Principles of Mining: Valuation, Organization and Administration

ation (Co

RUCTURAL CHARACTER OF THE DEPOSIT; SECONDARY ENRICHMENT

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 occasiona

ch 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 mu

ur 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 al

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 exten

which bear on such extension, and these are almost wholly dependent upon the character of

uctural character

ings in relation to

of the

the mine has alre

istrict for continuity and the

ating 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 aven

itted, as not being of sufficiently frequent occurren

ns above the pay limit are ore-bodies, or ore-shoots. The localization of

s created by structural movement,

of solutions from different sources, provided precipita

ks by:- Their solubility, and ther

a precipitating a

e as a source

ut fissures; in friable rocks, zones of brecciation; in slates or schistose rocks, linked lenticular open spaces;-thes

pon it. This is a difficult study into the physics of stress in cases of fractur

ear the surface, more

eposit is a necessary study, and even a digest of the

f values, ore-deposits may be roughly

rous beds, such as Lake Superior copper

e vein type, such as C

on deposits on the lines

of ore and of its metal contents. In the second, depending much upon the profundity of the earth movements involved, there is l

nts 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

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

n depth has necessitated a complete re?quipment. For instance, the Mt. Morgan gold mine, Queensland, has now become a coppe

ore-deposits may be consid

e dominating feature is oxidation a

, where the predominant feature is the depositi

here the special feature is the enrichmen

secondary alteration, where the

y are in evidence will depend, among other things, upon the amount and rapidity of erosi

he 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,

alteration is to raise slowly the concentrated metals toward the surface, and thus subject them to renewed attack and repeated migration. In this

ertain 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 extensi

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

rren "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

duction 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 combinati

idized 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 c

icles. 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

difficult of solution from ground agencies, as witness the great alluvial deposits, often of considerable geologic age. In consequence the phenomena of migration and enr

s from secondary alteration must be kept

reated more cheaply. This is not universal. Low-grade ores of lead, copper, and zinc may be treatable by co

richment at the base of the oxidized,

lly less pronounced enrichment below water level than copper. The rearrangement of the metals

iched in the oxidized zone, and at times ten

alue from the surface through the w

one of the oldest of miners' beliefs. It has some basis in experience, especially in fissure veins, bu

e 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 oft

pothesis 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 deepe

s 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 lowes

e 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, t

o 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 the