Lead mining was one of the earliest industries of Illinois. The early
settlers’ need for bullets for procuring food and for defense of their
lives and property made lead an important commodity, and the deposits of
lead ore in the northwestern corner of Illinois were quickly exploited.
The ore was the mineral galena (fig. 5), for which the city of Galena in
Jo Daviess County is believed to have been named.
[Illustration: Figure 5—Galena with cubic cleavage blocks in
foreground.]
Galena is a dark, shiny mineral that breaks readily into cubes or
combinations of cubes. It is composed of lead and sulfur (PbS). Galena
itself is not suitable for use as a metal; the lead must first be
separated from the sulfur.
The earliest method of recovering lead from galena was crude. A pile of
logs, smaller pieces of wood, and ore was built on sloping ground. Just
below it a pit was dug. When the wood was set on fire, the heat caused
the lead and sulfur to separate, and the molten lead trickled down into
the pit. The smelting process was later improved, and stone “furnaces”
were built to house the operations.
_Crevice Deposits and Residual Deposits._—Most of the lead ore mined in
the early days of the northwestern Illinois mining district came from
crevice deposits in the dolomite bedrock and from residual deposits at
or near the surface of the ground. The crevices were vertical narrow
joints or fissures. Ore was not continuously present along them but
occurred from place to place in “pods” (fig. 6 and fig. 7). Dimensions
of the pods varied, but typical ones were about 3 feet wide, 5 feet
high, and a few to a few hundred feet long. The galena, for the most
part, occurred in a mixture of clay and weathered dolomite that filled,
or partly filled, the crevices.
The residual deposits were found where the action of the weather for
many thousands of years had dissolved the dolomite from the outcropping
parts of a crevice deposit and left behind a residue of brown or red
clay containing galena.
Some of the crevice and residual deposits worked by the early miners
cropped out at the surface, but most of them were covered by earth.
Other crevices were exposed in the bluffs of the Mississippi River and
extended back into them for 1,000 feet or more.
[Illustration: Figure 6—Diagrammatic cross section of two crevice
deposits. A reaches ground surface and is filled with clay; B is
only partly clay filled. Galena coats parts of the walls and occurs
as pieces scattered through the clay. Typical crevices are about 3
feet wide and 5 feet high.]
SOIL
A
GALENA
CLAY AND ROTTED DOLOMITE
B
GALENA
CLAY AND ROTTED DOLOMITE
DOLOMITE
When the richer deposits of ore in the crevices were worked out, some
mines were deepened into the dolomite bedrock, but usually less rather
than more galena was found.
As the amount of galena decreased, however, another mineral, which had
been present before in only small amounts, was found in increasing
quantities. This was sphalerite—a yellow, brown, or black mineral of
resinous appearance that is composed of zinc and sulfur (ZnS). It does
not look like a metallic ore.
At first the sphalerite was not used because there were no smelters in
the area that could separate the zinc from the sulfur with which it was
combined in the ore. Between 1850 and 1870, however, smelters were built
in northwestern Illinois and southern Wisconsin and the ore was shipped
there. Sphalerite is now the principal ore mineral produced in
northwestern Illinois.
The sphalerite and galena of southern Illinois, described later, are
similar to that found in northwestern Illinois.
[Illustration: Figure 7—Crevice lead mine. Miner pries ore loose and
pushes it into car at bottom of crevice. Shiny galena layer occurs
above his head and another at level of his waist.]
_Mining and Milling._—One large mine is producing zinc and lead near
Galena at present. Smaller mines operate irregularly. The ore may occur
as pockets, irregularly shaped rather flat masses, vertical or inclined
veins (fig. 8), or small particles scattered through the dolomite. The
principal ore bodies that have been worked in recent years have been of
irregular shape, both horizontally and vertically, and usually have been
between 50 and 200 feet wide and from a few to as much as 100 feet high.
They lie at a depth of roughly 300 feet.
Blasting is required to loosen and break the ore. At the large mine the
ore is brought to the surface by a hoist. In some relatively shallow
mines an inclined tunnel has been driven to the ore body and the ore
brought to the surface in trucks.
[Illustration: Figure 8—Diagrammatic cross section of a zinc ore
deposit showing flats and pitches. The deposit occurs in limestone
and dolomite. Much of the ore is a mixture of dolomite, limestone,
calcite, and sphalerite. Typical dimensions of such a deposit are
about 30 feet high and 100 to 200 feet wide.]
ORE
PITCHES
FLATS
Because the ore consists of galena and sphalerite attached to and
scattered through dolomite, it must be milled to free and separate the
metals from the rock. Crushing, grinding, and other operations are
involved. The dolomite is discarded and the galena and sphalerite
concentrates are shipped away to be smelted. No smelters have operated
in the Galena area for some time.
_Aids to Prospecting._—Finding deposits of ore 300 feet underground is
not easy. Inspection of the surface usually tells little. To find and
outline a commercial ore deposit many holes often must be drilled to
explore the unexposed rock strata. Because this is a costly process,
every possible means is employed to drill the holes where ore is most
likely to be found. This is where geologists are useful—geologists of
the mining companies and of the Illinois Geological Survey. Three
examples of how their investigations help to find ore are given here.
It was noted early in the development of the northwestern Illinois
mining district that zinc ore deposits were most common along small
downfolds in the bedrock, called synclines, that were a few hundred feet
wide and a mile or so long. The synclines were associated with much
larger synclines that extended for several miles. A map prepared by the
Illinois Geological Survey shows the possible location and extent of
many of these downfolds and has had much practical use in the selection
of the most promising areas for test drilling to find ore.
The Survey also collects the records of borings made by companies and
individuals in their search for ore. The records are on permanent file
at the Survey offices and are valuable in several ways. Some indicate
where no ore was found and where it is, therefore, useless to drill
further; others show only traces of ore but suggest that more drilling
in the vicinity might discover a deposit large enough to be mined
profitably. Still other records are of borings that encountered rich ore
in which mines have been developed.
The third aid to prospecting is the study of ore bodies and their
minerals to determine how the deposits were formed. The ore bodies have
been and are being studied in the mines. Ore specimens are carefully
examined in the Survey laboratories. If geologists can learn how the
known deposits were formed, it may be possible to direct exploration
into promising new areas.