(_a_) Vein deposits, from which the enormous masses of
copper are taken out.
(_b_) Copper-bearing ash beds, of amygdaloidal diabase.
Chief mine, Quincy.
(_c_) Beds of conglomerate in which the cementing material
consists partly of copper. This last class of deposits
yields three-quarters of the Lake copper supply. Their
average copper content is 2·9 per cent. The chief mines
are the Calumet and Hecla, the Tamarack and the Atlantic,
all situated on one ore chute measuring 3 miles in
length, and worked to a depth of 4,000 feet.
=Sulphide Ores=: _Chalcopyrite_ (_Copper Pyrites_) is by far the most
widely distributed ore of copper, and furnishes the greater proportion
of the world’s supply.
The formula when pure is Cu_{2}S. Fe_{2}S_{3} (Cu 34·4, Fe 30·5, and S
35·1 per cent.), but usually the ore is not in this condition, being
mechanically mixed with large quantities of iron pyrites, and very
often with pyrrhotite. It occurs principally in the older crystalline
rocks, often in bedded veins.
The value of copper veins below the limit of surface decomposition is
nearly always due to chalcopyrite. Silver and gold are often carried,
as well as other metals. It occurs extensively in Montana, Arizona,
Tennessee, Canada, Chili, Japan, Spain, Cornwall, etc.
Chalcopyrite ores vary considerably in copper contents; thus Tennessee
ores contain about 2·5 per cent. of copper, Montana ores 5 to 5½ per
cent. (with gold and silver valued at about £11 per ton of copper),
whilst the Arizona ores vary, being often rich.
_Chalcocite_ (also known as copper glance or redruthite) is much less
important. The copper contents are 79·8 per cent. when pure, but such
a condition is rare, although the ore seldom contains less than 50
per cent. of copper. Below this proportion it often tends to pass
into bornite, and then to chalcopyrite. It is found in Montana, is
an important ore in Arizona (Clifton district), and occurs also in
Cornwall.