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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
this hypothesis.
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.
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.
The sample
descriptions are here (opens in a new window)
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
the samples.
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
inter-region comparisons.
Sample descriptions
(this study)
A comparison of
many worldwide magnetite samples from BIF and skarn is here
Additional
discussion of decrepitation in FeOxides at Mengku, China is here
A discussion
of decrepitation in haematite and magnetite is here
A study of the
Krivy Rog BIF deposits, Ukraine is here.
A discussion of
Fe oxide deposits worldwide is here