Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Decrepitation data on Pb-Zn deposits in Tunisia,
North Africa.
Samples were collected for baro-acoustic decrepitation analysis
at 3 different Pb-Zn deposits in northern Tunisia in 1987. These
deposits are carbonate hosted "mississippi valley" type (MVT).
They were formed at low temperature from sedimentary basin fluids.
Microscope observations showed the presence of hydrocarbons in
some fluid inclusions in these samples. Samples were collected
from the Bou Grine, Bou Jaber and Fedjal Joum deposits. (Fedjal
Joum is probably my misspelling of "Fej Lahdoum", "Fedj el Adoum"
or "Fedj el Hadoum" referred to in other publications).
A recent paper in the Journal of Geochemical Exploration has
described some nearby deposits :
Mineralization and fluid inclusion studies of the Aptian
carbonate-hosted Pb\Zn\Ba ore deposits at Jebel Hamra, Central
Tunisia, Jaloul Bejaoui, Salah Bouhlel,
Esteve Cardellach, Àngels Canals, Joaquim Perona,
Àngels Piqué , JGE
V128, p136-146, 2013. Also available
here at academia.edu (membership required)
Discussion
Baro-acoustic decrepitation is usually carried out on quartz
samples. But quartz is not present in these deposits and various
other minerals were used instead. Baro-acoustic decrepitation does
NOT require transparent host minerals and can be used on opaque
minerals. However, the decrepitation result is influenced by the
host mineral and data from these samples cannot be compared
directly with results from different mineral hosts. Consequently
the main interest in this data is to compare the decrepitation
within and between various minerals rather than to try an
determine fluid conditions and temperatures during deposition.
Limestone / calcite
Limestone host rocks often show intense decrepitation, as seen
here on the 3 samples from Bou Jaber. They begin to decrepitate at
temperatures from 200 to 350 C, which would normally be considered
to be the formation temperature. However, carbonate minerals are
ductile and easily recrystallized which alters the fluid
inclusions. It is thought that the fluid inclusions reflect the
latest recrystallization event, possibly a regional overprint,
rather than the primary deposition temperatures of the
mineralisation. (Additional
discussion of this at the Gejiu deposit in China is here)
(additional data from
carbonates in Nova Scotia is here)
Note that the 2 samples from Bou Grine show no significant
decrepitation at all and these samples are probably unaltered
limestone sediments.
Baryte
All of the samples of baryte came from the Bou Jaber mine. They
show intense decrepitation, which is common for baryte samples
based on results from the Roxby Downs mine in South Australia.
Because of the ductility and cleavage of baryte, which facilitates
post-entrapment modification of the fluid inclusions, it is
unclear what meaning can be derived from the decrepitation
temperature. Additional baryte
data is here.
Fluorite
All of the fluorite samples also came from the Bou Jaber mine. They
show intense decrepitation as do other fluorite samples (shown here) except for
synthetic fluorite intended for optical usage. The fluorite
decrepitation is almost identical to the baryte decrepitation. It is
suspected that the fluid inclusions in fluorite are easily modified
post entrapment and the meaning of the actual decrepitation
temperatures is unclear.
Low density minerals (quartz, limestone)
The 3 samples below contained a mixture of both heavy and light
minerals, and a density separation was carried out using
tetrabromoethane (TBE), density 2.97 g/cc. These low density
fractions, which would include quartz and limestone, show only low
intensity decrepitation. Based on the lack of decrepitation above
600 C, these samples are probably mostly quartz. The data suggests
that a siliceous fluid event at about 300 C occurred in this mine.
Galena
Because galena is ductile and has strong cleavage it is a poor host
for fluid inclusions and although its decrepitation response is
interesting it should not be relied upon to determine formation
temperatures. These results show major differences between galena
samples, even between the 2 samples from the same mine, Bou Jaber
(red and green curves). Additional
examples of decrepitation of galena are here.
Sphalerite
Sphalerite should be a good host for fluid inclusions and is
sometimes used for microthermometry because it is sometimes slightly
transparent. Three of these samples, 1110B, 1104A and 1110A have low
decrepitation suggesting a formation temperature of about 320 C.
This temperature is higher than expected for these MVT type
deposits.Sample 1113B (green curve) was a mixture of sphalerite with baryte and this peak is most likely dominated by decrepitation of the baryte component rather than sphalerite.
Sample 1104B (yellow curve) is surprisingly intense, but also at unusually high temperature and is probably not caused by baryte contamination. The meaning of this decrepitation response is unclear.
Additional data on decrepitation of sphalerite samples is here.
Pyrite
Pyrite is often useful in decrepitation studies and has been used at
Chessy in France,
and at Kalgoorlie
in Western Australia and other studies. Pyrite frequently gives intense decrepitation, but at Bou Jaber in sample 1111 the pyrite had only weak decrepitation. This was the TBE gravity separated heavy fraction of sample 1111. The decrepitation temperature seems much higher than any expected formation temperature for the pyrite and it is unclear if there has been a high temperature event in this area or if this decrepitation temperature is unrelated to the formation temperature.
Host mineral comparison
To show the differences between different mineral hosts, one sample
from each mineral species is plotted here. Note that 4 of the plots
have been multiplied to facilitate comparison at this scale.
All of this minerals can provide decrepitation data, but it is not
easy to interpret the results in minerals which may have allowed
post-entrapment modification of the primary fluid inclusions. To use
decrepitation on such mineral hosts it is necessary to only compare
and contrast data from the same mineral. 
Summary
The decrepitation results from these samples in Tunisia are difficult to interpret because of the many different mineral species collected and the unavailability of a single common mineral host in all the sample locations. Despite this, decrepitation does not usually start below a temperature of 300 C, except for baryte samples. Perhaps there has been a late stage fluid event or metamorphic event near this temperature. And the baryte may have deposited and be reflecting a lower temperature during retrogression. However other studies on these deposits generally infer a formation temperature closer to 150 C from hot basin fluids and do not support the idea of fluids or events as hot as 300 C.Further research is necessary to properly understand this decrepitation data and the formation temperature of these MVT deposits in Tunisia.