PROSPECTIVE VALUE.[*] EXTENSION IN DEPTH; ORIGIN AND STRUCTURAL
CHARACTER OF THE DEPOSIT; SECONDARY ENRICHMENT; DEVELOPMENT IN
NEIGHBORING MINES; DEPTH OF EXHAUSTION.
[Footnote *: The term "extension in depth" is preferred by many
to the phrase "prospective value." The former is not entirely
satisfactory, as it has a more specific than general application.
It is, however, a current miner's phrase, and is more expressive.
In this discussion "extension in depth" is used synonymously, and
it may be taken to include not alone the downward prolongation of
the ore below workings, but also the occasional cases of lateral
extension beyond the range of development work. The commonest instance
is continuance below the bottom level. In any event, to the majority
of cases of different extension the same reasoning applies.]
It is a knotty problem to value the extension of a deposit beyond
a short distance from the last opening. A short distance beyond
it is "proved ore," and for a further short distance is "probable
ore." Mines are very seldom priced at a sum so moderate as that
represented by the profit to be won from the ore in sight, and what
value should be assigned to this unknown portion of the deposit
admits of no certainty. No engineer can approach the prospective
value of a mine with optimism, yet the mining industry would be
non-existent to-day were it approached with pessimism. Any value
assessed must be a matter of judgment, and this judgment based on
geological evidence. Geology is not a mathematical science, and
to attach a money equivalence to forecasts based on such evidence
is the most difficult task set for the mining engineer. It is here
that his view of geology must differ from that of his financially
more irresponsible brother in the science. The geologist, contributing
to human knowledge in general, finds his most valuable field in the
examination of mines largely exhausted. The engineer's most valuable
work arises from his ability to anticipate in the youth of the mine
the symptoms of its old age. The work of our geologic friends is,
however, the very foundation on which we lay our forecasts.
Geologists have, as the result of long observation, propounded for
us certain hypotheses which, while still hypotheses, have proved
to account so widely for our underground experience that no engineer
can afford to lose sight of them. Although there is a lack of safety
in fixed theories as to ore deposition, and although such conclusions
cannot be translated into feet and metal value, they are nevertheless
useful weights on the scale where probabilities are to be weighed.
A method in vogue with many engineers is, where the bottom level
is good, to assume the value of the extension in depth as a sum
proportioned to the profit in sight, and thus evade the use of
geological evidence. The addition of various percentages to the
profit in sight has been used by engineers, and proposed in technical
publications, as varying from 25 to 50%. That is, they roughly
assess the extension in depth to be worth one-fifth to one-third
of the whole value of an equipped mine. While experience may have
sometimes demonstrated this to be a practical method, it certainly
has little foundation in either science or logic, and the writer's
experience is that such estimates are untrue in practice. The quantity
of ore which may be in sight is largely the result of managerial
policy. A small mill on a large mine, under rapid development,
will result in extensive ore-reserves, while a large mill eating
away rapidly on the same mine under the same scale of development
would leave small reserves. On the above scheme of valuation the
extension in depth would be worth very different sums, even when the
deepest level might be at the same horizon in both cases. Moreover,
no mine starts at the surface with a large amount of ore in sight.
Yet as a general rule this is the period when its extension is most
valuable, for when the deposit is exhausted to 2000 feet, it is
not likely to have such extension in depth as when opened one hundred
feet, no matter what the ore-reserves may be. Further, such bases
of valuation fail to take into account the widely varying geologic
character of different mines, and they disregard any collateral
evidence either of continuity from neighboring development, or from
experience in the district. Logically, the prospective value can
be simply a factor of how _far_ the ore in the individual mine
may be expected to extend, and not a factor of the remnant of ore
that may still be unworked above the lowest level.
An estimation of the chances of this extension should be based
solely on the local factors which bear on such extension, and these
are almost wholly dependent upon the character of the deposit.
These various geological factors from a mining engineer's point
of view are:--
1. The origin and structural character of the ore-deposit.
2. The position of openings in relation to secondary alteration.
3. The size of the deposit.
4. The depth to which the mine has already been exhausted.
5. The general experience of the district for continuity and
the development of adjoining mines.
THE ORIGIN AND STRUCTURAL CHARACTER OF THE DEPOSIT.--In a general
way, the ore-deposits of the order under discussion originate primarily
through the deposition of metals from gases or solutions circulating
along avenues in the earth's crust.[*] The original source of metals
is a matter of great disagreement, and does not much concern the
miner. To him, however, the origin and character of the avenue
of circulation, the enclosing rock, the influence of the rocks
on the solution, and of the solutions on the rocks, have a great
bearing on the probable continuity of the volume and value of the
[Footnote *: The class of magmatic segregations is omitted, as
not being of sufficiently frequent occurrence in payable mines to
warrant troubling with it here.]
All ore-deposits vary in value and, in the miner's view, only those
portions above the pay limit are ore-bodies, or ore-shoots. The
localization of values into such pay areas in an ore-deposit are
apparently influenced by:
1. The distribution of the open spaces created by structural
movement, fissuring, or folding as at Bendigo.
2. The intersection of other fractures which, by mingling of
solutions from different sources, provided precipitating
conditions, as shown by enrichments at cross-veins.
3. The influence of the enclosing rocks by:--
(a) Their solubility, and therefore susceptibility to replacement.
(b) Their influence as a precipitating agent on solutions.
(c) Their influence as a source of metal itself.
(d) Their texture, in its influence on the character of
the fracture. In homogeneous rocks the tendency
is to open clean-cut fissures; in friable
rocks, zones of brecciation; in slates or schistose
rocks, linked lenticular open spaces;--these
influences exhibiting themselves in miner's terms
respectively in "well-defined fissure veins,"
"lodes," and "lenses."
(e) The physical character of the rock mass and the
dynamic forces brought to bear upon it. This
is a difficult study into the physics of stress in
cases of fracturing, but its local application has
not been without results of an important order.
4. Secondary alteration near the surface, more fully discussed
It is evident enough that the whole structure of the deposit is
a necessary study, and even a digest of the subject is not to be
compressed into a few paragraphs.
From the point of view of continuity of values, ore-deposits may
be roughly divided into three classes. They are:--
1. Deposits of the infiltration type in porous beds, such as
Lake Superior copper conglomerates and African gold bankets.
2. Deposits of the fissure vein type, such as California quartz veins.
3. Replacement or impregnation deposits on the lines of fissuring
In a general way, the uniformity of conditions of deposition in
the first class has resulted in the most satisfactory continuity of
ore and of its metal contents. In the second, depending much upon
the profundity of the earth movements involved, there is laterally
and vertically a reasonable basis for expectation of continuity
but through much less distance than in the first class.
The third class of deposits exhibits widely different phenomena
as to continuity and no generalization is of any value. In gold
deposits of this type in West Australia, Colorado, and Nevada,
continuity far beyond a sampled face must be received with the
greatest skepticism. Much the same may be said of most copper
replacements in limestone. On the other hand the most phenomenal
regularity of values have been shown in certain Utah and Arizona
copper mines, the result of secondary infiltration in porphyritic
gangues. The Mississippi Valley lead and zinc deposits, while irregular
in detail, show remarkable continuity by way of reoccurrence over
wide areas. The estimation of the prospective value of mines where
continuity of production is dependent on reoccurrence of ore-bodies
somewhat proportional to the area, such as these Mississippi deposits
or to some extent as in Cobalt silver veins, is an interesting
study, but one that offers little field for generalization.
THE POSITION OF THE OPENINGS IN RELATION TO SECONDARY ALTERATION.--The
profound alteration of the upper section of ore-deposits by oxidation
due to the action of descending surface waters, and their associated
chemical agencies, has been generally recognized for a great many
years. Only recently, however, has it been appreciated that this
secondary alteration extends into the sulphide zone as well. The
bearing of the secondary alteration, both in the oxidized and upper
sulphide zones, is of the most sweeping economic character. In
considering extension of values in depth, it demands the most rigorous
investigation. Not only does the metallurgical character of the ores
change with oxidation, but the complex reactions due to descending
surface waters cause leaching and a migration of metals from one
horizon to another lower down, and also in many cases a redistribution
of their sequence in the upper zones of the deposit.
The effect of these agencies has been so great in many cases as
to entirely alter the character of the mine and extension in depth
has necessitated a complete reŽquipment. For instance, the Mt.
Morgan gold mine, Queensland, has now become a copper mine; the
copper mines at Butte were formerly silver mines; Leadville has
become largely a zinc producer instead of lead.
From this alteration aspect ore-deposits may be considered to have
1. The zone near the outcrop, where the dominating feature
is oxidation and leaching of the soluble minerals.
2. A lower horizon, still in the zone of oxidation, where the
predominant feature is the deposition of metals as native,
oxides, and carbonates.
3. The upper horizon of the sulphide zone, where the special
feature is the enrichment due to secondary deposition
4. The region below these zones of secondary alteration, where
the deposit is in its primary state.
These zones are seldom sharply defined, nor are they always all
in evidence. How far they are in evidence will depend, among other
things, upon the amount and rapidity of erosion, the structure and
mineralogical character of the deposit, and upon the enclosing
If erosion is extremely rapid, as in cold, wet climates, and rough
topography, or as in the case of glaciation of the Lake copper
deposits, denudation follows close on the heels of alteration,
and the surface is so rapidly removed that we may have the primary
ore practically at the surface. Flat, arid regions present the
other extreme, for denudation is much slower, and conditions are
most perfect for deep penetration of oxidizing agencies, and the
consequent alteration and concentration of the metals.
The migration of metals from the top of the oxidized zone leaves
but a barren cap for erosion. The consequent effect of denudation
that lags behind alteration is to raise slowly the concentrated
metals toward the surface, and thus subject them to renewed attack
and repeated migration. In this manner we can account for the enormous
concentration of values in the lower oxidized and upper sulphide
zones overlying very lean sulphides in depth.
Some minerals are more freely soluble and more readily precipitated
than others. From this cause there is in complex metal deposits a
rearrangement of horizontal sequence, in addition to enrichment at
certain horizons and impoverishment at others. The whole subject
is one of too great complexity for adequate consideration in this
discussion. No engineer is properly equipped to give judgment on
extension in depth without a thorough grasp of the great principles
laid down by Van Hise, Emmons, Lindgren, Weed, and others. We may,
however, briefly examine some of the theoretical effects of such
Zinc, iron, and lead sulphides are a common primary combination.
These metals are rendered soluble from their usual primary forms
by oxidizing agencies, in the order given. They reprecipitate as
sulphides in the reverse sequence. The result is the leaching of
zinc and iron readily in the oxidized zone, thus differentially
enriching the lead which lags behind, and a further extension of
the lead horizon is provided by the early precipitation of such
lead as does migrate. Therefore, the lead often predominates in
the second and the upper portion of the third zone, with the zinc
and iron below. Although the action of all surface waters is toward
oxidation and carbonation of these metals, the carbonate development
of oxidized zones is more marked when the enclosing rocks are
In copper-iron deposits, the comparatively easy decomposition and
solubility and precipitation of the copper and some iron salts
generally result in more extensive impoverishment of these metals
near the surface, and more predominant enrichment at a lower horizon
than is the case with any other metals. The barren "iron hat" at the
first zone, the carbonates and oxides at the second, the enrichment
with secondary copper sulphides at the top of the third, and the
occurrence of secondary copper-iron sulphides below, are often
most clearly defined. In the easy recognition of the secondary
copper sulphides, chalcocite, bornite, etc., the engineer finds a
finger-post on the road to extension in depth; and the directions
upon this post are not to be disregarded. The number of copper
deposits enriched from unpayability in the first zone to a profitable
character in the next two, and unpayability again in the fourth,
Silver occurs most abundantly in combination with either lead,
copper, iron, or gold
. As it resists oxidation and solution more
strenuously than copper and iron, its tendency when in combination
with them is to lag behind in migration. There is thus a differential
enrichment of silver in the upper two zones, due to the reduction
in specific gravity of the ore by the removal of associated metals.
Silver does migrate somewhat, however, and as it precipitates more
readily than copper, lead, zinc, or iron, its tendency when in
combination with them is towards enrichment above the horizons of
enrichment of these metals. When it is in combination with lead
and zinc, its very ready precipitation from solution by the galena
leaves it in combination more predominantly with the lead. The
secondary enrichment of silver deposits at the top of the sulphide
zone is sometimes a most pronounced feature, and it seems to be
the explanation of the origin of many "bonanzas."
In gold deposits, the greater resistance to solubility of this
metal than most of the others, renders the phenomena of migration to
depth less marked. Further than this, migration is often interfered
with by the more impervious quartz matrix of many gold deposits
Where gold is associated with large quantities of base metals,
however, the leaching of the latter in the oxidized zone leaves the
ore differentially richer, and as gold is also slightly soluble,
in such cases the migration of the base metals does carry some of
the gold. In the instance especially of impregnation or replacement
deposits, where the matrix is easily permeable, the upper sulphide
zone is distinctly richer than lower down, and this enrichment is
accompanied by a considerable increase in sulphides and tellurides.
The predominant characteristic of alteration in gold deposits is,
however, enrichment in the oxidized zone with the maximum values
near the surface. The reasons for this appear to be that gold
resistance to oxidation and wholesale migration gives opportunities
to a sort of combined mechanical and chemical enrichment.
In dry climates, especially, the gentleness of erosion allows of
more thorough decomposition of the outcroppings, and a mechanical
separation of the gold from the detritus. It remains on or near
the deposit, ready to be carried below, mechanically or otherwise.
In wet climates this is less pronounced, for erosion bears away
the croppings before such an extensive decomposition and freeing
of the gold particles. The West Australian gold fields present an
especially prominent example of this type of superficial enrichment.
During the last fifteen years nearly eight hundred companies have
been formed for working mines in this region. Although from four
hundred of these high-grade ore has been produced, some thirty-three
only have ever paid dividends. The great majority have been unpayable
below oxidation,--a distance of one or two hundred feet. The writer's
unvarying experience with gold is that it is richer in the oxidized
zone than at any point below. While cases do occur of gold deposits
richer in the upper sulphide zone than below, even the upper sulphides
are usually poorer than the oxidized region. In quartz veins
preŽminently, evidence of enrichment in the third zone is likely
to be practically absent.
Tin ores present an anomaly among the base metals under discussion,
in that the primary form of this metal in most workable deposits
is an oxide. Tin in this form is most difficult of solution from
ground agencies, as witness the great alluvial deposits, often of
considerable geologic age. In consequence the phenomena of migration
and enrichment are almost wholly absent, except such as are due
to mechanical penetration of tin from surface decomposition of
the matrix akin to that described in gold deposits.
In general, three or four essential facts from secondary alteration
must be kept in view when prognosticating extensions.
Oxidation usually alters treatment problems, and oxidized ore
of the same grade as sulphides can often be treated more cheaply.
This is not universal. Low-grade ores of lead, copper, and zinc
may be treatable by concentration when in the form of sulphides,
and may be valueless when oxidized, even though of the same grade.
Copper ores generally show violent enrichment at the base of the
oxidized, and at the top of the sulphide zone.
Lead-zinc ores show lead enrichment and zinc impoverishment in
the oxidized zone but have usually less pronounced enrichment
below water level than copper. The rearrangement of the metals
by the deeper migration of the zinc, also renders them
metallurgically of less value with depth.
Silver deposits are often differentially enriched in the oxidized
zone, and at times tend to concentrate in the upper sulphide zone.
Gold deposits usually decrease in value from the surface through
the whole of the three alteration zones.
SIZE OF DEPOSITS.--The proverb of a relation between extension
in depth and size of ore-bodies expresses one of the oldest of
miners' beliefs. It has some basis in experience, especially in
fissure veins, but has little foundation in theory and is applicable
over but limited areas and under limited conditions.
From a structural view, the depth of fissuring is likely to be more
or less in proportion to its length and breadth and therefore the
volume of vein filling with depth is likely to be proportional to
length and width of the fissure. As to the distribution of values,
if we eliminate the influence of changing wall rocks, or other
precipitating agencies which often cause the values to arrange
themselves in "floors," and of secondary alteration, there may be
some reason to assume distribution of values of an extent equal
vertically to that displayed horizontally. There is, as said, more
reason in experience for this assumption than in theory. A study
of the shape of a great many ore-shoots in mines of fissure type
indicates that when the ore-shoots or ore-bodies are approaching
vertical exhaustion they do not end abruptly, but gradually shorten
and decrease in value, their bottom boundaries being more often
wedge-shaped than even lenticular. If this could be taken as the usual
occurrence, it would be possible (eliminating the evident exceptions
mentioned above) to state roughly that the minimum extension of an
ore-body or ore-shoot in depth below any given horizon would be
a distance represented by a radius equal to one-half its length. By
length is not meant necessarily the length of a horizontal section,
but of one at right angles to the downward axis.
On these grounds, which have been reŽnforced by much experience among
miners, the probabilities of extension are somewhat in proportion
to the length and width of each ore-body. For instance, in the A
mine, with an ore-shoot 1000 feet long and 10 feet wide, on its
bottom level, the minimum extension under this hypothesis would
be a wedge-shaped ore-body with its deepest point 500 feet below
the lowest level, or a minimum of say 200,000 tons. Similarly,
the B mine with five ore-bodies, each 300 hundred feet long and
10 feet wide, exposed on its lowest level, would have a minimum of
five wedges 100 feet deep at their deepest points, or say 50,000
tons. This is not proposed as a formula giving the total amount of
extension in depth, but as a sort of yardstick which has experience
behind it. This experience applies in a much less degree to deposits
originating from impregnation along lines of fissuring and not at
all to replacements.
DEVELOPMENT IN NEIGHBORING MINES.--Mines of a district are usually
found under the same geological conditions, and show somewhat the same
habits as to extension in depth or laterally, and especially similar
conduct of ore-bodies and ore-shoots. As a practical criterion, one
of the most intimate guides is the actual development in adjoining
mines. For instance, in Kalgoorlie, the Great Boulder mine is (March,
1908) working the extension of Ivanhoe lodes at points 500 feet
below the lowest level in the Ivanhoe; likewise, the Block 10 lead
mine at Broken Hill is working the Central ore-body on the Central
boundary some 350 feet below the Central workings. Such facts as
these must have a bearing on assessing the downward extension.
DEPTH OF EXHAUSTION.--All mines become completely exhausted at
some point in depth. Therefore the actual distance to which ore
can be expected to extend below the lowest level grows less with
every deeper working horizon. The really superficial character of
ore-deposits, even outside of the region of secondary enrichment
is becoming every year better recognized. The prospector's idea
that "she gets richer deeper down," may have some basis near the
surface in some metals, but it is not an idea which prevails in
the minds of engineers who have to work in depth. The writer, with
some others, prepared a list of several hundred dividend-paying
metal mines of all sorts, extending over North and South America,
Australasia, England, and Africa. Notes were made as far as possible
of the depths at which values gave out, and also at which dividends
ceased. Although by no means a complete census, the list indicated
that not 6% of mines (outside banket) that have yielded profits,
ever made them from ore won below 2000 feet. Of mines that paid
dividends, 80% did not show profitable value below 1500 feet, and
a sad majority died above 500. Failures at short depths may be
blamed upon secondary enrichment, but the majority that reached
below this influence also gave out. The geological reason for such
general unseemly conduct is not so evident.
CONCLUSION.--As a practical problem, the assessment of prospective
value is usually a case of "cut and try." The portion of the capital
to be invested, which depends upon extension, will require so many
tons of ore of the same value as that indicated by the standing
ore, in order to justify the price. To produce this tonnage at
the continued average size of the ore-bodies will require their
extension in depth so many feet--or the discovery of new ore-bodies
of a certain size. The five geological weights mentioned above
may then be put into the scale and a basis of judgment reached.
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