Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Brief decrepitation summary of samples from the 20 IGES field trip F1
Samples were collected at these mines during the field trip associated with the Geochemical Exploration Symposium. This data is a selection of relevant results.
Escondida
Sample descriptions, Escondida
The Escondida deposit is unusual for a porphyry
copper
in that it has an abundance of quartz veins - at least in the
area of
the
pit we visited. (Quartz is the preferred sample medium for
fluid
inclusion decrepitation.) However, the quartz veins contained
abundant
sulphides and the decrepitation results are strongly influenced
by
inclusions
within the sulphides and perhaps by sounds accompanying
oxidation of
the
sulphides during the analysis. All of the response above 600 C
is due
to
sulphides, and some peaks below 600 C, particularly the narrow
ones,
are
due to sulphides. The decrepitation begins just below 400 C and
from
this
I infer that these quartz veins formed at a little below 400 C.
I am
unable,
in these samples, to see the 3 different hydrothermal stages
described
by Padilla Garza, Titley and Pimental (paper handed out during
the
visit)
due to the interference from sulphides. Microscope observation
of these
samples shows that separating the quartz from the sulphide would
be
impossible,
even by chemical attack, as much of the sulphide is very fine
grained
and
encased within quartz grains of about 300 microns across (the
analytical
size fraction). However, we can see there is no decrepitation in
the
temperature
range 200 to 350 C, where we would see evidence of CO2
were it present. Porphyry copper deposits would not be expected
to
contain
CO2 rich fluids and there are none at Escondida. We
did not
visit enough separate localities within the pit for me to see if
it is
possible to discern temperature gradations within the ore body.
Chuquicamata
Sample descriptions, Chuquicamata
Chuquicamata is a more typical porphyry Cu
deposit,
with
only rare quartz veins present. Samples h1459 (red) and h1465 (magenta)
are
of quartz and they do not decrepitate above 600 C, except for
some
minor
included sulphides. Samples 1451 (green) and 1453 (blue) were of
porphyry
and these continue decrepitating up to 800 C due to fluid
inclusions in
the feldspars. Decrepitation begins at about 400 C, so it
seems
that
these veins may have formed at a temperature slightly higher
than the
veins
in Escondida. Again we see no low temperature decrepitation and
deduce
that there were no CO2 rich fluids involved in this
system.
Although not shown here, two samples of chrysocolla from the
exotic
Mina
Sur pit gave no decrepitation at all, which is to be expected
from a
deposit
of supergene origin with fluids of only atmospheric temperatures
and
pressures.