Charles George Warnford Lock.

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platinum as potassium-platinum chloride. The whole is boated to
17 6"^ F. and left to stand for some days. The sodium chloride ii
added because the double chloride is more insoluble in a solution of it
tli«n in a menstruum of ammonium chloride. The rhodium present

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remaiDS in solution as a double salt. When the precipitate has com-
pletely settled, it is washed repeatedly with a Kolution of ammonium
chloride and finally with water. As some rhodium may still be
present, the chloride is dried and mixed with potassium bisutphate
containing a small quantity of ammonium sulphate, and the whole is
then heated in a platinum dish until the platinum is completely
reduced. The rhodium remains as a bisulphate which can be removed
by water, leaving behind pure platinum sponge. This is melted in a
lime-furnace consisting of two cylindrical pieces of quicklime hollowed
out to receive the charge of metal, then fitted together, and the whole
encircled by bands of iron. The roof is slightly arched, and has a
conical opening through which is passed the tube by which the fuel
is introduced. The hearth is flat, and the sides are curved to meet
the arch above ; it should be of such a width that the melted metal
will have a thickness not exceeding 3-4 mm., and in the lower part
is provided with a lip which serves as a vent for the flame and as a
tap-hole for the metaL The fuel used is illuminating gas mixed with
oxygen in order to cibtain the highest posnible temperature. Hydn*-
gen was formerly used instead of illuminating gas, but, although
^iving a greater heat than the latter, its use has been abandoned on
account of its greater cost. In this furnace the platinum is both
melted and refined, any iron or silicon present being absorbed by the
lime, while osmium is volatilised as oxide. The c«»st of preparing
crude metallic platinum is variously stated at 1«. 3<2.-2«. 6(2. an oz.

Commerce, — The qualities which render platinum so valuable are
its reeistance to oxygen, simple acids, and sulphur (but not to alkalies),
and its infusibility ; it is largely employed in chemical apparatus,
standards of measurement, &c., dentistry, and latterly most extensively
in electric lamps, telegraphic instruments, non-magnetic watches,
Btylographic pens, photography, and many other minor applicationn.
The market value of the prepared metal fluctuates between 30«. and
21. an oz. approximately, but is always uncertain. Besides the figures
of production incidentally given, it may be mentioned that Russia's
output amounts to 6000-10,000 lb. yearly, according to official
returns, but probably one-third more evades being taxed and re-

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Although seleninm* does not occnr in great abundanoOt it is widdy
distributed over the globe. Native selenium occurs as liolite at
Culebras, Mexico. Mixed with sulphur, it is found as selen-sulj^nr
in Volcano, one of the Lipari iBlands. Of the native selemdes
naumannite, a selenide of silver, occurs in the Harz, and at Tuco,
Mexico. Eucairite, a selenide of silver and copper, in Sweden, Chili
and in the desert of Atacama. Grookesite, a selenide of copper and
thallium with a little silver, comes from Norway. Zorgite, a selenide
of copper and lead, from the Harz at Olasbach. Lehrbachite, a selenide
of lead and mercury, from the Harz. Clausthalite, a selenide of lead,
from Zinken and Clausthal in the Harz, Beinsberg in Saxony, Sio
Tinto in Spain, and from Mendoza in South America; it contains
28*11 per cent, selenium, 70*98 per cent, lead, together with a Uttle
oobiilt. Some selenites have also been found native. The iron pyrites
which is used in the manufacture of sulphuric acid contains a small
percentage of selenium, and the selenium found in the chamben comes
urom this source.

To prepare the selenium of commerce, the deposit from the sul-
phuric acid chambers, which contains besides selenium, sulpbnrt
arsenic, zinc, tin, lead, iron, copper and mercury, is mixed with uitro-
hydrochloric acid, heated, and allowed to stand for a dav. Dilute
sulphuric acid is now added, which precipitates the lead ; this is
filtered off and hydrogen sulphide is passed through the filtiate,
which precipitates the selenium, copper, mercury, tin and anenio as
sulphides. The yellowish precipitate is washed, and then boiled with
nitro-hydroohloric acid until everything except the sulphur is dis-
solved ; the solution is again filtered, the excess of acid is drivoi of
by evaporation, and the copper, tin, and part of the mercotyare
precipitated with caustic potash. The liqmd is then filtered, erapo-
rated to dryness, and ignited to expel the rest of the mercury. The
residue, while still hot, is mixed with ammonium chloride, and tbe
mixture is heated in a retort till the ammonium chloride is all vidati-
lise J. Some of the selenium sublimes in the upper part of the retort ;
the greater portion remains with the saline mass in the bottom uf
the retort. This is all placed on a filter, and the saline matter is
washed away with water. Selenium remains behind on the filter.

Another and quicker method of isolating the selenium fitxn the
seleniferous deposit is to dissolve the deposit in hot caustic potash
and allow it to stand in contact with air for &-7 hours. Potaasiam
hyposulphite is formed, and selenium is precipitated.

* A. J. Simons, **■ Seleuium and its Gompoands," Pliarm. Jour^ Jono 15, 1889.

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To prepare selenium from olauRtbalite, Simons powdered the
mineral and treated it with hydrochloric acid to free it from the
chalk and other carbonates with which it is associated in nature,
then roasted the residue with burnt tartar and charcoal in a hessian
TQcible, placed the mass on a filter, and washed it with water until
the filtrate was colourless ; the washings were exposed to the air,
^hen the selenium separated in the form of a crust on the top of the

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The •• white metal " is very widely distribated» and in many forms,
chiefly as follows : —

Native BiWer oontaiiiin|^

Argentite, siWer glance, Afr«8 .... „

Gerargyrite, horn silver, AJcUl
Stvphanite, brittle silver, 5Ag^.8b,
Proustite, light mby silver, 8Ag,S.i
Pyrargyrite, dark ruby silver, a/Lg^ Jb,S,


The ores are thus essentially snlphides, arsenides, antimonid^
and chlorides ; and while large masses of such ores are founds yet
the bnlk of the silver produced is won from mixtures of them
hilver minerals with the ores of copper, iron, lead, manganese, and

Australia's phenomenal silver mine, at Broken Hill, New Soath
Wales, is an illustration of the complexity of argentiferous ore deposits.
It has been described under Lead (see p. 515). The oxidised sur&oa
ores are much richer in silver than are the unaltered snlphides found
at lower levels. Occasionally ore assaying 1000 oz. silver per ton
in met with, but about 40 oz. is near the average, and much does not
exceed 20 oz. About half a million tons of the upper ores treated
gave a mean yield of 42 * 6 oz. silver per ton« According to the well-
known geologist C. S. Wilkinson, the silver-bearing lodee occur
chiefly in mica schists in the vicinity of granite dykes, and this
statement is apparently confirmed by the equally prominent scientiist
Kobert L. JacK. At &orook, New South Wales, rich and extensive
silvei -bearing reefs are found in belts of felspar-porphyry alternating
with beds of altered fossiliferous shales of Upper Devonian age. The
argentiferous minerals are much disseminated through the reef rock,
and are chiefly argentite, with some cerargyrite and proustite.

Bolivia has been a silver-producing country for many centuries,
and the most conservative estimate makes its total output up to 1890
exceed 1000 million oz. I'he workings of the Spanish conquerors
reached a maximum depth of 1700 ft. from the outcrops, with abso-
lutely no other means of raiHing the ore and water than the backs uf
their enslaved Indians. All the principal mining camps in Bolivia*
appear to be in either rhyolite or dacite (the ** trachyte " of Dan J
Forbes), generally the latter. Such is the case with the great
Huancnaca mine, and also with the no le^s remarkable mines of

♦ A. F. Wendt, «♦ The Potoei, Bolivia, SUver District/* Trana. Amer. Inrt. Mia-
Kngs., Sopt 1890.

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Colqnechaca; wliile the Omro mines are also prbbably in daoite,
although some of the specimens seem to leave a doubt, and possibly
there has aUo Heen an eruption of rhyolite. Fossils prove that all
the eruptive rocks of this part of S. America are post-Tertiary, and
not post-Jurassic and pre-Tertiary, as supposed by Forbes. The most
remarkable silver deposit in Bolivia lies in the conical mountain Cerro
de l^otosi, a section of which is shown in Fig. 163: a, argentiferous
veins ; &, rhyolite ; c, shales ; d^ sandstones and shales. At the
sumnodt, thermal action has so changed the rhyolite that it is a practi-
cally pure quartz (95 per cent, silica), the felspar having been
dissolved out. It has been long supposed that the veins of Potosi
were of great length but much faulted, whereas Wendt's investiga-
tions prove that the veins constitute a stockwork, with, as a rule,
fairly well defined foot walls, but no precise limit of ore towards the
hanging-wall, nor indeed any real hanging wall. What few faults
do exist have a general N.E.-S.W. strike, dip slightly N., and are very
plainly marked, but never throw the veins more than 4-5 ft., and are
no serious hindrance in mining. The percolating waters which
brought in the silver salts in solution evidently passed through a

Fio. 163.— 81LVEB Deposits : Gebbo db Potosl

mass of shattered rock and not a simple fissure. The stock works have
been extracted bodily in areas over 100 ft. across. In depth, the great
veins of the mountain have, in Wendt's judgment, remained just about
the same as they were on the surface, with this notable diflferenoe, that
whereas on the surface the various ramifications formed a stockwork
which oould be worked at a profit, such a condition of things has
ctaned to exist in the harder and denser rock 2000 ft. below the top
of the mountain. The fissures still continue, but only the strong
veins, that is, those of sufficient width to be worked by themselves,
can be made profitable. Furthermore, the ores bein^ *' negrillos " or
dense sulphurets of iron, are more expensive to mine and reduce.
There should be an enormous body of ore where the E.- and W.-
dipping veins form a juncture, when that depth is reached. The
surface-ores (•' paces") were wonderfully rich, and composed prind-
pally of chlorides mixed with native silver. They were also much
more silicious than the deeper ores. With depth or penetration the
" paces " become ** mulattos,** containing iron oxide and sulphur (e. g.
24 per cent, silica, 18 iron, 4 sulphur), and eventually " negrillos,"
compact, hard iron pyrites, carrying a little copper pyrites, and

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sometimeB zino-blende, and rarely ^lena. The silYer oocnn as
tetrahedrite, yielding about 700 oz. silver per ton. An almost oqd-
Btant aooompaniment of the silver ores of Potoni, and of a great muj
of the silver ores of the plateau of Bolivia, is tin, in the shape of grev
or yellow oxide. Some of the silver veins are very rich in tin oiude.
notably so the Tajo*polo and the Yeta Estano. Formerly, the old
miners left the tin standing in the veins ; but of late years a profit-
able tin industry has been based upon the oocurrenoe of these oroi.
The following average analysis of the pay-shute in the Gotamitos mine
gives a fair indication of the character of the ores : — Iron, 44^ pa
cent. ; sulphur, 82 ; silica, 18 ; tin, Z\ ; copper, 2^. The approxinttte
average assay in silver is 60 oz. a ton, b^oine also a trace of goUL
It seems as if the whole mass of the mountam of Potosi had been
subjected to thermal action, and it is difficult to find any mineral in
the mountain that does not contain a trace of silver ; the hard quartz
rock at the top all coutains a few (generally under 8) oz. to the toe;
the pyrites crystals disseminated through the rhyolite oommaDlr
asHay | oz. to the ton. A cross-cut driven 50-100 ft. anywhere in
the mountain will be sure to cut a number of veins more or less wi<k
Any of these veing, if followed a sufficient distance, will connect with
others that will as surely lead to some pay-shute or bonanza. Tbii
great multiplicity of veins, branches, and spurs is the bane of the
miner, for it is extremely difficult to determine where and when t>
stop the work of prospecting. But it is noteworthy that when the
veins leave the rhyolite they become poorer in silver. The preiait
product from one vein only is about 400,000 oz. yearly.

The Colquechaca* veins are fissures occurring in daoite and rhyolit^
the main lode being traceable a distance of over 2 milea. It dips
into the mountain from about 75^ to nearly verticaL The vein q>IitB
in the Aullagas mine, but is undivided throughout the othens. Both
branches of the split have carried rich ore of the same character u
the main vein, and no enrichment has been noted either at or bek>v
the junction. It is a pocket fissure, and, as is usual in such veins, tbt
ore IS very irregular in value. The thickness varies generally betwcea
2 and 12 in., the richer ore occurring in zones or belts sepamted bf
ground either nearly barren, or carrying lower grade ore. Then
variations occur both horizontally and vertically ; m some placet tb«
irregularities are abrupt and frequent, forming successions of pod^^'i
1-3 ft. or more wide. The vein matter is usually banded in stmotnre,
often containing cavities or *' vugs '' lined with beautiful ciystsb of
pyrargyrite and quartz with wire silver. It is clearly defined, va^
breaks away easily from the country rock. Slickensides and ekj
gouge are of frequent occurrence. Two classes of ore are dislifi*
guided: ** broza-guia," or first class, and '^broza,*' or second cIml
The first consists mainly of pyrargyrite, native silver, and argaitite,
sometimes accompanied by high-grade tetrahedrite. Associated with
these minerals are sphalerite and a little galena. Mnsscs of Ueode
and galena, with ruby, and interlaced with filigree silver, and lar^
pieces of native silver ore, in wire and filigree form, are of oomoMa
occurrence. The greater part of the value of the ore is carried m

♦ R. Peele, *• The Silver Minea of Colqaechacn, Bolivia,** En. and Min. Jl.

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pure massive, orystalline, mby silver. Large and perfeotlv termi-
Qated crystals of pyrargyrite are often found. The second class con-
sists of the poorer ore associated in the pockets with the first class,
ind of the lower grade material carried by the vein in the portions
lying between the pockets. The values of the two classes vary
^eatlv ; the rich ore may be said to run from 500 to 5000 oz., and the
second class firom 100 to 200 oz. per ton.

Honduras* possesses a remarkable argentiferous deposit known as
the Bosario vein, San Juancito. As seen in Fig. 164, it lies partly
n sedimentary rocks a of Triassio
)r Jurassic age, and partly in erup-
tive rock h which is a compact
'hyolite composed of crystals of
)ligocla8e in a felsitic matrix. In

iie vicinity of the vein there is no '

mch definite contact between the
tlate-ehales and intrusive rhyolite
IS exists to a much greater extent
n other parts of the mountain,
lore, the two interlock in a sort of
ringed edge, the rhyolite pene-
rating between the strata and rais-
ng them. The stringers of slate-
ihales pinch out, however, as deptl^
nto the mountain is attained. This
argillaceous shale frequently shades
Dto slate on and near the surface,
hewing all its characteristics ex-
ept that of cleavage ; but with
lepth it becomes a typical black
hale, somewhat baked through the
ofluence of the near eruptive rock.

The regular course of the vein
brough the sedimentary and erup-
ive rock shows its origin to have

eon somewhat later than the up- Fio. 164.— Silver Deposits, San
eaval of the latter. The vein is Juancito, Honduras.

lassed as a ** diaclase-" or " fissure-

mlt,*' cutting through sedimentary and eruptive rock alike, and
raceable for over 6000 ft. It strik^ N. 74° 30* E. for more than
hree-fourths of its length, and despite numerous local curves and
ends, its average course is very constant. Its average dip is N. 63° ;
nd this it holds with great regularity as distance or depth is attained.
?he vein pinches and widens : in some places it is only 2 ft. wide, at
thers (usually in ore-bodies) 16 ft. between walls ; its mean width
lay be placed at 4j^ ft The walls are well-defined, but seldom
qually hard and firm. The foot-wall usually, though sometimes
oth, is much decomposed and broken in the wider stopes, and a clay
arting often runs a foot or two inside of this wall parallel to the

* T. H. Leggett, ** The Bosario Mine at San Juancito, Honduras, C.A.,*' Trans.
Liner. Inst. Min. Engs., xvii. 435.

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vein, necessifatiug dose stuUing till the stope can be filled witbi
waste. The vein presents most of the oharacteristiGS of a ^ true fis-
sure " lode, in some places giving beautiful instances of banded
structure. Its chief peculiarity is a tendency to split into two distinct
veins in the more barren ground, and unite into one consolidated veia
where the ore-bodies occur. Here its width is often further incmsed
by the joining of feeders, that usually enter from the hanging-wall
The ore extracted is chiefly from the oxidised zone, and is a thoroughly
decomposed sulphuret-ore, carrying much native silver and ohlonde^
with free gold and frequent streaks of argentite and other rich ailvei
sulphides. The lower levels show the unchanged sulphides of iron^
oopper, lead and 2dnc. The gangue is quartz, carrying in the orei
bodies occasional clay streaks, heavily stained with the hydraied
oxides of iron and manganese. Other accompanying minerals, CoudJ
less frequently, are polybasite, embolite, pyromorphite, wulfe&ite^
cerussite, malachite, azurite, limonite, manganite and pyroliudte.

The mining belt of Peru * is made up of rocks of Jurassic and Cro
taceous ages. The great silver region is in the Gerro de Paaco^ anc

Fio. 165. — Sn<VBR Deposits : Gkbro db Pasoo.

limestones bound the argentiferous formation on the N., and K, an^
in part on the S. A general section is shown in Fig. 165 : a» lim^
stone conglomerate ; 6, slates ; c, unaltered andesite ; d, altered andc
site ; e, covered by bog ; /, argentiferous formation, highly metamod
phosed ; g^ slates, sandstones, and limestones of argentiferous fonnatioi3
very much less metamorphosed ; A, limestone. In the Cretaoeons limi^
stone are numerous veins, some striking N.-S., and some E.— W., readj
ing 6-10 ft. wide, and carrying rich tetrahedrite and other sulphide^
The eruptive jandesite is composed essentially of distinct crystals li
plagioclase and biotite. The argentiferous formation lies between
the limestones on the E. and the andesites on the W., with an area li
li X I mile. Following current phraseology, it maybe divided int
"surface-deposits" or ground above the water-level, and •*de€
deposits " or ground below the water-level ; and the dividing liiK
between these two classes may be taken roughly as occurring nei
the general level of the Quiulacocha tunnel, which, driven wit
varying grades and now caved in places, imperfectly drains many v
the mines. This tunnel does not reach the northern or eastern di^

i tricts, and the " deep deposits " occasionally rise nearly to the surface


^ ♦ A. D. Hodges, "Topography and Geology of the Cerro de Pasco, Pcrnj
'raus. Amer. lost. Miu. £iig8., zvi. 72il.

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Above the water-level, the formation consists of a bigUy metamor-
phoeed and greatly oxidised material, of constantly varying structure,
colour, and composition. It is everywhere very silicious, always
yields a considerable percentage of slimes when crushed wet, and
everywhere contains at least traces of silver, of pyrites and of carbonate
of lead (and of lime). Very rarely is the sUver visible, even with
the aid of the magnifying glass, and then principally in small native
scales in connection with quartzite. Very hard and very soft rocks
often adjoin, and large bodies of solid pyrites in a chalcedonic matrix
are found at varying depths, and generally in close proximity to
greatly oxidised materiaL The rock along the western slope is rather
harder, more quartzose, more ferruginous, and on the whole more
uniform in its silver contents than elsewhere, and forms what may
be called the ** cascajo zone," the term cascajo being applied broadly
to those ores which are worked by the patio process. East of this
zone the silver is more concentrated in pockets; the percentage of
lead carbonate is somewhat greater, and coppery ores are more fre-
quent. But the formation is very much richer in copper in the
district southerly and southwesterly of Yanacancha; and the lead-
ores occur chiefly at the north-east.

A typical analysis of large masses of cascajo is: — 72 per cent,
silica, 13^ iron peroxide, 6^ alumina, 2 iron sulphide, Ij^ lime and
magnesia carbonates, 1^ lead carbonate, *55 manganese peroxide, *5
iron protoxide, '4 zinc, '3 sulphur, '25 antimony, '05 copper, traces
3f arsenic, and from traces upwards of silver. A hard pyritio ore
&om deeper deposits shows 40 per cent, silica, 26J^ iron, 26]^ sulphur,
2| copper, 2^ antimony, * 13 silver, and traces of arsenic and nickel.
Grold and thallium are faintly present. Of the silver, 50-85 per cent.
a metallic, the remainder occurring as sulphides or antimonides, or
with lead carbonate ; argentite and stembergite occur now and then.
The pyrites are iron pyrite, chalcopyrite, rarely bornite and mispickel,
»ften a mixture of two or more of these varieties. Pure pyrite is
Lsually poor in silver, but not always. The pyritio material is often
associated with silver sulphides, and passes into tetrahedrite, which
<xia8ionally is very rich. The sulphurets of silver, copper and iron
ELst mentioned are common to the formation both above and below
he water-level, and to the veins in the limestone. Bich masses
ccor in pockets of greater or less size and irregularly disposed. The
animon idea that the deposits consist of three great parallel veins
oes not seem to be borne out, and Hodges regards them as much
roken and altered sedimentary rocks (slates, sandstones, and lime-
tones) metamorphosed by andesitic eruptions, and impregnated by
ubeequent silicious and metalliferous solutions.

The United States have many productive silver-mining districts.
Q Arizona, the Silver King mine is a central mass or chimney of
aartz, with innumerable radiating veinlets of the same, carrying
ich silver ores and native silver, in a great dyke of felspar porphyry,
rith associated granite, syenite (Blake), porphyry, gneiss, and slates,
LI of Archa&an age. The veinlets ramify through the strongly
Itered porph} ry, and form a stock work, which furnishes the principal
res. In the region are also Paleozoic strata, whose upper limestone

2 Q 2

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beds are referred by Blake to the Carboniferous. The minerals at
the mine are native silver, stromeyerite, argentite, sphalerite, galenite,
tetrahedrite, bomite, chalcopyrite, pyrite, quartz, calcite, siderite,
and, as an abundant gangue, barite. In the Tombstone district, &
great porphyry dyke up to 70 ft. wide, cutting folded Paleoosoio
strata, and itself extensively faulted and altered, carries above the
water line, in numerous vertical joints or partings, quarts with horn
silver, free gold, and a little pyrite, galenite, and lead carbonate.
Connected with other fissures parallel to this dyke^ are bedded
deposits in the limestone.

At Calico,* California, most of the ore-deposits oocnr in liparite
(rhyolite) or in its tufas, as veins along fractures and dislocations of
a more or less regular character; as simple, onoe open and sub-
sequently filled fissure-veins; as impregnations along oomfdex
fissure-systems, or filling and cementing more or less extensivdy
fractured zones. The gangne is predominantly barite with jasper;

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 67 of 76)