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.

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)

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