"Copper Agates" is a personal nickname I
gave to any agates that are found in the copper mine dumps
throughout the Houghton County of Michigan and it's easier
for me to say "Copper Agates" than properly named "Copper
Replacement Agates" even most agates don't have any copper
inclusions. The agates are generally pink to white sometimes
with orange hues and often include prehnite, chlorite,
datolite, calcite inclusions, silver and especially copper
which "Copper Replacement Agates" are named after. They
typically come in small sizes and the largest one ever found
was a fist-sized from C & H #21 mine but with little
copper.
Besides St.Louis mine, the agates were
brought up from the underground copper mines penetrating the
Kearsage Lobe and the Wolverine #2 produced some of the best
Copper Replacement Agates seen so far. It can be notoriously
difficult to find any agates that had not been fractured by
the blasting processes needed to break the hard basalt beds
in order to to extract copper. It's quite hard to extract
the agates out of hard basalt hosts without damaging them.
Personally, I do not like to refer "Lake
Superior Agates" to those kind of agates mainly because the
"Copper Agates" are in several ways too different from the
Lake Superior Agates found along the beaches. Being a broad
term, I refer "Lake Superior Agates" only to any agates that
have been picked from the Lake Superior beaches, rivers, and
glacial gravel pits without proven or clear in-situ sources.
One idea is that the calcite inclusions in the
agates may have been dissolved away leaving behind the gaps
for copper to fill in. That idea seem to be supported by the
fact that few nodules have been found with interlaying of
agate and calcite (sometimes chlorite, adularia or prehnite)
and some agates were found with gaps left by dissolved
minerals. That's just an idea that have yet to be proven or
disproved and there might be other ideas out there.
