Important deposits may be expected at or about the line of unconformability where slates, shales, quartzites, sandstones, limestones, schists and other sedimentary deposits are pierced by intrusive masses of igneous rocks.
Veins filling the cracks that once existed between two differing rocks
are known as contact veins. Such veins are often very rich. Curiously
enough large masses of true igneous rock rarely contain valuable
deposits of mineral, but where such intrusive masses cut dikes or
walls of porphyry, or diorite, the region is worthy of careful
In an open country the prospector should keep to the hill tops if on
the lookout for veins, as the outcrops show more distinctly on the
bare ridges, but alluvial deposits are only found in valleys and along
the borders of streams. In any case, much of the northern part of this
continent can only be prospected by following the streams, on account
of the dense growth of forest with which the soil is covered. The true
line of strike of a vein can be determined only on a level stretch.
The line of strike and the line of dip are always at right angles to
one another; the outcrop may follow the strike or it may not.
A pick, shovel, and pan, are absolutely necessary to a prospector's
proper equipment. A good pocket lens, cheesecloth screen, and small
iron pestle and mortar are often useful. The pan is the most essential
part of the outfit, and is always bright from use.
The regular gold miner's pan is 13¾ inches in diameter across the
top, 10 inches across the bottom and 1/8
inches deep. The best are made of sheet iron and have a joint around
the bottom rim which is of some assistance in retaining the spangles
A more primitive instrument than the pan is the batea. This requires
more skill than the pan, and is much in favor with South American
miners. It is made of hard wood, 20 inches in diameter, 2½ inches
deep in the center, inside measurement, and sloping gradually to
nothing at the sides.
The horn spoon has been handed on from antiquity. It is made from a
black ox horn, at least a black one is the best as it shows the gold
better; it is eight to ten inches long by three inches wide, cut off
When gold is suspected in quartz, but there is visible to the naked
eye more or less iron, copper, and other base metals, it is well to
crush the quartz into coarse fragments. Roast on a shovel or other
convenient tool over a hot fire, and finally pulverize in the mortar.
If panned it will now reveal much of its gold, while, had these
measures not been taken, the sample might have given negative results
and been declared valueless.
After pulverizing, the ore should be passed through the cheese cloth
screen before panning. If the approximate value of the ore is sought,
the sample must be dried and weighed before crushing; and the
resulting gold weighed. Thus:
Sample is to 2,000 lbs. as gold found is to Ans.
About 13 cubic feet of quartz weigh a ton before being disturbed; when
broken to medium sized lumps 20 cubic feet may be taken as
representing a ton. Although experience teaches the miner to estimate
very closely the value of his sample, it is better for the tyro to
have a small pair of scales with grain weights. A grain of gold, if
tolerably pure, is equal to four cents. Above all things avoid the too
common error of panning the pick of the rock, as a false estimate is
bound to follow and only too probably eventual loss.
A yard of gravel before being dug makes one and a half yards
afterwards. A pan of dirt is usually about 20 pounds, although it is
not well to fill quite full in actual work.
Many a valuable mine has been found by following up "float" ore. Float
is detached fragments of the vein or gangue, and it becomes more and
more abundant as the lode is approached until it finally ceases
abruptly. This indicates that the vein has been reached or passed, and
a trench dug throughout the alluvial soil at right angles to the
assumed line of the vein will probably reveal it. The float and
mineral of course drift down hill; if the side of the mountain be
saddle-shaped the float will spread out like a fan as it washes down,
but if concave the force of gravity will concentrate it within a
narrow space in the ravine. Float found at the foot of a hill has
come, as a rule, from that hill. The nearer the vein the less worn
will be the edges of the float and mineral. The gangue or vein-rock in
which the metal is found may be calcite or calc spar, fluor spar,
heavy spar or baryta, or quartz. Gold is almost always found in this
last matrix. The upper parts of most quartz lodes are usually
oxidized, that is to say, the atmosphere has acted upon the iron
pyrites, freeing the sulphur and staining the quartz yellow, red, or
brown, by oxide of iron. This is known as "gossan" or the "iron hat."
Such quartz is frequently honeycombed and rotten. Below the water
level these veins run to sulphides in which decomposition has not set
in, and the gold contained in the quartz is no longer "free milling,"
i.e. will not give up its gold to mercury without a preliminary
Certain minerals are likely to be found associated. Cassiterite goes
with boron and tourmaline, topaz, fluor spar and lithia-mica; all
containing fluorine. It is also found with wolfram, chlorite and
arsenical pyrites. Magnetite is often accompanied by rocks containing
garnet, epidote and hornblende. Zinc blend and galena may occupy the
same vein, which is likely to be of baryta or heavy spar. Much galena
carries silver. Gold is associated with many metallic sulphides such
as iron, magnetic, and copper pyrites, mispickel, galena, blend,
stibnite and tetrahedrite. Gypsum accompanies salt.
Surface indications may be described as: Form of ground, color,
outcrop, decomposed and detached mineral, mineral deposits from
springs, altered or peculiar vegetation and other similar guides. A
hard quartz outcrop often stands up like a wall and is traceable for
miles. The Rainbow silver bearing lode of Butte, Montana, stood 20
feet above the surface. Soft minerals, such as clay, are cut into and
sunk below the surrounding level. Deposits of Kaolin or China clay are
usually so found.
Any special bright coloration of the rocks of a district merits
investigation. Copper gives green, blue, and red stains; iron, red or
brown; manganese, black; lead, green, yellow or white; cobalt, pink;
cinnabar or quicksilver, vermilion. The nickel deposits of New
Caledonia were made known to the world by the explorer Garnier in
1863, his curiosity having been aroused by the delicate green coating
given the rocks by an ore containing water, quartz, nickel and
Hard beds of shale decompose on the surface into soft clay, and a
still more noticeable change is the conversion of ores containing
sulphur into oxides. This chemical change causes the gossan or "iron
hat," for which token of underlying wealth the prospector should be
eternally watchful. This alteration may extend downward four or five
hundred feet from the surface, but in such cases the true weathering
has ceased long before the limit of discoloration is reached, and the
change of substance is due to the filtering of surface waters through
Gossan varies greatly in its nature. Galena becomes anglesite,
cerussite, pyromorphite and mimetite. Copper pyrite changes into
native copper, melaconite, cuprite, malachite, chessylite, or perhaps
into a phosphate, arsenate, or silicate of the metal. Carbonate of
manganese gives the black oxides and silver sulphide ores are, after
weathering, known as native silver, kerargyrite and embolite.
The ore in the gossan is very generally more valuable than it will be
below, and this is especially true of gold and silver ores. The gold
having been set free from the close embrace in which the iron pyrite
held it previous to the latter's oxidation, it is now readily caught
by quicksilver. Silver under similar conditions becomes chloride, and
likewise amalgamates without difficulty.
Seams containing native sulphur often show no trace of that element on
the surface, having weathered into a soft, white, gray or
yellowish-white granular, or pulverulent, variety of gypsum.
Veins of asbestos often decompose into a white powder found in the
crevices of the rocks; fibrous asbestos existing in the interior.
Petroleum shows in an iridescent film upon still pools, and the odor
is a sure guide to its nature.
A "dipping-needle" is valuable to the prospector on the lookout for
iron ore; by its use he may discover masses of magnetic ore and trace
their extent. As he carries the compass over the ground the needle
dips toward any iron mass he approaches; directly over the ore it