There is an abundance of
information in the technical literature about fluid inclusions
in porphyry Cu, Cu-Au and related hydrothermal vein systems. A
common and distinctive feature of fluid inclusions in this type
of mineralizing system is the presence of halite daughter
crystals indicating > 25% salinity in the fluids. With few
exceptions, finding halite daughter crystals in fluid inclusions
can be used to infer that a porphyry system is nearby. So if you
are in the business of porphyry deposit exploration, looking for
such saline fluid inclusions ought to be high up on the list of
exploration techniques.
So
WHY do exploration
geologists completely ignore fluid inclusion techniques, even
when they are such an obviously useful technique in exploring
for porphyry deposits? Surely this behaviour is diabolical.
A comment based upon exploration at the Cadia mine,
NSW, Australia
This lack of understanding of fluid inclusions is highlighted in
a recently published and very thorough discussion of the 20
years of exploration of the Cadia Mine area, NSW, Australia. But
not by what is stated in the article, rather by what is does not
say. It says absolutely nothing about fluid inclusions
whatsoever!
A comprehensive, 2 part feature article has recently been
published in the SEG newsletter, "Discovery of the Cadia
Deposits, NSW, Australia" By Dan Wood. (
SEG
Newsletter
#88,
January 2012 and
#89,
April 2012.) This information is not freely available on
the web but these 2 Newsletter issues can be purchased and
downloaded at the SEG website.
A shorter and
older paper presented by Holliday et. al. is available.
An
additional
pdf presentation about cadia by Collet is also available,
but comprises slides from a presentation, without explanation.
The Cadia mine area is located some
260 Km west of Sydney, NSW and is operated by Newcrest Mining
Ltd. A small mining operation had been running in the area and
in 1991 it was decided they needed to find additional resources
as existing reserves were minimal. Low grade Au-Cu
mineralization had been identified in volcanics in an old open
cut, and also in monzonite porphyry in old mine workings 500
metres away. This led to speculation that there might be a an
economic Au-Cu porphyry deposit present. A resource was
first outlined at Cadia Hill, where the mineralisation
outcropped near monzonite. The location of additional
resources was far more difficult as they did not outcrop and
were covered by postmineralization Silurian sediments and also
recent basalts in places. It required good geological
understanding of porphyry systems and alteration patterns as
well as great persistence and tenacity to pursue an exploration
program involving very many, often quite deep, drillholes to
discover the several additional mineralization pods now known.
At various stages the team used soil sample geochemistry,
Induced polarization (IP) and helicopter borne magnetics in
their search efforts. But extensive drilling and good
understanding of alteration patterns were a key component of
their exploration. They also tried comprehensive oriented
core measurements of vein orientation as a guide to higher grade
zones, although this was unsuccessful. In summary, they had a
good understanding of porphyry copper systems and models and
used almost every available technique in their search efforts.
But, without trying to detract from their excellent work,
there is absolutely no indication
that they ever looked at a fluid inclusion!
The following images of the pit and some of the resource
outlines are from the paper by
Collet.
Why do
industry geologists fail to understand fluid inclusions?
This lack of application
of fluid inclusion methods is particularly disappointing
because these types of porphyry deposits are associated with
very diagnostic highly saline fluids which can be easily
identified in fluid inclusions and used for exploration
targeting.
I suggest that a major cause of this lack of
understanding lies not with the industry geologists
themselves, but with the academic course work, which
fails to clearly explain how this data is directly relevant
in exploration programmes, and also completely ignores the
explanation of simple fluid inclusion methods because of a
focus on expensive instrumental methods and excessive and
unnecessary precision, rather than practical methods.
There are plenty of papers in the literature
which deal with fluid inclusion data at porphyry deposits.
But a common problem with these publications is the
collection of large amounts of fluid inclusion data,
including temperatures, salinities and the
details of the complex inclusion parageneses and internal phase
relations. While this information is important in a research
program, the overload of information merely conceals and
obfuscates the few critical items which are of use to industry
geologists. Consequently, the fluid inclusion data is simply
skipped over and regarded as far too complex for use in mineral
exploration. In addition, the data is collected using
heating-freezing stages on microscopes, requiring liquid
nitrogen, as well as on raman microprobes and various other
exotic equipment which conceals the fact that mere room
temperature observation of halite daughter crystals in crushed
grains mounted in oil is adequate to provide important salinity
information to guide exploration. None of the expensive and
complex laboratory equipment is actually necessary! One can even
make the observations on a cheap biological microscope rather
than a polarizing petrological microscope, and you don't need
petrographic doubly polished sections, as grain mounts in an RI
liquid such as clove oil are quite adequate! I myself have done
this and recognized that the Telfer deposit in Western Australia
was associated with a porphyry intrusive long before the
intrusion was located and later described in the literature!
But I am not aware of any academic course which actually points
out that this simple method works well, or includes this
technique in course work.
There is such a strong focus on an teaching understanding of the
complexity of fluid inclusion phase relationships using
expensive analytical equipment (to justify its purchase), that
we have lost sight of an important real purpose of this
research; that of applying the knowledge of fluid inclusions to
the location of new mineral resources, using the simplest
techniques which work satisfactorily.
The extreme detail and accuracy of most reported fluid inclusion
work, where temperatures are measured to the nearest 0.1 C,
serves only to conceal the real truth that mineral deposits form
over a very wide range of temperatures. Almost all papers
compile their temperature measurements into histograms, and
conclude that deposits usually formed over a temperature range
of 100 C or more. The high precision temperature measurements
are little more than an academic laboratory exercise. Even
knowing the formation temperature of a deposit is of limited
use. To find a mineral deposit you need to recognize differences
between proximal and distal samples, and mineralized and
background samples and this typically requires a spatial array
of data points including remote and background samples. But
almost no research includes an adequate number of these
important distal and background reference samples.
A very extensive study which measured over 5000 homogenisation
temperatures from auriferous and barren quartz from Sovetskoye,
Russia by
Tomilenko
et. al. (Econ Geol, 2010, 105-2, 375-394) showed
vein formation over a range from 225 to 400 C, with no
discernible temperature difference between auriferous and barren
veins. Clearly the measurement of temperatures to the nearest
0.1 C is of little use in a mineral exploration programme.
This
data is has also been discussed in a presentation, shown here.
A very simple yet very effective fluid-inclusion technique which
does assist in exploration for porphyry deposits is to look for
halite daughter crystals in the fluid inclusions. This can be
done on a simple microscope on crushed grain samples, without
the need for even for preparation of a petrographic this
section.
A
discussion of this as an exploration tool is here.
At the left is a fluid inclusion containing a gas
bubble (dark, round) and a halite daugther crystal (cubic,
transparent) adjacent and to the right of the bubble. There are
several other daughter crystals present in this complex
inclusion.
This simpler inclusion has a vapour bubble V , and a halite
daughter crystal h .
The cubic shaped halite crystals are easy to identify.
Halite daughter crystals within fluid inclusions are easy to
recognize with simple microscope methods, are are an extremely
useful exploration tool for porphyry deposits as they indicate
you are within the core of the system.
Summary
Although mineral exploration geologists are remiss in failing to
use fluid inclusion information in their work when it is
appropriate, a large part of the blame for this is due to a
failure to point out concise diagnostic criteria and
appropriately simple measurement methods in research papers and
in course work. Few industry geologists understand the relevance
of the abundant FI results in research papers because the data
is extremely detailed to the point of obfuscating the few
critical points of relevance to their work. And there is so much
emphasis on using the latest expensive, high resolution
laboratory equipment that simple methods are ignored completely
even though they are sometimes all that is required to derive
data relevant to an exploration programme. Of course the high
resolution laboratory equipment is necessary and important as
the basis for cutting-edge advances of fundamental scientific
principles. But that does not necessarily make it appropriate
for routine field exploration problems. That would be like
trying to use a Formulae-1 race car as an offroad exploration
field vehicle - stupid!
A review of using fluid
inclusions in exploration for porphyry deposits was prepared
in 1981
An
additional
comment on this topic has been published by Bob Bodnar &
Bruce Yardley in the "Triple Point" column of Elements
magazine, V8 #4, 2012. (august 2012)
(This link is a pdf document which opens in a
new window)
Do you have a viewpoint on this or other fluid-inclusion
topics? Submit it for
inclusion on this website.