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Greisen samples from the Erzgebirge,
Germany-Czech
Conclusions
Greisens form within or closely
associated with granites, and the hydrothermal fluids involved are
almost certainly of granitic origin. This study shows that
such fluids are predominantly aqueous, with only minor involvement
of CO2 rich phases. A similar lack of CO2
rich fluids is seen in the granite hosted gold deposits of Hebei and Shandong in China. Although CO2 rich
fluids are commonly associated with gold deposits, most of them
form from metamorphic fluids. In contrast, the fluids involved in the formation of
greisen deposits like those at erzgebirge are dominantly aqueous
with only minor CO2 rich phases.
Discussion
The
Erzgebirge
mountains form the border between the Czech republic and
south-eastern Germany. The name means "Ore mountains" in
german, and mining started in these mountains in the 12th century.
The area was a major source of silver and tin as well as other
metals in medieval times. Geologically, the area is famous for its
tin greisen deposits and in fact the definition of greisen
comes from these deposits.
A suite of samples was collected from various tin localities (all
inoperative in 2010) for baro-acoustic decrepitation analyses. The
sample locations are shown on the following map with the sample
number. Brief sample descriptions are
here.
The samples from the Czech Republic were collected with the help of
Petr Rojik from Sokolov and those from Germany with the help of
Peter Suhr from Freiberg. Many thanks to these people for
their invaluable assistance.
Czech republic
Multiple samples from Prebuz are largely similar and lack any low
temperature decrepitation. However sample 2122G (H2423, dark blue)
has a distinctive peak at 350 C. This is probably due to a very
uniform hydrothermal pulse of CO2 rich fluids. One sample
from Geyer in Germany (some 40 Km to the NE) had the same
distinctive decrepigram. The 2 samples are plotted together for
comparison below.
The samples from Rolava were from loose waste rock at a millsite,
the actual mine being some 1.2Km further east and inaccessible.
There is a suggestion of CO2 rich low temperature
decrepitation in sample 2124c (dark blue), but overall CO2
is not common. The decrepigrams are similar to Prebuz (which is 5 Km
SW), but with lower decrepitation intensity.
At Rotava, about 10 Km SW from Prebuz, sample 2125a was of milky
white quartz and showed similar decrepitation to the samples above.
However, the other 3 samples at this site were described as
gray-white in colour and these had very weak and indistinct
decrepitation. In this case the subtle difference in quartz colour
is linked to a dramatic difference in the fluid inclusion population
of the samples. None of the samples show low temperature CO2
caused decrepitation.
Horni Slakov is a very large and deep abandoned mine pit. Most
of the 15 samples collected (next 2 graphs) show a decrepitation
peak at about 430 C, with various other peaks around 500 C also.
Clearly there are significant differences in the hydrothermal
character of these samples within the pit, which was 200-300m
across. Note that several samples still had intense decrepitation at
620 C which indicates that there must have been other mineral
phases, such as feldspar, present in the sample, as quartz
decrepitation rapidly returns to zero above the alpha-beta
transition temperature. (explanation
here)
The Druzba pit is an open cut coal mine near Sokolov and this sample
is not a greisen, but a vein of coarse white and mauve dog tooth
quartz. This is the only "barren background" sample I was able to
collect in the region. It is very different from the greisen
samples, with very weak decrepitation, but a distinct peak.
In general, the greisen samples lack any low temperature
decrepitation and it is concluded that CO2 rich fluids
were only a rare part of the fluid systems which caused these
greisens. However, a few samples do show signs of CO2
rich fluid involvement. The tin mineralisation in these greisens
does not seem to be formed from CO2 rich fluids .
Although there are significant variations in the decrepitation
between 400 and 600 C, many of the samples were from disturbed loose
rock at minesites rather than from undisturbed outcrop so it is not
possible to interpret the meaning of these variations based on these
samples. When CO2 rich fluids are present, they are of
limited spatial and/or temporal extent.
Germany
Samples 2129 to 2134 were from a collection of rock samples at the
geological survey of Saxony at Freiberg. They mostly lack low
temperature decrepitation, but several samples do show the presence
of CO2 rich
fluids, such as sample 2129 from Beierfeld (red).
At Zinnwald, two fragments of the same small hand specimen had quite
different decrepitation, showing the variation of fluid inclusion
populations over a quite small scale of just centimetres. One
sub-sample had a significant low temperature peak at 240 C while the
other did not. The CO2 rich fluids seem to have been of
very limited spatial and/or temporal extent.
Both sub-samples of the single sample 2134 from Ehrenfriedersdorf
had quite similar decrepitation, but there are still some
differences even at the centimetre scale.
The binge (collapsed mine?) at Geyer is a very large pit resulting
from a roof collapse of the old underground workings. The many
samples collected here over a diameter of some 400m are quite
similar and almost all of them lack low temperature CO2
decrepitation peaks. There are subtle differences in the
decrepigrams near 530 C, but these differences cannot be interpreted
on this data set. Sample 2135f (yellow) does have a distinct low
temperature peak at 340 C which is like that observed on sample
2122g from Prebuz. This sample is plotted together with the similar
Prebuz sample below.
Sample 2136 was of 2 quartz vein fragments occurring as veins in the
host rock on the margin of what seemed to be the mineralised area
within the binge. These show quite similar decrepitation to those
from the main pit area and this quartz was probably formed in
association with the mineralising event, rather than being quartz
from an earlier regional event.
At Breitenbrunn, a sample of magnetite was obtained from a skarn
occurrence, primarily for comparison with other magnetite samples
from Fe-oxide Cu-Au deposits. The sample was split into 2 fractions,
a magnetic one of primarily magnetite and a non magnetic, primarily
haematite one. The intense decrepitation is typical of skarn
magnetite, in contrast to BIF magnetite which has almost no
decrepitation. (See here for other
Fe-oxide data.) Note that there is substantial
decrepitation in the haematite also. This indicates that the
haematite is a primary mineral and is not a supergene weathering
product from the magnetite, as such weathering would have destroyed
the fluid inclusions.
The two distinctive CO2 rich
samples, one from Prebuz, Czech and the other from Geyer, Germany
are shown here together. The occurrence of such narrow and well
formed decrepitation peaks at 340 and 350 C is quite unusual. The
fluid event which caused this seems to have had a very uniform
temperature and composition and was perhaps a single, brief event.
These two samples have surprisingly similar decrepitation and
perhaps these two distant sites (40 Km apart) experienced the same
hydrothermal formation event?
As with the nearby greisen samples from Czech, there are only rare
CO2 rich fluids in these mineralising systems, which were
dominated by aqueous fluids. However there are a few occurrences of
CO2 rich fluids from Geyer, Zinnwald and Beierfeld.
Conclusions
Greisens form within or closely associated with granites, and the
hydrothermal fluids involved are almost certainly of granitic
origin. This study shows that such fluids are predominantly aqueous,
with only minor involvement of CO2 rich phases. A similar
lack of CO2 rich fluids is seen in the granite hosted
gold deposits of Hebei
and Shandong in China.
Although CO2 rich fluids are commonly associated with
gold deposits, most of them deposit from metamorphic fluids. In
contrast, the fluids involved in the formation of greisen deposits
like those at erzgebirge are dominantly aqueous with only minor CO2
rich phases.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
