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UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Brucite mineralization
in a hydrothermally
altered carbonate system,
in the marble of
Gåsgruvan Bergslagen area,
central Sweden
Andreas Wennerström
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
B895
Master of Science (120 credits) thesis
Göteborg 2015
Address
Geovetarcentrum
Guldhedsgatan 5A
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
Brucite mineralization in a hydrothermally altered carbonate system, in the
marble of Gåsgruvan Bergslagen area, central Sweden
Andreas Wennerström, University of Gothenburg, Department of Earth Sciences;
Geology, Box 460, SE-405 30 Gothenburg.
Abstract
Brucite mineralization has long been encountered in the marbles and limestones of
Bergslagen. They were first discovered in the nineteenth century and have been
explored in numerous carbonate rocks of Bergslagen to the present day. The
carbonate rocks have a tendency to occur as lenses in the 1.9-1.8 Ga felsic
metavolcanic rock.
The calcitic marble of Gåsgruvan in Bergslagen is currently mined for industrial uses
by SMA Mineral. Brucite mineralization can for them be a problem where the
hydroxide member interferes with the pH-value, yielding a worse product.
The objective of this thesis is to investigate the carbonate rocks of Gåsgruvan, trying
to interpret and understand the brucite mineralization and how it relates to other
minerals, hydrothermal alterations, and mafic intrusives.
Six marble samples were collected in a southeast trending profile in Gåsgruvan, with
varying content of brucite. The geochemistry of the samples was first analyzed with
ICP-MS & ICP-AES, done by ALS geochemistry service. They were also analyzed
with a scanning electron microscope at the University of Gothenburg, for subsequent
correlation with mapped and XRF-analyzed drill cores, done by SMA Mineral.
The studies show that brucite is furthermost related with hydrothermal magnesium
metasomatic processes in contact metamorphism of dolomite and hydration of
magnesium bearing minerals. Spinel and periclase indicate that the mafic intrusions
increased the temperature locally from greenschist facies into amphibolite facies
within the mine. These high-temperature minerals are an indication that the whole
system resembles a >600°C metamorphic event. Access to magnesium in the calcitic
marble is the controlling factor for brucite genesis, the magnesium enrichment was
derived from the wall rock of granitic and mafic intrusions.
Keywords: Brucite, Marble, Gåsgruvan, Bergslagen, SEM, REE, Metasomatic
alteration
Brucite mineralization in a hydrothermally altered carbonate system, in the
marble of Gåsgruvan Bergslagen area, central Sweden
Andreas Wennerström, University of Gothenburg, Department of Earth Sciences;
Geology, Box 460, SE-405 30 Gothenburg.
Sammanfattning
Mineraliseringar av brucit har länge påträffats i Bergslagens marmor och kalkstenar.
De upptäcktes för första gången på 1800-talet, och kom att påträffas i flera av
Bergslagens karbonater i framtiden. Karbonaterna tenderer att ligga som linser i de
1.9–1,8 miljarder år gamla felsiska metavulkaniter.
SMA mineral bryter en kalcitrik marmor i Gåsgruvan för industriellt bruk. För dem
kan brucit vara ett problem då mineralets hydroxidgrupp påverkar pH-värdet, vilket
resulterar i en sämre produkt.
Arbetets syfte är att undersöka Gåsgruvans karbonater för att försöka tolka och förstå
dess mineraliseringar av brucit och hur det relaterar sig gentemot andra mineral,
hydrotermala omvandlingar och mafiska gångbergarter.
Under detta arbete togs sex marmorstuffer i en sydostlig profil i Gåsgruvan med
varierande innehåll av brucit. Provernas geokemi analyserades med ICP-MS & ICPAES av ALS. De analyserades även i ett svepelektronmikroskåp från Göteborgs
Universitet, för att senare jämföras med karterade och XRF-analyserade borrtjärnor
gjorda av SMA Mineral.
Undersökningarna påvisar att brucit är ytterst relaterat till hydrotermal magnesiummetasomatos vid kontaktmetamorfos av dolomit och hydrering av magnesiumbärande
mineral. Spinel och periklas indikerar att de mafiska intrusionerna lokalt höjt
temperaturen från grönskifferfacies till amfibolitfacies i gruvan. Dessa
högtempererade mineral indikerar att hela systemet liknar ett metamorft event från
över 600°C. Tillgången till magnesium i den kalcitrika marmorn är den styrande
faktorn för brucit genesis, vars anrikning härstammar från de granitiska och mafiska
intrusionernas sidoberg.
Nyckelord: Brucit, Marmor, Gåsgruvan, Bergslagen, SEM, REE, Metasomatiska
omvandlingar
Contents
Introduction ............................................................................................................................... 1
Objectives.............................................................................................................................. 1
Background ........................................................................................................................... 2
Occurrence of Brucite ....................................................................................................... 2
Genesis of Brucite............................................................................................................. 2
Geological Background ............................................................................................................. 4
Regional Geology ................................................................................................................. 4
Tectonics of Bergslagen ................................................................................................... 7
Local geology & Site description.......................................................................................... 8
Mineralization ................................................................................................................. 10
Method..................................................................................................................................... 15
Sampling ............................................................................................................................. 15
Sample preparation ............................................................................................................. 17
Whole rock analysis preparation..................................................................................... 17
SEM rock slabs preparation ............................................................................................ 17
Analytical methods .................................................................................................................. 17
Whole rock chemistry analysis (ALS) ................................................................................ 17
ME-ICP06 Analysis ........................................................................................................ 17
ME-MS81 & ME-4ACD81 Analysis ............................................................................. 17
Scanning Electron Microscopy ........................................................................................... 17
XRF (Old boreholes, SMA) ................................................................................................ 18
Results & Interpretation .......................................................................................................... 18
Scanning Electron Microscope (SEM)................................................................................ 18
Sample GG10 SEM backscattered electron image (BSE) interpretation ........................ 18
Sample GG11 SEM backscattered electron image (BSE) interpretation ........................ 21
Sample GG12 SEM backscattered electron image (BSE) interpretation ........................ 24
Sample GG13 SEM backscattered electron image (BSE) interpretation ........................ 26
Sample GG14 SEM backscattered electron image (BSE) interpretation ........................ 28
Sample GG15 SEM backscattered electron image (BSE) interpretation ........................ 30
Geochemical Analysis, ALS ............................................................................................... 33
Variation Diagrams ......................................................................................................... 33
Rare Earth Elements ....................................................................................................... 35
XRF mapping, SMA Mineral ......................................................................................... 35
Discussion ............................................................................................................................... 38
Brucite ................................................................................................................................. 38
Metamorphism .................................................................................................................... 39
Hydrothermal Fluids ........................................................................................................... 40
Future studies ...................................................................................................................... 42
Conclusions ............................................................................................................................. 43
Acknowledgement ................................................................................................................... 44
Bibliography ............................................................................................................................ 45
Appendix 1: Middle Proterozoic rift system ........................................................................... 47
Appendix 2: ALS geochemistry whole rock analysis package ............................................... 48
Appendix 3: ALS geochemistry whole rock analysis data ...................................................... 49
Introduction
Objectives
The aim of this thesis is to investigate metamorphosed carbonate rocks which contain
brucite, trying to perceive its genesis and relationship to other minerals and
hydrothermal alterations. These objectives are pursued by making whole rock analysis
of 6 carbonate samples containing brucite of a different grade in a profile towards
metabasite, within the Gåsgruvan marble. These samples are furthermore analyzed in
the SEM and compared with mapped and XRF-analyzed drill cores from Gåsgruvan
for further interpretation.
This thesis aim is to investigate and discuss following questions:
 Why do some carbonates contain brucite and how is it formed and related to
other minerals?
 To what degree is it possible for a metabasite intrusion to raise the temperature
from greenschist facies into amphibolite facies in marble, and can the
observed mafic sills and dikes give rise to brucite mineralization?
 Why is there in general not much magnesium in the mine, even though
enrichments of magnesium minerals are observed at specific locations?
1
Background
Mined carbonate rock from Gåsgruvan is sent to OMYA on the Persberg Peninsula
for the enrichment of calcite. Brucite is a problem for the floatation procedure in
which they try to eliminate silicates to produce clean calcite powder for industrial use.
The hydroxide member of brucite interferes with the pH value and the process
balance, resulting in a poor yield of calcite. For other pulverized products, the brucite
is not a problem, other than it yields a darker color for products which are supposed to
be white. On the contrary, products used in agriculture can contain carbonates with
brucite because higher magnesium content is helpful.
Occurrence of Brucite
Limestone and marble with disseminated brucite have been encountered many times
within Bergslagen of Filipstad. The first person who observed these unusual rocks
was L. J Igelström in 1858. He spotted them at three limestone mines (Igelström,
1858). A.E. Törnebohm remarked that he had found brucite-carrying limestone in
Värmland in 1878 with isotropic cores of periclase. A Sjögren later found brucite in
seven different mines in Bergslagen of Filipstad. Magnusson (1925) anticipated that
the brucite genesis in limestone is associated with younger eruption magmas, such as
the granite of Filipstad. At Gåsgruvan, one can observe that marble, rich in brucite
appears in the southwestern part of the mine and that an embankment of dolomite is
transcended into marble with brucite. The transition between the two is always very
rapid, making the amount of brucite in the marble constant. This observation suggests
that brucite mineralization in marble is related to isochemical contact metamorphism
of dolomite (Magnusson, 1925).
1918 A.F Rogers found some occurrence of calcite-brucite rocks in California, which
correspond well with those in Bergslagen of Filipstad. All of these occur near an
erupted granitic or mafic contact in dolomite, Rogers also indicates a core of periclase
in the brucite (Rogers, 1918). The term skarn is often mentioned in this thesis and
refers to as calc-silicate mineralization formed by metasomatic processes in contact
between a marble and an intrusive rock or from hydrothermal deposits.
Genesis of Brucite
Magnusson (1925) suggested that brucite in marble may be related to contact
metamorphism of dolomite. In geological literature, brucite is considered the product
of hydration of Mg-minerals in alkaline hydrothermal conditions, or in surface waters.
There are a handful of scientists who during the past have interpreted the genesis and
PT-conditions. An apparent phase diagram was produced by Bucher and Frey (2002)
of a metamorphic contact aureole in dolomite marble at a constant pressure of 2 kbar.
According to them, brucite may form by retrograde hydration of periclase (Reaction
1), where periclase first formed by thermal dedolomitization of dolomite (Reaction 2)
in contact with an igneous body. Brucite could also be formed directly from dolomite,
skipping the intermediate stage of periclase (Reaction 3):

MgO+H2O→Mg(OH)2
Periclase

CaMg(CO3)2→CaCO3+MgO+CO2
Dolomite

(1)
Brucite
Calcite
CaMg(CO3)2+H2O →CaCO3+ Mg(OH)2+CO2
Dolomite
(2)
Periclase
Calcite
Brucite
2
(3)
Figure 1. T-XCO2 phase diagram in dolomite marbles containing excess dolomite and calcite at 2 kbar
constant pressure (Bucher & Frey, 2002).
It is also obvious from figure 1, that both periclase and brucite can only form from the
interaction of dolomite-marble with an H2O-rich fluid (Bucher & Frey, 2002).
Another remarkable way for brucite genesis is the hydrothermal alteration of forsterite
from ultrabasic igneous rocks and pyrometasomatic contact aureoles established in
dolomite. Steinberg was the first person who related serpentinization of forsterite to
formation of brucite in 1960 (Ionescu, 1998), following (reaction 4):

4Mg2(SiO4) + 3H2O → Mg6[(OH)8|Si4O10] + 2Mg(OH)2
Forsterite
Serpentine
(4)
Brucite
Alteration of forsterite into serpentine minerals requires a low content of SiO2 in the
hydrothermal fluids. The field of equilibrium limits for magnesium minerals
concerning SiO2-factor at PH2O 1 kbar can be seen in figure 2 (Ionescu, 1998).
3
Figure 2. Limits of equilibrium between chrysotile (Serpentine), forsterite, brucite and periclase, at 1
kbar of PH2O (Ionescu, 1998).
Brucite may additionally form by deposition from late postmagmatic hydrothermal
solutions at 200°C and ground water which dissolves mg-carbonates from dolomites.
There are further ways brucite genesis may take place, which are pretty much
associated with hydration and metasomatism of additional magnesium minerals
(Ionescu, 1998).
Geological Background
Regional Geology
The Bergslagen region is very rich in various metallic mineral deposits, which is the
southernmost mining district in Sweden. The region is one of many major igneous
provinces of the Fennoscandian shield, which was formed, deformed and
metamorphosed during the Svecokarelian orogeny between 1.9 and 1.8 Ga (Stephens
et al. 2009).
The mineral deposits correspond well with the felsic metavolcanic rocks in the
northern and western part of Bergslagen (Figure 3).
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Figure 3. Geologic bedrock map of Bergslagen area (Stephens et al. 2009).
The region is dominated by submarine, felsic metavolcanic rocks with subaerial and
submarine intervals, lavas and intermediate to basic volcanic rocks. The metavolcanic
rocks were divided by Oen et al. (1982) into three groups, the lower-, medium- and
upper leptite groups (Table 1). An age of 1.9 to 1.87 Ga has been determined by
40Ar/39Ar dating and the magmatic rocks are calc-alkaline (Löfgren, 1979).
Hydrothermal solutions altered the unconsolidated volcanites through metasomatism
into two major chemical groups, sodium- and potassium-rich leptite (Griffin &
Helvaci, 1983).
Limestone and sedimentary deposits occur at some places in the metavolcanic rocks,
where they appear as layers or lens-shaped bodies with carbonates which vary in
magnesium and calcium content. Irregular skarn horizons have been developed
through metamorphism in contact zones between leptite and carbonate rock. These
skarn zones are often associated with iron and sulfide mineralizaton (Magnusson,
1973). The underlying basement rock is not exposed, but old detrital zircons (2.7-1.95
Ga) in thick quartzites occur locally (Lundqvist, 1987). These quartzites suggest that
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an older basement of granite lies below, exposed to erosion near the Bergslagen
region. Various trace element and isotopic studies of leptites in the western
Bergslagen reveal an even older basement felsic rock, of late Proterozoic and possibly
Archean age (Vivallo & Richard, 1984). Most of the Bergslagen region comprises
lower to upper amphibolite facies rock, with a higher grade in the north (Figure 4).
There is one small region to the west which was only exposed to greenschist
metamorphism, in which Gåsgruvan is located (Allen et al, 1996).
Figure 4. Map of Bergslagen with related metamorphic facies, faults and shear zones. Location of
Gåsgruvan is within the black circle (Stephens et al. 2009).
The metavolcanic formations were originally thought to be undeformed and
horizontal, but they have in fact endured a strong deformation and folding during the
Svekokarelian orogeny (Magnusson, 1973).
The supracrustal rocks were later intruded by some generations of calc-alkaline
magmas. The first generation intruded the rocks of central Sweden during the
Svekokarelian orogeny between 1.89-1.85 Ga. Other granite intruded the complex
suite between 1.8-1.6 Ga (Oen et al. 1982). Mafic magma dikes also intruded the
rocks during this time, which later were metamorphosed to metabasite.
6
Table 1. Table of stratigraphy for the Filipstad-Grythyttan-Hjulsjö region (Oen et al. 1982).
Observations of stratigraphic and lithological data resulted in the development of a
model for western Bergslagen. It concludes that in the early Proterozoic, a continental
rift formed a large basin in which felsic volcanoclastic material was deposited (Oen et
al, 1982).
Tectonics of Bergslagen
There have been many theories about the formation of the metavolcanic rocks and
associated skarn and mineral assemblages in Bergslagen. Even today, its genesis and
processes are not fully understood. Hietanen submitted a theory in 1975, suggesting
that the metavolcanic rock series formed in a volcanic island arc system above a
subduction zone (Hietanen, 1975). Some years later Böstrom et al, (1979) stated that
the deposition at Långban may have formed at a spreading center below sea level,
possible in a small basin.
Oen et al, (1982) suggested another more promising model (Appendix 1), where
Gåsgruvan is located just south of the Horrsjö block and to the west of Saxån basin.
They reflect that the supracrustal series correspond to a Middle Proterozoic aulacogen
rift system, where deposition suites and processes can be seen in table 1.
7
Allen et al, (1996) support Oen et al, (1982) and state that his model of continental rift
volcanism in western Bergslagen appears to be right. They, however, suggest that the
whole region was probably placed in an active continental margin and may be
categorized as a continental back-arc region (Allen et al, 1996).
Local geology & Site description
Gåsgruvan belongs to the Bergslagen ore district, which stretches through the
provinces Värmland, Närke, Västermanland, Dalarna, Uppland, and Gästrikland.
The marble mine which is a part of the Gåsgruvefältet is located approximately 500
meters west of Lake Lyngen, approximately 5 km northeast of Filipstad, central west
Sweden (Figure 5).
Figure 5. Map of western Bergslagen with the location of Gåsgruvan and extension of Bergslagen
region (shaded dark green in the inserted image of Sweden),(maps.google.se) (Ekomuseum
Bergslagen, 2015) (inserted image).
Gåsgruvefältet refers to all the mines within the Gåsgruvan marble and its
surrounding skarn mineralizations, as well as Hållplatsgruvan and Liselundsgruvan
(Figure 6). Gåsgruvan consists of an approximately 1400m long and 200m thick
metamorphosed limestone lens. The marble formation dips relatively steeply, roughly
70° towards the southwest. The marble is embedded in fine-grained felsic
8
metavolcanic rock, called leptite. This rock is bordered in the west by a younger
Svecokarelian granite suite, granite of Filipstad. The leptite is also bordered in the
northeast by an older granite (> 1.85 Ga old), the Horrsjö Granite. This red to brown
fine- medium-grained rock appears in a huge area in the Bergslagen of Filipstad
(Åberg et al. 1983). An uneven pyroxene-garnet contact skarn has been formed
between the marble and leptite, which can be seen as (skarn och järnmalm) in figure
6. This zone grows thinner into the marble, where it appears as lenses, veins, and
spots.
Figure 6. Map of Gåsgruvan and mines of Gåsgruvefältet (Magnusson, 1925).
The marble, skarn, and surrounding leptite are intruded by three metamorphosed and
deformed mafic intrusions with varying width, which strike to north-south to
northwest-southeast. In addition, a diabase is intruding the field. Several mineral
deposits have been explored within the skarn areas, mainly for iron but also for zinc
and lead (Magnusson, 1925). There is a fissure or shear zone (Figure 13) with varying
9
width west of the mine, on the contact between leptite and marble, interbedded with
skarn (SMA Mineral). This fissure zone may have acted as a transport passage for
hydrothermal fluids from below, contributing to alteration of the rocks.
Mineralization
Gåsgruvan marble
Gåsgruvan marble contains low magnesium, which is very unusual. In the central
part, north of Igeltjärn (Figure 6), excluding some silica-rich inclusions, the marble
contains 96-98% pure CaCO3 (Analysis 6, Table 2).
Table 2. Analyzes of different carbonate rock at western Bergslagen. 1: Analysis of dolomite,
Långban. 2: Average result of 6 analyzed dolomite from Långban. 3: Dolomitic limestone from
Limbergsåsen, 4: poorly dolomitic limestone, Gåsborns limestone. 5: Dolomitic limestone, Persbergs
odalfield. 6: Analyzed marble from Gåsgruvan. 7: Analysis of limestone, Limkullen (Magnusson,
1925).
Despite the lack of magnesium, dolomite still exists, often as small inclusions or as
decimeter-size enclaves in calcitic marble. As mentioned earlier, the marble contains
disseminations of brucite. The brucite tends to be evenly distributed through the
calcite-brucite segments as round 1-2 mm brown-black grains. Figure 7 may illustrate
the appearance of the marble with disseminations of brucite.
Dolomitization, hydration, and metasomatism gave rise to dolomite fragments and
magnesium-rich mineralization in zones of higher magnesium (Allen et al, 1996). The
mineralization of brucite may be related to this alteration, this matter will be further
discussed later in the report.
10
Figure 7. Relatively clean calcite marble with an interval of black brucite dots. The picture is taken
from an old drill core within the marble (SMA Mineral).
Skarn
The following mines have been mined within skarn areas of Gåsgruvan and can be
seen in figure 6. Observations were made by investigating waste rocks piles of each
mine.
Järnvägsgruvan in the north is rich in marble with more or less regular bands of
magnetite, serpentine, and pyroxene. The skarn is made of pyroxene, garnet, and
amphiboles.
The old concept Gåsgruvorna consisted of several mines in the eastern parts of the
marble. The most northern mine was Mellangruvan and Bankobergsgruvan in the
south with several mines between. The waste rocks of these show an enrichment of
marble carrying magnetite, pyroxene, and a yellowish serpentine. Yellow serpentine
is remarkably common in fissures in the marble. Brucite rich marble is also
encountered.
The waste rocks of Liselundsgruvorna in the west contain marble with magnetite,
pyroxene and idiomorphic garnet.
Blankagruvan to the north is very much the same as Gåsgruvorna, but pyrite was also
located in fissures in the skarn. Metabasite has also been found in the waste rocks.
Ladugruvan more to the south shows an enrichment of marble with brucite, in
addition, a serpentine-diopside bearing skarn was observed. The brucite tends here to
fill fissures in magnetite.
Observations at Kobergs iron ore mine showed that the skarn consisted mostly of
pyroxene and garnet, but also a dark serpentine, mica, and tremolite skarn. Kobergs
zinc mine is located further to the south, which mainly contains sphalerite with some
minor galena, chalcopyrite, and arsenopyrite. This mineralization sits as thin stripes in
marble. The skarn consists of diopside, spinel, and mica.
The garnet and pyroxene skarn of Hållplatsgruvan in the north is very rich in fissures
and is intruded by granite with pegmatite and aplite, which often merge into quartz
veins. Figure 8 may illustrate how magnesium skarn-altered marbe in Gåsgruvan
could appear.
11
Figure 8. Highly magnesium skarn-altered marble with distinct black brucite dots and dark magnetite
bands. Two dolomite cores are present in the upper left and middle of the picture, with haloes of green
epidote, chlorite and yellowish serpentine (antigorite). The picture is taken from an old drill core within
the marble (SMA Mineral).
Three skarn horizons west of Lake Lyngen can be observed in figure 9 where the one
closest to the beach is the highest stratigraphic unit. The one connecting with the
marble of Gåsgruvan further to the left is the stratigraphically lowest unit, which
means it is located in an anticlinal suite (Magnusson, 1925).
12
Figure 9. Map showing skarn and limestone horizons within Persbergs ore district, scale 1:40000
(Magnusson, 1925).
Fissure zone
The fissure zone (Figure 13) west of the marble neighboring the leptite is interbedded
with skarn. The marble close to this zone tends to be rather altered and clayish and is
often rich in green and yellow hydrous minerals as epidote, chlorite, and serpentine.
The absence of brucite is due to high silica levels. The crushed marble weathers into a
red to brown, even more breakdown and clayish mass (Figure 10), which is rich in
hydrous limonite, epidote, chlorite and contain traces of sulfides. The clayish mass
results in lots of drill core losses, as the material is highly porous (SMA Mineral).
13
Figure 10. Greenish marble in contact with a more red to brown hydrous altered mineralized rock in
the fissure zone. The picture is taken from an old drill core near the marble-fissure zone boundary
(SMA Mineral).
Leptite
The leptite surrounding Gåsgruvan has undergone metasomatic alteration in
connection with skarn formation. The sodium-altered lower leptite (Figure 11)
contains albitised plagioclase, chlorite, quartz, cordierite, gedrite and rare
cummingtonite (Magnusson 1925).
Figure 11. Felsic sodium altered leptite with chlorite. Picture is taken from an old drill core within the
leptite, west of the marble (SMA Mineral).
Metabasite
The three mafic dark-green intrusions within the marble consist of mainly plagioclase
interbedded in a matrix of hornblende (Figure 12). The plagioclase consists of
Ab50An50 and is possessed with muscovite and epidote. The rock is at some places
associated with magnetite, apatite and grains of pyrite (Magnusson 1925).
14
Figure 12. Mafic intrusion in the marble with younger felsic veins (SMA Mineral).
Method
Sampling
Six marble samples were collected from the bottom of the open pit mine, at a level of
~50 meters depth. Five of these (GG10-GG15) were taken in a close to southeast
pointing profile with an interval of ~40 meters, approaching an intruding metabasite
body at a low angle (Figure 13).
15
Figure 13. Map showing sample localities, old boreholes, fissure zone and the metabasite in
Gåsgruvan, made in ArcGIS 10.1. (The metabasite body wall, south of sample GG14 was roughly 2
meters away from sample GG15 when the samples were taken in May 2014).
16
Sample preparation
The six marble samples (GG10-GG15) were first cut in half with a rock saw and one
half was used for whole rock analysis. The remaining half was prepared for SEM
analysis. All the sample preparation was done by the author at the University of
Gothenburg, except the geochemical whole rock analysis of the pulverized marble
samples, which were sent to ALS geochemistry service.
Whole rock analysis preparation
The six samples were first crushed in a cast iron pounder to fine gravel. Each gravel
sample was then milled for 3 to 4 minutes in a vibratory disc mill in order to produce
a homogenized sample with correct grain size, which is necessary for a precise ICPMS geochemistry analysis.
SEM rock slabs preparation
Each rock slab was sawn against the foliation, to 3-5mm thick, 4cm long and 2cm
wide. It was then polished on a glass plate with 200 mesh (74µm) aluminum powder
for 5 minutes, then 400 mesh (37µm) for another 4 minutes. The slabs were then
mounted under a weight and polished for 1h in a vibrating polishing machine, using
aluminum powder grit of 600 mesh (10µm). Before they could be analyzed with the
scanning electron microscope, they had to be carbon coated. This is done in order to
prevent charge build-up within the sample during analysis.
Analytical methods
Whole rock chemistry analysis (ALS)
The pulverized samples sent to ALS geochemistry service were analyzed with
inductively coupled plasma mass spectroscopy and atomic emission spectroscopy
(ICP-MS & ICP-AES). A complete package of 5 different analytical methods yielding
65 elements in total (appendix 2) was ordered with a sample cost of 95 US dollar per
sample. The chemical data analysis done by ALS can be seen in appendix 3.
ME-ICP06 Analysis
A lithium metaborate & lithium tetraborate flux of 0.9 g was mixed well with the
prepared sample of 0.2 g, before fusion in a furnace at 1000°C. 100 mL of 4% nitric
acid and 2% hydrochloric acid was then used to dissolve the melt after cooling. The
solution was then analyzed with ICP-AES.
ME-MS81 & ME-4ACD81 Analysis
0.2 g of each sample was added to a lithium metaborate flux of 0.9 g, mixed and fused
in a furnace at 1000°C. After the melt had cooled, it was dissolved in 100 mL of 4%
HNO3 and & 2% HCl3 solution. The solution was then ready for ICP-MS analysis. It
is not ideal to analyze base metal content with this method, but this can be done by
digesting the samples with four acids (ME-4ACD81) (alsglobal.com).
Scanning Electron Microscopy
All six rock slabs were analyzed with the scanning electron microscope (SEM) at the
University of Gothenburg, using a Hitachi S-3400N machine. It operates at high
vacuum and a specimen current of 6.0nA at 20kV and a working distance of 9.6 mm.
The instrument is calibrated with an Oxford instrument energy-dispersive X-ray
analytical system. All samples were analyzed using the backscatter electron detector.
17
XRF (Old boreholes, SMA)
Representative old XRF-analysis data from mapped boreholes in the mine is used to
produce various major element diagrams, in order to appreciate chemical patterns and
characteristics of the marble. These interpretations are then compared and related with
the recently analyzed ALS- and SEM-data.
Results & Interpretation
Scanning Electron Microscope (SEM)
Sample GG10 SEM backscattered electron image (BSE) interpretation
Site 1 in figure 14 consists of mostly a matrix of calcite and small grains of dolomite,
derived by dolomitization of calcite in contact with magnesium-rich sea water. The
small dolomite crystals are scattered throughout the calcite matrix meaning the
recrystallization took place at an early stage. The sample contains retrograde
antigorite with cores of forsterite. This means the rock has been experienced a high
temperature to form forsterite, followed by cooling and hydration to form antigorite
through serpentinization (Kelemen, 2012). Magnetite crystals are present where some
may originate from serpentinizaton of olivine.
Site 2 (Figure 15) displays retrograde serpentinization as a pseudomorph mesh texture
of a big single olivine crystal in a matrix of calcite with associated dolomite. This
alteration also contains some minor magnetite. Site 3 (Figure 16) is more zoomed out
and display several forsterite crystals being altered to antigorite in a matrix of calcite.
It also contains forsterite with a rim of spinel. This spinel reaction rim indicates that
its forsterite core wasn´t in equilibrium with the surroundings (enriched in Al and
depleted in SiO2), meaning that the serpentinization could not continue. Figure 17
shows site 4 which contains retrograde clinochlore in calcite where some chlorite is
situated as a rim of spinel. This chlorite aureole may have been altered by spinel
releasing Mg and Al during retrograde metamorphism in the presence of MgO- and
SiO2-rich fluids (Kimball, 1990).
18
Figure 14. Site 1 of sample GG10 illustrates antigorite (serpentine) rims surrounding forsterite cores in
a matrix of calcite associated with minor dolomite. The bright mineral in the middle is magnetite in
contact with antigorite.
Figure 15. Site 2 of sample GG10, displays pseudomorph retrograde antigorite formed by
serpentinization of forsterite. Magnetite crystals are also present.
19
Figure 16. Site 3 from sample GG10 shows cores of forsterite with corona rims of antigorite in calcite,
with a spinel rim in contact with antigorite.
Figure 17. Site 4 from sample GG10 with both apatite and spinel crystals within clinochlore.
20
Sample GG11 SEM backscattered electron image (BSE) interpretation
GG11 (Figure 18-23) has undergone a stronger dolomitization with further
recrystallization, although the rock is still dominated by calcite. The sample is also
very low in silica.
Figure 18 shows site 1 where brucite was found in contact with large dolomite
(pseudomorphs) and as rims in magnetite grains. Just a little forsterite and antigorite
were present here.
Site 2 (Figure 19) displays dolomite in calcite containing brucite with a core of
periclase. The brucite has small inclusions of magnetite, which may be due to olivine
breakdown with small parts of iron (fayalite) content. The rock may have been
affected by a higher temperature, judging from the periclase grain in the brucite
(Bucher, 1981). A strong serpentinization of forsterite is located in site 3 (figure 20)
which is a sign of hydration with decreasing temperature. Brucite is even here located
as rims around magnetite, inside antigorite and in calcite. Magnetite inclusions in
brucite are not evident. Site 4 in figure 21 shows a big brucite grain with magnetite
inclusions in contact with dolomite in a matrix of calcite. Some antigorite is also
present in contact with dolomite. Site 5 (figure 22) is another image displaying brucite
in the rim and cleavage of a magnetite grain in calcite. Brucite in the rims and
cleavage of spinel is present in site 6 (figure 23). Brucite is also here located in
contact with dolomite and calcite, but only one grain of brucite contains apparent
inclusions of magnetite. Some antigorite is also present.
Figure 18. Site 1, sample GG11 detector. Brucite in contact with dolomite and magnetite in calcite.
Antigorite and forsterite sit together in the lower right corner.
21
Figure 19. A closed zoomed brucite crystal in dolomite at site 2 from sample GG11. The brucite got
traces of small magnetite inclusions and it´s likely possessing a core of periclase.
Figure 20. Sample GG11 at site 3, shows antigorite surrounding forsterite crystals adjacent to dolomite
and magnetite. Brucite formations occur as in rims of magnetite and inside antigorite.
22
Figure 21. Large brucite crystal in dolomite (pseudomorph) from sample GG11 at site 4. The brucite
holds small inclusions of magnetite. Antigorite next to dolomite.
Figure 22. Brucite formations in the rim, core and cleavage of a magnetite crystal from sample GG11,
site 5.
23
Figure 23. Site 6 from sample GG11, illustrating brucite next to dolomite, in the cleavage and rim of
spinel. White minerals are magnetite.
Sample GG12 SEM backscattered electron image (BSE) interpretation
GG12 (Figure 24-26) illustrates skarn inclusions in the marble with a high and
variable augite content. This sample does contain higher silica content, thus no
periclase or brucite are located. All the Mg is situated in augite and dolomite doesn’t
occur. Titanite can be observed through the sample. The skarn may have originated
from hydrothermal fluids from fissures or from contact metamorphism by an
intruding metabasite. Tremolite, forsterite and water most likely altered to augite and
antigorite at lower temperatures and albite-rich plagioclase started to grow when
fluids were enriched in sodium.
24
Figure 24. Site 1 from sample GG12 consists of augite (diopside-hedenbergite) and some albite in
contact with augite in a matrix of calcite. All white smaller crystals are titanite.
Figure 25. Site 2 from sample GG12, illustrating augite with connected albite in calcite. Associated
titanite is present (white minerals).
25
Figure 26. Site 3 from sample GG12 consists of calcite in a matrix of augite with a more frequent
titanite mineralization (bright minerals).
Sample GG13 SEM backscattered electron image (BSE) interpretation
Figure 27–29 illustrates a matrix of calcite, which has endured dolomitization and
forsterite serpentinization. The high content of silica is probably the main factor why
brucite formation is excluded here. Site 1 in figure 27 shows a matrix of calcite with
dolomite and antigorite with minor of forsterite. Magnetite is also present as
inclusions in antigorite, where iron sited in forsterite were released through
serpentinization. Clinochlore is most certainly a product of retrograde alteration of
forsterite related to mafic hydrothermal liquids. Site 2 (Figure 28) contains dolomite,
forsterite, and clinochlore in a matrix of calcite. A sphalerite grain was found which
may have been mobilized through hot fluids in fissures from nearby skarn ores.
Figure 29 shows an image of site 3 where dolomite and clinochlore can be located in
a calcite matrix, thus no forsterite or antigorite are present. Sphalerite and magnetite
still occur.
26
Figure 27. Sample GG13, site 1 Contains dolomite and small forsterite inclusions in antigorite.
Clinochlore mineralization in calcite also exists.
Figure 28. Site 2 from sample GG13. The picture shows dolomite in calcite, and clinochlore adjacent
to forsterite. The bright mineralization is sphalerite.
27
Figure 29. Dolomite and clinochlore in calcite from sample GG13, site 3. Sphalerite mineralization lay
in contact with dolomite and magnetite in calcite.
Sample GG14 SEM backscattered electron image (BSE) interpretation
Signs of dolomitization of calcite in sites 1-2 (Figure 30-31) are present and the
matrix consists of calcite. At some places, one can see more magnesium rich clusters
of forsterite surrounded by antigorite which evidences retrograde serpentinization of
forsterite. Even this sample lacks brucite due to high silica. An additional source of
aluminum gave rise to spinel in site 2 (Figure 31) which with calcite and forsterite
formed clinochlore together with dolomite at a lower temperature (Bucher, 1981).
28
Figure 30. Sample GG14, site 1 showing forsterite in antigorite (pseudomorph mesh texture) along
with clinochlore. Bright minerals are magnetite and the gray small dots in calcite are dolomite.
Figure 31. Site 2 from sample GG14 illustrates clinochlore surrounding forsterite crystals in its rims
and cleavage. Dolomite and spinel are also current.
29
Sample GG15 SEM backscattered electron image (BSE) interpretation
This rock sample (Figure 32-35) shows no signs of dolomitization and the matrix
dominates of calcite. Very little forsterite is present (Figure 35, site 4), it has largely
been replaced by antigorite, which means the rock has been strongly serpentinized at
a relatively low temperature. This rock may have been affected by a more felsic
intrusion as frequent inclusions of clinochlore occurs (Figure 33-35, site 2-4).
Clinochlore is most likely a product of chloritization of forsterite at a decreasing
temperature. This is probably the reason for Mn-ilmenite (pyrophanite) and magnetite
(Figure 32-33, site 1-2) formation as forsterite releases iron with associated
manganese and titanium during its breakdown (Fleet, 2006).
Figure 32. Site 1 of sample GG15 displaying antigorite in calcite with prehnite, pyrophanite, and
magnetite. The dark-grey dotted area around the calcite to the right consists of small inclusions of antigorite in
calcite.
30
Figure 33. Sample GG15 site 2, elongated magnetite crystals in calcite and pyrophanite inclusions in
clinochlore.
Figure 34. Clinochlore and antigorite in calcite from sample GG15 at site 3.
31
Figure 35. Site 4 from sample GG15 showing forsterite and clinochlore in a matrix of calcite, with
associated apatite in the top right.
32
Geochemical Analysis, ALS
Variation Diagrams
Figure 36. Variation diagrams showing the relation of major oxides, considerable sulfides, and the
more mobile REE lanthanum plotted against SiO2 in Gåsgruvan. Diagram a-e are plotted from marble
samples GG10-GG15 with data analyzed by ALS. In addition, diagram e) contains data from old
boreholes analyzed with XRF by SMA Mineral.
Looking at figure 36a, one can see that the Al2O3 content increases as SiO2 does, and
they are well connected with each other. The occurrence of clinochlore and albite
(GG12) are dependent on aluminum and silica content. The MnO content (Figure
36b) seems to clearly decrease with increasing silica, thus it should be associated with
magnetite. The sulfides plotted in figure 36c show an obvious positive trend with the
increase of silica. These sulfides may have been mobilized through post hydrothermal
solutions derived from younger intrusives. Plotted lanthanum (Figure 36d) being the
most mobile REE shows an increasing trend with a higher amount of silica. La is
widely dispersed in trace quantities in minerals such as pyroxene, feldspar, biotite and
apatite and may originate from the granite intrusives (Salimen, Plant, & Reeder,
33
2006). The diagram in figure 36e contains more than 1500 data points located all over
the mine. It seems to illustrate two different trends for alteration. One is silica poor
with significant increases in Mg that can be interpreted as dolomitization, and one is
silica-rich interpreted as magnesium-skarn altered marble. Thus, this statement is
rather uncertain where variable silica content could be either due to original quartz or
clay minerals or to the hydrothermal addition of silica with associated Mg. Only one
sample has brucite (Figure 18, GG11), namely the one with the lowest silica and
highest magnesium content. The high amount of silica in sample GG12 is very
apparent as augite in figure 26.
Figure 37. Variation diagrams showing the relation of some major oxides with barium and strontium
plotted against MgO in Gåsgruvan. Diagram a-d are plotted from marble samples GG10-GG15 with
data analyzed by ALS.
The relationship between CaO and MgO is relatively comprehensible where
magnesium enrichment seems to lower the calcium content of the marble, except for
sample GG13 (Figure 37a). This is thought to be highly related to early
dolomitization and other magnesium alteration. The content of Fe2O3 (total) in all
samples but GG12 (Figure 37b) seems related with the content of MgO, where higher
magnesium increases the iron content. The iron seems primarily bounded in
magnetite, often in connection with brucite (Figure 21). Some iron is also situated in
olivine crystals as fayalite. Figure 37c & 37d appear to be rather similar except Sr
content is higher. Their content decreases with an increasing magnesium value. Ba2+
and Sr2+ tend to replace Ca2+ in calcite, due to their smaller ionic potential. Ba2+ is by
Goldschmidt´s rule less abundant in calcite than Sr2+ (Faure, 1998). They also
substitute for Ca in plagioclase, hence the high content in GG12 (Figure 37c & 37d)
(Winter, 2001).
34
Rare Earth Elements
Figure 38. Chondrite-normalized (Evensen, 1978) REE and Y patterns calculated for samples GG10GG15. All the REE and Y data for the marble is averaged in the logarithmic scale. Sample G1B is
taken from another location in Bergslagen from an altered dolomite in contact with a gabbro intrusion
(Hogmalm et al. 2012).
The REE and Y abundance patterns for Gåsgruvan marble (Figure 38) are rather
similar with the exception of sample GG12. They show a relatively low content of
rare earth elements in all the samples, hence REE appears not affected by
hydrothermal alteration. The patterns also exhibit an enrichment of light REE, relative
to heavy ones and contain negative Eu anomalies (Boulvais et al. 2000). The negative
Eu anomaly could be connected with the substitution of Eu2+ for Ca in plagioclase.
Thus the hydrothermal liquid was at one time in equilibrium with former plagioclase,
possibly originated from intruded granites (Winter, 2001). The positive Tm anomalies
should be an analytical error. The sample (GG11) containing brucite with the most
magnesium and least silica holds the smallest REE content with the lowest Eu
anomaly.
The patterns look similar compared with a strongly altered dolomite sample (G1B)
from a magnesite deposit (30km east of Gåsgruvan) affected by a gabbro intrusion
(Hogmalm et al. 2012). The REE patterns don´t relate with seawater interaction.
Therefor seawater interference generally affects the REE with a fingerprint of a
progressive enrichment of heavy REE, relative to the light REE. They also exhibit a
depletion of Ce and an enrichment of La (Piper & Bau, 2013).
XRF mapping, SMA Mineral
Following graphs represent one borehole each, illustrate measured major elements in
weight percent (marble only). Attached drill-core mapping is located below each
graph and location for each hole can be seen in figure 13. XRF-analysis and drill-core
mapping were done by SMA Mineral.
35
Figure 39. XRF data with mapping, from borehole 535. It’s drilled towards the northeast with a 45°
dip, which probably goes below the interpreted metabasite intrusion (Figure 13). (MgO, Fe2O3, Al2O3
and MnO are plotted against the secondary axis).
Figure 40. XRF data with mapping from borehole 544. It´s drilled far out, southwest of the marble,
dipping 45° towards the northeast with its approximately 300 meter length (Figure 13). (MgO, Fe2O3,
Al2O3 and MnO are plotted against the secondary axis).
36
Figure 41. Diagram made from XRF data with mapping from borehole 621 (Figure 13). This borehole
is drilled horizontally towards the southeast and it´s almost 100 meters long. (MgO, Fe2O3, Al2O3 and
MnO are plotted against the secondary axis).
Figure 42. XRF data with mapping from borehole 628 (Figure 13), which is dipping 45° towards
southeast. This borehole is approximately 225 meters long. (MgO, Fe 2O3, Al2O3 and MnO are plotted
against the secondary axis).
37
What first can be noted by observing the diagrams (Figure 39-42) is the pattern of
MgO and CaO, which tend to be the complete opposite of each other. This may be
controlled entirely by calcite dolomitization. Dolomite often occurs together with
olivine and serpentine, which at some places is due to positive SiO2 and MgO
anomalies, even though silica and Mg don’t appear that correlated. Another obvious
observation is that Al2O3 is highly related to SiO2. This either reflects alteration by
silica- and Al-rich fluid, or more likely the original content of clay minerals in the
limestone. Positive Fe2O3 (total) anomalies tend to follow MgO as olivine breakdown
releases iron which oxidizes to iron oxides.
Different amount of silica in the mapped marble and marble with brucite areas reflect
magnesium silicate zones within the marble, such as olivine, serpentine, and chlorite.
An important observation is that brucite mineralization is found mainly in high
magnesium rocks and seems highly related with dolomitization.
Discussion
Brucite
Brucite was only situated in one of the taken samples (GG11), which reflected the
most Mg-rich and probably strongest dolomitized sample. GG13 & GG14 containing
almost as much magnesium as GG11 could have the potential of exhibit brucite,
although the higher content of silica (Figure 36e) probably compromised the
magnesium through skarn alteration. Here brucite was in contact with mainly large
dolomite crystals, but also forsterite and serpentine. Some brucite was even situated in
rims and cleavage of magnetite and spinel mineralization.
Most of the brucite in contact with dolomite show signs of small magnetite inclusions.
These crystals may be related with retrograde olivine serpentinization, where
forsterite hydrates to serpentine and brucite when it comes in contact with water- and
silica-rich hydrothermal fluids around 370-400°C (Figure 44, reaction 4). During
olivine breakdown, it releases iron from its endmember fayalite, which oxidizes to
magnetite (Winter 2001).
Only one single brucite crystal was seen to have a core of periclase (Figure 19). Such
brucite with a core of periclase in limestone is exactly what A.E Törnebohm found in
Värmland in 1878. Hydration of periclase to brucite (Figure 43, reaction 5 & Figure
44, reaction 2) is thought to take place below about 600°C at 1kbar in conditions of
low silica and XCO2 ≤ 0.05 (Gerdes et al. 1999).
This temperature correlates with amphibolite facies, which is too high for the regional
metamorphic grade of greenschist facies (Figure 4). This is strong evidence that some
areas within the marble have endured local contact metamorphism of amphibolite
grade due to the mafic intrusions within the marble.
Space in the cleavage of spinel and magnetite has been filled with brucite (Figure 22
& 23) crystallized from late postmagmatic fluids rich in water and magnesium. This
can occur in a relatively low-temperature environment (200°C), where brucite tends
to act as a fracture filling mineral (Magnusson, 1925). They may also have formed
directly from dedolomitization (Figure 43, reaction 9) at very low CO2 partial pressure
and high H2O partial pressure between 470-600°C (Gerdes et al. 1999). This process
may be hard to prove by looking at the SEM pictures, but these brucite crystals are
probably not derived from forsterite serpentinization, because they don’t contain any
magnetite inclusions. If however iron was released, it may have been absorbed by
existing magnetite crystals around the brucite.
38
In aluminous dolomite poor in silica, they often contain various amounts of Mgchlorite (clinochlore), which is stable in greenschist and amphibolite facies. This
clinochlore reacts with dolomite at 4 kb around 610°C and 680°C to form forsterite +
spinel + calcite + H2O + CO2 (Figure 44, reaction 5).
Figure 43. T-XCO2 phase diagram for a siliceous dolomite in a CaO–MgO–SiO2–H2O–CO2 system,
calculated using the Holland & Powell (1990) thermodynamic database (Gerdes et al. 1999).
The occurrence of spinel is also evidence for higher temperatures in amphibolite
facies. The newly formed forsterite may have cooled down to around 400°C, where in
contact with water it hydrates to form antigorite, and later brucite when remaining
liquid is poor in silica and CO2. The early periclase however shows that some brucite
has its origin in the high-grade event, formed by retrogression as the temperature fell.
The marble sample GG11 is the only sample in which clinochlore is absent. This
could mean that the stability field of the assemblage chlorite + dolomite has been
exceeded, either because of a too high temperature or because of an external
compositional control of the thermal liquids by surrounding granite. The marble
samples (GG13 & GG15) are by contrast lacking spinel, meaning the stability field of
clinochlore + dolomite has not yet been exceeded (Bucher, 1981).
Metamorphism
There is no secret that a combination of forces during the Svecokarelian orogeny have
influenced the area and made it rather complex and tricky to interpret. Two
generations of granitic magmas intruded the region during this orogeny contributing
with hydrothermal liquids and deformation. At the same time, mafic sills and dikes
intruded the region and three known intrusions penetrated the marble of Gåsgruvan.
These mafic sills and dikes intruded the rocks, which gave rise to high-temperature
skarn alterations through metasomatic processes. The pressure while these intrusions
took place is thought to have been relatively low, around 1kbar. The whole system
resembles a retrograde metamorphic event affecting a higher grade rock, (i.e. higher
39
“grade” minerals to lower ones, due to a decrease in temperature). Serpentine
pseudomorphs of forsterite (Figure 30) and lower grade chlorite rims around spinel
and forsterite (Figure 17 & 31) are great evidence of retrograde metamorphism.
Prograde spinel and periclase formed at the highest temperature, over 600°C (at given
pressure). Granitic magmas do not commonly generate such high temperatures, but
they could be achieved by mafic intrusions. Sample GG12 (Figure 24-26) contains a
lot of diopside, which could be explained by the retrograde reaction (6) tremolite +
forsterite + H2O = diopside + antigorite when the temperature has cool down to
around 525°C (Figure 44, reaction 6). This is the highest temperature where antigorite
is stable and first introduced in the system. When the altered skarn has cooled down
even more (370-400°C), forsterite begins to hydrate to antigorite and brucite. They
become unstable below 260°C and create chrysotile (Figure 44, reaction 7), which is a
serpentine less rich in magnesium than antigorite (Winter 2001).
Figure 44. Summary of hydrothermal brucite alterations in the marble of Gåsgruvan. Pressure is set to
1kb.
Hydrothermal Fluids
Magnusson (1925) pointed out that genesis of brucite is related with hydrothermal
fluids from younger erupted granitic magmas, such as granite of Horrsjö in the north
and granite of Filipstad in the west. He also stated that brucite mineralization is
related with contact metamorphism of dolomite (Magnusson, 1925). His statements
seem correct where intruding granites appear to have enriched the marble in
magnesium through metasomatic processes, leading to dolomitization.
Further magnesium development and high-temperature hydrothermal fluids emerged
once the mafic sills and dikes intruded the dolomitized marble. Fissures in the marble
40
acted as fluid passages leading to stronger hydrothermal alteration in greater
extension. The hydrothermal fluids generated by the mafic rocks must as well have
been enriched in silica, aluminum and iron looking at the mineralization of the marble
samples. These elements and others got leached out from wall rocks of the intrusions
continuing path through the crust, felsic basement granites, leptites and finally the
overlying gray and black slate formations.
The eroded overlying slate formations containing clay minerals may have been the
substantial source of aluminum and silica as the intrusions hydrothermal solutions
leached them out.
The taken samples show a relatively low content of REE elements for being exposed
by pretty strong hydrothermal alterations. It´s rather uncertain the Eu anomalies
(Figure 38) are derived by hydrothermal fluids from plagioclase-bearing intrusives or
basically inherited from the marble protolith, or a combination of both (Whitney &
Olmsted, 1998).
Hogmalm et al. (2012) investigated magnesite replacement formation in a calcitic or
dolomitic precursor rock, which was intruded by a gabbro intrusion in Bergslagen.
This gabbro gave rise to medium temperature hydrothermal fluids associated with
seawater, which probably gave rise to magnesite alteration. The REE plot of their
dolomite (G1B, Figure 38) is relatively similar to the marble samples at Gåsgruvan,
except their dolomite has higher REE (Hogmalm et al. 2012). It is hard to point out
the reason for this. The small enrichment of light REE (LREE) in sample G1B may be
due to that REE rather substitutes in calcium than in magnesium. Hence, the calciterich marble of Gåsgruvan has higher (LREE) enrichment.
The liquid for diagenetic dolomitization in the samples may originate from seawater
interaction, however, the low amounts of Sr (<73 ppm, Figure 37d) may indicate
otherwise as it should be of hydrothermal origin. Dolomitization associated with
seawater interaction should possess Sr-values of several hundred ppm (Gasparrini,
Thilo, & Boni, 2006). The REE pattern (enriched in LREE, relative HREE) is also not
in favor for seawater interaction.
Every mentioned alteration seems very magnesium and dolomite dependent, where
everything is being controlled by the content of magnesium and silica, as well as the
pressure of water and carbon dioxide. The marble contains areas with pure calcite
with no magnesium alteration whatsoever. Then there are disseminations of dolomite
with calcite and skarn alteration. If conditions of low silica & CO2-pressure and high
H2O-pressure & alkalinity are present, brucite may form by hydration of magnesium
minerals.
This has also been recognized in the past by Oen et al. in 1982 where they saw
alterations of tremolite and chlorite-serpentine skarn in contacts between limestone
and metabasite, which they called “sköl-areas”. They often found sköl areas with
sulfide impregnations along shear and fracture zones. Outside “Sköl-areas” marble
and metabasite instead show a sharp contact without any magnesium skarn alterations
(Oen et al. 1982).
The measured loss on ignition (LOI, Appendix 3) reflects the content of volatiles
(CO2+H2O) present in the marble samples. The average LOI values of all the samples
are 42.1%, excluding sample GG12 (Figure 26), which acquired 36% due to the high
silica content. This indicates high volatile content and the significance of high
carbonate content since it is equal with the development of carbon dioxide after
heating over 900 °C in a furnace (Onimisi, 2013).
41
Future studies
The samples may not have been placed at the perfect locations for the purpose of
studying the brucite alteration affected by the high-temperature mafic intrusion. The
mafic sills and dikes are most certainly somewhere below the surface of the sample
areas, making it hard to interpret their influence. What can be done for future work is
to take carbonate samples containing brucite, more proximal to a single known
metabasite intrusion, without risking interference with other sills and dikes. The
interval between samples shouldn’t be greater than one meter. Multiple samples
should also contain brucite, not just one. Performing a calcite-dolomite
geothermometer could help determine the temperature of calcite dolomitization,
which is helpful with the interpretation alteration processes. Possessing 18O/16O and
87
Sr/86Sr isotope ratios would be a useful compliment in the attempt to interpret the
fluid source of alteration. Geologists of the mine could use a handheld XRF to
analyze the rock walls in order to detect any unwanted mineral and alteration zone.
42
Conclusions
 Brucite mineralization is related with hydrothermally Mg-metasomatic
processes in contact metamorphism of dolomite and hydration of various Mgbearing minerals.
 Brucite tends to be stable at a differential range of temperatures between 600
and 260°C at 1 kbar, with high Mg- and low SiO2-content related with H2Orich fluids.
 Mineralization of spinel and periclase are good evidence that local mafic
intrusions in the marble raised the metamorphic grade from greenschist facies
into amphibolite facies, and gave rise to low XCO2 fluids. These mafic sills
and dikes could also have generated brucite by hydration of high-temperature
periclase.
 The system then underwent retrograde reactions of higher grade rock with a
decrease in temperature, where serpentine pseudomorphs of forsterite and
chlorite corona texture around spinel and forsterite are a great indication of
this.
 The calcite dolomitization got its magnesium from granitic and mafic intrusive
wall rocks through metasomatic leaching as they ascended through the crust,
near to metavolcanic and overburden sedimentary rocks.
43
Acknowledgement
I would like to thank my supervisor at SMA Mineral, Magnus Johansson for
providing me with this opportunity and letting me take samples and work with their
drill cores, and providing me with the desired counseling. I also want to thank my
supervisor, Johan Hogmalm, and examiner, David Cornell at the University of
Gothenburg for their dedication and time spent on this project, and for contributing in
dedicated discussions and criticisms.
44
Bibliography
Allen, L. (1996). Facies Analysis of a 1.9 Ga, Continental Margin, Back-Arc, Felsic Caldera
Province with Diverse Zn-Pb-Ag- (Cu-Au) Sulfide and Fe Oxide Deposits,
Bergslagen Region, Sweden. Economic Geology, v. 91, 979-1008.
Boulvais, P., Fourcade, S., Moine, B., Gruau, G., & Cuney, M. (2000). Rare-earth elements
distribution in granulite-facies marbles: a witness of fluid–rock interaction . Elsevier,
117-126.
Bucher, K. (1981). Petrology of chlorite-spinel marbles from NW Spitsbergen (Svalbard).
LITHOS, 203-213.
Bucher, K., Frey, M. (2002). Petrogenesis of Metamorphic Rocks. Berlin: Springer-Verlag
Berlin Heidelberg.
Böstrom, K., Rydell, H., and Joensuu, O. (1979). Långban - an exhalative sedimentary
deposit? Econ. Geol, 1002-1011.
Ekomuseum Bergslagen. (2015, March 26). Retrieved from Stiftelsen Ekomuseum
Bergslagen: http://ekomuseum.se/?page_id=1336
Evensen N,M,, Hamilton P,J,, and O'Nions R,K. (1978). Rare-earth abundances in chondritic
meteorites. Geochimica et Cosmochimica. v42, 1199-1212.
Faure, G. (1998). Principles and applications of geochemistry. New Jersey: Prentice Hall.
Filipstads Bergslag. (2011, 02 22). Bergshanteringen i Filipstads Bergslag. Retrieved from
Filipstads Bergslag:
http://www.filipstadsbergslag.com/gruvor/persberg/gasgruvan.html
Fleet, M. E. (2006). Rock forming minerals: Micas. London: The Geological Society.
Gasparrini, M., Thilo, B., & Boni, M. (2006). Massive hydrothermal dolomites in the
southwestern Cantabrian Zone (Spain) and their relation to the Late Variscan
evolution. Marine and Petroleum Geology, 1-26.
Gerdes, M, Baumgartner, L, Valley, J. (1999). Stable Isotopic Evidence for Limited Fluid
Flow through Dolomitic Marble in the Adamello Contact Aureole, Cima Uzza, Italy.
Journal of petrology (40), 853-872.
Griffin, W, Helvaci, C. (1983). Metamorphic feldspathization of metavolcanics and
granitoids, Avnik area, Turkey. Mineral. Petrol, 309-319.
Hellingwerf, R. H. (1984). Paragenetic Zoning and Genesis of Cu-Zn-Fe-Pb-As Sulfide Skarn
Ores in a Proterozoic Rift Basin, Gruvfsen, Western Bergslagen, Sweden. Economic
Geology, 696-715.
Hietanen, A. (1975). Generation of potassium-poor magmas in the northern Sierra Nevada
and the Svecofennian of Finland. U. S. Geol. Surv, 681--645.
Holland, T. J. B. & Powell, R. (1990). An enlarged and updated internally consistent
thermodynamic database with uncertainties and correlations: the system K2O–Na2O–
CaO–MgO–MnO–FeO–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2. Journal of
Metamorphic Geology 8, 89-124.
Igelström, L. J. (1858). Om ett för Sverige nytt mineral. Övers. af Kongl. Vek. Ak. Förh.
Ionescu, C. (1998). The Genesis of Brucite - Mg(OH)2. A general review. Geologia, XLIII, 1,
97-102.
K. Johan Hogmalm , Rob Hellingwerf , David H. Cornell & Friedrich Finger. (2012). An
epigenetic magnesite deposit in Bergslagen area, central Sweden. GFF, 134:1, 7-18.
Kelemen, P. B. (2012). Reaction-driven cracking during retrograde metamorphism: Olivine
hydration and carbonataion. Earth and planetary science letters, 81-89.
Kimball, K. L. (1990). Effects of hydrothermal alteration on the compositions of chromian
spinel. Contributions to Mineralogy and Petrology, vol.105, 337-346.
Lindström, M, Lundqvist, J, Lundqvist, T. (2000). Sveriges geologi från urtid till nutid. Lund:
Studentlitteratur.
Lundqvist, T. (1987). Early Svecofennian stratigraphy of southern and central Norrland,
Sweden, and the possible existence of an Archean basement west of the
Svecokarelides. Precambrian Research, v. 35, 343-352.
Löfgren, C. (1979). Do leptites represent Precambrian island arc rocks? Lithos, 159-165.
45
Magnusson, N. H. (1925). Persbergs malmtrakt och berggrunden i de centrala delarna av
Filipstads Bergslag. Stockholm: Victor Pettersons Bokindustriaktiebolag.
Magnusson, N. H. (1973). Malm i Sverige 1, Mellersta och södra Sverige. Almqvist &
Wiksell AB.
Oen, I.S., Helmers, H., Verschure, R.H. & Wiklander, U. (1982). Ore deposition in a
Proterozoic incipient rift zone environment: A tentative model for the FilipstadGrythyttan-Hjulsjö region, Bergslagen, Sweden. Geol Rundschau, 182-194.
Onimisi, M., Obaje, N., & Daniel, A. (2013). Geochemical and petrogenetic characteristics of
the marble deposit in Itobe area, Kogi state, Central Nigeria. Advances in Applied
Science Research ,4(5), 44-57.
Piper, D. Z., & Bau, M. (2013). Normalized Rare Earth Elements in Water, Sediments, and
Wine: Identifying Sources and Environmental Redox Conditions. American Journal
of Analytical Chemistry, 69-83.
Rogers, A. (1918). American occurance of periclase. American Journal of Science.
Salimen, R., Plant, J., & Reeder, S. (2006). Geochemical atlas of Europe. Part 1, Background
information, methodology and maps. Geological Survey of Finland.
Stephens, M.B., Ripa, M, Lundström, I., Persson, L., Bergman, T., Ahl, M., Wahlgren, C-H.,
Persson P-O., and Wickström, L. (2009). Synthesis of the bedrock geology in the
Bergslagen region, fennoscandian shield, south-central Sweden. Sveriges geologiska
undersökning, 259.
Turner, F. J. (1965). Note on the genesis of brucite in contact metamorphism of Dolomite.
Beiträge zur Mineralogie und Petrograpliie 11, 393-397.
Wellin, E. Wikander, U. & Kähr, A.M. (1980). Aradiometric study of a quartz-porphyritic Krhyolite at Hällefors, Örebro, county, Sweden. Lithos, 147-152.
Whitney, P., & Olmsted, J. (1998). Rare earth element metasomatism in hydrothermal
systems: The Willsboro-Lewis wollastonite ores, New York, USA . Geochimica et
Cosmochimica Acta (62), 2965-2977.
Winter, O. D. (2001). An Introduction to Igneous and Metamorphic Petrology. New Jersey:
Prentice-Hall Inc.
Vivallo, W & Richard, D. (1984). Early Proterozoic ensialic spreading subsidence: Evidence
from Carpenberg, central Sweden. Precambrian Research. v. 26, 203-221.
Åberg, G., Bollmark, B., Björk, L., & Wiklander, U. (1983). Radiometric dating of the
Horrsjö granite, south central Sweden. GFF, vol (105), 78-81.
46
Appendix 1: Middle Proterozoic rift system
47
Appendix 2: ALS geochemistry whole rock analysis package
48
Appendix 3: ALS geochemistry whole rock analysis data
Oxide
(%)
Element
(ppm)
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
Cr2O3
TiO2
MnO
P2O5
SrO
BaO
LOI
Total
Ba
Co
Cr
Cu
Zn
Hf
Sc
V
Y
Zr
Nb
Ni
Pb
Rb
Th
U
Sn
Sr
Ta
Tb
Cs
Dy
Er
Ga
Gd
Tm
W
As
Bi
Hg
Sb
Se
Te
Sample
GG10
1,45
0,2
0,38
50,8
3,11
<0.01
0,01
<0.01
0,02
0,17
<0.01
<0.01
<0.01
42
98,14
GG11
0,73
0,2
0,45
47,9
5,33
0,03
0,01
<0.01
0,01
0,12
<0.01
<0.01
<0.01
43,3
98,08
GG12
10,15
1,45
0,86
50,1
2,49
0,08
0,06
<0.01
0,04
0,05
<0.01
<0.01
0,01
36
101,29
GG13
2,17
0,44
0,67
51,3
5,41
<0.01
0,02
<0.01
0,02
0,1
<0.01
<0.01
<0.01
41,4
101,53
GG14
2,53
0,69
0,56
49,2
4,43
<0.01
0,02
<0.01
0,02
0,05
<0.01
<0.01
<0.01
41,4
98,9
GG15
1,84
0,4
0,43
50,4
3,41
<0.01
0,02
<0.01
0,02
0,12
<0.01
<0.01
<0.01
42,5
99,14
5,8
2
10
28
20
0,3
<1
<5
1
11
<0.2
<1
2
0,9
1,16
1,16
1
52,3
<0.1
0,03
0,03
0,17
0,1
0,4
0,24
0,03
1
0,2
0,02
<0.005
<0.05
0,6
0,02
3,4
2
<10
1
9
0,2
<1
<5
0,8
5
<0.2
<1
<2
0,7
0,48
0,36
<1
39,8
<0.1
0,02
0,04
0,13
0,08
0,3
0,12
0,03
1
<0.1
0,01
<0.005
<0.05
0,4
0,01
72,9
3
<10
31
52
0,6
1
<5
4,4
19
1,2
<1
<2
1,6
1,75
0,55
1
73,3
0,1
0,15
<0.01
0,71
0,46
2,1
0,84
0,07
1
<0.1
0,08
<0.005
0,18
0,4
0,02
2,8
3
<10
11
46
0,2
1
<5
1,5
7
<0.2
<1
<2
0,7
0,71
0,77
1
43,2
0,1
0,03
0,01
0,25
0,1
0,6
0,28
0,04
1
2,2
0,01
<0.005
0,07
0,4
0,01
3,5
2
<10
31
37
0,4
1
<5
2,1
12
<0.2
<1
<2
0,8
0,92
3,52
<1
50,9
0,1
0,05
0,02
0,35
0,2
1,1
0,28
0,04
1
1
0,02
<0.005
<0.05
0,3
0,01
4,7
3
<10
26
39
0,2
1
<5
1,6
6
<0.2
<1
3
0,7
0,7
0,3
1
47,8
<0.1
0,03
0,03
0,23
0,15
0,7
0,33
0,03
1
1,5
0,04
<0.005
<0.05
0,6
0,01
49
Ag
Cd
Li
Mo
Tl
Ge
In
Re
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
<0.5
<0.5
<10
<1
<0.02
<5
<0.005
<0.001
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Tm
Yb
Lu
2,1
3,5
0,42
1,6
0,26
0,04
0,24
0,03
0,17
0,03
0,03
0,08
0,01
1,4
2
0,21
0,9
0,14
<0.03
0,12
0,02
0,13
0,02
0,03
0,09
0,01
6,5
12,4
1,5
5,5
0,97
0,19
0,84
0,15
0,71
0,17
0,07
0,44
0,06
2,6
4,3
0,47
1,6
0,3
0,05
0,28
0,03
0,25
0,04
0,04
0,12
0,02
2
3,2
0,38
1,6
0,3
0,04
0,28
0,05
0,35
0,08
0,04
0,23
0,03
2,1
3,2
0,39
1,4
0,25
0,04
0,33
0,03
0,23
0,04
0,03
0,11
0,02
50