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A decrepitation study of tin bearing limestone
skarns, Gejiu, China.
August 2014
After the IAGOD conference in Kunming there was a field trip to
the Gejiu tin mining district in SW China. Samples were collected
from the operating tin mines and also from regional background
areas remote from the mines. There was no quartz associated with
the mineralisation and all samples were of various other minerals
and of limestone. Fluid inclusion decrepitation of these other
mineral species, particularly carbonates, is poorly understood,
which limits the interpretation of this data.
Conclusions
The interpretation of decrepitation data on mixed, non-quartz
mineral suites such as in this study is complex. However the
results show that limestones, garnet, tourmaline and iron-oxides
can all provide formation fluid information and discriminate
between potentially mineralised and barren areas.
It is likely that these deposits at Gejiu formed in the
temperature range from 350 to 450 C. There were very few samples
in this study and a larger study would be required to provide
better understanding and detail about the formation conditions and
fluids.
The supposed basalt sample from the Kafang mine is neither a
basalt nor of basalt derivation. Its lack of decrepitation
suggests an unaltered sediment origin, but this is inconsistent
with its mineralogy. Firm conclusions cannot be drawn from this
single sample, but the model proposing a basalt origin for the
local copper mineralization must be questioned more carefully.
See the summary for elaboration of these
conclusions
An enormous quantity of tin (in excess of 350,000 tons) has been
mined from the many deposits in this district. Mining has taken
place for several centuries and has been very intense over the
last 100 years. Alluvial tin mining commenced some 2000 years ago.
Ores in this district are mostly vein type mineralisation hosted
in Triassic carbonates over granite cupolas. The vein minerals
include tourmaline, garnet, diopside, epidote, phlogopite, beryl
and cassiterite. There are only minor sulphides present. Tin
grades are up to 1.2% Sn. Almost no quartz was observed in the
areas visited or samples collected.
Location of Gejiu in southern China, 250 Km south of Kunming.
These deposits are skarns within limestones, near to granites.
Samples were collected from 2 active working mines at Kafang
(underground) and Dadoushan (open pit) and also from an inactive
small adit entrance at Baishachong, which was unique as the contact
of the granite with the limestone skarn was visible here.
Location of mines and samples at Gejiu tin deposits. (1 minute grid
lines)
Three additional samples of unmineralised limestone were collected
from far distant outcrops which are not associated with any
mineralisation. These sample locations are near Dali and Lufeng,
west of Kunming.
Location of regional background samples. (1 degree grid lines)
Kafang underground mine
Within the Kafang mine a sample (2029A) of garnet from the skarn
shows substantial decrepitation continuing to 800 C. There was much
discussion about the other rock type, sample 2029B, which was stated
to be a "basalt". But it reacted strongly with acid and much
carbonate was present. The crushed grains were mostly of fine
grained limestone, with substantial garnet and some tourmaline.
There were no ferro-magnesian minerals observed. This sample is
clearly not a basalt at all and probably not even an altered basalt.
It may be a remnant sedimentary feature within the limestones, but
it is unclear why it forms a distinctive unit and why it has no
decrepitation. The lack of decrepitation is suggestive of an
unaltered sedimentary unit, but the mineralogy shows that alteration
has occurred.
The "basalt" is proposed to be the source of copper mineralisation
within the mine, but this rock unit cannot be the copper source.
Perhaps this individual sample was badly chosen, but it seems there
is a problem with mapping of the supposed basalt in this mine and
perhaps the entire district.
Dadoushan open pit mine
At the Dadoushan open pit, cassiterite occurs within a fine grained
skarn altered limestone which contains considerable tourmaline,
mostly present as thin, fine grained bands. There are occasional
large clumps of almost pure coarse grained tourmaline, and one of
these was sampled (2031). These samples were from the actively
worked ore zone.
The 2 samples of limestone show very intense high temperature
decrepitation, which is often seen on carbonates. These 2 samples of
limestone were collected within a few metres of each other and are
surprisingly different. Decrepitation in carbonate minerals is
possibly strongly affected by their ductility and cleavage and may
not reflect the conditions within the primary skarn alteration
fluid. Decrepitation of the tourmaline sample, 2031 (plotted below
in red), is very similar to one of the limestones. This is
unexpected if decrepitation in limestone is primarily a function of
cleavage and ductility, as tourmaline is brittle and lacks strong
cleavage, despite having well formed crystal structure. Tourmaline
should retain primary fluid inclusions and decrepitation should
provide a useful guide to the primary entrapment conditions of the
parental fluid. These results suggest an alteration fluid event at
about 350 - 380 C. The lower temperature of 300 C shown by sample
2030B may indicate a late stage event as the fluid system collapsed.
Baishachong workings (inactive)
This location was sampled because the contact between the granite
and limestone skarn is visible there. The samples collected were
from dumps at the entrance to a small adit and may have come from
widely separated locations within the workings. All of the samples
were very ferruginous. During sample preparation each sample was
separated into a magnetic and non-magnetic fraction, but only
sample 2033 had enough of the non-magnetic fraction to allow
analysis (2033B). All of the samples were only weakly magnetic,
much less than pure magnetite and maghaemite may be dominant.
Haematite and magnetite are quite good host minerals for fluid
inclusion decrepitation studies and have been used extensively (other examples here).
The decrepitation results show considerable variations between
samples, suggesting highly variable fluid conditions within the
deposit and a formation temperature of about 400 C.
The magnetic and non-magnetic fractions of sample 2033 show the host
mineral dependence of decrepitation data, with much lower
temperature decrepitation on the non-magnetic fraction. The
non-magnetic fraction is not merely quartz as decrepitation of
quartz does not continue above 600 C because of the transition from alpha
to beta quartz at 573 C. Observation of this sample showed it
to contain about 50% haematite grains with 15% quartz and 35%
translucent yellowish grains of greasy appearance, suspected to be
either andradite or epidote. There were also some rare garnet
grains.
Limestone samples from skarn and regional background areas
The skarn limestone samples give intense decrepitation, which is
a common feature of decrepitation of carbonate samples. In
addition, the unmineralised crystalline limestone from Mt
Changsha, near Dali, gave intense decrepitation. But the remote
background sedimentary limestone samples from Dali and Lufeng gave
no or only very small decrepitation.
It seems that the decrepitation response is strongly influenced
by the degree of crystal formation, with non-crystalline samples
having no decrepitation. However the crystal size in the Dadoushan
skarn samples was only about 40 microns and yet there is intense
decrepitation. Decrepitation of limestone samples is a function of
both the primary thermal and fluid history as well as subsequent
recrystallization and metamorphic events.
The 3 regional background limestone carbonates show very variable
decrepitation. Sample 2035 is an unmineralised coarse crystalline
marble and the decrepitation in this is due to the metamorphic
recrystallization. The other 2 samples have very little or no
crystals larger than about 10 microns and the low decrepitation is
as expected from an unaltered sedimentary rock. The intense
decrepitation often seen in carbonates may be caused by regional
metamorphism and not just by hydrothermal or skarn alteration
fluids.
Comparing decrepitation in different minerals
Garnet, tourmaline and magnetite (or mag-haematite) minerals are
good hosts for fluid inclusions and decrepitation of these
minerals can be useful in outlining alteration zones around
mineralisation. A range of formation temperatures between 350 C
and 450 C is suggested by the decrepitation of these minerals in
the Gejiu district. Although skarn altered limestone also contains
fluid inclusions and gives intense decrepitation it is difficult
to interpret because carbonate minerals are ductile and have
strong cleavage. The decrepitation is strongly influenced or even
completely controlled by these mineral characteristics and
it is not always obvious how to derive the original fluid
temperatures from decrepitation of such carbonate samples.
Summary
Fluid inclusion studies are usually performed on quartz host
minerals because normal microthermometric methods cannot be used on
opaque minerals. By using baro-acoustic decrepitation it is possible
to obtain some fluid inclusion data on the varied minerals in this
region, including opaque minerals. However, it is more difficult to
interpret decrepitation data from groups of dissimilar minerals such
as those in this study. In particular, carbonate minerals are easily
re-crystallized, which allows alteration of the fluid inclusions
post-entrapment. Inclusion stretching, which is commonly observed
within carbonate minerals and the relationship between crystallinity
and decrepitation observed in these results are both major concerns
which complicate the interpretation of baro-acoustic decrepitation
within limestones and carbonates.
It is curious that in the Dadoushan pit the decrepitation of
tourmaline in sample 2031 is similar to the nearby limestone sample
2030A. Tourmaline should preserve fluid inclusions well, in contrast
to limestone. The similarity of decrepitation in this case indicates
that, at least in some circumstances, limestone can preserve fluid
inclusions.
Ferruginous samples such as those from Baishachong show diverse
decrepitation patterns. Haematite / magnetite samples have been used extensively in other
decrepitation studies and usually provide useful data. The
variations seen at Baishachong probably indicate very variable
formation conditions. As these samples were collected from spill at
the adit entrance they could represent widely separated locations
within the extent of the adit.
Low temperature sedimentary rocks such as chert show no
decrepitation and this behaviour is also seen in the fine grained,
un-recrystallised limestones from far distant locations near Dali.
However, the coarse grained re-crystallized carbonate from Mt.
Changsha, near Dali, shows intense decrepitation which is presumed
to be related to fluids during a regional metamorphic event. Fluid
inclusions within carbonates are expected to be reset to the
conditions during the last thermal event.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
