There are two classes of minerals in which the prospector is interested, one may be called the "earthy" minerals, such as quartz, fluorspar, etc., associated with the precious ores; the other, the metallic minerals constituting the ores themselves. Both of these he wants to know at sight, or to determine with the simplest appliances. Generally speaking, his eyesight, his pocket-knife, his ore-glass and a little acid, will be all he needs, nor need he concern himself about a great number of minerals, if he only knows the commoner ones well. The earthy minerals form the gangue or veinstone of the vein in which the precious ores are distributed.
EARTHY GANGUE MINERALS:
These are principally quartz, calcite, dolomite,
fluorite and
barite, all of which we have already described among rock-forming
minerals. These crystals are nearly always to be found in the adjacent rock
as elements of that rock, and their more sparry condition in the gangue of
the vein is derived by solution from the enclosing country rock. Thus, a
vein running through granite, will contain mainly
quartz, though calcite and
fluorspar may be associated with it in small quantities. A vein passing
through limestone naturally carries calcite or fluorspar. Sometimes barite
is associated with the calcite, especially if near the limestone ore deposit
there are porphyries. Barite has been detected as an element of some
porphyries which are probably ore-bearing, and when prospecting, we have
found barite to be generally an indication of ore near by, whilst fluorspar,
or quartz, alone, may or may not be barren. The float, or loose surface
indications of ore-deposits at Aspen is commonly made up of fluorspar and
barite.
FLUORSPAR in
Colorado is generally confined to veins in the granitic rocks and in some of
the eruptive rocks. Its presence is a good sign of ore.
OXIDES OF IRON AND MANGANESE. These, often mixed together, form a
large element in the gangue matter of a vein or ore deposit. Manganese can
be recognized by its dark black color.
RHODOCROSITE A beautiful rose-colored carbonate of manganese,
rhodochrosite is occasionally found, associated with quartz and
metal in some veins.
CARBONATES OF COPPER are often associated with precious metal vein
gangue matter. It is readily distinguished by its bright green form (malachite)
or azure blue (azurite)
color. Vein "Float" is commonly rusty with iron-oxide streaked with stains
of copper carbonate.
SPATHIC IRON OR IRON CARBONATE OR SlDERITE occurs here and there in
the gangue of fissure veins.
Siderite is very like brown feldspar but heavier. These few common
minerals cover nearly all that are generally met with as indications of, or
in important connection with, ore deposits. As a rule most of these minerals
occur in a massive state rather than as individual crystals in a vein.
METALLIFEROUS MINERALS:
Through these gangues of various characters, the precious metals are
distributed in long, narrow patches or strings, or in large crystalline
masses, or in scattered crystals, or in decomposed masses. The gangue matter
is generally in the majority in a vein, and the ore thinly, sparingly, and
irregularly, distributed in it. When a vein is said to be ten or more feet
wide, it is not to be supposed, that ten feet of solid ore is meant, but
that this is the width of the gangue between walls. The ore body may be only
a few inches wide. The streak or main body of ore called the "pay streak"
has a tendency to keep near one wall or the other, or at times to cross from
wall to wall.
HIGH AND LOW GRADE
ORES:
In gold veins, flakes or wires of "free" or "native"
gold occur in the decomposed gangue; and sometimes in the
pure un-decomposed quartz, "native" silver is found in much the same way,
but more as specimens than as continuous bodies. Isolated patches of rare,
or valuable minerals, such as Ruby silver (Acanthite), Horn silver (chlorargyrite),
Silver glance, etc., occur locally in parts of the vein, sometimes coating
stalactites or crystals of a "vug" or cavity lined with quartz or other
crystals. An
assay from such picked specimens would give a very unfair average of
a mine or prospect. The bulk of the profits of a mine come from the
commoner minerals such as galena,
pyrite, or lead-carbonate, and from the average grade of the
mine. In the California gold mines, the average yield of gold per ton is
$16. In Dakota $6. In the silver and lead mines of Leadville, $40 per ton is
the average, and the ores are mostly low grade. A few mines of extraordinary
high grade may yield from $75 to $100 per ton, but these are exceptional.
Quantity of ore, facility for milling, cost of freight, the size of the
vein, and its facility for working and nearness to market give the offset.
DECOMPOSED MINERALS:
Sometimes the gangue matter contains a variety of decomposed ore in rich
secondary combination intimately mixed through its mass and rarely
discernible by the eye. Thus yellow mud from a mine may assay high, from the
presence of invisible chlorides or
sulfides of silver. No accurate estimate of the value of a
mine, or even of a piece of ore, can be found, without an assay or mill-run.
The reason for such richness in decomposed surface products, is, that nature
has been for ages leaching out, concentrating and combining in richer forms,
the essence, so to speak, of the vein.
GRAY COPPER (TETRAHEDRITE).
Besides the ordinary galena and pyrites common in most mines, we sometimes
find considerable bodies of gray copper in mines, or intermingled with other
ores. This is generally a rich silver bearing ore, running from 60 ounces to
some thousands per ton. It generally occurs massive, rarely showing its
pyramidal "tetrahedrite"
crystals. In appearance it is not unlike a freshly broken piece of bronze.
It is more common in fissure veins in granite and eruptive rocks than in
limestone. In Halls Valley, Colorado, it is associated with
barite in a vein in the gneiss. It occurs in the Georgetown veins in
granite. In the San Juan district it occurs also associated with barite in
the Bonanza mine; and an ore not identical with it in composition, but very
like it in appearance, called bismuthinite, consisting of bismuth, antimony,
copper and silver, is characteristic of that region and is rich in silver.
Bismuthinite has a more shiny tin-like appearance than gray copper, and the
red color which bismuth gives to charcoal under the blowpipe readily
distinguishes it from gray copper.
LOCAL VARIATIONS IN
VALUE OF ORES:
There are locally in different mining districts considerable differences in
the value of certain minerals and ores. In one district gray copper may
rarely exceed 60 ounces of silver, in another it is invariably over 100
ounces. A coarse
galena is generally poor in silver, while fine grained "steel
galena" is generally rich in silver, but the reverse may also be the case.
In some of the mines at Aspen, fine grained galena, especially near the
surface, is quite poor in silver, while in other mines in the same district
it is exceedingly rich. Localities occur also where coarse-grained galena
runs well in silver and is richer than fine-grained galena. This is the case
at the Colonel Sellers mine at Leadville. So one mining district or even one
mine is not a rule for another.
Continue on to:
Minerals For The Prospector, Part 2
Return To:
Gold And Silver Basic Prospecting Methods
