparent signs of mechanical
disturbances, during the long period that elapsed from the cooling
of the earth's surface to the deposition of the Silurian and Cambrian
systems, it is to be presumed that the internal igneous activity of the
earth's crust was in full force, so that on the inner side of it, in
obedience to the laws of specific gravity, chemical attraction, and
centrifugal force, a great segregation of silica in a molten state
took place. This molten silica continually accumulating, spreading,
and pressing against the horizontal Cambro-Silurian beds during a long
period at length forced its way through the superincumbent strata in all
directions; and it is abundantly evident, under the conditions of this
force and the resistance offered to its action, that the line it would
and must choose would be along any continuous and slightly inclined
diagonal, at times crossing the strata of the schists, though generally
preferring to develop itself and egress between the cleavage planes and
dividing seams of the different schistose beds."
He goes on to say, "Another argument to the same end (i.e., the igneous
origin) may be shown from the fact that the auriferous quartz lodes have
exercised a manifest metamorphic action on the adjacent walls or casing;
they have done so partly in a mineralogical sense, but generally there
has been a metamorphic alteration of the rock." Mr. Rosales then tells
his readers, what we all know must be the case, that the gold would be
volatilised by the heat, as would be also the other metals, which he
says, were in the form of arseniurets and sulphurets; but he fails to
explain how the sublimated metals afterwards reassumed their metallic
form. Seeing that, in most cases, they would be hermetically enclosed in
molten and quickly solidifying silica they could not be acted on to
any great extent by aqueous agency. Neither does Mr. Rosales's theory
account at all for auriferous lodes; which below water level are
composed of a solid mass of sulphide of iron with traces of other
sulphides, gold, calcspar, and a comparatively small percentage of
silica. Nor will it satisfactorily explain the auriferous antimonial
silica veins of the New England district, New South Wales, in which
quantities of angular and unaltered fragments of slate from the
enclosing rocks are found imbedded in the quartz.
With respect to the metamorphism of the enclosing rocks to a greater
degree of hardness, which Mr. Rosales considered was due to heat, it
should be remembered that these rocks in their original state were much
softer and more readily fusible than the quartz, consequently all would
have been molten and mingled together instead of showing as a rule
clearly defined walls. It is much more rational to suppose that the
increased hardness imparted to the slates and schists at or near their
contact with the lode is due to an infiltration of silica from the
silicated solution which at one time filled the fissure. Few scientists
can now be found to advance the purely igneous theory of lode formation,
though it must be admitted that volcanic action has probably had much
influence not only in the formation of mineral veins, but also on the
occurrence of the minerals therein. But the action was hydrothermal,
just such as was seen in course of operation in New Zealand a few years
ago when, in the Rotomahana district, one could actually see the growing
of the marvellous White and Pink Terraces formed by the release of
silica from the boiling water exuding from the hot springs, which water,
so soon as the heat and pressure were removed, began to deposit its
silica very rapidly; while at the Thames Gold-field, in the same country
hot, silicated water continuously boiled out of the walls of some of
the lodes after the quartz had been removed and re-deposited a siliceous
sinter thereon.
On this subject I note the recently published opinions of Professor
Lobley, a gentleman whose scientific reputation entitles his utterances
to respect, but who, when he contends that gold is not found in the
products of volcanic action is, I venture to think, arguing from
insufficient premises. Certainly his theories do not hold good either in
Australasia or America where gold is often, nay, more usually, found at,
or near, either present or past regions of volcanic action.
It is always gratifying to have one's theories confirmed by men whose
opinions carry weight in the scientific world. About seventeen years ago
I first published certain theories on gold deposition, which, even then,
were held by many practical men, and some scientists, to be open to
question. Of late years, however, the theory of gold occurrence by
deposition from mineral salts has been accepted by all but the "mining
experts" who infest and afflict the gold mining camps of the world.
These opine that gold ought to occur in "pockets" only (meaning thereby
their own).
Recently Professor Joseph Le Conte, at a meeting of the American
Institute of Mining Engineers, criticised a notable essay on the
"Genesis of Ore Deposits," by Bergrath F. Posepny. The Professor's
general conclusions are:
1. "Ore deposits, using the term in its widest sense, may take place
from any kinds of waters, but especially from alkaline solutions, for
these are the natural solvents of metallic sulphides, and metallic
sulphides are usually the original form of such deposits."
2. "They may take place from waters at any temperature and any pressure,
but mainly from those at high temperature and under heavy pressure,
because, on account of their great solvent power, such waters are
heavily freighted with metals."
3. "The depositing waters may be moving in any direction, up-coming,
horizontally moving, or even sometimes down-going, but mainly up-coming;
because by losing heat and pressure at every step such waters are sure
to deposit abundantly."
4. "Deposits may take place in any kind of waterways--in open fissures,
in incipient fissures, joints, cracks, and even in porous sandstone, but
especially in great open fissures, because these are the main highways
of ascending waters from the greatest depths."
5. "Deposits may be found in many regions and in many kinds of rocks,
but mainly in mountain regions, and in metamorphic and igneous rocks,
because the thermosphere is nearer the surface, and ready access thereto
through great fissures is found mostly in these regions and in these
rocks."
These views are in accordance with nearly all modern research into this
interesting and fruitful subject.
Among the theories which they discredit is that ore bodies may usually
be assumed to become richer in depth. As applied to gold lodes the
teaching of experience does not bear out this view.
If it be taken into account that the time in which most of our
auriferous siliceous lodes were formed was probably that indicated in
Genesis as before the first day or period when "the earth was without
form and void, and darkness was upon the face of the deep," it will be
realised that the action we behold now taking place in a small way in
volcanic regions, was probably then almost universal. The crust of the
earth had cooled sufficiently to permit water to lie on its surface,
probably in hot shallow seas, like the late Lake Rotomahana. Plutonic
action would be very general, and volcanic mud, ash, and sand would be
ejected and spread far and wide, which, sinking to the bottom of the
water, may possibly be the origin of what we now designate the azoic or
metamorphic slates and schists, as also the early Cambrian and Silurian
strata. These, from the superincumbent weight and internal heat, became
compacted, and, in some cases, crystallised, while at the same time,
from the ingress of the surface waters to the heated regions below,
probably millions of geysers were spouting their mineral impregnated
waters in all directions; and in places where the crust was thin,
explosions of super-heated steam caused huge upheavals, rifts, and
chasms, into which these waters returned, to be again ejected, or to
be the cause of further explosions. Later, as the cooling-process
continued, there would be shrinkages of the earth's crust causing other
fissures; intrusive granites further dislocated and upheaved the slates.
About this age, probably, when really dry land began to appear, came the
first formation of mineral lodes, and the waters, heavily charged with
silicates, carbonates of lime, sulphides, etc., in solution, commenced
to deposit their contents in solid form when the heat and pressure were
removed.
I am aware that part of the theory here propounded as to the probable
mode of formation of the immense sedimentary beds of the Archaic or
Azoic period is not altogether orthodox--i.e., that the origin of
these beds is largely due to the ejection of mud, sand, and ashes from
subterraneous sources, which, settling in shallow seas, were afterwards
altered to their present form. It is difficult, however, to believe that
at this very early period of geologic history so vast a time had elapsed
as would be required to account for these enormous depositions of
sediment, if they were the result only of the degradation of previously
elevated portions of the earth's surface by water agency. Glacial action
at that time would be out of the question.
But what about the metals? Whence came the metallic gold of our reefs
and drifts? What was it originally--a metal or a metallic salt, and if
the latter, what was its nature?--chloride, sulphide, or silicate, one,
or all three? I incline to the latter hypothesis. All three are known,
and the chemical conditions of the period were favorable for their
natural production. Assuming that they did exist, the task of
accounting for the mode of occurrence of our auriferous quartz lodes is
comparatively simple. Chloride of gold is at present day contained in
sea water and in some mineral waters, and would have been likely to be
more abundant during the Azoic and early Paleozoic period.
Sulphide of gold would have been produced by the action of sulphuretted
hydrogen; hence probably our auriferous pyrites lodes, while silicate
of gold might have resulted from a combination of gold chlorides with
silicic acid, and thus the frequent presence of gold in quartz is
accounted for.
A highly interesting and instructive experiment, showing how gold might
be, and probably was, deposited in quartz veins, was carried out by
Professor Bischof some years ago. He, having prepared a solution of
chloride of gold, added thereto a solution of silicate of potash,
whereupon, as he states, the yellow colour of the chloride disappeared,
and in half an hour the fluid turned blue, and a gelatinous dark-blue
precipitate appeared and adhered to the sides of the vessel. In a
few days moss-like forms were seen on the surface of the precipitate,
presumably approximating to what we know as dendroidal gold--that
is, having the appearance of moss, fern, or twigs. After allowing the
precipitate to remain undisturbed under water for a month or two a
decomposition took place, and in the silicate of gold specks of metallic
gold appeared. From this, the Professor argues, and with good show of
reason, that as we know now that the origin of our quartz lodes was
the silicates contained in certain rocks, it is probable that a natural
silicate of gold may be combined with these silicates. If this can be
demonstrated, the reason for the almost universal occurrence of gold in
quartz is made clear.
About 1870, Mr. Skey, analyst to the New Zealand Geological Survey
Department, made a number of experiments of importance in respect to
the occurrence of gold. These experiments were summarised by Sir James
Hector in an address to the Wellington Philosophical Society in 1872.
Mr. Skey's experiments disproved the view generally held that gold is
unaffected by sulphur or sulphuretted hydrogen gas, and showed that
these elements combined with avidity, and that the gold thus treated
resisted amalgamation with mercury. Mr. Skey proved the act of
absorption of sulphur by gold to be a chemical act, and that electricity
was generated in sufficient quantity and intensity during the process
to decompose metallic solutions. Sulphur in certain forms had long been
known to exercise a prejudicial effect upon the amalgamation of gold,
but this had always been attributed to the combination of the sulphur
with the quicksilver used. Now, however, it is certain that the
sulphurising of the gold must be taken into account. We must remember
that the particles of gold in the stone may be enveloped with a film
of auriferous sulphide, by which they are protected from the solvent
actions of the mercury. The sulphurisation of the gold gives no ocular
manifestation by change of colour or perceptible increase of weight,
as in the case of the formation of sulphides of silver, lead and other
metals, on account of the extremely superficial action of the sulphur,
and hence probably the existence of the gold sulphide escaped detection
by chemists.
Closely allied to this subject is the investigation of the mode in which
certain metals are reduced from their solutions by metallic sulphides,
or, in common language, the influence which the presence of such
substances as mundic and galena may exercise in effecting the deposit
of pure metals, such as gold, in mineral lodes. The close relation which
the richness of gold veins bears to the prevalence of pyrites has been
long familiar both to scientific observers and to practical miners. The
gold is an after deposit to the pyrites, and, as Mr. Skey was the
first to explain, due to its direct reducing influences. By a series of
experiments Mr. Skey proved that the reduction of the metal was due to
the direct action of the sulphide, and showed that each grain of iron
pyrites, when thoroughly oxidised, will reduce 12 1/4 grains of gold
from its solution as chloride. He also included salts of platina and
silver in this general law, and demonstrated that solutions of any of
these metals traversing a vein rock containing certain sulphides would
be decomposed, and the pure metal deposited. We are thus enabled to
comprehend the constant association of gold, or native alloys of gold
and silver, in veins which traverse rocks containing an abundance
of pyrites, whether they have been formed as the result of either
sub-aqueous volcanic outburst or by the metamorphism of the
deeper-seated strata which compose the superficial crust of the earth.
Mr. Skey also showed by very carefully conducted experiments that the
metallic sulphides are not only better conductors of electricity than
has hitherto been supposed, but that when paired they were capable of
exhibiting strong electro-motive power. Thus, if galena and zinc blende
in acid solutions be connected in the usual manner by a voltaic pair,
sulphuretted hydrogen is evolved from the surface of the former, and a
current generated which is sufficient to reduce gold, silver or copper
from their solutions in coherent electro-plate films. The attributing
of this property of generating voltaic currents, hitherto supposed to
be almost peculiar to metals, to such sulphides as are commonly found
in metalliferous veins, further led Mr. Skey to speculate how far the
currents discovered to exist in such veins by Mr. E. F. Fox might be
produced by the gradual oxidation of mixed sulphides, and that veins
containing bands of different metallic sulphides, bounded by continuing
walls, and saturated with mineral waters, may constitute under
some circumstances a large voltaic battery competent to produce
electro-deposition of metals, and that the order of the deposit of these
mineral lodes will be found to bear a definite relation to the order
in which the sulphides rank in the table of their electro-motive power.
These researches may lead to some clearer comprehension of the law which
regulates the distribution of auriferous veins, and may explain why in
some cases the metal should be nearly pure, while in others it is so
largely alloyed with silver.
The following extract was lately clipped from a mining paper. If true,
the experiment is interesting:--
"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.
I have long been of opinion that it is really so held in many of the
ferro-sulphides and arsenio-ferro sulphides. On this subject Mr. T.
Atherton contributed a short article in 1891 to the _Australian Mining
Standard_ which is worthy of notice. He says, referring to an occurrence
of a Natural Sulphide of Gold: "The existence of gold, in the form of
a natural sulphide in conjunction with pyrites, has often been advanced
theoretically, as a possible occurrence; but up to the present time
has, I believe, never been established as an actual fact. During my
investigation on the ore of the Deep Creek mines, Nambucca, New South
Wales, I have found in them what I believe to be gold existing as a
natural sulphide. The lode is a large irregular one of pure arsenical
pyrites carrying, in addition to gold and silver, nickel and cobalt. It
exists in a felsite dyke immediately on the coast. Surrounding it on all
sides are micaceous schists, and in the neighbourhood about half a mile
distant is a large granite hill about 800 feet high. In the lode and its
walls are large quantities of pyro-phyllite, and in some parts of the
mine there are deposits of pure white translucent mica, but in the ore
itself it is a yellow or pale olive green, and is never absent from the
pyrites.
"From the first I was much struck with the exceedingly fine state of
division in which the gold existed in the ore. After roasting and very
carefully grinding down in an agate mortar, I have never been able to
get any pieces of gold exceeding one-thousandth of an inch in diameter,
and the greater quantity is very much finer than this. Careful
dissolving of the pyrites and gangue so as to leave the gold intact
failed to find it in any larger diameter. As this was a very unusual
experience in investigations on many other kinds of pyrites, I was led
further into the matter.
"Ultimately, after a number of experiments, there was nothing left but
to test for gold existing as a natural sulphide. Taking 200 gr. of ore
from a sample assaying 17 oz. fine gold per ton, grinding it finely and
heating for some hours with yellow sodium sulphide--on decomposing the
filtrate and treating for gold I got a result at the rate of 12 oz. per
ton. This was repeated several times with the same result.
"This sample came from the lode at the 140 ft. level, whilst samples
from the higher levels where the ore is more oxidised, although carrying
the gold in exactly the same degree of fineness, do not give as high a
percentage of auric sulphide.
"It would appear that all the gold in the pyrites (and I have never
found any gold existing apart from the pyrites) has originally taken its
place there as a sulphide."
Professor Newbery, who made many valuable suggestions on the subject,
says, speaking of gold in pyritous lodes:
"As it (the gold salt) may have been in the same solution that
deposited the pyrites, which probably contained its iron in the form of
proto-carbonate with sulphates, it was not easy at first to imagine any
ordinary salt of gold; but this I find can be accomplished with very
dilute solutions in the presence of an alkaline carbonate and a large
excess of carbonic acid, both of which are common constituents of
mineral waters, especially in Victoria. This is true of chloride of
gold, and if the sulphide is required in solution, it is only necessary
to charge the solution with an excess of sulphuretted hydrogen. In this
matter both sulphides may be retained in the same solution, depositing
gradually with the escape of the carbonic acid."
Pyritic lodes usually contain a considerable proportion of calcareous
matter, mostly carbonates, and consequently it appears not improbable
that the gold may remain in some instances as a sulphide, particularly
in samples of pyrites, in which it cannot be detected even by the
microscope until by calcination the iron sulphide is changed to an
oxide, wherein the gold may be seen in minute metallic specks. The whole
subject is full of interest, and careful scientific investigation may
lead to astonishing results.