PROSPECTING FOR PLACER GOLD AND GOLD VEINS.

PROSPECTING FOR PLACER GOLD AND GOLD VEINS.

BY ARTHUR LAKES, 1895

Having given in preceding chapters a sketch of veins and
ore deposits in the rocks, it follows in order to speak of gold
placers, because these are derived from, the former by the
agencies of water, either in the form of glaciers of old, or of
ancient or modern streams.
The glaciers in olden times heavily mined the rocks and
the veins, by cutting broad gashes through them, thus
originating the canyons. In this way millions of tons of rock
were mined, together with the goldbearing veins in them, and
also the precious metals minutely diffused and scattered throughout
their masses. After the glaciers, the rivers took up the work, deepened
the canyons, broke up the boulders and sorted them, setting free the
gold and other metals they contained, and again sifted and sorted
them and deposited them along their banks and in their
beds.
Of the various metals thus handled by nature's jigging
process, many were dissolved and destroyed by various
acids in the waters, and by acids of vegetation and iron salts
percolating through the placer dumps after they had been
laid down. So with the exception of a few very hard
minerals, such as magnetite, diamonds, garnets, rubies, etc.,
little remained in the placer but the imperishable gold, and
even that appears to have been refined of its alloy of silver
which it contained in the original vein, for placer gold is
generally much purer and more valuable than that in the
original vein.
the placer appears to have been acted upon by certain salts,
such as the persalts of iron, and concentrated and amalgamated
into large nuggets. Some contend, however, that
these nuggets are only waterworn pebbles of gold, brought
direct from the vein, the result perhaps of concentration
there of the contents of large masses of gold-bearing pyrites ;
it is to be noted, however, that whilst gold-bearing nuggets
of various sizes are to be found, not uncommonly in gold
placers, they are very rarely found in gold veins.
With the gold in placers, is commonly found what is
called "black sand," which is composed of grains or pebbles
of magnetic iron ore, relics of the old gold-bearing pyrites
chemically changed. Being near in gravity to gold, and
originally associated with it, the two are generally found
together in a placer, and a prospector in surveying a bank
of placer-material made up of sand, pebbles and boulders,
generally looks for a streak of "black sand" as a likely
place for gold. Also by reason of the gravity of gold he is
inclined to look for it more down on bed-rock than in the
upper looser strata.
Ancient river beds as well as those of modern rivers may
be found gold-bearing, rivers that have long ceased to flow,
by reason perhaps of change in the configuration of the
country. In California and Australia many of these ancient
fold-bearing river-beds have at a period not long distant,
deluged and covered by lava, and the gold is extracted
by tunnelling beneath the lava-sheet or by shafting down
through it to the gravel below. These are called deep leads
whilst the ordinary uncovered gravels are called "shallow
placers."
Almost anywhere along ancient or modern water courses
not far from mountains, a prospector by panning, can get
colours of gold even on the pebbly " wash " covering the
surfaces of large portions of our plains, or even on the tops
of table lands that once were plains, over which broad rivers
and glaciers and large bodies of water distributed their
debris, but as a rule it will only pay to work where the
"wash" or "drift" or "alluvial" matter is plentiful and
thick, and more than this, only where water is accessible to
the work.

PROSPECTING.

A prospector hunting for a gold placer follows up the
water channels in which he finds specimens of all the rocks
in the neighbourhood. In Australia, the prospector looks
amongst these to find samples of granitic, porphyritic and
quartzose rocks or clay-slate as likely signs, and also pieces
of quartz honey-combed and rusty, which we have described
before as " float or blossom." Plenty of broken up quartz
he considers a good sign, but very pure, hard, dull white
quartz is generally considered as " hungry " or " barren ;""
the size of the fragments denotes his nearness or otherwise
to the reef, ie., the vein.
A prospector examines closely the fine sandy matter of
the stream bed especially where eddies and backwater have
been formed. A likely deposit should be scraped up, even
down into every crevice and depression in the bed rock or
solid rock bottom over which the river, modern or ancient,
has worn its channel. This material should be panned.
Gold, too, is often found on points and slopes of the bed rock
as well as in the deepest portion. Nuggets found on high
reefs above the level of the stream, imply that their weight
enabled them to remain in their position, during the deeper
erosion of the neighbouring streams, and that the original
vein from which they came, is not far off. As a rule, large
nuggets and coarse gold are found much nearer to the
source whence they came, than fine or " flour " gold, which is
often carried to unlimited distances away out on the plains.
The character of quartz veins and of their enclosing
rocks in the immediate vicinity, decides the character, too,
of gravels derived from them, hence sometimes a peculiar
pebble may be traced up to the peculiar rock whence it
came, and the gold vein be found near it in place.
It has been observed that " leads "following the course or
lines of a gold-bearing reef, maintain a more continuous
yield than those crossing a number of gold reefs at intervals.
Gold occurs in pockets and " shoots "
at intervals, with barren portions between, which accounts for
what we have stated above. In a country where the gold quartz
veins are small, though rich at wide intervals, the gravels will also
be small. In very deep ground where the " wash " is very heavy a
series of borings or even shafts are made to test the quality
of the bank. The following points have been observed as
worthy of note in prospecting for gold placers.
1. Streams crossing the lamina or stratification planes of
gold reefs at right angles are likely to be richest.
2. Gold is rarely found plentiful where there are indications
that the current was strong, but rather in the lee under
projecting points of rock, where beaches are usually formed
and the water was slack.
3. Gold in streams is deposited in crevices of the "bed
rock," which should be laid as dry as possible and picked
up to such depths as the sand descends between the laminations.
4. Terraces are shelf-like excavations and deposits upon
hill slopes above valleys, and are the remains of old glacier
or river beds. The prospector should discover the inlet and
outlet of the terrace and examine the gravel. The " wash "
sometimes contains gold in layers one above the other.
5. Whilst working up stream attention should be paid to
the banks on each side where sections are exposed so that
no outcropping vein be overlooked.
6. Alluvial gold should if possible be traced to its source
whence the "float " came. When the gold is large and
plentiful and the boulders large and angular the reef is
likely not far distant.
7. Sometimes there is a distinct peculiar feature in all the
veins of a district, such as a peculiar band of a definite colour.
8. Coarse alluvial gold is not always incompatible with fine
reef gold as a source, because the reef gold may be so fine in
general as to lend itself to very wide distribution when once
it is liberated, while the rarer coarse grains would not be
transported far.
9. Alluvial placers are richest where the current of the
stream is interrupted by diminution in fall, by sudden
change of direction, or by entrance of a tributary, also by
reefs, bars, eddies, etc. Absolute richness depends upon
local circumstances and the size and weight of floated
masses.
10. Creases, holes and fissures of bed-rock over which the
stream passed are favourite places.
11. The lowest layers of each separate period of deposition
are the richest.*
Sometimes several different periods of deposition have
succeeded each other.
12. The courses of present streams and of ancient channels
are placers.

" LOAMING "

is a form of prospecting. It is preliminary to
such prospecting as cutting experimental trenches, or sinking
trial shafts or boring. It consists in washing surface
prospects from the bases and slopes of the ranges, until
specks of gold, or specimens are found to be obtainable
with tolerable frequency, within certain limits. The prospector
then proceeds to trace the gold up hill to its source,
narrowing the limits of his work as by patient search he
approaches the vein, whence the gold has been derived..
When he can obtain surface prospects of gold up to a certain
point, or line, but no farther, he then proceeds by means of
trenching to search for the gold vein. The prospector has
often to work along a steep scrubby mountain side selecting
his prospects, numbering them, and placing samples in his
" loam bag." If he discovers prospects of gold, he finds his
way back to the spots the samples were taken from, so as to
continue his up-hill search, and trace the gold to its source
or vein. Sometimes there is no indication of a vein, soil
and bushes and debris covering its out-crop, but by learning,
the prospector ascertains its position, so as to expose it by
a trench not many feet in length. We remember an ingenious
way in which a valuable and long sought for vein was at last
discovered. Prospectors had long found very rich " float "
at the base of a hill whose surface was so deeply covered with
loose debris that no trace of the vein could be found. A prospector
found a small lake on top of this hill, and conceived the
idea of cutting a trench from this body of water to the
edge of the hill, and by damming up the trench, and then
suddenly letting out the water to full force, it cut a
deep trench through the loose debris down to bed rock
and the vein was discovered. This process is called " booming."
The cleavage of quartz is said to be freer, sharper and better
defined, in gold-bearing quartz than in that which is barren.
Pyrite is a good indication. A soft, fatty clay or gouge
often flanks the vein in its gold-bearing portions.
The mountain spurs should first receive attention for
veins ; if the quartz is hard, it stands up, if soft, as it more
commonly is, it will leave a streak-like depression. On finding
such, the prospector should first wash out some of the
decaying rock. If only a trace of gold is found in the quartz,
there is probably a gold vein in the neighbourhood, and
trenches should be dug and exploration systematically followed
up. Gold is generally near one wall of a vein, seldom
all through the stone. Quartz gold occurs in " shoots "
with barren spaces.
Before setting a valuation on a discovery, the facilities for
working the mine, such as we have alluded to, should be
considered. Placer mines as well as other mines are often
supposed to be "worked out." These are sometimes well
worth investigating and examining by cross-cuts or other
no means. Sometimes it happens that more gold is obtained
from " leader " veins that had been overlooked, than from
the main worked vein. Quite commonly, especially in the lower
part of a placer, the pebbles and sand are firmly cemented
together into a coarse conglomerate by infiltration of iron
oxide and clay. This may consolidate into a false-bottom and
not be true " bed rock." Generally two or three such false-bottoms,
with intervening strata of greater richness, alternate with
barren ones. So, many old diggings, thus supposed to have
been exhausted, may be worked again, the true bottom not
having been reached. These conglomerate bottoms may lie
just upon bed-rock, with a white clay rich in gold beneath
them. Gold occurs also in the conglomerate and must be
stamped out.


Taken from the Book

PROSPECTING FOR GOLD AND SILVER

BY ARTHUR LAKES, 1895

Late Professor of Geology at the State School of Mines,
Golden City, Colorado. Author of "GEOLOGY OF COLORADO AND WESTERN ORE DEPOSITS,'
"GEOLOGY OF COLORADO COAL DEPOSITS," Etc. SCRANTON, PA. THE COLLIERY ENGINEER CO.

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