the
mining camps of the western hemisphere. Locally, as at Butte, enargite
(copper-arsenic sulphide) is of great value. Other minerals of
considerable importance in some districts are chalcopyrite and bornite
(copper-iron sulphides), tetrahedrite (copper-antimony sulphide), and
covellite (cupric sulphide). Very commonly the copper sulphides are
associated with large quantities of the iron sulphide, pyrite, as well
as with varying amounts of lead and zinc sulphides and gold and silver
minerals.

The principal copper ores originate in the earlier stages of the
metamorphic cycle, in close association with igneous activity.
Katamorphism or weathering, in place, has played an important part in
enriching them. The processes of transportation and sedimentary
deposition, which have done so much toward making valuable iron ore
deposits, have contributed little to the formation of copper ores.

=Copper deposits associated with igneous flows.= The copper ores of the
Lake Superior district, and of a few small deposits in the eastern
United States, contain small percentages of native copper in
pre-Cambrian volcanic flows or in sediments between the flows. The ore
bodies have the form of long sheets parallel to the bedding, the copper
and associated minerals filling amygdaloidal openings and small fissures
in the flows, and replacing conglomeratic sediments which lie between
the flows. The copper was probably deposited by hot solutions related to
the igneous rocks, either issuing from the magmas or deriving heat and
dissolved material from them. Secondary concentration has not been
important. There is practically none of it near the present erosion
surface; but it appears in one part of the district near an older
erosion surface covered by Cambrian sediments, suggesting a different
climatic condition at that time.

The Kennecott copper deposits of Alaska have a number of resemblances to
the Lake Superior copper deposits, suggesting similarity in origin. The
Kennecott deposits occur exclusively in limestone, which rests
conformably on a tilted surface of igneous flows ("greenstones") not
unlike those of Lake Superior. The flows carry native copper and copper
sulphides in minutely disseminated form and in amygdules, but apparently
not in quantities sufficiently concentrated to mine. The flows are
supposed to be the original source of the copper now in the limestone.
The primary copper mineral in the limestone is chalcocite, in
exc