Visual display of the Supergene weathering at the Crandon deposit


Strong development of one of the destructive processes (or
moderate development of two) and weak to moderate
development of at least one more. Rocks may or may not be
strong enough to hold a back, depending upon which
destructive processes have been active.    Strong development
of argillization or fracturing would make the rock very
weak or unstable, whereas strong development of oxidation
or leaching may not seriously affect its ability to hold a
Moderate leaching with only minor other effects. The
compressive strength of the rock is not seriously reduced.
Weak development of leaching and/or oxidation. Rock
strengths are not affected.

Figure lA is a generalized cross section illustrating a typical
north-south weathering profile of Crandon deposit rocks. From this profile,
basic weathering characteristics which apply throughout the deposit can be
illustrated and discussed from figures 1B-1F.
The weathering profile and active destructive processes vary considerably
between footwall, Crandon formation and hanging wall. This is in response to
the primary chemistry of the rocks, and the physical-chemical conditions acting
upon those rocks.


In discussing the generalized cross sections, it must be kept in mind that
they represent basic weathering characteristics and principles applicable at
this deposit. The fact is that the supergene weathering system is very complex
and has many irregularities and variations from the generalized norm. While
the base of the weathering zones are shown relatively smooth on the
generalized section for illustrative purposes, in reality they are shaped much
like the base of a nolar tooth, with many root-like projections.
Two types of ore are present at the Crandon deposit. Massive ore is a
zinc-rich syngenetic ore type which is contained strictly within the Crandon
Unit.  Stringer ore is a copper-rich epigenetic ore type which is contained
within the footwall.   It is important to remember that only portions of the
Crandon Unit and footwall are of sufficient width and grade to be of commercial
The footwall rocks follow a relatively uniform weathering pattern compared
to the Crandon formation (Figure 1A). The lower boundaries of the various
weathering intensities are relatively horizontal, but contain many root-like
zones which penetrate deeper. The footwall weathering follows a relatively
uniform pattern because the rocks are relatively uniform in compo.sition, being
siliceous in nature. The primary destructive process is leaching.
In the strongly weathered zone, strong leaching has reduced the competency
of the rocks to the point where crushing and collapse has played an important
role in physically breaking up the rock.   The acidification of groundwater due
to the breakdown of sulfide minerals has further caused the breakdown of
silicate minerals, putting some into solution and altering others to clay