Download U-Pb Dating of Monazites from the Kiirunavaara and Rektorn Ore

UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
U-Pb Dating of Monazites
from the Kiirunavaara and
Rektorn Ore Deposits
Hydrothermal events affecting
the Kiruna Main Ore
Hannah Blomgren
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
B891
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
Abstract
The Kiirunavaara ore deposit is one of the largest production sites for metalliferous ore in the world
with a capacity of 2 billion tons of magnetite. It is also the type locality for the iron oxide apatite
“Kiruna-type” ore that can be found worldwide. The genesis of the ore has been a cause of extensive
debate during the last 100 years and any new information that can shed some light on the questions
“What has happened to the Kiirunavaara ore, and when did it happen?” is of both academic and
economic importance. In this study a sample from the Kiirunavaara ore deposit, as well as a sample
from an apatite vein crosscutting the Rektorn ore have been investigated by SEM and microscope. In
situ U-Pb dating of monazite and xenotime as well as trace element analysis of apatite have been
performed by Laser Ablation-ICP-MS.
The textural and trace element study shows that the apatite in the samples is altered with a LREE, Na,
Th, U, and Pb depleted rim around a monazite inclusion rich core. The inclusions of monazite display
a unidirectional growth along the apatite crystal axis caused by dissolution-reprecipitation. At the
grain boundaries of the apatite there are larger monazite crystals (~30µm) that are interpreted to have
formed as a product of apatite leaching by hydrothermal fluids percolating through micro-fractures and
possible nano-voids in the ore. U-Pb in situ dating of the monazites displays two separate groups with
different ages. The younger group yields an age of 1623±23 Ma and the older group yields an age of
1718±12 Ma. Monazites and xenotimes analyzed from the Rektorn ore deposit east of Kiirunavaara
yields a concordia age of 1721±19 Ma. The older age from Kiirunavaara and the concordia age from
Rektorn are considered to be connected to fracture mineralization at Malmberget dated at 1740 Ma.
The younger age found in the monazites at Kiirunavaara could also be related to fracture
mineralization at Malmberget dated to ~1617 Ma. The monazites investigated in this study are
interpreted to be caused by two hydrothermal events of regional scale possibly caused by late stage
activity of the Svecokarelian Orogen.
Keywords: Kiirunavaara, apatite-magnetite ore, U-Pb in situ dating, monazite, hydrothermal events.
1
Sammanfattning
Kiirunavaara är en av världens största produktionsplatser för metallmalm med en kapacitet av 2
miljarder ton magnetit. Avsättningen är även typ-mineraliseringen för järn-oxid-apatit ”Kiruna-typ”
malmen som återfinns världen över. Malmens ursprung har under de senaste 100 åren skapat livlig
debatt och all information som kan bringa ljus i frågorna ”Vad har hänt med Kiirunavaara-malmen och
när hände det?” har både akademisk och ekonomisk betydelse. I föreliggande studie har ett prov från
Kiirunavaara och ett prov från en apatitgång som korsar Rektorn undersökts med SEM och mikroskop.
In situ U-Pb datering samt spårämnesanalys av apatit har genomförts med hjälp av Laser AblationICP-MS.
Den textuella studien samt spårämnesanalysen visar att apatiten har en LREE-, Na, Th-, U- och Pburlakad kant kring en kärna som är rik på monazitinklusioner. Monazitinklusionerna växer med
samma orientering som apatitens kristallografiska axel och är orsakade av ”dissolutionreprecipitation”. I korngränserna mellan apatit förekommer större monazitkristaller (~30µm) som har
tillväxt som en produkt av urlakningen av apatit. Urlakningen har orsakats av hydrotermala vätskor
vilka perkolerat genom mikrosprickor och möjligtvis även nano-hålrum i malmen. U-Pb in situ
datering av monaziterna påvisar två grupper med olika åldrar. Den yngre gruppen visar en ålder av
1623 ±23Ma och den äldre gruppen ger en ålder av 1718±12Ma. Monaziter och xenotimer från
apatitgången i Rektorn, öster om Kiirunavaara, demonstrerar en concordia-ålder av 1721±19Ma. Den
äldre gruppen monaziter daterade i Kiirunavaara samt concordia-åldern från Rektorn anses här relatera
till sprickmineralisering i Malmberget som har daterats till 1740 Ma. Den yngre gruppen identifierad i
Kiirunavaara kan även den kopplas till en yngre sprickmineralisering i Malmberget daterad till ~1617
Ma. Monaziterna som har analyserats i den här studien tolkas ha orsakats av två hydrotermala
episoder, möjligen orsakade av sena faser i den Svekokarelska Orogenesen.
Nyckelord: Kiirunavaara, apatit-magnetitmalm, U-Pb in situ datering, monazit, hydrotermala episoder.
2
Table of contents
Abstract
1
Sammanfattning
2
Table of contents
3
Introduction
5
Aim and purpose
5
Background
6
Regional geology
6
Local geology of the Kiruna area
7
Kiruna ore district
8
Kiirunavaara ore deposit
8
Previous dating and constraints of the Kiirunavaara ore body
9
Apatite
9
Monazite and xenotime
9
Monazite dissolution and reprecipitation
10
Laser ablation inductive plasma mass spectrometry
10
LA-ICP-MS
10
Instrumental drift
10
The U-Th-Pb dating system
11
Apatite and monazite U-Th-Pb dating
12
Method
12
Monazite
12
Apatite
13
Results
14
Sample description
14
Sample description JH2
14
Sample description Rektorn
20
3
Monazite results
23
Monazite data from sample JH2
24
Monazite data from sample JH2b
26
Combined data from sample JH2 and JH2b
28
Rektorn
29
Apatite results
31
Discussion
34
U-Pb ages of monazite
34
Evaluation of data
35
Further studies
36
Conclusion
37
Acknowledgements
37
References
38
4
Introduction
Aim and purpose
The Kiirunavaara ore deposit is one of the largest production sites for metalliferous ore in the world
(Cliff, Rickard, & Blake, 1990), and it is also the type locality for the apatite iron oxide “Kiruna-type”
ore that can be found worldwide. The genesis of the ore has been a cause for extensive debate during
the last 100 years, and age determination of the ore has been limited to a constraining age of 19091880 Ma (Cliff et al., 1990). Recent studies, however, have dated zircon from the ore to an age of
1878 Ma and monazite to an age of 1628 Ma. The zircon age is fairly consistent with previous
constraints, but the monazite age is much younger than the previous age constraints. Furthermore,
oxygen isotopic measurements of the host rock and the ore exhibit varying isotopic ratios (Westhues,
Hanchar, & Whitehouse, 2014). It is evident that the Kiirunavaara ore has been subjected to some sort
of activity, and any new information that can shed some light on the questions; “What has happened to
the Kiirunavaara ore, and when did it happen?”, could have implications regarding the perception and
understanding of the genesis of the Kiruna-type ore, not only at Kiruna, but at all the important IOA
mineralization sites worldwide.
In this study 32 monazites from the Kiirunavaara ore deposit, as well as 3 monazites and 5 xenotimes
form the Rektorn ore, have been dated by in situ U-Pb dating with the aim to further investigate the
Kiirunavaara ore and contribute with information pertaining to the question “What has happened to the
Kiirunavaara ore?”, with emphasis on “When did it happen?” By regarding the results in the present
report in relation to previous dating of rocks in northern Sweden, this study further aims to establish if
the activity that has affected the Kiirunavaara ore has caused implications on a regional scale.
5
Background
Regional geology
The northern part of Sweden has bedrock of
different periods that constitute a suite
comprising of 2.8-2.68 Ga Archaean rocks,
2.4-1.96 Ga Karelian rocks, and 1.96-1.85 Ga
Svecofennian rocks (note that Svecofennian
will be used to describe the bedrock and
Svecokarelian when referring to the orogeny)
(Bergman, Martinsson, & Persson, 2002). The
tectonic history of the area is complex and is
thought to have evolved during different stages
(Billström, Bergman, & Martinsson, 2002).
The Svecokarelian Orogen had its peak around
1.9-1.8 Ga and affected Norway and Finland as
well as Sweden (fig. 1) (Lundqvist, Lundqvist,
Lindström, Calner, & Sivhed, 2011). The
bedrock associated with the orogen shows
varying structural directions throughout the
Svecofennian Province related to separate
domains of both brittle and ductile Figure 1. A map displaying the areas affected by different
orogenies in Sweden, Norway, and Finland. Modified from
deformation. It is possible that the presence of (Smith, Storey, Jeffries, & Ryan, 2009)
Archaean bedrock below the northern parts of
the Svecofennian region in Sweden is in part the cause of the differences within the area. The tectonic
development could have been affected by the underlying bedrock causing different grade of
deformational impact in different localities with mostly localized severe deformation. Furthermore, the
presence of the Archaean bedrock can have influenced the melts created in the northern region
(Bergman et al., 2002; Lundqvist et al., 2011).
Apart from the peak metamorphism around 1.9-1.8 Ga, a later event at 1.86-1.85 Ga is suggested
based on U-Pb dating on samples from the north eastern parts of Norrbotten County. The U-Pb dating
of metamorphic titanite and monazite yields ages of 1.86-1.85 Ga and 1.80-1.78 Ga. Furthermore, ArAr analysis was performed on mica and hornblende samples from the Karesuando-Arjeplog
Deformation Zone to determine metamorphic cooling ages in the area. The resulting ages of the
samples investigated were 1.78 Ga, suggesting a system resetting at this time, probably connected to
the1.8 Ga metamorphic event (Billström et al., 2002). Alteration assemblages with titanite have also
been found with a connection to epigenetic mineralization, and the U-Pb ages of the titanite suggest
two significant hydrothermal events around 1.88-1.86 Ga and 1.79-1.77 Ga (Billström & Martinsson,
2000).
The surface rocks of the northern Svecofennian region consists predominantly of felsic and mafic
volcanics that in many places are penetrated by granitoids associated with the early orogenic phases
(Lundqvist et al., 2011). The northernmost Svecofennian region differs from the overall area
characterized by the Svecokarelian Orogen in Sweden, especially in the north western parts of
Norrbotten County where there is evidence of a more extensive early (1.89-1.87 Ga) deformation of
the bedrock. The metamorphic evidence is rarely overprinted and a conclusion based on these
observations is that large scale metamorphism ended earlier in the northern regions than in the south.
It is also possible that there has been extensive deformation, foliation, and erosion early on in the
6
formation of the Svecofennian surface rocks. In addition to the evidence found for major metamorphic
deformation at, and previous to, 1.8 Ga, isotopic measurements in Norrbotten and northern Finland
show indications of low-grade metamorphism around 1.79-1.74 and 1.6-1.5 Ga (Lundqvist et al.,
2011). These younger dates have been found in fracture mineralization in the Malmberget iron ore
deposit, where U-Pb dating of monazite has yielded ages of 1740 Ma and titanite ages of 1620-1613
Ma. Furthermore, analyses have been made on the mineral stilbite that is of low thermal stability and
consequently can constrain cooling histories in an area when dated. The stilbite samples are also taken
from Malmberget and show an older generation with an approximate age of 1730 Ma, indicating that
the area stayed below or at the thermal stability of stilbite (150 °C). Due to the relatively slow
exhumation rates in the Malmberget area, the fracture mineralization investigated is not thought to be
related to the exhumation and cooling of the rock, but more likely evidence of tectonic reactivation
(Romer, 1996). Also, dating of uraninite grains from the Arjeplog-Sorsele area displays U-Pb and PbPb ages of 1767-1740 Ma (Hålenius, Smellie, & Wilson, 1995) indicating that the older deformation
event at 1760-1740 Ma might be of regional scale (Romer, 1996).
Local geology of the Kiruna area
The Kiruna area is placed within the
Svecofennian Province and is characterized by
tectonically north-south striking structures
(Lundqvist et al., 2011) of sedimentary,
volcanic, and volcaniclastic rocks forming a
monoclinical structure with rock that becomes
younger toward the eastern parts of the area
(Geijer, 1910). The Archean basement rock is
discordantly overlain by the conglomerates of
the lowermost stratigraphic unit in the area, after
which there are the Kiruna greenstones, the
Kurravaara
conglomerate,
the
Kiruna
porphyries, the Lower Hauki Formation, and the
Upper Hauki Formation. The sequence of rocks
is intruded by numerous generations of
granitoids and the apatite-magnetite ore
characteristically found in the Kiruna area
resides within the Lower Hauki Formation and Figure 2. A: Schematic map of the Baltic Shield
within the Kiruna porphyries (fig. 2) (Romer, displaying the placement of the Kiruna area in relation to
the margin of the Archean craton. B: Simplified
Martinsson, & Perdahl, 1994). Per Geijer (1910) geological map of the Kiruna area. C: Map of the Kiruna
has done an extensive description of the iron ores and their host rocks. 1: Upper Hauki Formation,
geological sequence found in the Kiruna area, 2:Lower Hauki Formation, 3:syenitic sill, 4=quartzand he describes the lower Kiruna porphyries to bearing porphyry, 5=apatite-bearing iron ores, 6= syenite
porphyry, 7=Kurravaara conglomerate, 8=Kiruna
primarily consist of felsic to mafic extrusive greenstone. The figure description and the image are
rocks, and in the western parts the porphyries taken from Romer, Martinsson & Perdahl (1994).
display amygdules and fluidal textures with
varying combinations of calcite, apatite, amphibole, magnetite, and titanite (Geijer, 1910).
Approximately 500 m above the Kiirunavaara ore body a major syenitic sill intrudes the porphyry. The
sill pre-dates the metamorphic events (Romer et al., 1994). In contrast to the lower porphyries, the
upper Kiruna porphyries are predominantly ryodacitic pyroclastic rocks with feldspar phenocrysts in a
feldspar-quartz matrix with minor magnetite and biotite (Parák, 1975). Different alterations affect the
porphyries, but alkali metasomatism is the dominating alteration type (Geijer, 1910).The Lower Hauki
7
Formation contains mafic volcanics and silicified felsic tuffs with hematite inclusions while the Upper
Hauki Formation displays a sequence of different rock types. The lowermost unit consists of phyllites,
conglomerates, and graywackes and the upper unit is predominantly quartzite (Frietsch, 1979).
Alterations affecting the Lower Hauki Formation are foremost sericitization and silicification (Parák,
1975).
Kiruna Ore District
The apatite-magnetite ores in the Kiruna area can be divided into two groups with respect to their
stratigraphical placement and their phosphorous content. The first group has phosphorous contents
below 1 % and is placed in the contact between the syenite porphyry and the quartz-bearing porphyry.
The second group displays higher phosphorous contents of 3-5 % and is located between the Kiruna
porphyry and the Lower Hauki Formation. While the first group constitutes the larger ore bodies of
Kiirunavaara and Luossavaara, the second group consists of smaller deposits called the “Per Geijer
Ores”, or; Nukutusvaara, Haukivaara, Henry, and Rektorn. All ore bodies are predominately lenticular
and are positioned conformable with the lithological boundaries of the host rocks (Romer et al., 1994).
Some authors have suggested that the two ore groups are related to two main stages of magnetite
emplacement, where the high content phosphorous group would constitute the younger (Cliff &
Rickard, 1992; Cliff et al., 1990).
Kiirunavaara ore deposit
The Kiirunavaara ore deposit mainly consists of
magnetite, with minor hematite, and up to 30 %
apatite. The apatite is predominantly
flourapatite and occurs with accessory
constituents of biotite, actinolite, diopside,
calcite, quartz, spehen, albite, and talc. There
are occurrences of sulphides as impregnations
within the magnetite and occasionally as larger
crystals within crosscutting veins (Parák,
1975). The ore body is a lenticular disc, 4 km
long and approximately 60 meter broad (Cliff
et al., 1990).
The stratigraphy in which Kiirunavaara resides
is as described in Figure 3. The basement
consists of greenstone, after which there is a
conglomerate that is overlain by syeniteporphyries. In contact with the syenite are the
magnetite ores, that in turn are overlain by
quartz-porphyry (Geijer, 1910). Overlaying the
Figure 3. Stratigraphy of the Kiruna Complex from Parák
porphyry are the Per Geijer ores followed by a
(1975).
phyllite and greywacke sequence, and at the top
of the stratigraphic sequence resides the Vakko sandstone (Parák, 1975). The footwall at Kiirunavaara
is syenite porphyry and the hanging wall is quartz porphyry (Geijer, 1910). The syenite porphyry
displays lava structures and in places phenocrysts and amygdules. At the contact between the
Kiirunavaara ore body and the syenite porphyry, there are interlayers of tuffaceous material that cuts
the lower ore boundary at 30-35⁰ angles. In contrast to the footwall porphyries, the quartz porphyry of
the hanging wall does not display clear lava patterns. The porphyry does, however, exhibit an
abundance of phenocrysts composed of microcline and albitic plagioclase while the matrix is
8
predominantly quartz, microcline, and plagioclase. A zone of 0.1-1.5 meter thickness at the ore
boundary displays kaolinization of the quartz porphyry. Furthermore, there are occurrences of
agglomoritic zones within the porphyry (Parák, 1975).
Previous dating and constraints of the Kiirunavaara ore body
Previously performed geochronological work related to the Kiruna ore has dealt with a time
constraining period for the ore by dating the host rock and crosscutting rock entities. The host rock has
been dated by Skiöld and Cliff (1984) to 1909 ± 18Ma, and a crosscutting and undeformed
granophyric dike has been dated to 1880 ± 35Ma (Cliff & Rickard, 1992). Other rock bodies in the
Kiruna area has been dated, and amongst the youngest are a suite of granitoids that have yielded U-Pb
ages of 1794 ± 24Ma (Skiöld, 1988) while whole rock Rb-Sr dating of the same granitoids gives ages
of 1530 ± 25Ma (Gulson, 1972). The differing dates have been explained by either a slow crustal
cooling or a resetting of the Rb-Sr system by late thermal activity (Cliff & Rickard, 1992). Previous
attempts at dating the Kiruna ore itself have been unsuccessful, but they have shown signs of “major,
later disturbance of the ore”(Cliff et al., 1990). Pyrite occurs as fine disseminations in primary
magnetite and has formerly been thought to be related to early sulphide mineralization and initial lead
isotope control. Cliff & Richard (1992) suggest, however, that the pyrite is related to hydrothermal
fluids depositing sulphide in the primary magnetite ore. By radiometric dating of ore and sulphide
samples from Kiirunavaara, a U-Pb age of 1560 ± 60Ma, a Sm-Nd age of 1490 ± 130Ma, and a Rb-Sr
age of 1450 ± 30Ma have been obtained. The different dates attained are considered to be evidence of
a major isotopic reequilibration at, or near, 1500 Ma. The authors also refer to age determinations of
intrusive rocks in northern Sweden that have yielded zircon ages of 1800 Ma while Rb-Sr whole rock
data gives ages closer to 1500 Ma, indicating regional resetting of the isotope systems in the rocks
(Cliff & Rickard, 1992). Furthermore, the sulfides are connected to saline fluid inclusions in apatite at
Kiirunavaara and is claimed to be a sign of recrystallization of the apatite in the ore. The 1500 Ma
hydrothermal event is also related to uplift of the region which could have caused a large amount of
fluid circulation (Cliff & Rickard, 1992).
Apatite
Apatite is the most common phosphate on Earth with the general formula Ca5(PO4)3(F,OH,Cl), and it
can substitute several cations in the place of Ca2+ into its crystal structure. The three main end
members are flourapatite, hydroxyl apatite, and chlorapatite (Hughes & Rakovan, 2002). The
phosphorous mineral can occur in sedimentary, igneous, metamorphic, and hydrothermal
environments (Rakovan, 2002).
In geological environments apatite can precipitate from melts, low temperature aqueous solutions, and
from concentrated hydrothermal brines. It can be formed in any system containing phosphorous and
fluoride due to the low affinity of those elements in most other minerals (Rakovan, 2002).
Monazite
Monazite is a compound included in the orthophosphate group together with xenotime. Monazite can
host most of the LREE in its crystal structure while xenotime incorporates the HREE. Also, monazite
crystallizes in the monoclinic system while xenotime crystalizes in a tetragonal structure. Both
monazite and xenotime can occur in a wide range of environments, and their existence in alluvial
sediments proves them highly resistant to weathering (Boatner, 2002).
As well as incorporating REE, monazite can also incorporate uranium and thorium into its crystal
structure. This ability has made it the primary mineral for commercial extraction of thorium as well as
a mineral of interest within the field of geochronology. All minerals of the anhydrous orthophosphate
9
group are highly durable and chemically insoluble in aqueous environments, which further is a reason
why they can be utilized in the field of geochronology (Boatner, 2002).
The melting temperature of monazite is in excess of 2000°C, and experiments have reported that the
mineral remains chemically stable close to the melting temperature after which some thermal
decomposition takes place (Boatner, 2002). The mineral stability could be attributed to the nine fold
coordination polyhedra structure that enables a relatively large amount of distortion. This is also the
reason why monazite can incorporate a large number of cations of different sizes into its crystal lattice
(Seydoux-Guillaume et al., 2002).
Monazite dissolution-reprecipitation in apatite
Experimental studies of the dissolution and reprecipitation of monazite within, and at the grain
boundaries of, apatite have been performed by Harlov et al. (2005). The study is inspired by the
observation of leached apatite with monazite inclusions in several Kiruna type apatite-magnetite ores.
In the study, what is referred to as nano-channels and nano-voids have been detected and are
considered to aid in the mass transport of elements by fluids percolating through the apatite. Through
these nano-voids hydrothermal fluids, or brines, can enter the apatite and partially dissolve the
mineral, primarily of Y+REE. When a supersaturation of REE is created in the fluid, nucleation of
monazite can initiate within a nano-void. During the experimental trial it was found that HCl was
more efficient than H2SO4 in the dissolution-reprecipitation process (Harlov, Wirth, & Förster, 2005).
The nucleation of monazite within apatite, and in an open system, could progress by the following
reactions (modified by Harlov et al (2005) from Pan et al (1993)):
Apatite (1) + (Ca2+, P5+) in a fluid = apatite (2) + monazite + (Si4+ and Na+) in a fluid
Apatite (1) is relatively enriched in LREE, Na and Si compared to apatite (2)
It is assumed that Ca and P are added in an open system where the apatite is depleted in LREE. This
assumption is partly based on the perception that part of the apatite is dissolved during the process.
Thereby, Ca and P is released in the system and space is cleared for monazite growth (Harlov et al.,
2005).
Laser ablation inductively coupled plasma mass spectrometry
LA-ICP-MS
The laser ablation inductively coupled plasma mass spectrometer attains material by using a high
energy laser pulse to evaporate the surface area of a solid sample. The ablations takes place in an
enclosed space called the ablation cell that is continuously filled and refilled with an inert gas, usually
argon or helium. The gas functions as an elemental transporter and carries the aerosol created above
the sample through tubes to the ICP-MS. It requires vaporization, atomization, and ionization of the
aerosol created by the laser pulse before any analysis can be made in the mass spectrometer. These
processes take place within the ICP. When isotopes have been created, the mass spectrometer can
determine composition and concentration of the ablated elements (Hattendorf & Gunter, 2014).
Instrumental drift
Several factors can cause instrumental drift in the LA-ICP-MS. Often a cause of drift is the thermal
equilibration of the laser or the ICP-MS. The laser energy output can be compromised by the laser
temperature and optical factors. The ICP-MS is also affected by temperature and changes can disturb
the vacuum interface and the electronics, ultimately changing the transmission proficiency. Lastly, the
composition of the gas used as transport medium can have a temporal drift due to washout of residual
10
air after an analysis. Most instrumental problems can be compensated for by simultaneously
measuring a reference sample to the actual analysis (Hattendorf & Gunter, 2014).
The U-Th-Pb dating system
Elements with high atomic number, and thereby a high proton count, such as uranium (92 protons) and
thorium (90 protons) are unstable and radiogenic. U and Th decay by emission of alpha particles into a
series of daughter isotopes with different half-lives. The relatively long lived isotopes of 238U and 235U
as well as 232Th all decay into daughter isotopes of Pb where the relationship can be summarized as
follows:
238
U→206Pb + 8α + 6β- + 47.4 MeV
235
U→207Pb + 7α + 4β- + 45.2 MeV
232
Th→208Pb + 6α + 4β- + 39.8 MeV
(Harrison, Catlos, & Montel, 2002)
Due to the low risk of the uranium isotopes 238U and 235U to fractionate in nature, their measured decay
systems can be plotted against each other in a Concordia plot that yields a geochronological history of
a mineral, if it has remained in a closed system. If the system has been disturbed at some later time
from closure, a discordia line can be created with a lower intercept age that marks the time of the
younger event, for instance metamorphism. The Pb loss or U gain at some later time from the mineral
formation will shift the values off the Concordia curve, creating a scatter of values of no probable
geological significance (Harrison et al., 2002).
In contrast to zircon, which is popularly used within in the field of geochronology due to its stable
nature under several environmental conditions, the phosphate minerals that can be used for dating are
not prone to damage by internal radiation. The high ability of monazite to incorporate U and Th into
its crystal lattice yields one of the most common radiogenic minerals on Earth and also the most
common one. The general crystal structure of monazite is ABO4, where the A-site is occupied by a
large cation and the B-site contains tetrahedrally coordinated small cations. In the A-site all REE can
be incorporated, but preferentially the LREE such as Ce and La, and it is also in these sites the U and
Th isotopes can be accommodated. The substitution of REE for U and Th can take place through two
substitution schemes:
Huttonic substitution: REE3+P5+=Th4+Si4+ → endmember Huttonite ThSiO4
Brabantitic Substitution: 2REE3+=Ca2+Th4+ → endmember Brabantite Ca0.5Th0.5PO4
(Harrison et al., 2002)
11
Apatite and monazite U-Th-Pb dating
The main issue when using the U-Th-Pb system to date apatite is the affinity for initial incorporation
of common lead in the apatite crystal. Also, apatite usually has low concentrations of U, Th and Pb
which can render the dating process difficult. Due to the often high common lead incorporation, the Pb
correction is of significance when dating apatite (Chew, Sylvester, & Tubrett, 2011).
The benefit of using monazite for dating of rocks from different environments is the low initial
incorporation of common lead into the crystal, and thereby all lead found in the monazite is the
radiogenic product of U and Th decay. Difficulties that can arise when dating monazite are lead
contamination of the samples as well as Th disequilibrium, Pb loss and grain zoning. The single grain
zoning can be caused by recrystallization, lead diffusion or multi-stage growth and is connected to
different Th/Pb ratios. Thus, an investigation of possible zonation is important to attain a correct
dating of the mineral (Scherrer, Engi, Gnos, Jakob, & Liechti, 2000).
Method
The samples investigated in this study are JH2 (thin sections JH2 and JH2b) from the apatitemagnetite Kiirunavaara ore deposit (level 935, crosscut 126, D5 type ore), and Rektorn (thin section
Rektorn) from an apatite vein crosscutting the Rektorn apatite-magnetite ore in the Kiruna Ore
District. Both specimens are randomly sampled from the ore deposits.
The thin sections were primarily investigated by a scanning electron microscope (SEM) and a regular
microscope in order to identify the different mineral phases. The thin sections were mapped in the
SEM and subsequently investigated with the laser ablation inductively coupled plasma mass
spectrometer (LA-ICP-MS). The data was lastly managed in excel-spreadsheets. The instruments
utilized were the HITACHI Scanning Electron Microscope Model S-3400N and the ESI New Wave
Research NWR213 Nd:YAG (213 nm) laser microprobe connected to an Agilent Technologies 8800
triple quadrupole ICP-MS.
Monazite
The LA-ICP-MS settings for time span of laser ablation and spot size were based on the size of the
monazites as well as the properties (thickness etc.) of the thin section. The intensity was set to 5 Hz
and the spot size to 10 µm.
The standard that was used for the monazite data evaluation was primarily Moacir. The Moacir
standard derives from the Itambé pegmatite district in Brazil dated to 504 Ma (Seydoux-Guillaume,
Wirth, Deutsch, & Schärer, 2004). Other standards used during the analysis were TMM, the
Thompson Mine monazite from Canada dated to 1766 Ma (Williams, Buick, & Cartwright, 1996), and
44069 derived from the Wilmington Complex in Delaware USA, dated to 425 Ma (Aleinikoff et al.,
2006). All standards have been thoroughly evaluated in the diploma thesis “Evaluation of U-Th-Pb
dating of monazite by LA-ICP-MS” by Marianne Richter (Richter, 2013).
The analysis was conducted by taking two spots on each of the standards before taking additionally 10
points on monazite crystals. Depending on the size of the crystal one or two spots were taken. The best
signals were then chosen for the age calculations based on their error and quality. Figure 4A is an
example of a “bad” signal where the disturbance is caused by an inclusion of another mineral within
the monazite. Figure 4A, however, displays a “good” signal with minimal disturbance in the analysis.
12
A
B
Figure 4. A) The diagram displays a poor quality signal during analysis by LA-ICP-MS. B) The diagram displays a
good quality signal during analysis by LA-ICP-MS.
After the 10 spots on the sample, another 6 spots on the standard samples were performed.
The data from the LA-ICP-MS analysis were extracted from GLITTER and data reduction and age
calculations were handled in Excel (in-house spreadsheet by Thomas Zack, modified by Eric Ackevall
(2015), Gothenburg University). The ages were determined using Isoplot 4 (Ludwig, 2011).
Sample JH2 and JH2b were analyzed at different times due to sample preparation. During run 1 the
JH2 monazite data and the trace element apatite data was acquired. The instrumental settings for the
monazite analysis were 53% output energy with a frequency of 5 Hz and 5.77 J/cm2 at a spot size of
10µm. During run 2, when the JH2b sample was analyzed, there was 35% output energy with 5 Hz
frequency and 5.65 J/cm2 at a spot size of 10µm. During both runs the carrier gas was at a flow rate of
600 ml He/min. The minor differences in the settings are not considered to have affected the
compatibility of the data from both thin sections.
Apatite
The apatites were primarily investigated using the scanning electron microscope to identify possible
zonation and textural relationships to other mineral phases. In the LA-ICP-MS the apatites were
analyzed for trace elements and REE, as well as major elements. The spot size and laser intensity were
determined based on the properties of the thin section and set to 5 HZ and 10 µm. The data was
standardized with NIST 610. The determination of Na content was performed by quantitative analysis
(100 sec. and cobalt calibration) in the SEM.
The data from the LA-ICP-MS analysis was examined in GLITTER, were peaks were filtered and
intervals chosen in reference to the quality of the signal. The filtered data was then extracted from
GLITTER and inserted into excel to provide chondrite normalized REE-plots according to Evensen et
al. (1978).
13
Results
Sample description
The textural description is based on both thin sections from the rock sample JH2 that in this report
have been labeled “sample JH2” and “sample JH2b” to distinguish between the data from the both thin
sections. The textural appearance is, however, common for both samples. The Rektorn sample will be
described separately.
Sample description JH2
In the hand sample the apatite appears as bands,
almost like a foliation within the broader
magnetite streaks (fig. 5). In some places the
apatite has a pinkish color in contrast to its
overall white appearance. In thin section the
streaks of magnetite and apatite can be
identified as darker (magnetite) and lighter
(apatite) bands that display almost a gneiss-like
appearance (fig. 6). In the scanning electron
microscope the apatite and magnetite appear as
massive streaks (fig. 7). There are, however,
spots of magnetite in the apatite streaks as well
as apatite crystals within the magnetite streaks.
Monazite occurs over the entire sample, but is
predominantly present in the apatite streaks.
Both dolomite and talc is present as minor
constituents (fig. 8). There is abundant monazite
within the apatite crystals (fig. 9) and microcracks in the sample are mostly filled with talc.
Figure 5. Hand sample of JH2 from the Kiirunavaara
magnetite-apatite ore.
Figure 6. Scanned thin section of the JH2 sample. The
dark phase constitutes magnetite and the light phase is
apatite.
14
Figure 7. BSE image of the apatite streak in sample JH2 where magnetite, apatite, monazite, dolomite, and talc is visible.
Ap
Mt
Dol
Mnz
Tlc
Figure 8. Cross polarized image of sample JH2b with
dolomite, apatite, magnetite, and talc.
Figure 9. Cross polarized image of a large apatite grain
with abundant monazite inclusions in sample JH2b.
Apatite
The apatite appears as disseminated grains within the magnetite streak. In the apatite streak the apatite
crystals are of varying size and the shape varies between euhedral and subhedral form. The grainsize
of the apatite varies and in some places it is difficult to discern a prominent boundary. Most of the
apatite crystals display zonation in BSE images with a lighter core and a darker rim, and throughout
the sample the thickness of the darker rim differs and is not necessarily evenly distributed around the
core. The apatite most often has abundant inclusions of monazite. The monazite grains that occur
15
within the lighter cores are small (~1-5µm). In between many of the smaller apatite grains a triple
junction is present (fig. 10A, B).
Figure 10. A) Visible triple junctions between apatite crystals in a plane polarized image from sample JH2b. B) Cross
polarized image of 120/120/120 degree junctions. C) Cross polarized image of an apatite grain with elongated monazite
inclusions growing in a preferred orientation along the apatite crystal axis.
Monazite
In the apatite streaks the monazite occurs as disseminated smaller (1-5µm) anhedral grains within the
apatite crystals (within the lighter core). Also, the monazite occurs as larger (40-100µm) anhedral to
subhedral grains at the grain boundaries between the apatites. The monazites within the magnetite
bands most often occur in contact with an apatite crystal and are generally sporadic. No zonation can
be detected in the monazite grains. The monazite grains in the larger apatite crystals in the sample are
elongated and grow in a preferred orientation (fig. 10C), following an apatite crystal axis. The
16
monazite inclusions within the larger apatites display a cluster appearance within the core of the
apatite grain (fig. 11). The rims are inclusion free.
Figure 11. Plane polarized image of an apatite crystal with a cluster of monazite inclusions at the center of the apatite
crystal.
Magnetite, dolomite, and talc
The magnetite consists of streaks as well as separate anhedral crystals. The magnetite has sporadic
inclusions of both apatite and monazite. Magnetite also occurs as inclusions in the apatite. When
viewing the magnetite in reflective light no alterations or damages in the crystals are visible (fig. 12).
The magnetite appears homogeneous, and the grains are mostly subhedral.
Dolomite occurs throughout the sample, mostly as separate anhedral grains ~50-300 µm, but also as
larger grains with inclusions of all other phases. Where the dolomite occurs in the apatite bands it is
filling spaces between apatite grains (fig. 13B). In the magnetite streak it occurs as separate grains.
Talc occurs in smaller euhedral grains both in the grain boundaries between apatite crystals and as
inclusions in the dolomite. It also dominates as fracture mineralization in the sample.
17
Figure 12.Reflective light image of magnetite in sample JH2b.
18
A
B
Tlc
Mag
Dol
Ap
Mnz
Figure 13. BSE images of A) Apatite zonation with a darker depleted rim. Also, visible abundant monazite inclusions in a
large apatite grain. B) Visible micro-cracks and mineral phases in sample JH2b. In order from lightest to darkest phase:
monazite, magnetite, apatite, dolomite, and talc.
19
Sample description Rektorn
The sample is derived from Rektorn, east of Kiirunavaara, from an apatite vein crosscutting the ore
(fig. 14). In thin section the sample displays an apatite mass with thin veins of dark iron-oxide (fig.
15). Some larger crystals of calcite/dolomite are visible. The minerals found in the sample are apatite,
monazite, xenotime, ilmenite/hematite, chalcopyrite, rutile, dolomite, calcite, and quartz. The
inclusions in the sample comprise of monazite in all other phases, ilmenite and chalcopyrite foremost
in the apatite and the dolomite. In the scanning electron microscope the apatite appears massive in
some areas and in others granular with prominent grain boundaries. Most notable in BSE images are
the abundant inclusions of lighter phases within the apatite and the iron-oxide veins crosscutting the
apatite.
Figure 14. Apatite vein cutting the Rektorn apatite-magnetite
ore.
20
Figure 15. Scanned thin section of the Rektorn sample.
Apatite
The apatite is both massive and granular throughout the sample. Where the grain boundaries of the
apatite are visible grains display zonation (fig. 16) with a lighter core and a darker rim. The apatite
contains abundant inclusions, mainly monazite and xenotime but also ilmenite.
Figure 16. Apatite zonation with abundant monazite inclusions and needle mineralization of chalcopyrite.
Monazite and xenotime
The xenotime occurs as a few larger crystals (100-200 µm), but mostly euhedral smaller grains (10-50
µm). The monazite also appears in this way, but is generally smaller (10-30 µm) than the xenotime. In
some places the xenotime and monazite grow together (fig. 17), both joint at one crystal surface and as
inclusions in one another. In some grains the monazite displays zonation (fig. 18). Both monazite and
xenotime appear as inclusions (~5-10 µm) within the apatite grains.
Figure 18. Zonation in a monazite grain from
Rektorn.
Figure 17. Monazite and xenotime growing together.
The lighter phase represents the monazite.
21
Calcite and dolomite
Calcite with dolomite exsolutions (fig. 19)
occurs in relatively small abundance and is
restricted to fewer larger crystals as well as
scattered thin veins across the sample. There is
no “pure” calcite or dolomite in the sample,
dolomite always appears as exsolutions within
the calcite.
Figure 19. Zoomed image of a calcite grain (light gray
phase) with exsolutions of dolomite (darker phase).
Hematite/Ilmenite, chalcopyrite, rutile, and quartz
The hematite and ilmenite occur in a solid solution state where the hematite appears as inclusions
within the ilmenite (fig. 20). The Fe-Ti oxide combination constitute thin veins crossing the apatite
mass. Also, there are inclusions of ilmenite within all phases in the sample as well as larger grains (50100 µm) that predominantly are euhedral.
The chalcopyrite occurs as needle-like crystals (fig. 16) between apatite grain boundaries in isolated
areas of the sample. A few larger crystals are present (~200µm). Furthermore, there are some
inclusions of the sulphide within the apatite.
The quartz is of relatively low abundance and occurs in association with calcite/dolomite. The crystals
are of no apparent crystal shape.
Rutile is also present in the iron oxide streaks. It often appears with needles of ilmenite/hematite
intruding the crystal and often as inclusions within the ilmenite/hematite (fig. 21).
Dol/Cal
Rt
Ilm
Rt
Mnz
Mnz
Hem
Figure 21. BSE image of iron-oxide streak in the Rektorn
sample with rutile. Some rutile occur with needle
inclusions of ilmenite/hematite.
Figure 20. BSE image of ilmenite with hematite in the
Rektorn sample.
22
Monazite results
The results of the in situ U-Pb dating of the monazite data is divided into three sections where the data
from the first run, the data from the second run, and the combined data from both runs are presented.
When plotting the calculated ages for the JH2 samples in a probability density plot, two peaks are
indicated at two separate ages (fig. 22).
Figure 22. Probability density plot of age calculated monazites from the Kiirunavaara ore (sample JH2 and JH2b),
displaying two age groups at ca. 1600 Ma and ca. 1730 Ma. Bad signal values are excluded from the plot.
Furthermore, when all calculated ages are plotted in one Concordia diagram no probable York-fit age
is found, and consequently the two peaks in the density plot are considered to be consistent with two
distinct groups of monazites. The groups are further on referred to as “α-group” (blue in Concordia
diagram ellipsoids) and “β-group” (black in Concordia diagram ellipsoids) where the α-group
represents monazites of a younger age relative to the β-group (please observe that the naming of these
groups are in no way related to any interpretation of their genesis or other association than U-Pb age).
The red values in the diagrams represent data points that were excluded from the age calculation due
to bad quality of the signal during the analysis. 32 monazites from the Kiirunavaara deposit, as well as
3 monazites and 5 xenotimes from the Rektorn ore body, were analyzed for Th, U, and Pb. All values
are presented with 1σ error and are displayed in Table 1 with 208Pb/232Th, 207Pb/235U, 206Pb/238U, and
207
Pb/206Pb ages and ratios as well as Th and U content and Th/U ratio. All investigated monazites,
except two grains (MLM01a and Rektorn m16), contains <1 wt% ThO2 (table 1.).
23
Monazite data from sample JH2
The U-Pb Concordia diagram from sample JH2 is displayed in Figure 23. The younger α-group
monazites (blue color) yields an upper intercept age of 1640±17 Ma with a MSWD = 0.45 and a
Probability of fit = 0.64. The older group of β-group monazites (black color) yields an upper intercept
age of 1720±12 Ma with a MSWD = 0.38 and a Probability of fit = 0.89. Both discordia lines have a
lower intercept that is within error of zero (-894±1300Ma and -837±1100Ma) which can be concluded
to not indicate a late lead loss.
Figure 23. U-Pb Concordia diagram with discordia lines of in situ monazite dating from sample JH2
(Kiirunavaara). The blue values represent the α-group and the black values represent the β-group. The
red values are excluded data points due to bad quality of the analysis.
In Figure 24 all age calculated monazites are marked by position in the sample. The α-group
monazites are marked by a blue dot and the β- group monazites are marked by a red dot. There is no
strong connection between the positioning of monazites in the different groups and the mineral
domains in the sample.
24
Figure 24. JH2 thin section with marked position of α-group monazites (blue) and β-group monazites (red). The dark
streak constitutes magnetite and the light streaks are predominantly apatite.
25
Monazite data from sample JH2b
The U-Pb Concordia diagram from sample JH2b is displayed in Figure 25 and shows an α-group with
an upper intercept age of 1603±24Ma with a MSWD=0.20 and a Probability of fit=0.98 (blue color).
The β-group yields an upper intercept age of 1712±27Ma with a MSWD=0.25 and a Probability of
fit=0.91(black color). There is no indication of late lead loss.
Figure 25. U-Pb Concordia diagram with discordia lines of in situ monazite dating from sample
JH2b (Kiirunavaara). The blue values represent the α-group and the black values represent the βgroup. The red values are excluded data points due to bad quality of the analysis.
In Figure 26 all age calculated monazites are marked by position in the sample. The α-group
monazites are marked by a blue dot and the β- group monazites are marked by a red dot. There is no
strong connection between the position of the different groups and the mineral domains in the sample,
only a slight tendency of the α-group monazites to occur within the magnetite streak.
26
Figure 26. JH2b thin section with marked position of α-group monazites (blue) and β- group monazites (red). The
dark streak constitutes magnetite and the light streaks are predominantly apatite.
27
Combined data from JH2 and JH2b
The combined data from sample JH2 and JH2b is joined in Figure 27. The combined diagram yields
an upper intercept α-group of 1624±23Ma and an upper intercept β-group of 1718±12Ma.
Figure 27. U-Pb Concordia diagram with discordia lines of combined data from sample JH2 and
JH2b. The blue values represent the α-group and the black values represent the β-group.
28
Rektorn
In the Rektorn sample 4 monazites and 5 xenotimes are investigated in 10 data points. The monazite
and xenotime results yield a concordia age of 1721±19 Ma with a probability of fit=0.99 and a
MSWD=0.00025 (fig. 28). Black ellipsoids represent monazite data points and blue ellipsoids
represent xenotime data points.
Figure 28. U-Pb Concordia diagram of in situ dating of monazite and xenotime from the Rektorn
sample. Black ellipsoids represent monazite data points and blue ellipsoids represents xenotime
data points.
29
Table 1. Th and U concentrations (ppm),Th/U ratio, and ThO2 content from sample JH2, JH2b and Rektorn. Also
208
Pb/232Th, 207Pb/235U, 206Pb/238U, and 207Pb/206Pb ratios, ages, and related errors used in the age calculation. The red
values represent excluded values in the age calculations.
30
Apatite results
The apatite analysis performed displays a concentration difference between the lighter cores and the
darker rims of the apatite. The rims are relatively depleted in LRRE in comparison to the cores, and all
sample spots demonstrate a Europium anomaly (fig.29). The data is chondrite normalized (Evensen,
Hamilton, & O'nions, 1978).
Figure 29. REE-diagram of trace element analysis of core and rim parts of apatite in sample JH (Chondrite
normalized).
In Table 2 both major and trace element concentrations from the apatite analysis are presented. The
red values represent those elements that display a varying content between rim and core parts of the
apatite. The elements concerned are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium
(Nd), and samarium (Sm). The table also demonstrated a difference in common Pb, Th, U, and As
between core and rim. The rim is relatively depleted also in these elements.
31
The rims of the apatites furthermore display sodium (Na) depletion relative to the core. In Figure 30
the Na2O content of the core and rim part of 8 apatite grains in sample JH2 is plotted.
Figure 30. Scanning Electron Microscope analysis of Na2O content (wt%) in rim and core parts of 8 apatite
grains at Kiirunavaara, sample JH2.
32
Table 2. LA-ICP-MS analysis of major (wt %) and trace element content (ppm) from core and rim parts of apatite in
sample JH2. The red values represent element concentrations which fluctuate between core and rim parts of the sample.
33
Discussion
U-Pb ages of monazites
The U-Pb ages obtained from this study indicate that the Kiirunavaara ore body has been subjected to
some kind of activity at a much later stage than the previous age constraints of the ore (~1.9-1.8 Ga)
have suggested. What this study also demonstrates is that there is a possibility of two separate events
affecting the ore, displayed in the younger α-group ages of 1624 Ma (which corresponds to the
monazite dating of Westhues et al. 2014) and the older β-group ages of 1718 Ma (new ages in the
Kiirunavaara ore). The β-group older age of 1718±12 Ma could possibly be related to the same event
that is indicated by fracture mineralization at Malmberget, where monazite dating has yielded ages of
1740 Ma (Romer, 1996). Titanite ages, also derived from fracture mineralization at Malmberget, have
further yielded ages of 1620-1613 Ma (Romer, 1996) which are within error of the α-group monazites
of this study (1624±23 Ma). The younger ages also corresponds to the monazite age presented by
Westhues et al. (2014). In other words, the ages produced in this report are not unique within the
Svecofennian Province of Sweden, they are, however, surprising regarding the Kiirunavaara ore body
that previously has been assumed to be related to the processes that produced the host rock (Cliff &
Rickard, 1992; Frietsch, 1978).
The Svecofennian surface rocks have shown isotopic evidence of low grade metamorphism at 1.6-1.5
Ga in Norrbotten that pertains to late stages of the Svecokarelian orogeny (Lundqvist et al., 2011). It is
possible that this metamorphism caused reactivation of faults and cracks while enabling hydrothermal
fluids to penetrate rocks in the area. The question is to what extent these hydrothermal fluids could
react and change the rock and the ore (assuming the ore was deposited pre-hydrothermal penetration).
From the textural study in this report the same alteration as earlier have been found by Harlov et
al.(2002) is identified, that is the apparent leaching of apatite causing a LREE, Na, Th, U, and Pb
depleted rim as well as monazite growth at grain boundaries. Additionally, monazite growth in larger
apatite grains are elongated and unidirectional which corresponds to the observations of Harlov et al.
(2005) and Pan et al. (1993), as well as the experimental study of dissolution-reprecipitation by Harlov
et al. 2005. All monazites investigated have a ThO2 content below 1wt% (except two monazites with
1.28 wt% and 1.45 wt% ThO2)which indicates that they are derived from hydrothermal activity
(Schandl & Gorton, 2004). The monazites with a higher ThO2 content is still well below the 3 %
boundary for igneous monazites (Schandl & Gorton, 2004) and are also considered to be of
hydrothermal origin.
Cliff et al (1990) and Cliff & Rickard (1992) provide data that strongly suggests hydrothermal activity
in the Kiruna area; “The isotope data presented suggest a postdepositional geochemical disturbance
which involved a wide range of elements on a scale larger than the ore-body itself.”(Cliff & Rickard,
1992). They have based this on isotopic studies that display a system resetting at this time, but they
also conclude that the resetting is more severe in the ore than in the host rock that is only partially
reset. If the isotopic data is poorly constrained, it is possible that the severe geochemical event that is
described by Cliff & Rickard(1992) to appear at 1500 Ma is connected to the α-group monazites found
in this study.
The Rektorn U-Pb concordia age of 1721±19 Ma adds additional probability to an event that can be
connected to both Kiirunavaara and Malmberget, thereby further implying an event of regional scale.
Romer (1996) has also suggested that the event causing the 1740 Ma fracture mineralization at
Malmberget is of regional scale due to dating of rocks with equivalent ages in the Arjeplog-Sorsele
area.
34
In the Rektorn sample some monazites display zonation that could indicate multi stage growth,
recrystallization or lead difusion (Scherrer et al., 2000). The zonation indicates that there has been
processes effecting the apatite vein after deposition and this is consistent with the zonation of the
apatite in the sample. The depleation of the apatite, as previously mentioned, is considered to be
related to hydrothermal leaching and the zonation of the apatite could indicate an extended period of
fluid activity causing multi stage growth of the monazite. It is probable that the same alterations found
in the apatitie vein also can be found in the ore body itself.
A closer look at the Th and U content as well as the Th/U ratio in the samples displays a higher Th/U
ratio for the Rektorn sample relative to the JH2 sample. The difference is caused by lower U and
higher Th contents in the Rektorn monazites. The uptake of U in monazite is generally limited and the
mineral more readily incorporates Th. When there is xenotime in the system the xenotime
preferentially incorporates U due to its crystal structure that more easily can incorporate the larger U
isotope. Furthermore, the Th isotope is more suitable for the monazite crystal structure (Catlos, 2013).
Hence, there is a difference between JH2 and Rektorn due to the lack of apparent xenotime in the JH2
system, but a quite large amount in the Rektorn sample.
A hydrothermal system necessary to account for a regional isotopic resetting, as well as severe
recrystallization and extensive monazite growth, would most likely require tectonic processes of
regional scale. Therefore, the timing of the monazite growth at Kiruna is probably contemporaneous
with other major emplacemetns in Sweden. It could for example be linked to the Trans Scandinavian
Igneous Belt (TIB), a belt of rhyolitic porphyries and granitic intrusions that was emplaced around
1.83-1.65 Ga (Gorbatschev & Bogdanova, 1993). The time span mentioned for TIB emplacement is in
the range of the ages discussed in the present study and there are TIB related rocks in the ArjeplogSorsele area. As previously mentioned, Romer (1996) connects his dating at Malmberget with dating
performed on rocks in the Arjeplog-Sorsele area by Hålenius & Smellie(1995). TIB has been
suggested to have three pulses of different ages; TIB 1: 1.81-1.77 Ga, TIB2: 1.7 Ga, and TIB 3:1.681.65 Ga (Larson & Berglund, 1992). TIB 2 and TIB 3 can be related to the two phases of hydrothermal
activity suggested to have affected the Kiirunavaara ore in this report.
When investigating a possible systematics in the occurrence of young versus old monazites and
mineral domains in the samples, no apparent trend can be discerned. There is a slight tendency of the
α-group monazites to occur in association with magnetite in sample JH2b, however, the number of
data points aquired in the sample is not regarded to satisfy a statistical certainty of a trend.
Evaluation of data quality
The thin section used for the analysis of JH2 was poorly polished and very thin. This could affect the
investigation in the SEM because small zonations in the monazite would be impossible to discern.
When evaluating the standards used for the monazite analysis during run 1 an instrumental drift was
discovered in Th and U contents. To compensate for the drift individual standards were picked for
each batch to minimize the differences in counts between the standards and the batch. The errors are
affected, but the results are considered to be valid because of the good quality of the sampled data
points from JH2.
During run 2 there was a disturbance, (primarily detected in the standard data) that was, after the
analysis, identified to be the lack of an expansion chamber. Because of this, elements preferentially
entered the ICP-MS according to weight, creating a cyclic appearance of the sampling signal. In many
standard points this caused a very high error percentage of up to 6 %. The quality of the standard
35
analysis decreased during the run and partly resulted in unusable data and over all caused relatively
high errors. However, the monazites from sample JH2b gave very good signals with low errors, and no
drift was detected during the run. Thereby, the results that are presented are considered to be reliable.
The monazites and xenotime analyzed in the Rektorn sample are some of the data points that are
affected by poor standards causing high errors, and when the age is calculated the MSWD becomes
low, indicating underestimated errors. It causes a higher uncertainty in the analysis, but as mentioned
above, the quality of the signals from the monazites and xenotimes are of good standard and are thus
regarded to be valid.
A few of the data points acquired during the analysis were excluded (red ellipsoids in discordia plots)
due to a poor data quality. This distinction was made by evaluating the error of the analysis and the
discordance of the data point. The same prerequisites were used on double data points from a single
monazite grain and the best sample point was used in the results.
Further studies
Early on in the apatite trace element analysis it was clear that there were not suffiicient U, Th, or Pb to
perform U-Pb dating of the apatite and this study has focused on the trace element analysis. However,
it would be of great interest to date both the apatite and the inclusions of monazite within the apatite
core (at present the monazites within the core are too small for LA-ICP-MS dating). This data could
present new understanding of the mechanisms that have affected the ore (possibly also related to the
deposition of the ore) and yield a tighter constraint on the timing of the hydrothermal events in the
area.
It would be useful to investigate, in further detail, the trace element concentration in the monazite of
Kiirunavaara to establish more traces of the hydrothermal fluids that deposited the minerals. There is
still ambiguity as to the processes that gave rise to the Kiruna type magnetite-apatite ores and the
varying ages that have been obtained both in this study and in others display a perhaps more intricate
history than previously expected.
36
Conclusion
The monazites investigated from the Kiirunavaara ore deposit in northern Sweden displays two
separate groups of different ages. The younger group yields an upper intercept age of 1624±23 Ma and
the older group yields an upper intercept age of 1718±12 Ma, the latter new to the Kiirunavaara ore
body. The monazites and xenotimes analyzed from an apatite vein in the Rektorn ore deposit yield a
concordia age of 1721±19 Ma that is considered to be related to the older event seen in the monazites
from Kiirunavaara, and possibly also 1740 Ma fracture mineralization at Malmberget. The younger
age found in the monazites at Kiirunavaara could further be related to titanite mineralization at
Malmberget dated by Romer (1996) to 1620-1613 Ma. The monazites investigated in this study are
interpreted to be caused by two hydrothermal events generated by large scale tectonic processes,
possibly by late stages of the Svecokarelian orogeny. The hydrothermal events can be connected to
fracture mineralization at Malmberget, as well as regional isotopic resetting, and are thereby
considered to be of regional scale.
Acknowledgements
Firstly I want to thank Johan Hogmalm for providing me with a project once again and Thomas Zack
for constructive comments and the examination of my report. I want to thank Ulf B. Andersson for
providing samples for this project, and Emily Whitehurst Zack for the thin section preparation of my
samples. Next I want to thank my lovely friends who always find the time in their hectic schedules to
listen to my problems and rants; Johanna Engelbrektsson, Hannah Berg, and Marika Sunesson, you are
and always will be fabulous! Last but not least, my amazing love Eric Ackevall, without your support
and help I would never have been able to finish this project!
37
References
Aleinikoff, J. N., Schenck, W. S., Plank, M. O., Srogi, L. A., Fanning, C. M., Kamo, S. L., &
Bosbyshell, H. (2006). Deciphering igneous and metamorphic events in high grade rocks of
the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered
electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite. Geological
Society of American Bulletin, 118, 39-64.
Bergman, S., Martinsson, O., & Persson, P.-O. (2002). U-Pb zircon age of a metadiorite of the
Haparanda suite, northern Sweden. In S. Bergman (Ed.), Radiometric dating results 5 (pp. 611). Uppsala: Sveriges geologiska undersökning C 834.
Billström, K., Bergman, S., & Martinsson, O. (2002). Post-1.9 Ga metamorphic, mineralization and
hydrothermal events in northern Sweden. GFF, 124(4), 228-228.
Billström, K., & Martinsson, O. (2000). Links between epigenetic Cu-Au mineralization and
magmatism/deformation in the Norrbotten county, Sweden. Abstract volume and field trip
guide, 2nd GEODE-Fennoscandian shiled field workshop on Paleoproterozoic and Archean
greenstone belts and VMS districts in the Fennoscandian Shield (Vol. 2000:06, pp. 6). Luleå
University of Technology: Department of Environmental Engineering Division of Applied
Geology.
Boatner, L. A. (2002). Synthesis, Structure, and Properties of Monazite, Pretulite, and Xenotime.
Reviews in Mineralogy & Geochemistry, PHOSPHATES-Geochemical, Geobiological, and
Materials Importance, 48, 87-116.
Catlos, E. J. (2013). Review: Versatile Monazite: resolving geological records and solving challenges
in materials science. Generalizations about monazite: Implications for geochronologic studies.
American Mineralogist, 98(5-6), 819-832.
Chew, D. M., Sylvester, P. J., & Tubrett, M. N. (2011). U–Pb and Th–Pb dating of apatite by LAICPMS. Chemical Geology, 280(1), 200-216.
Cliff, R. A., & Rickard, D. (1992). Isotope systematics of the Kiruna magnetite ores, Sweden; Part 2,
Evidence for a secondary event 400 my after ore formation. Economic Geology, 87(4), 11211129.
Cliff, R. A., Rickard, D., & Blake, K. (1990). Isotope systematics of the Kiruna magnetite ores,
Sweden; Part 1, Age of the ore. Economic Geology, 85(8), 1770-1776.
Evensen, N., Hamilton, P., & O'nions, R. (1978). Rare-earth abundances in chondritic meteorites.
Geochimica et Cosmochimica Acta, 42(8), 1199-1212.
Frietsch, R. (1978). On the magmatic origin of iron ores of the Kiruna type. Economic Geology, 73(4),
478-485.
Frietsch, R. (1979). Petrology of the Kurravaara area, northeast of Kiruna, northern Sweden (pp. 62):
Sveriges Geologiska Undersökning. Cited in Romer (1994).
Geijer, P. (1910). Igneous rocks and iron ores of Kiirunavaara, Luossavaara and Tuollavaara.
Economic Geology, 5(8), 699-718.
Gorbatschev, R., & Bogdanova, S. (1993). Frontiers in the Baltic Shield. Precambrian Research(64),
3-21. Cited in Andersen et al. 2002.
Gulson, B. L. (1972). The Precambrian geochronology of granitic rocks from northern Sweden.
Geologiska Föreningens i Stockholm Förhandlingar, 94, 229-244. Cited in Romer et al. 1994.
Harlov, D. E., Andersson, U. B., Förster, H.-J., Nyström, J. O., Dulski, P., & Broman, C. (2002).
Apatite–monazite relations in the Kiirunavaara magnetite–apatite ore, northern Sweden.
Chemical Geology, 191(1), 47-72.
Harlov, D. E., Wirth, R., & Förster, H.-J. (2005). An experimental study of dissolution–reprecipitation
in fluorapatite: fluid infiltration and the formation of monazite. Contributions to Mineralogy
and Petrology, 150(3), 268-286.
Harrison, T. M., Catlos, E. J., & Montel, J. (2002). U-Th-Pb Dating of Phosphate Minerlas. Reviews in
Mineralogy & Geochemistry, PHOSPHATES-Geochemical, Geobiological, and Materials
Importance, 48, 523-552.
Hattendorf, B., & Gunter, D. (2014). Laser Ablation Inductively Coupled Plasma Mass Spectrometry
(LA-ICPMS). In G. Gauglitz & D. S. Moore (Eds.), Handbook of Spectroscopy (2nd ed., pp.
647-697): Wiley-VCH Verlag GmbH & Co. KGaA.
38
Hughes, J. M., & Rakovan, J. (2002). The Crystal structure of Apatite, Ca5(PO4)3(F,OH,Cl). Reviews
in Mineralogy & Geochemistry, PHOSPHATES-Geochemical, Geobiological, and Materials
Importance, 48, 1-11.
Hålenius, U., Smellie, J. A. T., & Wilson, M. R. (1995). Uranium genesis within the ArjeplogArvidsjaur-Sorsele uranium province, norhtern Sweden. Luleå: Swedish Geological Company.
Cited in Romer (1996).
Larson, S. Å., & Berglund, J. (1992). A chronological subdivision of the Transscandinavian Igneous
Belt - three magmatic episodes? Geologiska Föreningens i Stockholm Förhandlingar, 114(4),
459-461.
Ludwig, K. R. (2011). Isoplot/Ex Version 4: A Geochronological Toolkit For Microsoft Excel.
Berkeley: Geochronology Center.
Lundqvist, J., Lundqvist, T., Lindström, M., Calner, M., & Sivhed, U. (2011). Sveriges Geologi- Från
Urtid till Nutid. Lund: Studentlitteratur AB.
Pan, Y., & Fleet, M. E. (1996). Rare earth element mobility during prograde granulite facies
metamorphism: significance of fluorine. Contributions to Mineralogy and Petrology, 123(3),
251-262.
Pan, Y., Fleet, M. E., & Macrae, N. D. (1993). Oriented monazite inclusions in apatite porphyroblasts
from the Hemlo gold deposit, Ontario, Canada. Mineralogical Magazine, 57(389), 697-708.
Parák, T. (1975). Kiruna iron ores are not" intrusive-magmatic ores of the Kiruna type". Economic
Geology, 70(7), 1242-1258.
Rakovan, J. (2002). Growth and Surface Properties of Apatite. Reviews in Mineralogy &
Geochemistry, PHOSPHATES-Geochemical, Geobiological, and Materials Importance, 48,
51-81.
Richter, M. (2013). Evaluation of U-Th dating of monazite by LA-ICP-MS. M.Sc., Johannes Gutenberg
University, Mainz, Germany.
Romer, R. L. (1996). U-Pb systematics of stilbite-bearing low-temperature mineral assemblages from
the Malmberget iron ore, northern Sweden. Geochimica et cosmochimica acta, 60(11), 19511961.
Romer, R. L., Martinsson, O., & Perdahl, J. A. (1994). Geochronology of the Kiruna iron ores and
hydrothermal alterations. Economic Geology, 89(6), 1249-1261.
Schandl, E. S., & Gorton, M. P. (2004). A textural and geochemical guide to the identification of
hydrothermal monazite: criteria for selection of samples for dating epigenetic hydrothermal
ore deposits. Economic Geology, 99(5), 1027-1035.
Scherrer, N. C., Engi, M., Gnos, E., Jakob, V., & Liechti, A. (2000). Monazite analysis; from sample
preparation to microprobe age dating and REE quantification. Schweizerische Mineralogische
und Petrographische Mitteilungen, 80(1), 93-105.
Seydoux-Guillaume, A. M., Wirth, R., Deutsch, A., & Schärer, U. (2004). Microstructure of 24-1928
Ma concordant monazites; implications for geochronlogy and nuclear waste deposits.
Geochimica et Cosmochimica Acta, 68 (11), 2517-2527.
Seydoux-Guillaume, A. M., Wirth, R., Nasdala, L., Gottschalk, M., Montel, J.-M., & Heinrich, W.
(2002). An XRD, TEM and Raman study of experimentally annealed natural monazite.
Physics and Chemistry of Minerals, 29(4), 240-253.
Skiöld, T. (1988). Implications of new U-Pb zircon chronology to Early Proterozoic crustal accreation
in northern Sweden. Precambrian Research, 38, 147-164.(in Romer et al. 1994).
Smith, M. P., Storey, C. D., Jeffries, T. E., & Ryan, C. (2009). In situ U–Pb and trace element analysis
of accessory minerals in the Kiruna district, Norrbotten, Sweden: new constraints on the
timing and origin of mineralization. Journal of Petrology, 50(11), 2063-2094.
Westhues, A., Hanchar, J. M., & Whitehouse, M. J. (2014). The Kiruna apatite iron oxide deposits,
Sweden - new ages and isotopic constraints. Goldschmidt Abstracts.
Williams, I. S., Buick, I. S., & Cartwright, I. (1996). An extended episode of early Mesoproterozoic
metamorphic fluid flow in the Reynolds Range, central Australia. Journal of Metamorphic
Geology, 14(1), 29-47.
39