Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Brief decrepitation summary of samples from the 20 IGES field trip F1
These samples all had significant decrepitation intensities, which implies that the fluid inclusions are dominantly liquid and that they formed at relatively high pressure at considerable depth. This is unusual for an epithermal deposit formed in a surface environment and El Penon would seem to be of very deep epithermal or perhaps mesothermal origin. Many samples also have a distinct early decrepitation peak near 350 C which is indicative of the presence of high internal pressure CO2 rich inclusions, which also points to a deep origin for this deposit.
There was no consistently recognizable difference between results from the 2 different vein systems and both these veins seem to have formed under similar or identical fluid conditions.
A plot of all these results on one graph shows 3 separate types
of sample, based on their CO2 levels as indicated by the
decrepitation near 350 C. 10 samples have distinct peaks caused by
substantial CO2, 5 samples have modest CO2
contents and only 2 have no detectable CO2. These
sample groupings are shown on the following diagram which accentuates
the low temperature decrepitation region.
The triple peak response from El Penon samples is
typical
of samples from Au deposits where CO2 rich fluids
occur.
In this case the lowest temperature peak at 350 C is quite high, based
upon which we infer that the partial pressure of CO2
in
this system is quite low. (In archaean Au systems this first peak is
often
near 250 C.) Samples from the Au deposits in Nevada lack CO2
and the consequent low temperature decrepitation peak, so the El Penon
deposit seems to be a high level system, but deeper than typical Nevada
systems. The peak at about 420 C does vary slightly between samples and
suggests it may be possible to map out palaeo-thermal gradients in
these veins, given a proper spatially located sample suite.
The temperature of the ore forming fluids was about 350 C. As all of
the
samples collected were from the "run of mine" dumps we inspected, it is
impossible to infer any spatial relationships from these samples.
Sample descriptions, El Penon comparison pair
Two of the samples collected showed strong quartz
zonation
with the fragment (about 2 cm across). Each of these samples was split
into
an early milky quartz and a late stage, clearer and more coarsely
crystalline
overgrowth quartz. In the above plot of samples 1824F and 1824G, these
two types of visually distinct but very closely associated quartz,
showed
no discernible difference in fluid inclusion content. (The other sample
pair showed the same self-similarity.) This confirms the
interpretation
that the El Penon system was formed during a single thermal event with
remarkably little thermal variation within the system either spatially
or temporally.