Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

. (page 53 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 53 of 76)
Font size
QR-code for this ebook


but where it crosses the slaty belt it is very rich in gold.*


Fig. 127 illustrates a series of flat veins in granite, at Woof'


Point: o, quart? veins or reefs; 6, soft granite; c, hard grwiia


d, slate walls.


♦ E. J. Dunn.






Digitized by Vj


oogle



METALLIFEROUS MINERALS, 469

On tHe terraces between Loyola and Eevington, the lodes have a
westerly strike nearly parallel with that of the enclosing strata (slates
Dd sandstones). The lode track is
long the axes of an anticlinal curve
1 the strata, and the quartz a forms
sort of saddle reef, or as it is
Kally termed '* boiler " reef (Fig.
28), while in other places it|forms
massiye segregation dose to the
ces and slighUy parallel to one
de (Pig. 129). The quartz is
ilky white, and in parts glassy,
ith darker lineations of fine slate
id pyrites. Along the lode track,
X)wni8h - grey quartzitic sand-



Fio. 126.— Gold Deposits, Mount Fw. 127.— Gold Defositb : Wood's

DoBAN. Ponrr.



Fios. 128, 129.— Gold Deposits, Loyola.

Dee alternate with slates ; it is generally between the slate and
dBtone walls that the richest quartz is found, and not where the

Digitized by VjOOQ IC



470 ECONOMIC MINING.

quartz seems reticulate through the sandstones or slates ; tJie latta
is frequently carbonaceous.*

. On the Clunes field, the Port Phillip mine carries many loda
(Fig. 130), the off-shoots in many cases proving richer than the mui
lodes. The country rock is Lower Silurian, a, basalt oap^ni^
6, diorite dyke ; c, west vein ; d, Bobinson's vein, having iw-est brand
e and east branch /; q^ old man vein ; \ east vein ; », weloome vein
2;, ofikhoots.t

At Fryer's Creek, the richer ^sometimes very rich) workingB ha^
been confined to depths ranging from 100 to 200 ft, and below 400 9k

nothing payable has bea
struck. On Collyer^s red
Fig. 131, are no clearly dc
fined leaders, but a aaxioi
very narrow veins (rich),
tersecting the natural s
at all kinds of angles,
soon worked out. TKe
formation comprises
hard sandstone, wfaic^
difficult to excavate ;
although the* yield of _
was extraordinarily good ^
far as the workings extended
there appears to be little o
no probability of farther e^



Fio. 130.— Gold DBPOsrre, Pobt Fig. 131.— Gold Deposits,

Phillip. Colltkb's Rbbp.

plorations. The total yield of gold from 22 tons of stone was 107 o«
averaging 4 oz. 17 dwt. to the ton. The quartz was of a broumi^
colour, ferruginous, and the gold occurred in the joints and small fnw
tures, and was very fine and pretty equally difi'used throughout ; n
foot or hanging wall^. The crushing stuff was of a con^omerate]
character, 1-4 in. thick, a, anriferoiis alluvial ; 6, sandstone
c, quartz. At Specimen Hill, thousands of ounces were got firw
less than 40 ft. deep; at 300 ft,, nothing. The Mosquito, Fig. \^

• J. Stirliqg. t R. Allan.



Digitized by



Google



METALLIFEROUS MINERALS. 471

aa proved remarkably rich down to 162 ft ; the general run of
nartz bears N. 14** W., and compriBes flat leaders and a saddle forma*
Ion ; at No. 11 shaft (a) is a conglomerated mass of quartz, slate,
od fluoan, with detached portions of floating sandstone, nearly all
eing payable ; the gold is both heavy and flne, and generally assooi-
ted with galena, iron and arsenical pyrites, and blackjack ; a. No. 11
laft ; 6, slate ; c, sandstone wall ; a, quartz ; e, rich quartz.*



Fia. 133.— Gk>LD DiPOBiTB,
Maldon.



Fia. 132.— OoLD DsFOSiTB, MoflQuiro.



At Maldon, dykes or «* bars " of granite a (Fig. 133) cut the quartz
3f8 fr, and generally at these intersections the lode is increasingly

On some of the Sandhurst mines, the strata have been bent and
Qtorted into anticlinal axes and synclinal troughs, in places (as at
Fig. 184) so sharply that the hard sandstone beds are arched with



A W\\\\V\\\\



Fig. 134. — Gold Deposits, Bandhubst.

udinB of 14 ft. only. The anticlinal axes are locally called '< centre
intry"; their general. direction is a few degrees W. of N., and,
^ther with the " pitch," determiues the strike of the reefs ; the
ter as a rule conform to the bedding planes 6. Veins of quartz c

* M. Amos. t £• J* Dunn.

Digitized by VjOOQIC



47^



ECONOMIC MINING.



iDteneci the sandstone d and slates e in many places, and are 1

as '* spurs " ; there are two sets : those dipping S. are buren ;

dipping N., auriferous. A width of 80 ft. of slate and sand

payable. At /occurred much galena, iron pyrites, and mispickd;!

g the reef was sprinkled with gold tbe

of currants, through its whole width of 2 I

and at J^, it was 18 ft. wide, and carried I

^ * 16 dwt. gold per ton.*

In Russell's reef. Fig. 135, the «' 1^8 '
inverted ; a, sandstone ; 6, reef.

In the Ballarat East mines. Fig. 136, 1
several vertical lodes a, but they are not
defined or regular, in some places ~ _
great width, and then pinching out to t^
small dimensions. These vertical lodes T
so far proved to be scarcely payable (exo
in one case where they have apf
made into one lode below the 300-ft. le
Nearly the whole of the gold won has been obtained from flatl
diagonal veins 6, which vary from the horizontal to a dip of 45** IL, i
in thickness from 1 in. to 10 fb. The slate country look e is



Fio. 135.— Gold Dbfosits,
Bubskll's Beet.



Fio. 136.— Oold Dkfobitb, Ballabat East.

vertical in the upper levels, but takes a westerly underlay in tl
deeper ones, and in the lowest levels is about 1 ft. in 6. There a|



• E. J. Dunn.



Digitized by



Google






METALLIFEROUS MINERALS. 473

Bevend " indicators " nmning with the strata, and where these inter-
sect the quartz veins large quantities of gold have been obtained,
iocluding nuggets up to 225 oz. These indicators (which are for the
most part thm veins of pyrites) continue through the whole length
}f the belt. In some cases, the flat veins have faulted the indicators
uid country rock, which would point to the conclusion that they are
>f more recent date. 6 is a heavy slide.*

At Wedderbum, the country rock is of yellow and grey soft sand-
ftone and clayey beds, having a strike of N. 5** W., dip 60^-80° E.
}ne of the beds of rock is of dark-grey to black colour, and 5-7 in.
ride, made up of thinly laminated unctuous clay. This is known to
he miners as the '* indicator." At intervals of a few inches to several
eet apart, are flat leaders of quartz, ^ in. to 1 ft. thick, dipping to
he N.W. at angles ranging from 20° to 80^ Where they intersect
he ^ indicator," the latter is generally displaced a few inches. At
nd near the intersections the quartz leaders become auriferous, and
ften richly so, the gold occurring in coarse nuggety pieces, and fre-
luentlj of crystalline character. The leaders are barren, except at
be intersections with the indicator, or of nearly vertical thin quartz
eina, oalled " droppers " by the miners.t

Western Australia is likely soon to materially increase the Austral-
sian gold output. As to its geological formation, A. F. Calvert states
lat the general character of the auriferous counl^ is a series of belts
allowing in the south the coast line which is a litUe W. of N. and E.
r S. The gold reefs traverse the lower Devonian schists, which are
ore or less altered by the action of the dioritic bosses which have
istorted them in many places. These schists lie directly upon the
ranite, which has lifted them to an angle of 30*^ to 45^. llie granites
) not appear in a mountainous form, but take the shape of low
nges, wnich in many districts have not outcropped more than suf-
rient to give the country an undulating character. It is these N.
id S. belts where the diorites and sometimes trachytes have played
eir pturt, which are of importance from an auriferous point of

Austria's small product of gold comes chiefly from Bathhausberge,
Salzburg, and from the antimony mines of Euttenberg, in Bohemia,
e gold being recovered as a bye-product. In Schemnitz and Krem-
fcK, the auriferous quartz lodes occur in eruptive rocks of Tertiary
e, chiefly in greenstone-trachyte, (propylite), porphyries of various
ids, diorite, and granite; a reddish quartz characteristic of the
strict is impregnated with blende, galena, and pyrites and is locally
led nnopd. Some of the TransyTvanian ores are highly telluri-

COS.

Canadian gold mines in the Lake of the Woods country are asso-
rted, with pyrites, mispickel, and galena, the country rock being
eifis, syenite, and diorite.

Tlie gold of Nova Scotia occurs in Cambrian rooks— compact
urtzites and sandstones (locally called ''whin"), frequently fel-
btliicy rarely calcareous, associated with argillaceous slates (some-
xnagnesian or chloritic).

♦ B. Allan. f E. J. Dann.



Digitized by



Google



474 ECONOMIC MINING.

Qold quartz in Nova Sootia * is distributed tlirough thoufiands of
feet vertically, but the workable deposits commence at about 2500 ft.
below the graphitic slates and cease at about 8000 fL, thus leaTing
an undulating productive belt of 5500 ft. Only those portions of the
truncated crests of the foldings which upheaval and denudation have
exposed oan be approached by the gold seekers ; consequently, nearly
all the mines are found to be in connection with anticlinals, and no

Cspeoting is attempted outside these productive rocks. Some of the
t mines happen to be on the dome-like masses caused by inteneo
tion of E.W. and N.S. anticlinals, and, although other good minee are
not so situated, yet it may well be that renewed igneous action
favoured continued segregations of gold in longer disturbed area.
Most of the gold is got from bedded veins (locally caUed "leads");
they are intercalated with the quartzites and slates, and follow botn
their strike and dip. They conform to the foldings of the uodnla-
tions, and lie at every angle with the horizon ; as, however, the pro-
ductive lodes are found at the anticlinals, the portions mined usnalir
dip at a steep angle. These lodes affect a grouping arrangement, and
usually the leads are numerous, though small in size, varying from i
to 12 or 18 in. Sometimes, and especially in the slates, they are n
closely grouped that they can be worked as one large lode, the inter-
vening ** whin " being picked out. In an anticlinal assemblage ^ere
is often a principal, or very persistent, lode, carrying much viabte
gold in flakes and grains, which varies from 8 to 12 in. average size.
The veins are closely grouped in slate belts, but are not continaoiis;
they squeeze out in length, and are again found farther on, wbik
parallel to the squeeze, and in close proximity, another ^'lead^a
developed. These branches vary from J to 12 in. in width. In vm
districts are immense lodes of quartz which are only faintly anrifer
ous. The walls of the bedded lodes when in dioritio quartdte an
fused into the quartz, so that it is difficult to separate the waste rock
and this makes extraction of such quartz expensive. Often «m
slate found on or near the wall ameliorates this. The walls si
definite in quartzite, and irregular in slate. Sometimes the lodes «?
corrugated or wrinkled (" barrel quartz ") remarkably, probably \
pressure during folding, and the leads associated with theee wrinkk
are often rich, especially in the broad part of the quartz oorrugatiooi
A few gash veins are met with, often well defined and usually rk^
but local, and recognised by containing lime carbonate. True veia
occasionally occur, sometimes rich, but not persistent. The gold «
the bedded lodes is always visible, and no vein is considered w»ti
prospecting that shows no " sights " ; it occurs in nuggets, large leaf
like flakes, small grains and strings in the quartz, but " leads " wit
much fine gold are rare. In nearly all the lodes portions of the Td
are rich in gold, whilst between them the quartz is worth but lit:j<
These shoots of ore are sometimes 300 ft. long, but are often ms^
shorter. The gold streaks are usually richest in the middle, dwisi
ling irregularly towards the margin. The gold occurs in translnotfi
or milky quartz, the richest quartz possessing a characteristic lei^

• Bronton Symons, "The Gold Fields of Nova Sootia," Proc. B. Ged »
Cornwall.



Digitized by



Google



METALLIFEROUS MINERALS. 475

hue, due to metallic sulphides — XDundic, mispickel, galena, blende,
chaloopyrite and molybdenite. The presence of sulphides seems
essentiad to a productive lode, and the miner always considers the
presence of galena and yellow ore an infallible sign of approaching
vudble gold. Arsenical pyrites is very plentiful in most lodes, and
varies in value fix>m a few pennyweights to several ounces of gold
per ton.

Germany's gold comes from the argentiferous galenas, notably
near Freiberg.

In India,* schistose rocks, superimposed on gneiss or other meta-
norpbic rocks, are the great gold carriers. Chloritic, talcose,
iematiticy micaceous and argillaceous schists are the prevailing
brms of the schistose series, and of these the chloritic and talcose
varieties have been found most favourable for gold. Hitherto gold
las only been encountered in workable quantities in a quartz matrix.
Quartz reefs are seen traversing the schist and the underlying gneiss,
rat it is only in the schists that they have been found commercially
iroductive, although a little gold has been met with in the reefs
raversing the gneiss. In South India the gold-bearing rocks are
Tjown as the Dharwars ; in North India, as the sub-metamorphics.
lykes of diorite and dolerite are met with on most of the gold fields,
atting through the schists and reefe, and displacing or fietulting the
fctter. In many parts of the country, the schists are covered over
)r miles with a coating of trap rock. On the Eolar field the quartz

of a bluish colour, and very hard and compact. A ribboned or
minated structure is apparent near the walls. The field is much
It with dykes of diorite, some of these being of great size ; the main
fke, running N.S., is over 200 ft. thick in some places, and has been
dced for 10 miles; Granitic cross-courses, running E.W., also occur.
> far the Champion lode alone has been found remunerative, and has
elded over 2,000,000/. worth of gold within the last few years. It
is been traced about 4 miles, and varies in width from a mere
read to massive '^ makings " of quartz 20-30 ft. wide. Its average
idth may be set down at 3 ft. The strike of the reef is nearly NJS.,
tt within this general direction its course may be said to be serpen-
le, and in places even to double back on itself, so that what may
pear from surface outcrops to be two distinct N.S. reefs and E. W.
anters, are really portions of the same reef. The '* makings" of
artz, too, are extremely irregular, and there is no uniformity in the
dth of the lode in successive levels. The strike of the reef is also
ilted by trap dykes and granitic cross-courses. The main run of
B dykes appears nearly parallel with the strike of the reefs — ^i. e.
S., and a trifle more easterly than the reefs. The ore is free-milling,
bre being very little pyrites present, so that crushing batteries and
lalgamators (plates and pans) suffice to secure the biUk of the gold,
t there is great difficulty in securing the finer particles of gold
lich are disseminated throughout the gangue. Owing to the in-
isely hard nature of the quartz and country rock, and irregularities

the strike and make of the lode, mining has b)sen an expensive
m, and averages for 1892 about 1/. 4^. 6(2. per ton ; milling costs
* Mcnryn Bmitli, " Gold MiniDg in India,'' Trans. Infit Min. and Met, L 313.



Digitized by



Google



476 ECONOMIC MINING.

9«. 6c2. ; and treatment of tailings about h%. The entire oatlay iih
eluding administration and home charges brings up the cost per ton
of ore to 2L 6«. Sd., or about 41 per oent. of the yield per ton. About
90 per oent. of the assay value of the gold is reoovered.

South Africa derives much of its gold from a " cement " or coo-
glomerate locally called hafikeL The geological formation in the
Witwatersrand field consists chiefly of sandstones, shales, quartates,
and cherts, tilted into a nearly vertical position (and more or leai
metamorphosed in places) by the intrusion of a mass of granite. Be-
tween the strata of sandstone, Ac., come the auriferous beds of oon-
glomerate, attaining a total thickness of 200 ft sometimes, and
extending for many miles in length. Above water line the rook li
reddish ; below, bluish, due to undecomposed pyrites.

On the Dekaap fields * are found large granitic areas, flanked '\i\

schistose and shaly rocks, containing auriferous beds. Dykes of troi

diorite occur all over the country, penetrating the granite, the tiltec

schists, the shales, and the sandstones, and influencing the aurifinoa

cbaracter of the deposits. A most remarkable feature is the almos

entire absence of lime. Quartz not only predominates as a constitnen

of the granite but occurs among the slaty rocks as segregated mtssn

interbedded with and replacing them to some extent, often indeei

assuming such dimensions as to constitute the larser part of the whoi

country-rock. Their shape is irregular, but rou^dy lenticular; tb^

are generally connected by very thin seams ; or suoceed one anothe

at fairly regular intervals, either in a straight line or on both sides d

and at no great distance from, such a line, which can be termed tli

axis of their strike, and which coincides with that of the endosiBj

rocks. Many are auriferous, and they are individually far more pes

sistent in depth than in length. Another class of interbedded quarti

deposits, which may be termed segregated quartz-veins, also occur i

these rocks, as a rule extending for considerable distances along the!

line of strike. They are often very auriferous, and have proved coi

tinuous to a limited depth ; their width, however, is very vaxiaUi

These and the large lenticular bodies are generally composed of

dark-blue and homogeneous (never distinctly orystalliiie) quart

which sometimes resembles quartzite, from its granulated appearance

where the quartz is milk-wbite, it shows traces of crystaUisatioi

The associated minerals are all sulphides, usually pyrite or pyrrhotit

more rarely chalcopyrite and arsenopyrite ; galenite in one or t^

places only. Samples from some distance below ''water-level

entirely undecomposed, often show large quantitieB of &ee gidd o

being washed. The sulphides are very rich, never, when &ee goi

also occurs, containing less than 5 oz. gold to the ton ; 30-60 oe. ai

not uncommon, and in one instance 760 oz. The shales or sditf

enclosing these veins are generally decomposed near the snr&oe £i

some distance on either side, and are often found to be as rich (ere

richer) in gold as the quartz itself. This decomposition is due 1

that of the pyrites, as below "water-level" auriferous pyrites ai

found to be disseminated for a limited distance on both sides of tl

veins. Throughout these tilted rocks are numerous inteiBtrati&e

♦ W. p. Purlonge, Trana. Amer. Inat Min. Engs.



Digitized by



Google



METALLIFEROUS MINERALS, 477

beds of quartz-TOck or quartzite, generallj of large size, and con-
tinuoDS for very long distances. Locally, they are termed "bars."
The quartz composing them is never crystalline, but compact and
amorphous, or greidually passing to roughly granular. In colour, it
wes fix)m black to pure white. Very often the whole bar has a
banded structure, dark and light ribs alternating, the dividing line
wrresponding to the strike of the enclosing rocks. Most of these
' ban " are due to the replacing of the shales or schists by silica ;
4hen probably were originally strata of sandstone, rendered homo-
^eoos by solutions which permeated them and dissolved their
tomponent parts, redepositing gelatinous silica. The ''bars" of
[uartzite have a very important practical value, inasmuch as the
nindpal gold-deposits are found in, or immediately adjoining, them,
hoQgh never without there being also some eruptive rock in close
noximity. The gold may occnr : — (a) in the body of the bar itself,
Q ^'shutes," or evenly disseminated through the whole of it, more
paringly. (5) Where the "bars" have been fissured or fractured,
Dother variety of quartz has been deposited in the cracks and
revicee, often differing but little from that composing the bar, but
asily distinguishable, being of a different colour and texture, and
lore pyritiferous. (c) Instead of these deposits of auriferous quartz,
1 and alongside the " bars " are others, of great extent, of iron oxides,
nitaining 80-40 per cent, iron ; all this ironstone carries gold. At
irfaoe, the iron-ores consist generally of limonite and hematite, with
>me magnetite ; in depth, the limonite disappears, and the ore then
msists of hematite and magnetite, the former always predominating ;
jrrite alflo is found in quantity. The gold contained in these ferru-
mou8 deposits varies from " traces " to 3-4 oz. per ton, and in the
tore extensive of the paying ones may be averaged at 18-20 dwt.
Br ton. The amount which can be extracted by ordinary milling
id plate-amalgamation usually does not exceed 7 dwt. The ferru-
inouB deposits are not confined to the "bars," but are also found
nongst the slaty rocks, and are known as "burnt leaders" (from
leir colour); they often "pan" well, and assay better, but are
either large nor continuous enough to be worked with profit Many
$d8 of argillaceous material, enclosing small " stringers " of quartz,
id (firom the decay of previously-contained pyrite) now a ferru-
nons red clay, contain paying amounts of gold. The beds of
nglomerate which are enclosed in the tilted strata are to a large
itent impregnated with pyrite. In some instances, the whole bed,
ben oxidised, shows by washing a paying amount of " free " gold ;
id, when pyritous, often assays over 1 oz. gold to the ton. The
iriferoos pyrite, and, consequently, the " free " gold, are evidently
later date than the beds themselves, and neither of them is ever
tmd in the quartz pebbles ; they are entirely confined to the cement-
g TPfrtftriAl, The so-called " idluvial " of these fields is really not
lavial gold at all, in the proper sense of the term. It is usually
and an the tops of rounded hills, and in a position where it could
xdly have been placed mechanically. Examination almost always
owa that the sou is simply a decomposed felsitic rock, probably a
i^te in situ, which contains the nuggets, with generally a large

Digitized by VjOOQIC



478 ECONOMIC MINING.

amount of quartz fragments also. There can be bat little qnesdon
that these quartz fragments oonstitnted originally the secondtfj
quartz which filled the cracks and crevices of the more deoompoie^
volcanic dykes.

Of the United States, California is foremost in gold prodxxctioA
Its veins of auriferous quartz are usually described * as s^regato:
veins, in slates and oilier metamorphic rocks, and more or les
parallel with the bedding. The quartz contains auriferous pyrite
free gold, arsenopyrite, chalcopyrite, tetrahedrite, galena, and blende
but pyrites is far the most abundant. Some telluridee have beo
noted by Silliman at Carson Hill, Calaveras County. The vein
approximate at times a lenticular sbape, which is less marked u
Cfalifomia than in some other regions, and which shows analogies o
shape with pyrites lenses and magnetite lenses. In such cases tb
fissure-vein character is somewhat obscure. Califomian veins occup
undoubted fissures in the slates ; the largest and best known u tii
so-called Mother Lode, a lineal succession of innumerable larser an
smaller quartz veins running parallel with the strike, but cutting th
steep dip of the slates at an angle of 10^ It was doubtless forme
by faulting in steeply dipping strata. The wall rocks of Califomiai
veins are serpentine, diabase, diorite, and jy^ranite, as well as slatj
The serpentine is probably a metamorphosed igneous rock, while tb
diabase and diorite form great dykes. Considerable calcite, dolomin
and ankerite occur with the quartz, and very often it is penetrate
by seams of a green chloritio silicate, which is provisionally oall^
mariposite, as it is probably not a definite mineral, but rather
infiltoktion of decomposition products. The quartz veins vary soi
what in appearance, oeing at times milk-white and massive (1(
called ''hungry," from its general barrenness), at times grea;^
darker, and again manifesting other differences, which are difficult
describe, although more or less evident in specimens. The ri("
quartz in many mines is somewhat banded, and is called "ribbonj



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 53 of 76)