In the U.S.A., lead ore was obtained principally from the States of Missouri, Idaho, Utah, Nevada and Colorado. In the south-eastern district of Missouri, galena occurs in the form of extensive impregnations in limestones of Ordovician age. Sandstones occur in the formation, but these contain no ore. The ore is remarkable for the fact that it contains practically no zinc minerals. It contains on the average about 7 per cent, of galena, and can be dressed by jigs to yield a product containing from 60 to 70 per cent, of the mineral.
In south-west Missouri, Joplin is the most important locality. Here the ore-bodies are sometimes several hundred feet thick, and the ore is found as veins or as breccia-cement in limestones of Lower Carboniferous age. The chief ore mineral is sphalerite, which is cadmium bearing, but galena also is found in abundance. The lead zinc ores of northern Arkansas may be referred to along with those of Missouri. They occur in limestones of Ordovician and Carboniferous age, partly in veins, and partly in breccias or in a disseminated condition; the chief ore minerals are galena and sphalerite, embedded in a gangue of calcite, dolomite, barite, and chert. The lead and zinc ores of the Missouri region are supposed to have arisen by a process of slow concentration of lead and zinc sulfides from the limestones, through which they are disseminated in small amounts. The operation was probably effected by the oxidation of disseminated sulfides to sulfates, their transportation in solution, and their re-precipitation as sulfides under the reducing influence of the organic matter in the limestones. Shale beds are supposed to have played an important part in the deposition of the ores, by arresting the circulation of the solutions prior to precipitation. Important deposits of argentiferous lead ore are mined at Coeur d'Alene, Idaho, where the geological formations consist of folded sandstones, slates, and quartzite. The ore occurs in veins. The chief ore mineral is argentiferous galena, and this is associated with siderite, quartz, and sphalerite. The workable ore-bodies contain from 5 to 25 percent of lead, and from 7 to 10 ounces of silver per ton.
Large deposits of
argentiferous lead ore are found in the Tintic district of Utah, in the
folded Palaeozoic limestones and associated Tertiary igneous rocks of that
area. The chief ore mineral is argentiferous galena, associated with
sphalerite,
pyrite, enargite, barite, and quartz. Other important
deposits of argentiferous lead ore in the U.S.A. are those of the Leadville
district of south-central Colorado. The sedimentary rocks of the region are
of Palaeozoic age and include thick beds of Carboniferous limestone. These
sediments are traversed by dikes of porphyry and are extensively interbedded
with igneous rocks of the same type. These porphyries are connected with the
plutonic masses of
granite that underlie the region. The rocks are extensively
folded and faulted as a consequence of the late-Cretaceous earth-movements
which produced the Rocky Mountains, and these earth-movements are supposed
to have had some influence in stimulating the deposition of the ores. The
ore-bodies occur frequently beneath the sills of porphyry where these are in
contact with the Carboniferous limestone. The chief ore mineral is
argentiferous galena, associated with calcite, barite, chert, and native
gold; but in the weathering zone,
cerussite and anglesite are abundant, associated with
limonite and
cerargyrite (silver chloride)
NEW MEXICO: The Kelley Lode. Oxidized lead ores, with some sphalerite,
calamine, forming a contact deposit between slates and porphyry. The ore
body is in the Magdalena Mountains, thirty miles west of Socorro, and has
supplied the Billings smelter at that point. Numerous other ore bodies along
the contact between sedimentary and eruptive rocks occur in the same region.
Lake Valley. Farther south in Dona Ana County, the mines of Lake Valley have
been worked upon deposits very closely analogous to those of Leadville,
Colorado. They contain less lead, hardly enough, in fact, to be classed as
lead-silver
ores, according to the recent valuable paper of Ellis Clark,
although earlier descriptions place greater emphasis on the presence of
carbonates of this metal. According to Clark, the geological section
involved includes quartzite and limestone, considered Silurian, 600 feet;
Lower Carboniferous, black shale, 100 feet; green shale, 60 feet; nodular
limestone, 48 feet; blue limestone, 24 feet; crinoidal limestone, 125 feet,
and overlying limestone, 50 feet; about 1,000 feet in all. These are
penetrated by four distinct eruptions of igneous rocks, hornblende-andesite,
rhyolite, obsidian and porphyrite. The obsidian is comparatively
unimportant, and of the others the porphyrite is most intimately associated
with the ore. The ore bodies are always connected with the blue limestone,
and lie along the contact of this, either with the porphyrite or the
overlying crinoidal limestone. They are in the nature of large chutes or
elongated contact deposits, very similar, as the figure will indicate, to
those at Leadville. The ores are of several varieties but the general
components, in addition to the silver, are silica, oxides of iron and
manganese, limestone, some galena at times, and some zinc. The varying
percentages of the silica and bases afford basic, neutral and siliceous
ores. In the bonanza called the Bridal Chamber, great masses of horn-silver
were found. Many ores, and interesting metals, such as vanadinite,
descloizite, etc., have made the district well known to mineral collectors.
Some favor the view that the leaching of the porphyrite (which is
argentiferous) during its exposure and erosion, by descending surface
waters, has been the source of the ore. An earlier view attributed it to
uprising currents.
COLORADO: Leadville. Bodies of oxidized lead silver ores,
passing in depth into sulphides, deposited in much faulted Carboniferous
limestone, in connection with dikes and sheets of porphyry. Leadville is
situated in a valley which is formed by the head waters of the Arkansas
River. The valley runs north and south, being confined below by the closing
in of the hills at the town of Granite. It is about twenty miles long and
sixteen broad, and even to superficial observation is seen to he the dried
bottom of a former lake. The mountains on the east form the Mosquito range,
a part of the great Park Range, while those on the west are the Sawatch, and
constitute the Continental Divide at this point. Leadville itself is on the
easterly side, upon some foothills of the Mosquito range. The eastern slope
of the Mosquito range rises quite gradually from the South Park to a general
height of 13.000 feet. The range then forms a very abrupt crest, with steep
slopes looking westward, which are due to a series of north and south faults
whose easterly sides have been heaved upward as much as 7,500 feet. The
faults pass into anticlines along their strike. The Mosquito Range consists
of crystalline Archean rocks, foliated granites, gneisses, and amphibolites,
and of over 5,000 feet of Paleozoic sediments and igneous rocks. The former
include Cambrian quartzite, 150 to 200 feet; Silurian white limestone, 160
feet, and quartzite, 40 feet; Carboniferous blue limestone, 200 feet (the
chief ore-bearing stratum) ; Weber shales and sandstones, 2,000 feet; and
Upper Carboniferous limestones, 1,000 to 1,500 feet. The igneous rocks are
generally porphyries. The sedimentary rocks were laid down in Paleozoic time
on the shores of the Archean Sawatch Island, and were penetrated by the
igneous rocks, probably at the close of the Cretaceous. They were all
upheaved, folded, and faulted in the general elevation of the Rocky
Mountains, about the beginning of the Tertiary period. The intrusion of the
igneous rocks was the prime mover in starting ore deposition, and the
solutions favored the under sides of the sheets, along their contacts with
the blue Carboniferous limestone. The early history of Leadville will be
subsequently referred to in speaking of auriferous gravels. The lead-silver
ores first became prominent in 1877, although discovered in 1874, and by
1880 the development was enormous. The region grew at once to be the largest
single producer of these ores, and has remained such ever since. The mines
are situated east of the city on the three low hills, Fryer, Carbonate and
Iron, but recently a deep shaft in the city itself has found the extension
of the ore chutes and opened up great future supplies. The ores have chiefly
come in the past from the upper oxidized portions of the deposits. Of late
years, however, the older and deeper workings have been showing the
unchanged
sulfides.
The ores are chiefly
earthy carbonate of lead, with chloride of silver, in a clayey or siliceous
mass of hydrated oxides of iron (limonite)
and manganese. In the Robert E. Lee mine silver chloride occurred without
lead. Some zinc is also found, and a long list of rare minerals. Where the
ore is in a hard, siliceous, limonite gangue it is called hard carbonate,
but where it is sandy and incoherent it forms a soft carbonate, or sand
carbonate. All the older mines produce small amounts of gold, but in some
newer developments the gold is of more importance than the silver. A few ore
bodies are found at other horizons than the Carboniferous. They also run in
instances as much as 100 feet from the contact, and may likewise be found in
the porphyry, doubtless replacing included limestone. They were all
deposited as sulphides, and, according to Emmons, when the rocks were at
least 10,000 feet below the surface. In 1891 and 1892 great interest
centered in the discovery and development of
gold ore bodies, whose values in gold much exceeded those in
silver, and which were situated further east from the city of Leadville than
the older silver mines. The gold output has now proved very considerable,
although limited to but few mines. The geological associations are much the
same as in the older workings, and indeed the gold ores occur on the
extended lines of the older chutes, when projected to the eastward.
UTAH: In Bingham and Big and Little Cottonwood Canyons, bedded
veins, often of great size, containing oxidized lead-silver ores above and
galena and pyrite below the water level, in Carboniferous limestones, or
underlying quartzite, or on the contact between the two. The mines are
situated in the Oquirrhand Wasatch Mountains, southwest and southeast of
Salt Lake City, in canons well up toward the summits. The region is much
disturbed, and there are great faults and porphyry dikes and knobs of
granite associated with the sedimentary rocks. The ores occur in belts,
extending considerable distances, and these in places have the rich chutes
or chimneys of oxidized products. In Bingham Canyon an immense bed of
auriferous quartz is found, overlying the lead zone and next the hanging.
Some peculiarity about the gold prevents its easy treatment, but much of the
rock is very low grade. Recently very extensive deposits of
copper ore have been found in the Highland Boy. Other fissure
veins in the massive rock of the region are known, but are of less
importance. The mines were the occasion of the first development of the
lead-silver smelters in the West, and have made Salt Lake City an important
center of the industry. The Telegraph group, the Emma, Flagstaff, and others
were famous mines in their day. As will appear, nearly all the Utah mines
are productive of lead silver ores.
NEVADA: At Eureka, Nevada, bodies of oxidized lead silver ores in
much faulted and fractured Cambrian limestone, with great outbreaks of
eruptive rocks near. The Eureka geological section is one of the most
interesting in the entire country, and involves some 30,000 feet of
Paleozoic strata, divided as follows: Cambrian quartzite. limestone, and
shale, 7,700 feet; Silurian limestone and quarztite, 5,000 feet; Devonian
limestone and shale, 8,000 feet; Carboniferous quartzite, limestone, and
conglomerate, 9,300 feet. These have afforded some extremely valuable
materials for comparative studies with homotaxial strata in the East. The
ore occurs especially in what is called the Prospect Mountain limestone of
the Cambrian, one smaller deposit being also known in Silurian quartzite.
The limestone has been crushed and shattered along a great fault, and
through its substance ore solutions have circulated, replacing it in part
with large bodies of sulphides which have afterward become oxidized to a
depth of 1.000 feet. The ore bodies were puzzling as regards their
classification, and a famous mining suit, with many interpretations from
various experts, resulted. The alteration of the ore has caused shrinkage,
and the formation of apparent caves over it. But there are many empty caves,
formed by surface waters long after the ore was deposited, and J. S. Curtis
very clearly shows that the ore bodies originated by replacement. All are
connected with more or less strongly marked fissures which formed the
conduits. Mr. Curtis made a careful series of
assays of the neighboring igneous rocks to find some
indication of the source of the ore. A quartz porphyry gave significant
results, and to this the metals are referred, the portions of the mass at a
great depth are considered to have furnished them. Eureka was one of the
first places in this country where the hypothesis of replacement was applied
to ores in limestone. The district is now far less productive than they once
were.
Return
to Metal Ores Page:
Precious and Base Metal Ores
