Charles George Warnford Lock.

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I immense quantities are produced also in the new processes of steel
anufEurtnre. Hence the utilisation of slag * is an important question,
arge quantities are used for making paving setts and as road metal,
id, in a very finely powdered condition, for cement manufacture,
nddle slag and heating cinder have been recently utilised at Boonton,
^«w Jersey, for paint. The process involves crushing the slag to an
npalpable powder with a Cyclone pulveriser, and then setting it
ith some sorting action in air chambers. Used directly and alone,
; affords a dark olive-green paint, which also makes an excellent
ody for other and brighter shades. In making reds, the coarser
rnshed material from the first treatment is mixed with sulphuric
cid and allowed to sweat, as it is called, for 4 days. This changes
be slag from silicate to sulphate of iron. It is then calcined to afford
be oxide, and reground. The mill is making 5-6 tons of paint stock
aily. Basic slag contains roughly 17 per cent, phosphoric acid and
per cent. lime. While not a suitable fertiliser for all soils (cal-
areons ones for instance), yet for sour, peaty, and clay soils it is of
Teat value, as is shown by the fact that all the 600,000 tons a year
lade in England are sold at 20-30«. a ton at the woijks. It is
Dteresting to note that the phosphoric acid is combined with the
ime in a difierent way from what has been found in nature, and
nstead of being a tri-basic phosphate it is a tetra-phosphate, readily
oluble in water, and thus only requires very fine grinding in order
be utilised by plants. At first the attempt was made to treat it
>J various chemical methods, the same as super-phosphate, but it
Kras not successful until applied simply in the ground state. Prof.



♦ G. Redgrave, " Utilisation of Slag," Soc. Arts Jl., Jan. 31, 1890.

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514 ECONOMIC MINING,

Scheibler's ingenious process for extracting the phosphoric acid is
therefore but little used. He first calcines the slag in an oxidisiDg
flame, then pulverises and sifts it. The powder is dissolved in hydro-
chlonc acid, and the solution is saturated with milk of lime. In tiiis
way a substance is produced which is said to contain 35-37 per cent
phosphoric acid, under the form of bi-basic phosphate of lime. A



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METALLIFEROUS MINERALS. 515



LEAD.

^HiL8 the distribution of lead ores is wide, both geographically and
;eologically, it has been assei-ted * that the period of their formation
as had considerable inflneuce upon the amount of silver associated
rith them. The principal ores and their content of lead when pure
re: —

Lead.
Per cent.

Galena, sulphide, PbS 86}

CerDssite, carbonate, PbCO, 77}

Angledte, snlphate, PbSO^ 67}

Pyromorphite, pboapbate and chloride, Pb,P,0„ PbCl, •. 75

In addition, various mixtures of these and limonite (iron oxide)
■© Worked.

The chief centres of lead production are Cumberland, Cornwall,
kd Derbyshire in England ; Fuy de Ddme and Bretagne in France ;
kzon Erzgebirge, Silesia, Harz and parts of Khenish Prussia in
armany; Carinthia in Austria; Linares and Cartagena in Spain;
beria, Ural, and the CaucasuM; the Rooky Mountains and Upper
ississippi Valley ; and recently, Broken HUl, Australia.

The production in the United Kingdom shows a steady decline
»m about 100,000 tons of ore worth 10/. a ton in 1870 to only 45,000
IS value 8/. 10s. a ton in 1890.

The plumbiferous region t of Puy de D6me is, roughly speaking,

nndnlating plateau of ciystalline schists, ranging from ohloritic
list to gneiss. The age of the lode formation is placed by Lodin
;ween the Miocene and Pliocene. Eruptive rocks are present in
>at variety, two kinds especially : — (a) acidic, usually occurring as
kee, and of age anterior to the lode formation ; (&) basic, generally
lava flows, of age posterior to the lodes, whose croppings they often
er. The acidic type is represented for the most part by granulite
pegmatite, sometimes rendered porphyritic by the presence of large
Htalfl of felspar. The dykes of this rock are very numerous. Their
ke is usually between N.N.E. and N.N. W., and their thickness varies
n a few inches to more than 60 ft. Their general course coincides
h that of the mineral belt. In addition to these there are others
lorpbyrite, far less considerable in number, and of an age later
a the lode formation. The connection between the lodes and the
Qulite dykee is most marked. The veins of ore consist of a barytic

quartzose filling, containing blende and galena, occurring in
aks of varying regularity in the midst of the dyke rock, which is
etimee brecciated and crushed, sometimes solid and unbroken.

• F. C. von Petersdorflf.

t T. A. Bkkard, ** Tbe Lodes of Pontgibaud," En. and Min. JL

2 L 2 .

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Si6 ECONOMIC MINING.

When the dyke diminishee in size, the streak of ore deoietses i
width ; when the lode fracture ceases to be accompanied by dj)
rock, and penetrates the enclosing gneiss or schist, then the veb
filling narrows and becomes .barren of ore.

Numerous veins of argentiferous galena occur in Devonian rod
in the vicinity of Brilon, Miisen, and Siegen, in Rhenish Westpbdi
In the sandstone of Bleiberg, in the Eiffel district of Dliren, are lu]
deposits of lead in the form of nodules or KikMen^ and the rock
which they belong is known as KnoUeinr%a%dai^n, The nodules cona
chiefly of galena, and more rarely of cerussite. They are spherio
concretions, usuallv smaller than a pea, and at Bleiberg constits
4 to 10 per cent, of the whole of the bed. The total production
lead ore in these provinces in 1890 was 82,400 tons ; and of this tot
the district of Commem-Ckm&ad (where the Meinerzhagener Bleiba
mine Mechemich is the chief producer) yielded 43,440 tons. In il
Hars, lead ores occur in the Devonian and Lower Carboniferous rod
of Clausthal and Zellerfeld ; in the Devonian rocks of Bammelsbei]
and in the Silurian rocks of St. Andreasberg and Haizgerode. H
Rammelsberg deposits consist of lenticular masses of ore chiefly ooi
posed of iron pyrites, copper pyrites, and galena. The productioo
concentrated ore in the Uarz for 1890 was: Clausthal, 7233 ten
Lautenthal, 1683; Orund, 4635; Andreasberg, 239: Rammelsbd
34,818. In the Er^zebirge the lodes occur in crystalline schists ai
in igneous rocks. The ^ief district is Freiberg, where tiiere i
about 90 lodes, chiefly in gneiss, both red and grey, which toward tl
west is overlain by mica schists and day-slates. The schistose stnl
are traversed by eruptive rocks, some of which have become convertj
into serpentine. The output of ore in the Freiberg district for 18J
was 31,502 tons. The deposits of Upper Silesia containing zinc u
lead are enclosed in beds of the lower *' Muschelkalk." Gkdena ooca
in the form of grains or in seamlike deposits in dolomite ; the this
ness of the be& is not more than 12 ft. The produce for 1891 ^
27,616 tons. The production of lead ore in Prussia in 1891 amonnti
to 140,112 metric tons.

The lead mines of the Spanish provinces of Murcia and Almeii^
exist in various geological formations. The north group, that'
Mazarron, occurs in tvpical trachytes, which are traversed by a m
work of powerful lodes of galena, often accompanied by bl^ide i^
pyrites; the Sierra of the Lomo de Bas has numerous smfdl veijtf'
clay slates, probably of Devonian age ; the groups of the Baladre tf
Charcon, and Jarabia, contain galena, blende, pyrites, and ^
spathic iron ; in the Sierra Almagrera tiie mines are very numeit)^
and many have proved highly remunerative from their considerab
percentage in silver, but they are worked under great diffioultiec,t
account of the intense heat underground, and from the large inflow <
water in the deeper workings. The Sierra Almagrera is fcHined \
slates and schists, more or less metamorphio and non-fo6silif«n70
being probably Silurian. At the foot of this mountain mass, in ^
plain below, at the Herrerias, is found an immense deposit of ii^
stone, a portion of which is manganiferous,and furnishes an excelled
* J. Deby, ** Argentiferous Lead Mines of Spain,*' indaislries.

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METALLIFEROUS MINERALS. 517

ore of iron ; while another portion contains sand, gravel, and frag-
ments of barytes, and abo silver, both in the native state and as a
chloride. Farther south, liie Bedar and the Beforma mines are
f^orked in a remarkable formation, consisting of a conglomerate, or
rather breccia, constituted by fragments of limestone united by coat-
ings and strings of galena, and which often attains a thickness of
150 ft. This rests upon Tertiary dolomitic limestone, which in turn
ies unconformably on mica schist. In some places tiie galena is re-
placed by the blue and green carbonates of copper. The production
>f the conglomerate is no doubt due to the intrusion in the neighbour-
lood of an immense body of basaltic rocks. The approximate total
)rodnction of lead in Spain jduring 1892 may be fixed at 160,000 tons,
►f which one-third only is argentiferous.

Gralena is common in many parts of India, but Cuddapah and
Barnaul are probably richest in the ore. In Cuddapah, at the village
•f Judgumrazpilly, close to the Nallamallay Hills, old lead workings
re conspicuous. A sample of ore from these regions yielded 78 per
ent. lead and 22 oz. silver to the ton of lead. Both these and the
Camaul mines would probably repay scientific exploitation hand-
amely. 'J'hree analyses of ore from uie latter yielded, respectively,
74 oz., 175 oz., and 165 oz. silver to the ton, which would render
bem well worth working. In Bengal galena has been found in the
onthal Pergunnahs and also in Bhagalpore. An analysis of ore
>tind in the latter district showed 78 per cent, lead and 103 oz. silver
> the ton.

In the United States the chief sources (80-85 per cent.) of lead in
ite years have been argentiferous ores, and considerable from zinc
res, bat a notable exception is found in S.E. Missouri,* where galena
xx>mpanied by nickeliferous pyrite is disseminated through mag-
esian limestone of Cambrian age. The Mines are at Bonne Terre,
Line la Motte, and Doe Bun. The strata lie almost horizontal, and
re known to carry lead through over 200 ft. in thickness. The pro-
active places fade out into barren rock, and appear to be local
irichments of the limestone, of which the galena forms an integral
xrt. At Bonne Terre, they are of enormous size, one working
inning 3000 ft., and being 100-200 ft. broad and 25-60 ft. high. No
nc, however, occurs with the lead, and the silver contents are very
aall, being about 4 oz. to the ton of lead. At Mine la Motte some
tpper is found, and considerable nickel and cobalt. Pyrite accom-
knies the galena, and carries the nickel and cobalt, which is obtained
a bye-product in the lead smelting. All the ore bodies are crossed
7 small faults, adjoining which the rock is invariably barren,
nobs of Archaean granite, containing diabase dykes, crop out near
e mines, but never penetrate the limestone, and were evidently in-
aded before it was laid down. The ore must have been deposited
itb the limestone, or have been introduced since the latter was
rmed, and by the percolation of ore-bearing solutions through the
ck, with no marked fissure vein development. It is a curious fact
at as the ore bodies are followed up to the faults they invariably
come lean or run out. Their place of formation has apparently
♦ J. F. Kemp, * Ore Deposits,' p. 158.



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Si8 ECONOMIC MINING.

some connection with low folds at right angles to the faults. The
ore bodies favour the anticlinal bends. This whole region of Cam-
brian and Lower Silurian rooks, over nearly 3000 sq. miles, conUiM
lead. The ore affords an average of about 8 per cent, galeoa.
Except at Mine la Motte, lead was also once obtaiDed from small gash
veins, but the workings were never commensurate with the preaent
mines of disseminated ore.

The gash veins and horizontal cavities (" flats ") of Wisooimn *
are limited to the Galena and Trenton limestones, and contain galena,
blende, pyrite (or marcasite), calcite, barite, and residual clay. The
Galena limestone is a dolomite, 250 ft. thick; under it lies the
Trenton, 40-100 ft. thick, in two portions — an upper, blue, dod-
magnesian, and a lower, buff, magnesian. The ore beds spedallv
favour the shallow synclinal depressions in the E.-W. folds, and
occur in crevices, which are alternately barren and productive.
Lead ores predominate in the Upper Gralena ; zinc ores in the Low«t
Galena and the Trenton. The upper deposits are mostly in vertical
gashes; the lower in "flats," which dip down at the ends ("pitoh^"!
and often connect with another flat. I'he ores were probably deposited
with the limestones.

The Missouri deposits occur in the Keokuk or Archimedes limestcfw
of the Lower Carboniferous, in "runs," 100-300 ft. long, 10-50 ft.
wide, and 5-50 ft hijih ; and even larger. As a general thing the
ore is in interstices of brecciated chert, but it is alno in limestooe
and dolomite, and associated with a siiicified form of the in»4nbl«
residue left by the solution of the limestone, which Dr. Jennej cali^
"cherokite." All the ores require concentration. Galena naoaliy
occurs near the surface, while blende is more abundant in depth.
Cadmium is at times present in the blende in notable amount.

In Wythe county, Virginia, are similar strata in limestone cff
dolomite, impregnated with lead and zinc

Ot by far the greatest importance are the Leadville (Coloiado
bodies of oxidised silver-lead ores, passing in depth into salphides
in much faulted Carboniferous dolomitic limestone, associated with
dykes and sheets of porphyry. The ores are chiefly earthy lead oar-
bonate, with silver chloride, in a clayey or silicious mass of hydrated
oxides of iron and manganese. Sometimes silver chloride oocan
without lead. Some zinc is also found, and many rare nuDonl^
Where the ore is in a hard, silicious, limonite gangue, it is called
'* hard '' carbonate ; but where it is sandy and incoherent, it fiors^
** soft " carbonate, or " sand " carbonate. All the mines produce sbsi2
amounts of gold, occasionally of more importance than the mlTef-
A few ore bodies are found at other horizons than the Carhoniforwa.
They also run in instances as much as 100 ft. from the ocmtact, aad
may likewise be found in the porphyry, doubtless replacing indadrf
limestone. According to 8. F. Emmons, the ore bodies were depostd
from aqueous solutions, and originally as sulphides, at a gn*
depth below the rock-surface (probably 10,000 ft.); by subseqno*
dynamic movements and by erosion, they have been brought to tl»^
present position near the surface; through secondary alteratk*
♦ T. C. Chamberlain, Wisoooain Geol. Survey, iv. 367.



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METALLIFEROUS MINERALS. 519

by surface- waters, they have been changed to oxides, carbonates and
chlorides ; that the process of deposition was a metasomatic inter-
change between the minerals brought in in solution and the lime-
8ton€^ — that is, they were not deposited in already existiDg open
cavities, but gradually replaced the limestone, from the channels
through which they reached it outwards; the wilutions or ore-
currents reached the present ioctia of the deposits directly from
above, and not from below ; and whatever may have been the ultimate
»mrce from which the minei*al components of the deposits came,
the observed facts point to the neighbouring eruptive rocks as the
immediate source.

On AKpen Mountain ♦ the ore bodies favour the contact between
the blue limestone and the brown dolomite. The former is very
pare, while the latter contains 20-28 per cent, magnesium carbonate.
The ore replaces and impregnates the blue limestone, often with very
little change in its appearance, but it fills the numerous cracks in the
more broken dolomite, coating larger and smaller blocks. The ore
occurs also in minor fissures. On Smuggler Mountain the ore espe-
cially follows the fissure veins.

At Bed Mountain, Ouray county, oxidised silver-lead ores, passing
into sulphides below, are met with in large and small cavities in
knobs of silicified andesite.

In the Coeur d'Alene, Idaho,t are very important and productive
bodice of galena and subordinate alteration products, in a mineralised
eone having a well-marked quartzite footwall and an impregnated,
brecciated hanging wall of the same rock. The ore is in large shutes,
i^hich fill innumerable small fractures in the rocks — quartzite and
thin beds of schists, much folded, along east and west axes, by which
they became faulted and shattered, and in the principal mineral belt
ifibrded an opportunity for the ore to deposit ; the gangue is siderite.

Most of the Utah argentiferous lead deposits are in blue limestone.
Alt the Horn silver mine is a great contact fissure vein between a
rhyolite hanging wall and a limestone footwall. At Carbonate mine
\ fissure vein occurs in homblende-andesite. At the Cane mine,
chambers in limestone carry limonite and oxidised silver-lead ore»
5-7 per cent, lead), chiefly valuable for fluxing.

The mines of Eureka, Nevada, are in Cambrian limestone.

The total production of the United States is 150,000-200,000 tons
mnually, and Mexico, 25,000 tons.

The most remarkable mines in the world are the Broken Hill
p-oup,} New South Wales, which have yielded metal to the value of
ibout 10 million Hterling in less than 10 years, and still afford weekly
>OO - 8O0 tons lead, and over 200,000 oz. silver, 'i'he geological featnres
ire metamorphosed clay-slates and talcose mica-schists, traversed by
uaasee and dykes of granite and diorite, generally N. E.-S.W., but
boDEietimes forming networks. The schists locally pass into gneiss,
md that into porpnyritic granite. The rocks strike generally N.W.,
ind dip N. W. about 63^, but the formation is much disturbed in parts.

• J. P. Kemp, op. dt t J. E. Clayton, En. ond Miu. Jl., Feb. 11, 1888.

t M. B. Jamiesoo and J. Howell. ^ Mining and Ore Treatment at Broken Hill,''
If in. Proo. Inti. Ciy. Enge., Papet No. 2U09. *



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520 ECONOMIC MINING.

The lode appears to be a mineralised bed without regular and well*
defined walls. There is evidence of the existence of an ticlines at Tarioiis
points, and the whole may come together again beneath, althong^ it
IS quite as probable that the lode now being worked is only one of ft
series of ** saddle reefs." The surface outcrop consists of masses of
manganese oxide, gossan, altered schist, garnet-rock, quartz, and
quartzite, with some felspar; rich specimens of iodide, chloride,
and chloro-bromide of silver also have been found on the surfiwe in
some places. Lead carbonate did not appear in any quantity at tk
surface, but underneath both carbonate and phosphate were found in
great quantities, containing silver in a variety of combiuations. The
small quantity of iron pyrites in the mine does not account for th«
presence of the large amount of ferrous oxide in the upper porti(m of
the lode by the decomposition of iron sulphides. The ore bodi^
dassed according to the methods of treatment applicable to them,
are : — (a) suitable for blast-furnace treatment, chiefly lead carboDtW
and ferrous oxide, carrying silver ; (6) concentrating, both oiidised
and sulphide, carrying lead and a high percentage of silica ; (c) con-
taining silver in combination with chlorine and bromine, which,
after chloridising and roasting, are suitable for leaching ; (j^ fit for
amalgamation, containing silver as chloride, chloro-bromiae, aod
metaDic, associated with a small quantity of lead ; (e) areentifenms
lead and zinc sulphides requiring concentration, and which, of low
grade, exist in enormous quantities below the permanent water-level
Another classification is : — (a) iron and manganese oxides, contaiidiig |
22-160 oz. silver per ton; (o) lead carbonate, oontaioing 10-55 per
cent, lead, and 7-11 Ooz. silver per ton; (c) kaolin, containing l2-7(^'
oz. silver per ton, usually metallic, or as chloride, iodide, chlvrr^
bromide, or bromide, with or without a small percentage of lead u
cerussite ; (d) copper carbonate and oxide, containing lead in the fom
of silicate and carbonate, with 30-200 oz. silver per ton, freqnentlr
associated with massive silver chloride and native copper ; (e) garnrt
rock, being crystals of manganese-iron garnet, carrying 8-70 oz. ailvtf
per ton, and a small amount of lead ; (/} sulphide oree, chieflj d
lead and zinc, with 7-80 oz. silver per ton, and an average of aboQt
30 per cent, silica and garnet, about 26 per cent, lead, and 21 per
cent. zinc. The ore-bodies vary much in size, the greatest width
yet disclosed being 316 ft., least 15 ft., and average about K^fi
Manganese-iron ore may be said to form the capping of the kde
throughout, and the greatest depth to which the iron ore may b«
said to exist exclusively is 300 ft. from the surface, although it ba«
been discovered at 400 ft. Underneath the iron, the greatest exta;
of ore exists as lead carbonate and masses of kaolin ; copper carbouAt*
is found in a horizontal seam within the lead carbonate and kaoli
ore-bodies. Underneath the lead carbonate and kaolin lie enennoo
bodies of sulphides. At several points, at varying depths, the lod
is split into two legs by ** horses " of barren rock, more or leas as.
tinuously throughout its length. In places the lode stands alizM
vertical, but, on the whole, the greatest dip is to the west Tl>
average level of the sulphide zone is about 300 ft from the sorfece
the nearest point to the surface at which sulphides have been fboB*

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METALLIFEROUS MINERALS, 521

A about 110 ft., whilst the greatest depth at which oxidised ores
lave yet been found is about 515 ft. At some places thoroughly
)xidi8ed and almost pure sulphide ores are found side by side to as
preat a depth as 200 ft.

Dresnng. — The process to be chosen for extracting the lead from
ts ores will depend upon several conditions— the composition and
rield of the ore, character of gangue, influence of foreign matters,
Inx and fuel supplies — but practically all ores require to be first
Iressed, both to remove undesirable impurities, and to enrich the ore
D metallic contents. Chief among the foreign substances is silver,
rhich all lead ores contain, but only those which afford it in quanti-
166 sufficient to repay extraction are called argentiferous. While
ilver facilitates smelting, and adds value when abundant enough, it
ften complicates the dressing process, especially when its combina-
ions possess less specific gravity than the lead ore. All other metal-
tferous foreign matters may be regarded as injurious— zinc blende,
dbnite, iron, copper, and arsenical pyrites— must be removed, so that
ressing becomes a highly important operation. Generally speaking,
iad dressing fallows the principles and employs the appliances
Iready described under Concentration (pp. 133-52), and repetition
I not needed here ; but a few typical installations merit attention.

As an instance of the application of simple methods, reference may
e made to the rectangular inclined plane made of wood lined with
lieet iron (in Wales), or of stone (in Persia), on which, by some degree
r skill, zinc carbonate may be washed away from lead carbonate by
mply raking the mixture against a steady stream of water. But
res amenable to such easy treatment are rare.

The ores treated at Clausthal, in the Harz, consist of low-grade
'gentiferous galena, somewhat finely scattered through a gangue of
despar and baryta, and mixed with both copper and iron pyrites,
arcasite, and zincblende. The works are among the largest and
06t extensive in the world, their capacity being about 650 tons a
ly. in erecting and arranging the works, advantage has been
ken, in the usual way, of the slope of the hillside, the ore entering
pon its course of treatment on the highest, and leaving it on the
west level. The water used for dressing, and for driving a part of
te machinery, is brought by a ditch to the place where it is first
>€ded, whence it descends; after having been used on the higher
vel, it is allowed to cUar in tanks before being used again on the
»xt. In its downward course it passes through several series of
volving screens, settling boxes, jigs of all classes, stamp-batteries,
iddles, tables, &c ; it also drives several turbines and one overshot
heel before it finds rest in the slime pits on the lowest leveL It
metimee happens that all the water available is necessary for the



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 58 of 76)