LODE OR REEF PROSPECTING
The preceding chapter dealt more especially with prospecting as
conducted on alluvial fields. I shall now treat of preliminary mining on
lodes or “reefs.”
As has already been stated, the likeliest localities for the occurrence
of metalliferous deposits are at or near the junction of the older
sedimentary formations with the igneous or intrusive rocks, such as
granites, diorites, &c. In searching for payable lodes, whether of gold,
silver, copper, or even tin in some forms of occurrence, the indications
are often very similar. The first prospecting is usually done on the
hilltops or ridges, because, owing to denudation by ice or water which
have bared the bedrock, the outcrops are there more exposed, and thence
the lodes are followed down through the alluvial covered plains, partly
by their “strike” or “trend,” and sometimes by other indicating
evidences, which the practical miner has learned to know.
For instance, a lesson in tracing the lode in a grass covered country
was taught me many years ago by an old prospector who had struck good
gold in the reef at a point some distance to the east of what had been
considered the true course. I asked him why he had opened the ground at
that particular place. Said he, “Some folks don’t use their eyes. You
stand here and look towards that claim on the rise where the reef was
last struck. Now, don’t you see there is almost a track betwixt here and
there where the grass and herbage is more withered than on either side?
Why? Well, because the hard quartz lode is close to the surface all the
way, and there is no great depth of soil to hold the moisture and make
the grass grow.”
I have found this simple lesson in practical prospecting of use since.
But the strike or course of a quartz reef is more often indicated by
outcrops, either of the silica itself or ironstone “blows,” as the
miners call them, but the term is a misnomer, as it argues the easily
disproved igneous theory of veins of ejection, meaning thereby that the
quartz with its metalliferous contents was thrown out in a molten state
from the interior of the earth. This has in no case occurred, and the
theory is an impossible one. True lodes are veins of injection formed by
the infiltration of silicated waters carrying the metals also in
solution. This water filled the fissures caused either by the cooling of
the earth’s crust, or formed by sudden upheavals of the igneous rocks.
Sometimes in alluvial ground the trend of the reef will be revealed by a
track of quartz fragments, more or less thickly distributed on the
surface and through the superincumbent soil. Follow these, and at some
point, if the lode be continuous, a portion of its solid mass will
generally be found to protrude and can then again be prospected.
There is no rule as to the trend or strike of lodes, except that a
greater number are found taking a northerly and southerly course than
one which is easterly and westerly. At all events, such is the case in
Australia, but it cannot be said that either has the advantage in being
more productive. Some of the richest mines in Australasia have been in
lodes running easterly and westerly, while gold, tin, and copper, in
great quantity and of high percentage to the ton, have been got in such
mines as Mount Morgan, Mount Bischoff, and the Burra, which contain no
lodes properly so-called.
Mount Morgan is the richest and most productive gold mine in Australasia
and amongst the best in the world.
Its yield for 1895 was 128,699 oz. of gold, valued at £528,700.
Dividends paid in 1895, £300,000.
This mine was opened in 1886. Up to May 31, 1897, the total yield was
1,631,981 ozs. of gold, sold at £6,712,187, from which £4,400,000 have
been paid in dividends. (See _Mining Journal_, for Oct. 9, 1897).
Mount Morgan shareholders have, in other words, divided over 43½ tons
of standard gold.
The Burra Burra copper mine, about 100 miles from Adelaide, in a
direction a little to the east of north, was found in 1845 by a shepherd
named Pickett. It is singularly situated on bald hills standing 130 ft.
above the surrounding country. The ores obtained from this mine have
been chiefly red oxides, very rich blue and green carbonates, including
malachite, and also native copper. The discovery of this mine,
supporting, as it did at one time a large population, marked a new era
in the history of the colony. The capital invested in it was £12,320 in
£5 shares, and no subsequent call was ever made upon the shareholders.
The total amount paid in dividends was £800,000. After being worked by
the original owners for some years the mine was sold to a new company,
but during the last few years it has not been worked, owing in some
degree to the low price of copper and also to the fact that the deposit
originally operated upon apparently became exhausted. For many years the
average yield was from 10,000 to 13,000 tons of ore, averaging 22 to 23
per cent. of copper. It is stated that, during the twenty-nine and a
half years in which the mine was worked, the company expended £2,241,167
in general expenses. The output of ore during the same period amounted
to 234,648 tons, equal to 51,622 tons of copper. This, at the average
price of copper, amounted to a money value of £4,749,224. The mine
stopped working in 1877.
Mount Bischoff, Tasmania, has produced, since the formation of the
Company to December 1895, 47,263 tons of tin ore. It is still in full
work and likely to be for years to come.
Each of these immense metalliferous deposits was found outcropping on
the summit of a hill of comparatively low altitude. There are no true
walls nor can the ore be traced away from the hill in lode form. These
occurrences are generally held to be due to hydro-thermal or geyser
action.
Then again lodes are often very erratic in their course. Slides and
faults throw them far from their true line, and sometimes the lode is
represented by a number of lenticular (double-pointed in section)
masses of quartz of greater or less length, either continuing point to
point or overlapping, “splicing,” as the miners call it (Fig. 6). Such
formations are very common in West Australia. All this has to be
considered and taken into account when tracing the run of stone.
[Illustration: FIG. 6. LENTICULAR VEIN.]
The tyro also must carefully remember that in rough country where the
lode strikes across hills and valleys, the line of the cap or outcrop
will apparently be very sinuous owing to the rises and depressions of
the surface. Many people even now do not understand that true lodes or
reefs are portions of rock or material differing from the surrounding
and enclosing strata, and continuing down to unknown depths at varying
angles. Therefore, if you have a north and south lode outcropping on a
hill and crossing an east and west valley, the said lode, underlying
east, when you have traced its outcrop to the lowest point in the
valley, between the two hills, will be found to be a greater or less
distance, according to the angle of its dip or underlie, to the east of
the outcrop on the hill where it was first seen. If it be followed up
the next hill it will come again to the west, the amount of apparent
deviation being regulated by the height of the hills and depth of the
valley.
A simple demonstration will make this plain. Take a piece of half-inch
pine board, 2 ft. long and 9 in. wide, and imagine this to be a lode;
now cut a half circle out of it from the upper edge with a fret saw and
lean the board say at an angle of 45° to the left, look along the top
edge, which you are to consider as the outcrop on the high ground, the
bottom of the cut being the outcrop in the valley, and it will be seen
that the lowest portion of the cut is some inches to the right; so it is
with the lode, and in rough country very nice judgment is required to
trace the true course.
For indications, never pass an ironstone “blow” without examination.
Remember the pregnant Cornish saying with regard to mining and the
current aphorism, “The iron hat covers the golden head.” “Cousin Jack,”
put it “Iron rides a good horse.” The ironstone outcrop may cover a
gold, silver, copper, or tin lode.
If you are searching for gold, the presence of the royal metal should be
apparent on trial with the pestle and mortar; if silver, either by sight
in one of its various forms or by assay, blowpipe or otherwise; copper
will reveal itself by its peculiar colour, green or blue carbonates, red
oxides, or metallic copper. It is an easy metal to prospect for, and its
percentage is not difficult to determine approximately. Tin is more
difficult to identify, as it varies so greatly in appearance.
Having found your lode and ascertained its course, you want next to
ascertain its value. As a rule (and one which it will be well to
remember) if you cannot find payable metal, particularly in gold “reef”
prospecting, at or near the surface, it is not worth while to sink,
unless, of course, you design to strike a shoot of metal which some one
has prospected before you. The idea is exploded that auriferous lodes
necessarily improve in value with depth. The fact is that the metal in
any lode is not, as a rule, equally continuous in any direction, but
occurs in shoots dipping at various angles in the length of the lode, in
bunches or sometimes in horizontal layers. Nothing but actual exploiting
with pick, powder, and brains, particularly brains, will determine this
point.
Where there are several parallel lodes and a rich chute has been found
in one and the length of the payable ore ascertained, the neighbouring
lodes should be carefully prospected opposite to the rich spot, as often
similar valuable deposits will thus be found. Having ascertained that
you have, say, a gold reef payable at surface and for a reasonable
distance along its course, you next want to ascertain its underlie or
dip, and how far the payable gold goes down.
As a general thing in many parts of Australia--though by no means an
inflexible rule--a reef running east of north and west of south will
underlie east; if west of north and east of south it will go down to the
westward and so round the points of the compass till you come to east
and west; when if the strike of the lodes in the neighbourhood has come
round from north-east to east and west the underlie will be to the
south; if the contrary was the case, to the north. It is surprising how
often this mode of occurrence will be found to obtain. But I cannot too
strongly caution the prospector not to trust to theory but to prove his
lode and his metal by following it down on the underlie. “Stick to your
gold” is an excellent motto. As a general thing it is only when the lode
has been proved by an underlie shaft to water level and explored by
driving on its course for a reasonable distance that one need begin to
think of vertical shafts and the scientific laying out of the mine.
A first prospecting shaft need not usually be more than 5 ft. by 3 ft.
or even 5 ft. by 2 ft. 6 in., particularly in dry country. One may often
see in hard country stupid fellows wasting time, labour, and explosives
in sinking huge excavations as much as 10 ft. by 8 ft. in solid rock,
sometimes following down 6 inches of quartz.
When your shaft is sunk a few feet, you should begin to log up the top
for at least 3 ft. or 4 ft., so as to get a tip for your “mullock” and
lode stuff. This is done by getting a number of logs, say 6 inches
diameter, lay one 7 ft. log on each side of your shaft, cut two notches
in it 6 ft. apart opposite the ends of the shaft, lay across it a 5 ft.
log similarly notched, so making a frame like a large Oxford picture
frame. Continue this by piling one set above another till the desired
height is attained, and on the top construct a rough platform and erect
your windlass (Fig. 7). If you have an iron handle and axle I need not
tell you how to set up a windlass, but where timber is scarce you may
put together the winding appliance described in the chapter headed
“Rules of Thumb.”
[Illustration: FIG. 7. DOUBLE WINDLASS WITH LOGGED UP BRACE.]
If you have “struck it rich” you will have the pleasure of seeing your
primitive windlass grow to a “whip,” a “whim,” and eventually to a big
powerful engine, with its huge drum and Eiffel tower-like “poppet heads”
or “derrick,” with their great spindle pulley wheels revolving at dizzy
speed high in air.
“How shall I know if I have payable gold so as to save time and trouble
in sinking?” says the novice. Truly it is a most important part of the
prospector’s art, whether he be searching for alluvial or reef gold or
any other valuable metal.
I presume you know gold when you see it?
[Illustration: PLATE III.--WHIN. _To face p. 28._]
If you don’t, and the doubtful particle is coarse enough, take a needle
and stick the point into the questionable specimen. If gold the steel
point will readily prick it; if pyrites or yellow mica the point will
glance off or only scratch it.
The great importance of the first prospect from the reef is well shown
by the breathless intensity with which the two bearded, bronzed pioneer
prospectors in some trackless Australian wild bend over the pan in which
the senior “mate” is slowly reducing the sample of powdered lode stuff.
How eagerly they examine the last pinch of “black sand” in the corner of
the dish. Prosperity and easy times, or poverty and more “hard graft”
shall shortly be revealed in the last dexterous turn of the pan. Let us
hope it is a “pay prospect.”
The learner, if he be far afield and without appliances of any kind, can
only guess his prospect. An old prospector will judge from six ounces of
lode stuff within a few pennyweights of what will be the yield of a ton.
I have seen many a good prospect broken with the head of a pick and
panned in a shovel, but for reef prospecting you should have a pestle
and mortar. The handiest for travelling is a mortar made from a mercury
bottle cut in half, and a not too heavy wrought iron pestle with a
hardened face. To be particular you require a screen in order to get
your stuff to regulated fineness. The best for the prospector, who is
often on the move, is made from a piece of cheesecloth stretched over a
small hoop.
If you would be more particular take a small spring balance or an
improvised scale, such as is described in Mr. Goyder’s excellent little
book, p. 14, which will enable you to weigh down to one-thousandth of a
grain. It is often desirable to burn your stone before crushing, as it
is thus more easily triturated and will reveal all its gold; but
remember, that if it originally contained much pyrites, unless a similar
course is adopted when treated in the battery, some of the gold will be
lost in the pyrites.
Having crushed your gangue to a fine powder you proceed to pan it off in
a similar manner to that of washing out alluvial earth, except that in
prospecting quartz one has to be much more particular, as the gold is
usually finer. The pan is taken in both hands, and enough water to
cover the prospect by a few inches is admitted. The whole is then
swirled round, and the dirty water poured off from time to time till the
residue is clean quartz sand and heavy metal. Then the pan is gently
tipped, and a side to side motion is given to it, thus causing the
heavier contents to settle down in the “corner” or angle. Next the water
is carefully lapped in over the side, the pan being now tilted at a
greater angle until the lighter particles are all washed away. Nearly
all the water is got rid of, and the pan is then once more righted, and
the small amount of remaining water is passed over the pinch of heavy
mineral a few times, when the gold will be revealed in a streak along
the bottom. In this operation, as in all others, only practice will make
perfect, and a few practical lessons are worth whole pages of written
instruction.
To make an amalgamating assay that will prove the amount of gold which
can be got from a ton of your lode, take a number of samples from
different parts, both length and breadth. The drillings from the
blasting bore-holes collected make the best test. When finely triturated
weigh off one or two pounds, place in a black iron pan (it must not be
tinned), with 4 ozs. of mercury, 4 ozs. salt, 4 ozs. soda, and about
half a gallon of boiling water; then, with a stick, stir the pulp
constantly, occasionally swirling the dish as in panning off, till you
feel certain that every particle of the gangue has come in contact with
the mercury; then carefully pan off into another dish so as to lose no
mercury. Having got your amalgam clean squeeze it through a piece of
chamois leather, though a good quality of new calico previously wetted
will do as well. The resulting pill of hard amalgam can then be wrapped
in a piece of brown paper, placed on an old shovel, and the mercury
driven off over a hot fire; or a clay tobacco pipe, the mouth being
stopped with clay, makes a good retort (see “Rules of Thumb,” pipe and
potato retorting). The residue will be retorted gold, which, on being
weighed and the result multiplied by 2240 for a 1 lb. assay, or by 1120
for 2 lb., will give the amount of gold per ton which an ordinary
battery might be expected to save. Thus 1 grain to the pound, 2240 lbs.
to the ton, would show that the stuff contained 4 oz. 13 dwt. 8 gr. per
ton.
If there should be much base metal in your sample such as say stibnite
(sulphide of antimony), a most troublesome combination to the
amalgamator--instead of the formula mentioned above add to your mercury
about one dwt. of zinc shavings or clippings, and to your water
sufficient sulphuric acid to bring it to about the strength of vinegar
(weaker, if anything, not stronger), place your material preferably in
an earthenware or enamelled basin if procurable, but iron will do, and
intimately mix by stirring and shaking till the mercury has taken up all
it can. Retort as before described. This device is my own invention.
Never use the same pan for mercury and for prospecting, as the mercury
hides the gold by coating it.
The only genuine test after all is the battery, and that, owing to
various causes, is often by no means satisfactory. First, there is a
strong, almost unconquerable temptation to select the stone, thus making
the testing of a few tons give an unduly high average; but more often
the trouble is the other way. The stuff is sent to be treated at some
inefficient battery with worn-out mortars, shaky foundations, and uneven
tables, sometimes with the plates not half amalgamated, or coated with
impurities, the whole concern superintended by a man who knows as little
about the treatment of auriferous quartz by the amalgamating or any
other process as a dingo does of the differential calculus. Result: 3
dwt. to the ton in the retort, 30 dwt. in the tailings, and a payable
claim declared a “duffer.”
When the lode is really rich, particularly if it be carrying coarse
gold, and owing to rough country, or distance, a good battery is not
available, excellent results in a small way may be obtained by the
somewhat laborious, but simple, process of “dollying.” A dolly is a one
man power single stamp battery, or rather an extra sized pestle and
mortar (see “Rules of Thumb,” p. 152).
Silver lodes and lodes which frequently carry more or less gold, are
often found beneath the dark ironstone “blows,” composed of
conglomerates held together by ferric and manganic oxides; or, where the
ore is galena, the surface indications will frequently be a whitish
limey track, sometimes extending for miles, and nodules or “slugs” of
that ore will generally be found on the surface from place to place.
Most silver ores are easily recognisable, and readily tested by means of
the blowpipe or simple fire assay. Sometimes the silver on being tested
is found to contain a considerable percentage of gold, as in the great
Comstock lode in Nevada. Ore from the big Broken Hill silver lode, New
South Wales, also contains an appreciable quantity of the more precious
metal. A natural alloy of gold, termed electrum, contains 20 per cent.
silver.
Tin, lode, and stream, or alluvial, occurs only as an oxide, termed
cassiterite, and yet you can well appreciate the compliment one Cornish
miner pays to another whose cleverness he wishes to commend, when he
says of him, “Aw, he do knaw tin,” when you look at a representative
collection of tin ores. In various shapes, from sharp-edged crystals to
mammillary-shaped nuggets of wood-tin; from masses of 30 lbs. weight to
a fine sand, like gunpowder, in colour black, brown, grey, yellow, red,
ruby, white, and sometimes a mingling of several colours, it does
require much judgment to know tin.
Stream tin is generally associated with alluvial gold. When such is the
case both the gold and the tin can be saved, for the yellow metal is
much heavier. As the tin ore is an oxide which is not susceptible to
amalgamation, the gold can be readily separated by means of mercury.
Lode tin sometimes occurs in similar quartz veins to those in which gold
is got, and is occasionally associated with gold. Tin is also found, as
at Euriowie, in dykes, composed of quartz crystals and large scales of
white mica, traversing the older slates. A similar occurrence takes
place at Mount Shoobridge and at Bynoe Harbour, in the Northern
Territory of South Australia; indeed, one could not readily separate the
stone from these three places if it were mixed. As before stated tin
will never be found far from granite, and that granite must have white
mica as one of its constituents. It is seldom found in the darker
coloured rocks, or in limestone country, but it sometimes occurs in
gneiss, mica schist, and chlorite schist. Numerous other minerals may be
mistaken for tin, such as common tourmaline or schorl, garnet, wolfram
(a tungstate of iron with manganese), rutile or titanic acid, blackjack
or zinc blende, together with magnetic, titanic, and specular iron in
fine grains.
The readiest way of determining whether the ore is tin is by weight, and
by scratching or crushing, when, what is called the “streak” is
obtained. The colour of the tin streak is whitey-grey, which, when once
known, is not easily mistaken. The specific gravity is about 7·0.
Wolfram, which is most like it, is a little heavier, from 7·0 to 7·5,
but its streak is red, brown, or blackish-brown. Rutile is much lighter,
4·2, and the streak light-brown; tourmaline is only 3·2. Blackjack is
4·3, and its streak yellowish-white. I have seen several pounds’ weight
to the dish got in some of the New South Wales shallow sinking
tin-fields, and, as a rule, payable gold was also present. Twenty-three
years ago I told Western Australian people that the neighbourhood of the
Darling range would produce rich tin, which it has done lately; there is
promise of a great development of the tin industry here. The tin “wash”
in question is reported to yield payable gold.
Metals are easily distinguished from non-metals by their lustre,
toughness, fusibility, opaqueness, conductivity, and rusting. Most
metals can be bent, twisted, drawn, and hammered to a degree not
possible in non-metals.
Sodium, potassium, lithium, and in a somewhat less degree, calcium,
strontium, and barium, will rust almost immediately when exposed to
moist air, and their white rusts quickly dissolve in water. Another
group of metals, zinc, lead, magnesium, and antimony, have white rusts
which are not soluble in water. Their rusts form a thin, adherent
coating, which gives the surface of the metal a dull appearance without
altogether concealing it. At higher temperatures than ordinary, if the
metals are finely divided, the chemical energy of rusting is so great
that the metals burn with a vivid light and give off a dense white
smoke. The permanency of these rusts and their protective character are
utilised in white paints.
A third group of metals have coloured rusts, _e.g._, silver, copper, and
iron. A fourth group never rust, such as gold and platinum, which occur
as metals in the gangue, not as ore from which the metal is produced. In
the case of the other metals it is an advantage that they are found in
the rust or ore condition, as they can be manufactured much more easily
than native metal.