A decrepitation study of magnetite in Bergslagen, Sweden, 2013
In August 2013, a suite of magnetite samples was collected from old minesites in the Bergslagen area in southern Sweden. The aim was to determine if decrepitation could recognize differences between magnetite samples despite the regional metamorphism.
The Bergslagen region in southern Sweden has been a major mining
area since the 15th century. This region was a major source of
iron in early times because there were many deposits of magnetite
which was reduced to iron using charcoal from the local forests.
Silver and copper were also mined. In the 20th century there were
many magnetite mines supplying iron ore, but most of these have
now closed. The focus now is on base metal deposits, usually in
skarns and often associated with magnetite.
Fluid inclusions within magnetite could assist in exploration for
the hydrothermal systems which have formed the numerous skarn
deposits, which contain base metals and gold. Baro-acoustic
decrepitation is able to provide fluid inclusion data in magnetite
although magnetite is unsuitable for conventional
microthermometric methods of studying fluid inclusions because it
is opaque. However, the Bergslagen area studied has been
regionally metamorphosed to greenschist facies which may have
overprinted the fluid inclusions related to earlier mineralising
events. To investigate this, magnetite samples were collected from
numerous old mines and workings and analyzed by baro-acoustic
decrepitation. If regional metamorphism had "reset" the fluid
inclusion assemblages, then all the decrepitation results across
the province should be practically identical.
The results of this study confirm that the regional
Sveconorwegian metamorphism does not erase or conceal the
original fluid inclusion signatures in magnetite because
there are numerous and varied decrepitation responses across this
suite of samples, despite the uniform greenschist facies
metamorphism in this area.
The variations observed in this study can be matched with
expected variations in the type of deposit, either BIF or skarn or
apatite type and these decrepitation patterns match type examples
from deposits around the world, which are documented on this
website. (See the links after the summary at
the end of this page) This suggests the possibility that the
decrepitation signatures can assist in categorising the magnetite
ore deposit type, although it would be advisable to use this with
caution in the absence of additional confirmatory evidence.
Baro-acoustic decrepitation of magnetite samples could provide
important information to aid mineral exploration. The
multiple samples collected at each locality show interesting
variations which suggests that it should be possible to map out
local scale decrepitation patterns. These may well be related to
economic mineralisation distribution, but this study did not
include sufficient samples or any chemical analyses to confirm
Although fluid inclusions cannot be seen in magnetite because it
is opaque, the decrepitation method can provide some information
on the fluid inclusions and thus on the origin and hydrothermal
history of magnetite, and other opaque minerals. This
information can be useful in mineral exploration by providing
"fingerprints" of the hydrothermal events and mapping out
local variations within these hydrothermal cells, which can help
to identify potentially mineralised cells and to target
economically mineralised zones within these cells.
Location of the Bergslagen region (BR) in southern Sweden
The Bergslagen area sampled in this study has been affected by 2
regional metamorphic events with contact metamorphism and skarn
formation associated with granite intrusions occurring between
these 2 regional events. The regional metamorphic grade of the
younger metamorphism is upper greenschist to amphibolite. In the
area of this study (which was limited to eastern Bergslagen) this
regional metamorphic grade is greenschist facies.
This geological description is from the SGA excursion guidebook, August 2013, Rodney Allen, Nils Jansson & Magnus Ripa (eds.)
The Bergslagen region in central Sweden contains an ore district that has been a major metal producer for well over 1 000 years and which contains more than 6 000 registered ore deposits and mineral prospects (Stephens et al. 2009, SGU mineral and bedrock resource data base). The major part of the Bergslagen region is situated in the 2.0–1.8 Ga Svecokarelian orogen. However, the westernmost part of the region, containing abundant ductile shear zones operative under greenschist facies metamorphic conditions, is situated in the frontal part of the Sveconorwegian orogen with 1.0–0.9 Ga tectonic reworking. The Bergslagen ore district (Fig. 4) refers to the intensely mineralised, arc-shaped area in the north-western part of the region, where a metamorphosed, Palaeoproterozoic (1.91–1.87 Ga), predominantly felsic magmatic province dominates in the near-surface realm (Figs. 4 and 5). This province belongs to the second cycle of magmatic activity, sedimentation and deformational-metamorphic events described in the previous overview of the Fennoscandian Shield in Sweden. The rocks are inferred to have formed along an active continental margin in a convergent plate boundary setting, when a period of retreating subduction and extensional or transtensional tectonic regime was followed by advancing subduction and transpression. The Bergslagen ore district contains a diverse range of ore deposit types; banded iron formation, skarn- and carbonate-hosted iron ore, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and stratabound polymetallic base metal sulphide ores, W skarn and REE deposits. In addition, Bergslagen is a major exporter of industrial minerals, including dolomite, calcite, feldspar and garnet. Most of the ore deposits are associated with skarn, crystalline carbonate rock and metamorphosed, hydrothermally altered volcanic rock. Skarn is extremely common in Bergslagen and the word “skarn”, which is used here non-genetically as a reference to calc-silicate or Mg-silicate mineral assemblages, originates from this region.
Distribution of the two principal types of metallic mineral deposits and the non-metallic mineral deposits in the Bergslagen region.
The deposits are presented on a simplified bedrock geological map of the region. (Map from: M.B. Stephens et al, Swedish Geological Survey publication ba58)
Samples were collected from 11 locations. At each location,
multiple sub-samples were collected to assess local versus
regional variability. Each sub-sample was analysed
individually, using an alphabetic suffix of the locality sample
number. For example, at sample site 2237, Pershyttan, near Nora, 4
sub-samples were collected and analysed as 2237A, 2237B, 2237C and
2237D. These sub-samples were collected within a radius of 5 to 15
metres. Samples were crushed and sieved to -420+200 microns.
Each sample was then separated magnetically. All except one sample
(2232, Grangesberg) were dominantly magnetic. Sample 2232 had
enough non-magnetic fraction that both the magnetic and
non-magnetic fractions were analysed. Each sample was also
tested for reaction with 10% HCl to see if carbonates were
present. If a reaction was observed, that sample was immersed in
10% HCl for >2 hours and then rinsed and dried prior to
analysis. Such acid treated samples are labeled "acid" in the
graphs and "AW" in the sample descriptions.The sub-samples from
each sample location are plotted together in the following graphs.
The sample locations are shown here:
View Bergslagen magnetite sample locations 2013 in a larger map
Samples were collected from Dannemora, Sala, Klackbergs,
Garpenberg, Idkerberget, Tuna-Hastberg, Grangesberg, Stallbergs,
Riddarhyttan, Stora and Pershyttan.
descriptions are here (opens in a new window)
The Sala deposit is a carbonate skarn and was mined for silver. Carbonate was present and the sample was cleaned with acid to remove it. However it is still likely that some of the decrepitation is due to skarn minerals attached to magnetite grains. The intense high temperature decrepitation seen here is quite typical of skarn deposits and minerals.
The Dannemora deposit is currently being mined for iron and is a manganiferous skarn with crystalline carbonate, haematite and magnetite. Two of the 4 samples contained carbonate and were cleaned with acid. (sub-samples 2226B (green) & 2226C (blue)) These 2 samples had the most intense decrepitation, perhaps suggesting that other skarn minerals may be contributing to the decrepitation.
Sample 2266A (analysis h2677, red line) was examined microscopically. The grains were composites with about 80% magnetite and 20% milky quartz. The magnetite was microcrystalline with hackly texture and crystals only about 20 microns in size. Quartz would not contribute to the peaks at 650 or 750 C and this decrepitation is probably entirely due to magnetite.
The Klackbergs deposit is considered to be a BIF type. None of these samples contained carbonate. BIF magnetite would not be expected to contain high temperature fluid inclusions or to decrepitate. The high decrepitation intensity of most of these samples, particularly 2228E (cyan) is unexpected for an unaltered BIF. It seems that there has been some major late-stage fluid processes and recrystallisation at this deposit.
The Garpenberg deposits are of stratabound skarn type and zinc, lead, silver and copper are the primary metals produced in the current mines nearby. But these samples were from old pits several Km away and are thought to have been worked for iron rather than base metals.
These samples were collected from the waste around shallow pits some 3 Km south of the present Ryllshyttan development. All of these sub-samples originated from within a 10 metre radius, with better spatial control than any other samples in this study. The similarity of the decrepitation or all these sub-samples is consistent with this. But there are also significant variations in intensity and temperature differences of up to 40 C in the peak locations.
Most sub-samples contained carbonate and were cleaned in acid. Only 3 sub-samples (2229A (green), 2229B (blue) and 2229C (magneta)) did not require acid cleaning.
Sample 2229F (h2688, yellow-green circles) was observed microscopically. The grains comprised about 60% magnetite with 40% of milky white mineral. Much of this seemed to have a cleavage and was probably not quartz. But it was not carbonate or feldspar either. The magnetite is micro-crystalline with crystals about 20 microns across with a hackly texture.
The decrepitation differences from this group of closely spaced samples indicates that regional metamorphism has not erased or overprinted the original local variations in this group of samples. And it is also apparent that there is a degree of consistency in the decrepitation of closely spaced samples.
The Idkerberget (and also the following Aventyrsgruvan and Grangesberg) deposits are apatite type iron deposits. The origin of this type of deposit is uncertain and both magmatic and hydrothermal mechanisms have been proposed.
None of these samples contained carbonate. Sample 2230B has intense decrepitation and is divided by 10 in this plot, unlike its associated sub-samples. There may be some quartz and other silicates contributing to this result. These were all spill sub-samples from the old shaft and they may be from very different parts of the mine. These samples mostly (except 2230A, red) show evidence of fluid events and contain differing populations of fluid inclusions.
The Aventyrsgruvan mine is also an apatite type deposit. Two of these sub-samples (2231A (green) and 2231C(blue)) contained carbonate and were cleaned in acid. Again there is evidence for a fluid event in the formation of the magnetite of samples 2231B and 2231C.
Grangesberg was one of the largest iron mines in Bergslagen and is an apatite type deposit. None of these sub-samples contained carbonate.
The decrepitation is of only low intensity and there are significant differences between each of the magnetic samples. The peak at 600 C on sub-sample 2232D might be due to quartz, but it also has decrepitation at 680 C and 740C which is due to magnetite and not quartz.
Sample 2232C was split into a magnetic (red, H2699) and a non-magnetic (green, H2666) fraction. The difference between these results highlights the reason that decrepitation studies should be done on mono-mineralic samples to avoid complications caused by different fluid inclusion assemblages and behaviour in different minerals within the same sample.
The Stallbergs deposit is of uncertain type, but is close to Grangesberg and may be an apatite type deposit. All of these sub-samples contained carbonate and were cleaned in acid.
The intense decrepitation is unusual and it is still increasing at 800 C. This intense decrepitation is unlike apatite type deposits and suggests that this is actually a skarn type deposit instead. This decrepitation pattern may be due to oxidation rather than inclusion decrepitation, and perhaps there was some acid insoluble siderite present (although none was observed microscopically), which can give this type of decrepitation response. These samples were collected from spillage near the old buildings.
These additional samples from the Stallbergs deposit were from the perimeter of a very large open pit about 100 metres from the old mine buildings. All these sub-samples contained carbonate and were cleaned in acid. And like the other Stallbergs sample (2233), decrepitation is intense and continuing beyond 800 C, possibly due to oxidation or the presence of siderite, although none was observed during microscope examination of sample 2234B.
Sample 2234B was observed microscopically. This sub-sample was comprised of about 90% magnetite with just 10% of very clear quartz or other silicate (apatite?). The magnetite crystals were typically 400 microns across, the full size of the sample grain. This was the most uniform and coarse-grained magnetite of any observed in this study.
The Riddarhyttan deposit is a high magnesian skarn type of deposit. None of these sub-samples contained carbonate. These 3 samples show radical differences, probably because they are from very different locations within the mine. Note the relatively intense decrepitation of sample 2235A, which has been divided by 4 in this plot to facilitate comparisons. The low overall decrepitation intensity indicates that this deposit is probably not a skarn. Compare this with the Dannemora manganiferous skarn results above, which has decrepitation intensities of 1000 to 5000 counts.
Sample 2235B was observed microscopically. Almost all of the sample grains were composites with magnetite and completely transparent quartz or other silicate. Magnetite comprised only about 50% of the sample volume. The magnetite crystals were about 20 microns across.
The Stora mine is near Strassa, and is probably a BIF type deposit. None of these sub-samples contained carbonate.
Two of the samples have very low decrepitation which is consistent with a BIF, but sample 2236C shows modest decrepitation suggesting a later fluid event.
The Pershyttan deposit is probably also a BIF type deposit based on its location near Nora. None of these sub-samples contained carbonate. Every one of the sub-samples is different, despite being collected from a waste dump which was thought to have come from a single mine. Note the surprisingly high decrepitation of samples 2237A and 2237C, which suggests that this may not be a BIF type deposit and may be a skarn instead. Perhaps these 4 samples originate from 2 different mines of different deposit types, a BIF and a skarn.
Sample 2237C was observed microscopically. Magnetite comprises about 80 to 90 % of the sample volume, with the remainder being very clear quartz. The magnetite is usually the full 200-400 micron size of the grains and has a conchoidal fracture surface.
For comparison, these 2 analyses are replicates of the same sample from the Kiruna apatite type iron deposit in northern Sweden. The sample was collected from the working ore face in 2012. The modest decrepitation intensity indicates limited fluid interaction superimposed upon a magmatic or BIF type deposit.
The baro-acoustic decrepitation method can be applied to
magnetite samples and it provides vital information about the
fluid inclusion populations present and the fluid events which
have caused or subsequently affected the magnetite.
There is considerable variation of decrepitation between sample
locations and regional metamorphism has clearly NOT obscured local
variations in the fluid systems at different locations. There is
also substantial variation between sub-samples at some of the
locations, indicating that small scale variations are preserved in
Decrepitation intensities of the skarn type deposits such as
Garpenberg are usually intense. Although this decrepitation is
probably due to inclusions within magnetite, there is possibly
some contribution from other skarn minerals, as the samples
analysed often had significant amounts of silicates attached to
magnetite within the grains. Because the magnetite crystal size is
quite small it is difficult to obtain mono-mineralic magnetite
samples from most of these deposits.
The BIF type deposits often have much lower decrepitation
intensities, as is expected for a sedimentary type deposit, in
which high temperature fluid inclusions should be absent. But many
BIF samples have significant and complex decrepitation patterns,
indicating that there have been later hydrothermal events
affecting and recrystallising these samples. This study is unable
to determine if these later events were part of the regional
metamorphism, or were more localised events.
The Grangesberg apatite type deposit shows similarities with a
sample collected from the Kiruna apatite type deposit in northern
Sweden. These deposits seem to have low level decrepitation of
only a few hundred counts maximum, and a broad high temperature
decrepitation pattern. It is interesting that such far distant
examples of this same type of deposit show similarities, although
decrepitation is not the preferred method to make such
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