The baro-acoustic decrepitation method

The Baro-acoustic decrepitation method

What is it and why use it?

The baro-acoustic decrepitation method is a rapid and economical means of using fluid inclusion information as an exploration aid. It is an alternative to the microthermometric technique which is too slow and costly for routine exploration useage. The technique is applicable to either transparent or opaque minerals and provides information on the sample formation temperature, the abundance of fluid inclusions and the occurrence of CO₂ rich fluid inclusions in the sample. The results are completely objective because the analyses are performed on a computer controlled instrument without operator intervention. Each analysis takes just half an hour and uses only 0.5g of sample which is merely crushed and sieved, avoiding the need for expensive thin section preparation.

In exploration the technique can be used in three different ways: To characterise ("fingerprint") known mineralised systems for comparison with other nearby systems as a means of locating extensions to known mineralisation; to ascertain the presence of CO₂ rich fluid inclusions which are often an indicator for gold mineralisation; or to map out temperature zonation within a single thermal system in order to locate the best mineralised areas.

Characterisation:
By comparing the decrepigrams from a known mineralisation or vein with similar unknown occurences nearby it is possible to rank the various occurrences or to determine genetic relationships between the occurrences. This is particularly useful in areas of complex or poorly outcropping geology or where multiple mineralisation events are suspected. In using this method it is necessary for the between-event differences to be greater than the within event variations and this may not be so on strongly growth zoned or telescoped mineralisations.

CO₂ rich fluid inclusions:

CO₂ rich fluids are commonly associated with gold mineralisation and although the relationship is suspected to be indirect, the mapping out of zones of CO₂ enrichment can be a useful guide in exploration for gold in areas of low metamorphic grade. (CO₂ is ubiquitous in metamorphic fluids of amphibolite grade or higher) The presence of CO₂ rich fluids in samples gives rise to decrepigrams with a prominent low temperature peak or a peak skewed towards low temperature. (less than 350`C)

Temperature zonation:

Within a single thermal system, where the fluid compositions are similar, it is (theoretically) possible to map out temperature variations and thus locate thermal centres with which the best grades of mineralisation may be associated. Both lateral and vertical zonations may be defined. Because variations of only 20`C seem to be important it is necessary that there be no growth zoning or telescoping as these effects would mask the lateral or vertical variations. It can be difficult to use decrepitation in this manner but an example is shown from data at the Malanjkhand Cu deposit in India.

Most of the interpretation is empirical and it is thus preferable to conduct a small orientation survey to examine the variations both within and between thermal systems and to carry out microscope observations on a few samples for control purposes. Useful observations may be done on grain mounts in refractive index oil to avoid the cost of thin section preparation.

In addition to these exploration applications, the technique can be of use as an adjunct in conventional microthermometric studies by aiding in the selection of samples and providing information from a large, statistically meaningful number of inclusions in each sample.

Many minerals can be used in decrepitation, the most commonly used being quartz, carbonate, feldspars, pyrite, pyrrhotite, magnetite and haematite. Note especially that silicified zones in host rocks can also be used, not just vein quartz. The analytical samples should be monomineralic if possible, although mixed mineral samples can sometimes be used. Small lump samples of about 2 cm across are ideal as this ensures that an adequate quantity of the -420+200 micron grainsize fraction will be available after crushing. Much smaller samples (as little as 1g), such as offcuts from drillcore or from thin section preparation, can be used if necessary. If monomineralic samples are not feasible, mineral seperations can often be done. Some care needs to be taken with carbonate contamination of non-carbonate samples as carbonates decrepitate intensely and can obscure the result from the desired mineral. Trace amounts of carbonate contamination can usually be easily removed by washing in acid.

Quartz is the most commonly used sample medium and typically has 3 decrepitation peaks. The lowest temperature peak (250`-350`C) has always been found to be due to the presence of CO₂ rich fluid inclusions. (This is a consequence of high internal pressures in the inclusions and is explained in detail seperately) The mid temperature peak (350-550`C) often correlates with the homogenisation temperature of primary inclusions in the sample and can, under favourable conditions and perhaps with some microscope control, be used to estimate the mineral formation temperatures. The highest temperature peak at 580`C is due to preferential inclusion decrepitation during weakening of the quartz lattice as it transforms from the alpha to beta phase at 573`C. This peak is not usually useful in interpretation of the results.

Interpretation in other minerals is less well understood and normally restricted to empirical comparisons of suites of carefully collected samples

K. Burlinson, 2003, 2009

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