The arguments against the theory that mineral veins have been produced
by the leaching of superficial _igneous_ rocks are in part the same as
those already cited against the general theory of lateral secretion.
They may be briefly summarized as follows:
1. Thousands of mineral veins in this and other countries occur in
regions remote from eruptive rocks. Into this category come most of
those of the eastern half of the Continent, viz., Canada, New England,
the Alleghany belt, and the Mississippi Valley. Among those I will refer
only to a few selected to represent the greatest range of character,
viz., the Victoria lead mine, near Sault Ste. Marie, the Bruce copper
mine on Lake Huron, the gold-bearing quartz veins of Madoc, the Gatling
gold vein of Marmora, the Acton and the Harvey Hill copper mines of
Canada, the copper veins of Ely, Vermont, and of Blue Hills, Maine, the
silver-bearing lead veins of Newburyport, Mass.; most of the segregated
gold veins of the Alleghany belt, the lead veins of Rossie, Ellenville,
and at other localities farther South; the copper bearing veins of
Virginia, North Carolina, and Tennessee; the veins carrying
argentiferous galena in Central Kentucky and in Southern Illinois; the
silver, copper, and antimony veins of Arkansas; and the lead and zinc
deposits of Missouri and the Upper Mississippi.
In these widely separated localities are to be found fissure,
segregated, and gash veins, and a great diversity of ores, which have
been derived, sometimes from the adjacent rocks--as in the segregated
veins of the Alleghany belt and the gash veins of the Mississippi
region--and in other cases--where they are contained in true fissure
veins--from a foreign source, but all deposited without the aid of
superficial igneous rocks, either as contributors of matter or force.
2. In the great mineral belt of the Far West, where volcanic emanations
are so abundant, and where they have certainly played an important part
in the formation of ore deposits, the great majority of veins are not in
immediate contact with trap rocks, and they could not, therefore, have
furnished the ores.
A volume might be formed by a list of the cases of this kind, but I can
here allude to a few only, most of which I have myself examined, viz.:
_(a.)_ The great ore chambers of the San Carlos Mountains in Chihuahua,
the largest deposits of ore of which I have any knowledge. These are
contained in heavy beds of limestone, which are cut in various places by
trap dikes, which, as elsewhere, have undoubtedly furnished the stimulus
to chemical action that has resulted in the formation of the ore bodies,
but are too remote to have supplied the material.
_(b.)_ The silver mines of Santa Eulalia, in Chihuahua, from which
during the last century one hundred and twelve millions of dollars were
taken, opened on ore deposits situated in Cretaceous limestones like
those of San Carlos, and apparently similar ore-filled chambers; an
igneous rock caps the hills in the vicinity, but is nowhere in contact
or even proximity to the ore bodies. (See Kimball, _Amer. Jour. Sci,_.
March, 1870.)
_(c.)_ The great chambers of Tombstone, and the copper veins of the
Globe District, the Copper Queen, etc., in Arizona.
_(d.)_ The large bodies of silver-ore at Lake Valley, New Mexico;
chambers in limestone, like _c_.
_(e.)_ The Black Hawk group of gold mines, the Montezuma, Georgetown,
and other silver mines in the granite belt of Colorado.
_(f.)_ The great group of veins and chambers in the Bradshaw, Lincoln,
Star, and Granite districts of Southern Utah, where we find a host of
veins of different character in limestone or granite, with no trap to
which the ores can be credited.
_(g.)_ The Crismon Mammoth vein of Tintic.
_(h.)_ The group of mines opened on the American Fork, on Big and Little
Cottonwood, and in Parley's Park, including the Silver Bell, the Emma,
the Vallejo, the Prince of Wales, the Kessler, the Bonanza, the Climax,
the Piñon, and the Ontario. (The latter, the greatest silver mine now
known in the country, lies in quartzite, and the trap is near, but not
in contact with the vein.)
_(i.)_ In Nevada, the ore deposits of Pioche, Tempiute, Tybo, Eureka,
White Pine, and Cherry Creek, on the east side of the State, with those
of Austin, Belmont, and a series too great for enumeration in the
central and western portions.
_(j.)_ In California, the Bodie, Mariposa, Grass Valley, and other
mines.[1]
_(k.)_ In Idaho, those of the Poor Man in the Owyhee district, the
principal veins of the Wood River region, the Ramshorn at Challis, the
Custer and Charles Dickens, at Bonanza City, etc.
[Footnote 1: See Redmond's Report _(California Geol. Survey Mining
Statistics, No 1),_ where seventy-seven mines are enumerated, of which
three are said to be in "porphyritic schist," all the others in granite,
mica schist, clay, slate, etc.]
In nearly all these localities we may find evidence not only that the
ore deposits have not been derived from the leaching of igneous rocks,
but also that they have not come from those of any kind which form the
walls of the veins.
The gold-bearing quartz veins of Deadwood are so closely associated with
dikes of porphyry, that they may have been considered as illustrations
of the potency of trap dikes in producing concentration of metals. But
we have conclusive evidence that the gold was there in Archæan times,
while the igneous rocks are all of modern, probably of Tertiary, date.
This proof is furnished by the "Cement mines" of the Potsdam sandstone.
This is the beach of the Lower Silurian sea when it washed the shores of
an Archæan island, now the Black Hills. The waves that produced this
beach beat against cliffs of granite and slate containing quartz veins
carrying gold. Fragments of this auriferous quartz, and the gold beaten
out of them and concentrated by the waves, were in places buried in the
sand beach in such quantity as to form deposits from which a large
amount of gold is now being taken. Without this demonstration of the
origin and antiquity of the gold, it might very well have been supposed
to be derived from the eruptive rock.
Strong arguments against the theory that the leaching of superficial
igneous rocks has supplied the materials filling mineral veins, are
furnished by the facts observed in the districts where igneous rocks are
most prevalent, viz.: (1.) _Such districts are proverbially barren of
useful minerals_. (2.) _Where these occur, the same sheet of rock may
contain several systems of veins with different ores and gangues._
The great lava plain of Snake River, the Pedrigal country of eastern
Oregon, Northern California and Mexico are without valuable ore
deposits. The same may be said of the Pancake Range and other mountain
chains of igneous rock in Nevada, while the adjacent ranges composed of
sedimentary rocks are rich in ore deposits of various kinds. A still
stronger case is furnished by the Cascade Mountains, which, north of the
California line, are composed almost exclusively of erupted material,
and yet in all this belt, so far as now known, not a single valuable
mine has been opened. In contrast with this is the condition of things
in California, where the Sierra Nevada is composed of metamorphic rocks
which have been shown to be the repositories of vast quantities of gold,
silver, and copper. Cases belonging to this category may be found at
Rosita and Silver Cliff, where the diversity in the ores of the mines
already enumerated can hardly be reconciled with the theory of a common
origin. At Lake City the prevailing porphyry holds the veins of the Ute
and Ulay and the Ocean Wave mines, which are similar, and the Hotchkiss,
the Belle, etc., entirely different.
We have no evidence that any volcanic eruption has drawn its material
from zones or magmas especially rich in metals or their ores, and on the
contrary, volcanic districts, like those mentioned, and regions, such as
the Sandwich Islands, where the greatest, eruptions have taken place,
are poorest in metalliferous deposits.
All the knowledge we have of the subject justifies the inference that
most of the igneous rocks which have been poured out in our Western
Territories are but fused conditions of sediments which form the
substructure of that country. Over the great mineral belt which lies
between the Sierra Nevada and the front range of the Rocky Mountains,
and extends not only across the whole breadth of our territory, but far
into Mexico, the surface was once underlain by a series of Palaeozoic
sedimentary strata not less than twenty to thirty thousand feet in
thickness; and beneath these, at the sides, and doubtless below, were
Archæun rocks, also metamorphosed sediments. Through these the ores of
the metals were generally though sparsely distributed. In the
convulsions which have in recent times broken up this so long quiet and
stable portion of the earth's crust (and which have resulted in
depositing in thousands of cracks and cavities the ores we now mine),
portions of the old table-land were in places set up at high angles
forming mountain chains, and doubtless extending to the zone of fusion
below. Between these blocks of sedimentary rocks oozed up through the
lines of fracture quantities of fused material, which also sometimes
formed mountain chains; and it is possible and even probable that the
rocks composing the volcanic ridges are but phases of the same materials
that form the sedimentary chains There is, therefore, no _a priori_
reason why the leaching of one group should furnish more ore than the
other; but, as a matter of fact, the unfused sediments are much the
richer in ore deposits. This can only be accounted for, in my judgment,
by supposing that they have been the receptacles of ore brought from a
foreign source; and we can at least conjecture where and how gathered.
We can imagine, and we are forced to conclude, that there has been a
zone of solution below, where steam and hot water, under great pressure,
have effected the leaching of ore-bearing strata, and a zone of
deposition above, where cavities in pre-existent solidified and
shattered rocks became the repositories of the deposits made from
ascending solutions, when the temperature and pressure were diminished.
Where great masses of fused material were poured out, these must have
been for along time too highly heated to become places of deposition; so
long indeed that the period of active vein formation may have passed
before they reached a degree of solidification and coolness that would
permit their becoming receptacles of the products of deposition. On the
contrary, the masses of unfused and always relatively cool sedimentary
rocks which form the most highly metalliferous mountain ranges (White
Pine, Toyabe, etc.) were, throughout the whole period of disturbance, in
a condition to become such repositories. Certainly highly heated
solutions forced by an irresistible _vis a tergo_ through rocks of any
kind down in the heated zone, would be far more effective leaching
agents than cold surface water with feeble solvent power, moved only by
gravity, percolating slowly through superficial strata.
Richthofen, who first made a study of the Comstock lode, suggests that
the mineral impregnation of the vein was the result of a process like
that described, viz., the leaching of deep-seated rocks, perhaps the
same that inclose the vein above, by highly heated solutions which
deposited their load near the surface. On the other hand, Becker
supposes the concentration to have been effected by surface waters
flowing laterally through the igneous rocks, gathering the precious
metals and depositing them in the fissure, as lateral secretion produces
the accumulation of ore in the limestone of the lead region. But there
are apparently good reasons for preferring the theory of Richthofen:
viz., first, the veinstone of the Comstock is chiefly quartz, the
natural and common precipitate of _hot_ waters, since they are far more
powerful solvents of silica than cold. On the contrary, the ores
deposited from lateral secretion, as in the Mississippi lead region, at
low temperature contain comparatively little silica; second, the great
mineral belt to which reference has been made above is now the region
where nearly all the hot springs of the continent are situated. It is,
in fact, a region conspicuous for the number of its hot springs, and it
is evident that these are the last of the series of thermal phenomena
connected with the great volcanic upheavals and eruptions, of which this
region has been the theater since the beginning of the Tertiary age. The
geysers of Yellowstone Park, the hot springs of the Wamchuck district in
Oregon, the Steamboat Springs of Nevada, the geysers of California, the
hot springs of Salt Lake City, Monroe, etc., in Utah, and the Pagosa in
Colorado, are only the most conspicuous among thousands of hot springs
which continue in action at the present time. The evidence is also
conclusive that the number of hot springs, great as it now is in this
region, was once much greater. That these hot springs were capable of
producing mineral veins by material brought up in and deposited from
their waters, is demonstrated by the phenomena observable at the
Steamboat Springs, and which were cited in my former article as
affording the best illustration of vein formation.
The temperature of the lower workings of the Comstock vein is now over
150°F., and an enormous quantity of hot water is discharged through the
Sutro Tunnel. This water has been heated by coming in contact with hot
rocks at a lower level than the present workings of the Comstock lode,
and has been driven upward in the same way that the flow of all hot
springs is produced. As that flow is continuous, it is evident that the
workings of the Comstock have simply opened the conduits of hot springs,
which are doing to-day what they have been doing in ages past, but much
less actively, i.e., bringing toward the surface the materials they have
taken into solution in a more highly heated zone below. Hence it seems
much more natural to suppose that the great sheets of ore-bearing quartz
now contained in the Comstock fissure were deposited by ascending
currents of hot alkaline waters, than by descending currents of those
which were cold and neutral The hot springs are there, though less
copious and less hot than formerly, and the natural deposits from hot
waters are there. Is it not more rational to suppose with Richthofen
that these are related as cause and effect, rather than that cold water
has leached the ore and the silica from the walls near the surface? Mr.
Becker's preference for the latter hypothesis seems to be due to the
discovery of gold and silver in the igneous rocks adjacent to the vein,
and yet, except in immediate contact with it, these rocks contain no
more of the precious metals than the mere trace which by refined tests
may be discovered everywhere. If, as we have supposed, the fissure was
for a long time filled with a hot solution charged with an unusual
quantity of the precious metals, nothing would be more natural than that
the wall rocks should be to some extent impregnated with them.
It will perhaps illuminate the question to inquire which of the springs
and water currents of this region are now making deposits that can be
compared with those which filled the Comstock and other veins. No one
who has visited that country will hesitate to say the hot and not the
cold waters. The immense silicious deposits, carrying the ores of
several metals, formed by the geysers of the Yellowstone, the Steamboat
Springs, etc., show what the hot waters are capable of doing; but we
shall search in vain for any evidence that the cold surface waters have
done or can do this kind of work.
At Leadville the case is not so plain, and yet no facts can be cited
which really _prove_ that the ore deposits have been formed by the
leaching of the overlying porphyry rather than by an outflow of heated
mineral solutions along the plane of junction between the porphyry and
the limestone. Near this plane the porphyry is often thoroughly
decomposed, is somewhat impregnated with ore, and even contains sheets
of ore within itself; but remote from the plane of contact with the
limestone, it contains little diffused and no concentrated ore. It is
scarcely more previous than the underlying limestones, and why a
solution that could penetrate and leach ores from it should be stopped
at the upper surface of the blue limestone is not obvious; nor why the
plane of junction between the porphyry and the _blue limestone_ should
be the special place of deposit of the ore.
If the assays of the porphyry reported by Mr. Emmons were accurately
made, and they shall be confirmed by the more numerous ones necessary to
settle the question, and the estimates he makes of the richness of that
rock be corroborated, an unexpected result will be reached, and, as I
think, a remarkable and exceptional case of the diffusion of silver and
lead through an igneous rock be established.
It is of course possible that the Leadville porphyries are only phases
of rocks rich in silver, lead, and iron, which underlie this region, and
which have been fused and forced to the surface by an ascending mass of
deeper seated igneous rock; but even if the argentiferous character of
the porphyry shall be proved, it will not be proved that such portions
of it as here lie upon the limestone have furnished the ore by the
descending percolation of cold surface waters. Deeper lying masses of
this same silver, lead, and iron bearing rock, digested in and leached
by _hot_ waters and steam under great pressure, would seem to be a more
likely source of the ore. If the surface porphyry is as rich in silver
as Mr. Emmous reports it to be, it is too rich, for the rock that has
furnished so large a quantity of ores as that which formed the ore
bodies which I saw in the Little Chief and Highland Chief mines,
respectively 90 feet and 162 feet thick, should be poor in silver and
iron and lead, and should be rotten from the leaching it had suffered,
but except near the ore-bearing contact it is compact and normal.
Such a digested, kaolinized, desilicated rock as we would naturally look
for we find in the porphyry _near the contact_; and its condition there,
so different from what it is remote from the contact, seems to indicate
an exposure to local and decomposing influences, such indeed as a hot
chemical solution forced up from below along the plane of contact would
furnish.
It is difficult to understand why the upper portions of the porphyry
sheet should be so different in character, so solid and homogeneous,
with no local concentrations or pockets of ore, if they have been
exposed to the same agencies as those which have so changed the under
surface.
Accepting all the facts reported by Mr. Emmons, and without questioning
the accuracy of any of his observations, or depreciating in any degree
the great value of the admirable study he has made of this difficult and
interesting field, his conclusion in regard to the source of the ore
cannot yet be insisted on as a logical necessity. In the judgment of the
writer, the phenomena presented by the Leadville ore deposits can be as
well or better accounted for by supposing that the plane of contact
between the limestone and porphyry has been the conduit through which
heated mineral solutions coming from deep seated and remote sources have
flowed, removing something from both the overlying and underlying
strata, and by substitution depositing sulphides of lead, iron, silver,
etc., with silica.
The ore deposits of Tybo and Eureka in Nevada, of the Emma, the Cave,
and the Horn Silver [1] mines in Utah, have much in common with those of
Leadville, and it is not difficult to establish for all of the former
cases a foreign and deep seated source of the ore. The fact that the
Leadville ore bodies are sometimes themselves excavated into chambers,
which has been advanced as proof of the falsity of the theory here
advocated, has no bearing on the question, as in the process of
oxidation of ores which were certainly once sulphides, there has been
much change of place as well as character; currents of water have flowed
through them which have collected and redeposited the cerusite in sheets
of "hard carbonate" or "sand carbonate," and have elsewhere produced
accumulations of kerargyrite, perhaps thousands of years after the
deposition of the sulphide ores had ceased and the oxidation had begun.
In the leaching and rearrangement of the ore bodies, nothing would be
more natural than that accumulations in one place should be attended by
the formation of cavities elsewhere.
[Footnote 1: The Horn Silver ore body lies in a fault fissure between a
footwall of limestone and a hanging wall of trachyte, and those who
consider the Leadville ores as teachings of the overlying porphyry would
probably also regard the ore of the Horn Silver mine as derived from the
trachyte hanging wall; but three facts oppose the acceptance of this
view, viz., let, the trachyte, except in immediate contact with the ore
body, seems to be entirely barren; 2d, the Horn Silver ore "chimney,"
perhaps fifty feet thick, five hundred feet wide, and of unknown depth,
is the only mass of ore yet found in a mile of well marked fissure; and
3d, the Carbonate mine opened near by in a strong fissure with a bearing
at right angles to that of the Horn Silver, and lying entirely within
the trachyte, yields ore of a totally different kind. Both are opened to
the depth of seven hundred feet with no signs of change or exhaustion.
If the ore were derived from the trachyte, it should be at least
somewhat alike in the two mines, should be more generally distributed in
the Horn Silver fissure, and might be expected to give out at, no great
depth.
If deposited by solutions coming from deep and different sources, the
observed differences in character would be natural; it would accumulate
as we find it in the channels of outflow, and would be as time will
probably prove it, perhaps variable in quantity, but indefinitely
continuous in depth.]
Another question which suggests itself in reference to the Leadville
deposits is this: If the Leadville ore was once a mass of sulphides
derived from the overlying porphyry by the percolation of surface
waters, why has the deposit ceased? The deposition of galena, blende,
and pyrite in the Galena lead mines still continues. If the leaching of
the Leadville porphyry has not resulted in the formation of alkaline
sulphide solutions, and the ore has come from the porphyry in the
condition of carbonate of lead, chloride of silver, etc., then the
nature of the deposition was quite different from that of the similar
ones of Tybo, Eureka, Bingham, etc., which are plainly gossans, and
indeed is without precedent. But if the process was similar to that in
the Galena lead region, and the ores were originally sulphides, their
formation should have continued and been detected in the Leadville
mines.
For all these reasons the theory of Mr. Emmons will be felt to need
further confirmation before it is universally adopted.
From what has gone before it must not be inferred that lateral secretion
is excluded by the writer from the list of agencies which have filled
mineral veins, for it is certain that the nature of the deposit made in
the fissure has frequently been influenced by the nature of the adjacent
wall rock. Numerous cases may be cited where the ores have increased or
decreased in quantity and richness, or have otherwise changed character
in passing from one formation to another; but even here the proof is
generally wanting that the vein materials have been furnished by the
wall rocks opposite the places where they are found.
The varying conductivity of the different strata in relation to heat and
electricity may have been an important factor. Trap dikes frequently
enrich veins where they approach or intersect them, and they have often
been the _primum mobile_ of vein formation, but chiefly, if not only, by
supplying heat, the mainspring of chemical action. The proximity of
heated masses of rock has promoted chemical action in the same way as do
the Bunsen burners or the sand baths in the laboratory; but no case has
yet come under my observation where it was demonstrable that the filling
of a fissure vein had been due to secretion from igneous or sedimentary
wall rocks.
In the Star District of Southern Utah the country rock is Palæozoic
limestone, and it is cut by so great a number and variety of mineral
veins that from the Harrisburg, a central location, a rifle shot would
reach ten openings, all on as many distinct and different veins (viz.,
the Argus, Little Bilk, Clean Sweep, Mountaineer, St. Louis, Xenia,
Brant, Kannarrah, Central, and Wateree). The nearest trap rock is half a
mile or more distant, a columnar dike perhaps fifteen feet in thickness,
cutting the limestone vertically. On either side of this dike is a vein
from one to three feet in thickness, of white quartz with specks of ore.
Where did that quartz come from? From the limestone? But the limestone
contains very little silica, and is apparently of normal composition
quite up to the vein. From the trap? This is compact, sonorous basalt,
apparently unchanged; and that could not have supplied the silica
without complete decomposition.
I should rather say from silica bearing hot waters that flowed up along
the sides of the trap, depositing there, as in the numerous and varied
veins of the vicinity, mineral matters brought from a zone of solution
far below.
To summarize the conclusions reached in this discussion. I may repeat
that the results of all recent as well as earlier observations has been
to convince me that Richthofen's theory of the filling of the Comstock
lode is the true one, and that the example and demonstration of the
formation of mineral veins furnished by the Steamboat Springs is not
only satisfactory, but typical.
* * * * *
[NATURE.]