Charles George Warnford Lock.

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the present ores are haloid salts of silver, hydrosilicate and carbonate
of copper, resulting from primary rich silver sulphides and copper
pyrites. Another class, closely connected with the former, occur as
irregular surface-deposits in tufa or (rarely) in liparite, in the former
case often approximately following planes of bedding. Still another
class of little-investigated deposits are found as veins in the Pliocene
sandstone or on its contact with a liparitic breccia. They diffiar firom
the rest in carrying a larger quantity of lead-ores. The principal
deposits are genetically connected with fractures and dislocaticHis,
and their origin is that of usual fissure- veins formed by precipitation
or deposition from aqueous solutions. The smaller, irregular surface-
deposits, usually occurring in the tufa, carry exactly the same ores as
the fissures and are rarely, if ever, found far from l^em ; always, too,
on a lower level. The only difference between the two classes of
deposits is that the latter carry little or no barite. The most natural
explanation is to consider them as having been deposited by the
solutions filling the fissures and overflowing from them. The ores
are almost exclusively chloride and chlorobromide of silver, usually
accompanied along the fissures by a gangue of barite. Calcite is not
very frequent; manganese minerals (pyrolusite, &c.) are common;
finely distributed rich sulphides occur sparingly. Small masses of a
black sulphide of silver, copper and lead have been found. There is
a remarkable absence of base metals, and the bullion is usually very
fine. A black earthy mass, locally called " black oxide," proves to be
a mixture of barite and chloride-ore stained with manganese. The
cerargyrito and emboli te mostly occur as thin coatings on joints and
cracks, but often also imbedded in the barite. In almost all the
mines the ore has grown poorer or disappeared before any great
depth has been reached ; in none of the few exceptions from this vule
have the explorations extended more than 200 feet below the c
On the other hand, the gangue continues apparently without
ishing or changing. Fig. 106 illustrates the formation : a,
h, sandstones and clays ; c, liparite tufa ; d, liparite ; e, ore de]

♦ W. LiiidgTcn. " The Silver Mioee of Calico, California," Trans, hf^^n. }mIl
Mil). Engs., XT. 717.

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The ore deposits of Ouray County,* Colorado, are either in the
Tertiary eruptives or, confined to a relatively narrow zone, in the
sedimentary beds just beneath them. The ore-
deposits in the igneous formation would be
classed as fissure-veins. In addition to these,
however, there is a form of fissure locally called
"chimneys." In the sedimentary formation, the
orenieposits are either fissures, usually along xi^-iicxviv'!^
fault-planes, or intercalated beds between lime-
stone (foot-wall), and a quartzite or shale (hang-
ing-wall). The ore-body of greatest commercial
value occurs in the Carboniferous crystalline
limestone, at its contact with the overlying pink
quartzite. The minimum thickness of the vein
material is 3 ft. ; but it extends downward into
fche limestone for a very variable distance, being
nearly 100 ft. thick in some workings, and even
reaches nearly to the base of the limestone. The
^eatest longitudinal extent of these bodies of
rein-materiad is in a direction parallel with the
itrike of the enclosing rocks. The rich ore-
X)die8 in the vein-material are very irregular,
ind the line of demarcation between rich ore
ind low-grade vein-material is exceedingly in-
[efinite. There is, however, an apparent ten-
lenoy of the richer argentiferous iron-ores to
ssume a horizontal position, while the light-
oloured kaolin bands with which the lead-vaTues
re associated assume a position nearer vertical
long the original fracture-planes of the lime-
tone. The pyrite and the great mass of the
ein material is low grade in silver, containing
om traces up to 20 oz. per ton. The silver
dsts as sulphide, rarely native, and masses of
ilphide are found almost pure (over 20,000 oz.
Iver per ton). Fig. 167 shows the formation :
andesite ; 6, upper quartzite : c, pink quartz-
e; d, limestone; e, lower quartzite; /, vein

The Aspen mining district,t Colorado, ex-
bits an impregnation or mineralisation of the
untry rock in place, either blue limestone or
lomite, in the neighbourhood of stratified or
Qlted contacts. The method by which the
ineralisation has been produced differs in the
ses of the two rocks. The dolomite does not
^m to have been more readily decomposed by
e mineral-bearing solutions, but owing to its

* Q. £. Kedzie, TraDs. Amer. Inst. Min. Engs., xvi. 570.

t W. B. Newberry, ** Geology of the Ajspen Mining rHstrict," Trans. Amer. Inat.

0. Bogs., xviii. 277.

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short Btmctnre, and many cleavage planes, it has been more readilv
permeable by them, so that much of the brown limestone ore con-
sists of small cubes of nearly unaltered dolomite, witii the TalnaUe
mineral deposited mainly in its cleavage-planes. The dolomite his,
however, in many cases, thoroughly lost its structure, and is ofleB
reduced to the condition of a dolomite sand, impregnated witk
valuable mineral. The mineralisation of the blue limestone seems to

Fio. 167.— SiLVBB Deposits: Oubat.

have been accomplished by the replacement of a portion of the calcite
without materially altering its structure. In many cases the lock
retains its appearance, and the mineralisation can only be disoorered
by assay. !Even in such cases the rock usually appears more ciystil-
line and porous than when barren. The faulted contact is character-
ised by a steeper pitch than the stratified contact, and by the great
quantity of wall-rock contained by it, and the strong evidence of
motion. There seems to be a general system of ore-shutes in these
limestones, having a southerly direction, often interrupted by fitnlts,
but probably in many cases continuous along the fault-planes. From
the developments so far made, the trend of these ore-shutes seenw to
be about S. 66° W., swinging slightly to S. with increased depth.
The gangue of the Aspen ores is generally either limestone or
sulphate of baryta, or both, passing from ores containing as high as
90 per cent, carbonate of lime, to others containing 70 per cent
sulphate of baryta. The amount of silica in the ores is generally
small, rarely running over 20 per cent. The bulk of the ore is very
low in lead, that containing 25 per cent, being a rarity ; an average
would contain not far from 55 oz. silver per ton, with less than 5 per
cent, lead, and 12-15 per cent, sulphate of baryta.

Custer County, Idaho, contains several well-known mines. The
Bamshom is in metamorphic slates on a fissure vein that has rich
shoots of high-grade silver ores in a siderite gangue. Other minefl
have veins in porphyry, with quartz gangue ; and others again are in
granite areas. Enormous masses of ruby silver sometimes occur.

At Silver Islet, Lake Superior, is a fissure vein carrying native

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silver, argentite, tetrahedrite, galena, blende, and some nickel and
oobolt Gomponnds in a gangue of calcite, in flags and shales of tbe
Animikie (Cambrian) system, and outting a large trap dyke, witbin
wbich alone tbe vein is productive. Silver Islet is or was originally
little more tban a bare rock some 90 ft. square, lying off tbe north
shore of Lake Superior jnst outside of Thunder Bay, and witbin the
Canadian boundaries. Native silver was detected outcropping; beneath
the water. The vein was productive to a depth of 800-1000 ft., but
below this it yielded little. The trap dyke has usually been called
diorite, but is pronounced to be norite by Wadsworth and gabbro by
Irving. Some 600,0002. was obtained from the mine, yet the expenses
were so great in keeping up the surface works against winter gales
and ice wiat but little profit was realised. The vein has been traced
9000 ft., but is nowhere else productive. Considerable graphite has
been found in the workings; and according to W. M. Courtis,* the
silver deposits of the district are found only where the country rock
containfl carbon in some form ; and the absence of the carbonate spars
in the barren parts of the veins seems to indicate that carbon or
hydrocarbons played an active part in the deposition of the silver
and the formation of the veins. Some of the vugs, both at Silver
Islet and the Duncan, contained hydrocarbon gas under great

Montana is one of the richest silver producing states, notably in
the Butte city region. In the basic granite, and north of the copper
zone, is a belt carrying sulphides of silver, lead, zinc, and iron in a
silioious gangue, but abundantly associated with manganese com-
pounds of various sorts, especially rhodochrosite. No manganese is
Known in the copper belt, nor any copper in the silver belt — ^most
striking phenomena in veins in tne same wall rock. There are as
many as 4 distinct veins. All the mines show that the ore and
gangue have replaced the granite along a shattered strip, for cross
sections exhibit alternations of quartz with ore, rhodochrosite, crushed
wall rock, residual clay, occasional horses of granite, &c. In the
more silicious granite west of the butte is another silver belt with
the same ores. Pig. 164 shows a cross section t of a vein 40 ft wide :
a, granite country ; 6, softened granite full of quartz veinlets ; c, clay
wall with decomposed granite; d^ quartz, broken and seamed;
e, quartz and manganese spar — " curly ore " ; /, quartz and ore —
'*hard vein." The cross-veining of the granite by quartz seains is
venr extensive, especially at the 300 ft. level. The veinlets intersect
each other and form a network, enclosing blocks of granite 1-3 ft.
diam. or more. The central portions of all these blocks are hard,
dark-ooloured, and compact unchanged granite, not affected by the
veins, while the rock next to the veins and seams is softened and
partially kaolinised, turning a light grey colour. The veins seem to
have affected the rock on both sides. These narrow veins extend for
many feet with an even width, and do not fault or throw each other
at the intersections. The vertical veinlets, 1-6 in. thick, are filled

* Trans. Amer. Inst. Min. Engs., xt. 673.

t W. P. Blake, "Rainbow Lode, Butte, Montana," Trans. Amer. Inst. Min.
Engs,, xYi 65.

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with snlpliides, while the highly inclined or horizontal seamfl are
nearly harren. Below 200 ft., the quartz in the lodes is white, and
the metalliferous contents are bright and glancing, giving a brilliaiLt
metallic ore, carrying iron pyrites, galena, zinc blende, and silphide
of silver. All these minerals are disseminated in the quartz in a
granular, sub-crystalline condition, and in general are in dcee
mixture and association, the exception being the iron pyrites, whidi
is frequently disseminated in the gangue, a little apart from the
galena and blende. Wire-silver occurs not only in the quartz t^-
stone, or with the well defined ore, but sometimes in the granitk
rock of the horses or divisions of the lode, apparently disoomiected
from any vein-stone. Masses of the ^ blue ores " are often covered
with such filaments and ¥dres of silver, so thickly matted and corkd
over the surface, as to resemble a woolly covering. Theee filauaentg
are often several inches long. They are, in general, not much thicbr
than horse-hair; but specimens have been taken out which exhUa;

Fia. 168.— SiLVEB Deposits: Butte, Moktaka.

the silver in curled masses \ in. thick. Gold is always whu^-^
with the native silver. There are some good reasons to support tb?
opinion, that the silver and gold are not disseminated in the balk d
the ore, but chiefly in certain layers or portions of it, and in a oks-
paratively free state, not enveloped or combined with the bift?
metals or their sulphides. This suggests the jwssibility of workiE^
the ores without roasting. Some of the massive sulphides of le»i
and zinc, almost free of quartz, contain but little silver. Otl^
portions are extremely rich.

In Deer Lodge County, Montana, are very rich silver depo^tE ii
fissure veins in eruptive granite. The pay shoots do not reii
within 100 ft. of the surface. A broad zone of mineralised limat®'
follows the edge of the granite, and contains large bodies of mangins*
ore carrying more or less silver. The Drumlumu
are on the contact between a granite knob and the
morphic schists.

In Nevada, the ores are in general silver-lea<
veins with sulphides in quartzite and granite. A
Pine County, the ore bodies occur, according to
four forms, all in Devonian limestone: — (1) in fie
anticlinal axis; (2) in contact deposits between 1

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shales; (3) in beds or ohambers in the limestone parallel to the
stratification ; (4) in irregular vertical and oblique seams across the
bedding. The ore is chiefly chloride of silver in quartz gangue. It
ifl thought by Hague to have probablj come up through the main
cross fissure, and, meeting the impervious shale, to have spread
through the limestone in this way. The ore bodies of the Reese
River district are in a hill of biotite granite pierced by a rhyolite
dyka At Tuscarora, the veins lie in decomposed homblende-andesite.

The great Ck>mstock Lode * is a fissure vein, 4 miles long, forked
into two branches above, along a line of faulting in eruptive rocks of
the Tertiary age, chiefly andesites. In the central part of the vein,
the displacement has been about 3000 ft, shading out, however, at
the ends. The ores are high-grade silver ores in quartz, and occur in
;reat bodies, called '* bonanzas,*' along the east vein. Over 60,000,0002.
in gold and silver has been extracted, in the ratio of two of the
'ormer to three of the latter. This remarkable vein strikes N.-S.,
iips £. about 43^ and is usually 20-60 ft. thick, swelling to several
lundred feet in places. The ore bodies are soft and irregular ; and the
emperature of the workings is excessive (air 126° F. and water as
ligh as 170*^ F.).

Some of the Utah silver mines have been mentioned under Lead,
rhe Ontario mine,t ^^ Summit County, is on a vein 4-23 ft. (average

I ft.) wide, in quartzite, which has been worked on a length of 6000 ft. ;
he best parts of the mine have quartzite walls ; the ores are silver
:lanoe, grey copper, galena, blende, &o. Similar veins occur in the
eighbonrhood. In riute County are a number of mines in quartz-
orphyry or between limestone and

uartzite. At Silver Eeef are en-

)untered deposits of native silver,

rgentite and cerargyrite impreg-

ating Triassic sandstones. Above

drmanent water line the ore is horn

Iver; below, argentite. The im-

regnation is probably due to igneous

itbreaks subsequent to the deposi-

on of the sandstone. Even where Fio. 169.— 8n,vBR Deposits:

> sUver is visible in the rock it may ^"'^ ^^^ ^^^"•

Donnt to 30 oz. per ton. The for-

ation exhibits much irregularity and faulting, as s^n in Fig. 169 :

impiiegnated sandstone (ore) ; 6, barren sandstone.

Treatfoeid, — The extraction of silver from its ores and combina-
>n8 is effected by a number of processes, which may be grouped
ider three heads : — (a) Amalgamation, in which the silver is caught

mercury ; {hi) Smelting, where a lead or copper bath is the reoep-
cle of the silver ; and (c) Lixiviation, or solution in alkaline liquors,
finetiines a combination of two or more processes is employed.

Amalgamation is only adapted to the free-milling ores, generally
ose found near the surface, and which are free from sulphur. If
Iplmr is present, it must be removed by a preliminary roasting,

II oh is accomplished in furnaces of several types. The older forms

• J. F. Kemp, • Ore Depoeite,' p. 233. t Ibid., p. 225.

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are the long hand-worked reverberatories (see p. 439), the Howell-
White and Brtiokner revolving cylinders (see p. 439), and later the
Stetefeldt shaft, to be now described. More recently, we have Pearoe'i
turret furnace (see p. 440), and others of that type.

The Stetefeldt furnace. Fig. 170, consists of a contracting shaft o,
heated by two fireplaces h ; the flue c which carries off the waste gases
and dust is heated by a supplementary fireplace <i The charge it
admitted by the iron hopper e, provided with a draw-valve /, and
surmounted by a compbcated mechanism for regulating the feed.

Fig. 170. — Stetefeldt Fubmagb.

The fire gases from h enter the shaft a at ^, and a supply of free air
is admitted at h under proper control. These ascend together and
meet the falling stream of powdered ore (30-40 mesh). At t are peep-
holes, and at A;, openings ^r the insertion of tools to remove slagged
ore from the furnace walls if any should form. The roasted ore
collects in the pit Z, and is drawn away at nt into ears n. The dust
flue c has a number of working doors o, and delivers its accumulations
into the pits ^, which discharge into cars r. The fire gases from ^
lay over the contents of jp. O^mmon dimensions are : shaft, 30-3o

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up to 50 ft. high, and 4-6 up to 6 ft. square; walls, of brick, 8 in.
thick and 3 in. apart, the interval being filled with sand or ashes to
conserve the heat. In working silver ores it is usnal to give a
'* chloridising " roast, salt being mixed with the ore before charging ;
it decomposes in the ftumace and attacks the silver, forming an arti-
ficial chloride. This reaction is favoured by the greatest heat per-
missible short of sintering the ore, and is prolonged by keeping the
ore hot after it has been drawn from the fdmaoe. When running
imder a strong draught, the capacity of the furnace is large, and
fally half the ore passes over into the dust flue c, about 80 per cent,
falling in the first pit j? ; the usual total amount of dust is 30-50 per
cent. By leaving the dust for a long time (some days) in the flue,
chlorination is perfected. At 9 the gases pass to a chimney, which
should be 50-100 ft. high and 4-5 ft. square, and is best built on high
ground. The capacity of the furnace varies chiefly with the ore, say
from 30 tons per 24 hours with compact sulphurets to 65-70 tons
with loose oxidised ores. The loss of volatilised silver (augmented
when antimony, arsenic, or zinc is present) is said to be much less
than with reverberatory hearths. The consumption of salt varies
much, and can only be determined by experiment in each case. A
sufficiency should always be used ; but where its cost is very high, it
will form a serious item. The Butte ores require 15 per cent., which
has amounted to IO9. per ton of ore chloridised. The labour for a
fornace treating 25 tons per 24 hours is 1 fireman and 2 feeders per
shift. The fuel consumption varies with the ore, but averages 2\
cords good dry wood per 24 hours for 20-25 tons ; it may be as little
«8 \\ cord for very oxidised ore, and as much as 5 cords for sulphides.
A furnace to treat 40-100 tons per 24 hours will require 5000 fire-
brick, 250,000-275,000 common brick, 3000-4000 cub. ft. of stone,
and about 20 tons of ironwork. Ores varying from 30 to 800 oz.
sQver per ton are roasted in this furnace, and the chlorination effected
ranges from 85 to 95 per oent.

The *' Freiberg " process is also known as the " barrel " process of
amalgamation. The following description supposes its application in
conjunction with the reverberatory hearth. Points to be kept in
view are : — (a) The ore must not contain more than 4 per cent, lead
or 1 per cent, copper, {b) There must be 34 per oent. iron pyrites
present, (c) The ore is mixed with 10 per cent, salt and very care-
fully calcined; a double-bedded furnace is the best, being of such
construction that the mineral can be gradually raked from the coolest
portion of the upper bed to the hottest, and thence through a hole on
to the coolest portion of the lower bed, and ultimately to the hottest
portion of the lower bed ; the heat should be very carefully applied,
80 as to avoid the clotting of the ore, a mishap to which it is liable on
account of the large amount of sulphur present ; it should also be
borne in mind that at a strong red heat silver chloride volatilises.
((f) The calcined ore must be carefully sifted, so as to separate the
clotted portions, if any ; these latter being crushed and recaldned
with a fresh charge of ore. The sifted portion should be intimately
mixed with 2-3 per cent, salt, and again calcined, (e) The twice-
calcined ore is ground, and passed through a fine sieve. (/) Then

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put into the barrels, the charge for each barrel being 10 cwt. ealemed
ore, 3 cwt. water, and 80 lb. wronght-iron scraps in pieces as amall
and regular as possible. In charging, the water is introduced firet
then the ore, and finally the iron. The barrels are now set rotating,
and a thorough incorporation of the mass is attained ; this is one c^
the most important points of the process, as the iron reduces tk
silver chloride (formed during the calcining) to metallic silver, a
process which otherwise the mercury, subsequently added, will h&re
to perform with much loss of mercury in the form of calomel (meicoij
chloride) ; after having been rotating for about 2 hours, the contend
of the barrels are examined, and a proper consistency is arrived at bj
adding more water or calcined ore, as the case may be. {g) 5 cvt
mercury is added, and the rotation is continued for about 18 hours,
the barrels revolving about 16 times a minute ; the contents are then
examined, and if amalgamation is found to have taken place satii-
factorily, the barrels are filled up ^ith water, and after being rotated
slowly for about 2 hours, the amalgam is drawn ofL, and subseqaendr
the slime, the latter being carefally washed to collect small globules
of mercury that may be disseminated through the mass ; the matter
washed away should be passed over a slime table. The rate of rota-
tion should be neither so rapid as to cause the mercury to stick to tlie
sides of the barrels, nor so slow as to allow it to collect in the lowest

The Patio (Mexican), the Cazo (Chilian), and the Fondo (Bdiyian)
processes * are virtually one, and are the basis of the practical metal-
l^irgy of silver in all Spanish-speaking countries ; yet they present
some differences worth noting. The ore is usually first hand-sorted
according to its richness or to the gangue, and sometimes imdergoei
a preliminaiy pile-roasting to remove excess of sulphur ; it is tki
pulverised in stamp batteries, Chilian mills, rolls, or breakers, b
next passes to the artaxiTa^ a rude yet efficient machine, oonsistini
essentially of a shallow circular space 12-14 ft. diam., paved witfc
slabs of hard stone, and having stone walls 20 in. high, and famished
with a central post carrying 4 arms, each of which drags a heavj
hard stone as it rotates, driven by mule, water, or steam power. The
duty varies from 600 to 1200 lb. per 24 hours, sufficient menaiy
being added to amalgamate all the free gold and silver present, acd
water to make a pulp. The rubbing in presence of mercury renden
this a most effective machine, though slow ; and the absence of irQ&*
work eliminates one of the sources of trouble encountered in modem
machines designed to replace it, when working on acid ores.

Amalgamation in the arrastra is only used when the ore carriee
free gold or silver, or bromides, chlorides, or iodides of the latter
Otherwise, it is simply a fine grinding process in preparation for tb
j^tio. This is a large court with a paved and slightly inclined flocc
on which the liquid pulp from the arrastra is spread to dry till it !»•
a thick muddy consistence, when it is piled in extemporised en-
closures of 15-160 tons each, called torto«, in a bed about 10 in. thid;
and 4-5 per cent, salt is evenly scattered over it, and incorporated bj
spading and more particularly by a laborious prooess of treading bj

* T. Egleston, ' The Patio and Gaze Proceas of Amalgamating Silfor Oraa'

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nanles, wbich bo maoliinery has efficiently superseded. The next
$tep is to add the mcigistral, an uncertain mixture of copper and iron
nilpbates and oxides with many impurities, obtained by calcining
jopper and iron pyrites, but the efficiency of which depends solely

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 68 of 76)