Newest
Topics:
For the latest news, see the NEWEST TOPICS page.
Google is too dumb to let me put the list of news in this column and falsely claims that all my pages are self-duplicates.
Google-NONSENSE
Google's so-called "Artificial Intelligence" is an abuse of the concept of intelligence!
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
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration.