ng above
of a leached portion and below of oxides and carbonates of copper in a
gangue of quartz or clay; second, a zone of secondary sulphide
enrichment, characterized by chalcocite coatings, chalcopyrite, and
pyrite, with a gangue of quartz and igneous rock or limestone; and
third, a zone of primary deposition with similar gangue, characterized
by chalcopyrite, and at Butte by enargite and chalcocite. The oxide zone
as a whole may be rich or lean in values, depending on the nature of the
associated gangue material and country rock. When these are more soluble
than the copper--as is commonly the case in limestone--the copper may be
residually concentrated, notwithstanding the fact that much copper
originally present has been carried off in solution. When the associated
gangue and country rock are less soluble than the copper--as is common
with quartz and igneous rocks--the oxide zone is likely to be depleted
of values.

The zones formed by weathering and secondary enrichment are extremely
irregular, both in distribution and depth, in any one deposit, and they
overlap and grade into one another in a very complex fashion. In many
places the primary zone is too lean to be mined to commercial advantage;
but in other places, as at Butte, and in the limestone deposits of
Bingham, the primary ores are of considerable importance.

When evidence of secondary sulphide enrichment was first recognized
there was a tendency to magnify its effectiveness, and to assume that in
most cases the values were due to this process; that the primary zones
would be found to be valueless. In recent years the emphasis is being
somewhat changed because of the recognizing in many camps of rich
primary zones. While some chalcocite is clearly the result of secondary
enrichment from above, other chalcocite seems to have been related
closely to the primary deposition. The quantitative discrimination of
the two is a matter of great difficulty.

It has come to be recognized that the zonal arrangement caused by
enrichment from the surface has been imposed usually on a zonal
arrangement caused by the primary hot solutions and not related to the
surface but to the source of the solutions. In some districts, as
illustrated by Butte and Bingham, the copper-bearing minerals seem to
have been deposited nearest the igneous source, while the lead, zinc,
gold, and silver minerals have been deposited farther away,--suggesting
the cooling of the solutions with in