GOLD PROSPECTING--ALLUVIAL AND GENERAL
It is purposed in this chapter to deal specially with the operation of
searching for valuable mineral by individuals or small working parties.
It is well known that much disappointment and loss accrue through lack
of knowledge by prospectors, who with all their enterprise and energy
are often very ignorant, not only of the probable locality, mode of
occurrence, and widely differing appearance of the various valuable
minerals, but also of the best means of locating and testing the ores
when found. It is for the information of such as these that this chapter
is mainly intended, not for scientists or miners of large experience.
All of us who have had much to do with mining know that the majority of
the best mineral finds have been made by the purest accident; often by
men who had no mining knowledge whatever; and that many valuable
discoveries have been delayed, or, when made, abandoned as not payable,
from the same cause--ignorance of the rudiments of mineralogy and
mining. I have frequently been asked by prospectors, when inspecting new
mineral fields, what rudimentary knowledge will be most useful to them
and how it can be best obtained.
If a man can spare the time a course of lessons at some accredited
school of mines will be, undoubtedly, the best possible training; but if
he asks what books he should read in order to obtain some primary
technical instruction, I reply: First, an introductory text-book of
geology, which will tell him in the simplest and plainest language all
he absolutely requires to know on this important subject. Every
prospector should understand elementary geology so far as general
knowledge of the history of the structure of the earth’s crust and of
the several actions that have taken place in the past, or are now in
operation, modifying its conditions. He may with advantage go a few
steps further and learn to classify the various formations into systems,
groups, and series: but he can acquire all that he need absolutely know
from this useful little 2_s._ 6_d._ book. Next, it is advisable to learn
something about the occurrence and appearance of the valuable minerals
and the formations in which they are found. For all practical purposes I
can recommend Cox and Ratte’s “Mines and Minerals,” one of the Technical
Education series of New South Wales, which deals largely with the
subject from an Australian standpoint, and is therefore particularly
valuable to the Australian miner, but which will be found applicable to
most other gold-bearing countries. I must not, however, omit to mention
an admirably compiled _multum in parvo_ volume prepared by Mr. G.
Goyder, jun., Government Assayer and Assay Instructor at the School of
Mines, Adelaide. It is called the “Prospectors’ Pocketbook,” costs only
one shilling, is well bound, and of handy size to carry. In brief, plain
language it describes how a man, having learned a little of assaying,
may cheaply provide himself with a portable assay plant, and fluxes, and
also gives considerable general information on the subject of minerals,
their occurrence and treatment.[1]
[1] Another excellent and really practical book is Prof. Cole’s
“Practical Aids in Geology” (second edition), 10_s._ 6_d._
It may here be stated that twenty-one years ago the author did a large
amount of practical silver assaying on the Barrier Hill, which was not
then so accessible a place as it is now, and got closely correct results
from a number of different mines, with an extemporised plant almost
amusing in its simplicity. All I took from Adelaide were a small set of
scales capable of determining the weight of a button down to 20 ozs. to
the ton, a piece of cheese cloth to make a screen or sieve, a tin ring
1½ in. diameter, by 1/2 in. high, a small brass door knob to use as a
cupel mould, and some powdered borax, carbonate of soda, and argol for
fluxes; while for reducing lead I had recourse to the lining of a
tea-chest, which lead contains no silver--John Chinaman takes good care
of that. My mortar was a jam tin, without top or bottom, placed on an
anvil; the pestle a short steel drill. The blacksmith at Mundi Mundi
Station made me a small wrought iron crucible, also a pair of bent tongs
from a piece of fencing-wire. The manager gave me a small common red
flower pot for a muffle, and with the smith’s forge (the fire built
round with a few blocks of talcose schist) for a furnace, my plant was
complete. I burned and crushed bones to make my bone-dust for cupelling,
and thus provided made nearly forty assays, some of which were
afterwards checked in Adelaide, in each instance coming as close as
check assays generally do. Nowadays one can purchase cheaply a very
effective portable plant, or after a few lessons a man may by practice
make himself so proficient with the blowpipe as to obtain assay results
sufficiently accurate for most practical purposes.
Coming then to the actual work of prospecting. What the prospector
requires to know is, first, the usual locality of occurrence of the more
valuable minerals; secondly, their appearance; thirdly, a simple mode of
testing. With respect to occurrence, the older sandy and clay slates,
chlorite slates, micaceous, and hornblendic schists, particularly at or
near their junction with the intrusive granite and diorite, generally
form the most likely geological country for payable mineral lodes,
particularly gold, silver and tin. But those who have been engaged in
practical mining for long, finding by experience that no two mineral
fields are exactly alike in all their characteristics, have come to the
conclusion that it is unwise to form theories as to why metals should or
should not be found in certain enclosing rocks or matrices. Some of the
best reef gold got in Victoria has been obtained in dead white,
milky-looking quartz almost destitute of base metal. In South Australia
reef gold is almost invariably associated with iron, either an oxide, as
“gossan;” or ferruginous calcite, “limonite;” or granular silica,
conglomerated by iron, the “ironstone” which forms the capping or
outcrop of many of our reefs, and which is often rich in gold.
But to show that it is unsafe to decide off-hand in what class of
matrix metals will or will not be found, I may say that in my own
experience I have seen payable gold in the following materials:--
Quartz, dense and milky, also in quartz of nearly every colour and
appearance, saccharoidal, crystalline, nay, even in clear glass-like
six-sided prismatic crystals, and associated with silver, copper, lead,
arsenic, iron as sulphide, oxide, carbonate, and tungstate, antimony,
bismuth, nickel, zinc, lead, and other metals in one form or another; in
slate, quartzite, mica schist, granite, diorite, porphyry, felsite,
calcite, dolomite, common carbonate of iron, siliceous sinter from a hot
spring, as at Mount Morgan; as alluvial gold in drifts formed of almost
all these materials; and once, perhaps the most curious matrix of all, a
small piece of apparently alluvial gold, naturally imbedded in a shaly
piece of coal. This specimen, I think, is in the Sydney Museum. One
thing, however, the prospector may make sure of: he will always find
gold more or less intimately associated with silica (quartz) in one or
other of its many forms, just as he will always find cassiterite (oxide
of tin) in the neighbourhood of granite containing muscovite (white
mica), which so many people will persist in terming talc. It is stated
to be a fact that tin has never been found more than about two miles
from such granite.
From what has been said of its widely divergent occurrence, it will be
admitted that the Cornish miners’ saying with regard to metals generally
applies with great force to gold: “Where it is, there it is”: and
“Cousin Jack” adds, with pathetic emphasis, “and where it is generally,
there I ain’t.”
I have already spoken of the geological “country rock” in which reef
gold is most likely to be discovered--_i.e._, the junction of the slates
and schists with the igneous or metamorphic (altered) rocks, or in this
vicinity. Old river beds formed of gravelly drifts in the same
neighbourhood may probably contain alluvial gold, or shallow deposit of
“wash,” or hillsides and valleys will often carry good surface gold.
This is sometimes due to the denudation, or wearing away, of the hills
containing quartz veins--that is, where the alluvial gold really was
derived from such veins, which, popular opinion to the contrary, is not
always the case.
Much disappointment and loss of time and money would often be prevented
if prospectors would realise that _all_ alluvial gold does not come from
the quartz veins or reefs; and that following up an alluvial lead, no
matter how rich, will not inevitably develop a payable gold lode.
Sometimes gold, evidently of reef origin, is found in the alluvial; but
in that case it is generally fine as regards the size of the particles,
more or less sharp-edged, or crystalline in form if recently shed; while
such gold is often of poorer quality than the true alluvial which occurs
in mammillary (breast-like) nuggets, and is of a higher degree of purity
as gold.
The ordinary non-scientific digger will do well to give credence to this
view of the case, and will often thereby save himself much useless
trouble. Sometimes also the alluvial gold, coarser in size than true
reef-born alluvial, is derived almost _in situ_ from small quartz
“leaders,” or veins, which the grinding down of the face of the slates
has exposed; these leaders in time being also broken and worn, set free
the gold they have contained, which does not, as a rule, travel far, but
sometimes becomes waterworn by the rubbing over it of the disintegrated
fragments of rock.
But the heavy, true alluvial gold, in great pure masses, mammillary, or
botryoidal (like a bunch of grapes) in shape, have assuredly been formed
by accretion on some metallic base, from gold salts in solution,
probably chloride, possibly sulphide or silicate.
Nuggets, properly so-called, are never found in quartz lodes; but, as
will be shown later, a true nugget having all the characteristics of
so-called waterworn alluvial may be artificially formed on a small
piece of galena, or pyrites, by suspending the base metal in the loop of
a thread in a weak solution of chloride of gold in which a few hard-wood
chips are thrown.
Prospecting for alluvial gold at shallow depths is a comparatively easy
process, requiring no great amount of technical knowledge. Usually the
first gold is got at or near the surface and then traced to deep leads,
if such exist.
At Mount Brown Gold-field, N.S.W., in 1881, I saw claimholders turning
out to work equipped only with a small broom made of twigs and a tin
dish. With the broom they carefully swept out the crevices of the
decomposed slate as it was exposed on the surface, and putting the
resulting dust and fragments into the tin dish proceeded to dry blow it.
In “dry blowing” the operator takes the dish about half full of dirt,
and standing with his back or side to the wind, if there be any, begins
throwing the stuff up and catching it, or sometimes slowly pouring it
from one dish to another, the wind in either case carrying away the
finer particles. He then proceeds to reduce the quantity by carefully
extracting the larger fragments of rock, till eventually he has only a
handful or so of moderately fine “dirt” which contains any gold there
may be. If in good sized nuggets it is picked out, if in smaller pieces
or fine grains the digger slowly blows the sand and dust aside with his
breath, leaving the gold exposed. This process is both tedious and
unhealthy, and of course can only be carried out with very dry surface
dirt. The material in which the gold occurred at Mount Brown was
composed of broken slate and alluvium with a few angular fragments of
quartz. Yet, strange to say, the gold always had a waterworn appearance,
probably due to erosion by drifting sand as is so often the case in
Westralian so-called alluvial.
Dry blowing is now much in vogue on the West Australian fields owing to
the scarcity of water; but the great objection is first, the large
amount of dust the unfortunate dry blower has to carry about his person,
and secondly, that the peck of dirt which is supposed to last most men a
lifetime has to be made a continuous meal of every day.
[Illustration: FIG. 4. PUDDLING TUB.]
[Illustration: PLATE I.--TUB PUDDLING _To face p._ 18]
For wet alluvial prospecting the appliances, besides pick and shovel,
are puddling tub (Fig. 4 and Pl. I.), tin dish, and cradle (Fig. 5 and
Pl. II.); the latter, a man handy with tools can easily make for
himself.
[Illustration: FIG. 5. SECTIONAL SKETCH OF CRADLE.]
In sinking, the digger should be careful (1) to avoid making his shaft
inconveniently small, and (2) not to waste his energy by sinking a huge
“new chum” hole, which usually starts by being about three times too
large for the requirements at the surface, but narrows in like a funnel
at 10 feet or less. A shaft, say 4 feet by 2 feet 6 inches and sunk
plumb, the ends being half rounded, is large enough for all requirements
to a considerable depth, though I have seen smart men, when they were in
a hurry to reach the drift, get down in a shaft even less in size.
The novice who is trying to follow or to find a deep lead must fully
understand that the present bed of the surface river may not, in fact
seldom does, indicate the ancient watercourses long since buried either
by volcanic or diluvial action, which contain the rich auriferous
deposits for which he is seeking; and much judgment and considerable
underground exploration are often required to decide on the true course
of leads. Only by a careful consideration of all the geological
surroundings can an approximate idea be obtained from surface inspection
alone; and the whole probable conditions which led to the present
contour of the country must be carefully taken into account.
How am I to know true bottom when I see it? asks the inexperienced
digger. Well, nothing but long experience and intelligent observation
will prevent mistakes at times, particularly in deep ground; but as a
general rule, though it may sound paradoxical, you may know the bottom
by the top.
That is, we will assume you are sinking in, say, 10 to 12 feet ground in
a gully on the bank of which the country rock is exposed, and is, say,
for instance, a clay slate or sandy slate set at a certain angle; then,
in all probability--unless there be a distinct fault or change in the
country rock between the slate outcrop and your shaft--the bottom will
be a similar slate, standing at the same angle; and this will very
probably be overlaid by a deposit of pipeclay, formed by the
decomposition of the slates.
From the crevices of these slates, sometimes penetrating to a
considerable distance, you may get gold, but it is a useless attempt to
sink through them. If the outcropping strata be a soft calcareous
(limey) sandstone or soft felspathic rock, and that be also the true
“bottom,” great care should be exercised, or one is apt to sink through
the bed rock, which may be very loose and decomposed. I have known
mistakes made in this way when many feet have been sunk, and driven
through what was actually bed rock, though so soft as to deceive even
men of experience. The formation, however, must be the guide, and except
in some specially difficult cases, a man can soon tell when he is really
on bed rock or “bottom.”
On an alluvial lead the object of every one is to “get on the gutter,”
that is, to reach the lowest part of the old underground watercourse,
through which for centuries the gold may have been accretionising from
the percolation of the mineral-impregnated water; or, when derived from
reefs or broken down leaders, the flow of water has acted as a natural
sluice wherein the gold is therefore most thickly collected. Sometimes
the lead runs for miles and is of considerable width, at others it is
irregular, and the gold-bearing “gutter” small and hard to find. In many
instances, for reasons not readily apparent, the best gold is not found
exactly at the lowest portion of these narrow gutters, but a little way
up the sides. This fact should be taken into consideration in
prospecting new ground, for many times a claim has been deserted after
cleaning up the “bottom,” and another man has got far better gold
considerably higher up on the sides of the gutter. For shallow alluvial
deposits, where a man quickly works out his 30 by 30 feet claim, it may
be cheaper at times to “paddock” the whole ground--that is, take all
away from surface to bottom, but if he is in wet ground and he has to
drive, great care should be taken to properly secure the roof by means
of timber. How this may best be done the local circumstances only can
decide. In loose or treacherous ground careful attention should be given
to timbering, _i.e._, securing the ground to prevent caving in.
[Illustration: PLATE II.--CRADLING]