Geochemistry, mineral chemistry and pressure–temperature conditions of the Jõhvi magnetite quartzites and magnetite­rich gneisses, NE Estonia

. The Jõhvi magnetite quartzites (MagQ) occur as subvertical beds with a complicated structural outline in biotite­garnet­ cordierite and pyroxene gneisses which in places also contain high concentrations of iron. Drill core study shows that the complex of MagQ and magnetite­rich gneisses may be up to 100 m thick. The MagQ provide a wide range of chemical composition: SiO 2 ranges between 40.3 and 60.1 wt%, Al 2 O 3 between 1.7 and 19.7 wt% and total iron between 15 and 45.2 wt%. This study also revealed unusually high manganese contents of 1–6 wt%. The rare earth element (REE) patterns of MagQ and the surrounding gneisses partly overlap. Cutting granitoids form two different REE patterns. Magnetite occurs as anhedral grains elongated along rock fabric, as rounded inclusions in other minerals or as tiny platelets along grain edges and along cleavage planes of amphibole and biotite. Sulphides are present as pyrite, pyrrhotite and other minor sulphide minerals (chalcopyrite, galena and sphalerite). Analysis of the magnetite grains from drill core J­1 shows that classifying Jõhvi magnetites into a certain deposit type is not unambiguous. The garnet


INTRODUCTION
The crystalline basement of Estonia represents one of the least known parts of the East European Craton (Soesoo et al. 2004(Soesoo et al. , 2006;;Bogdanova et al. 2015).It consists of Palaeoproterozoic highgrade metamorphic (amphibolite to granulite facies) and igneous rocks that are covered by 100-700 m of Palaeozoic and Neoproterozoic sediment ary rocks.Accordingly, the available geological informa tion on the crystalline basement in Estonia comes mostly from studies of drill core material and geophysical measurements.During the 1950s-1980s, the Estonian basement rocks were extensively drilled and studied for ore perspective.
From a series of magnetic anomalies established in the Estonian basement the most striking is the Jõhvi mag netic anomaly discovered already in 1924 (Fig. 1).These anomalies are mainly caused by magnetiterich rocks (Jõhvi and Sakusaare anomalies) and sulphide-graphite bearing gneisses (Uljaste, Haljala and Assamalla anomalies).The first two drill holes, J1 (505.03 m) and J2 (721.5 m), were drilled into the Jõhvi magnetic anomaly during 1937-1939 by the company 'Magna'.The studies of drill cores revealed a thick iron ore formation whose extent, unfortunately, remained unknown.During the Soviet times, several exploratory drill holes were drilled into the Jõhvi magnetic anomaly.However, most of the research stopped in the late 1980s.
All these Jõhvi drill cores but J1 and J2 have been lost due to fire in a drill core shed.On the basis of previous information (now more than 40 years old) it was established that the Jõhvi magnetite quartzites (MagQ) occurred as subvertical beds in garnetcordierite and/or pyroxene gneisses.The surrounding gneisses may in places also contain a high concentration of iron.
Earlier drillings have shown that the complex of mag netiterich rocks including MagQ and gneisses (ironrich gneisses) is about 100 m thick, and the reserves of iron ore (Fe over 25%) are about 355 million tonnes if calculated to a depth of 500 m, 629 million tonnes if calculated to a depth of 700 m and 1500 million tonnes if calculated to a depth of 1000 m (Soesoo et al. 2004).These estimates, however, are based on very limited data and need to be assessed by drilling in the future.During the last 30 years, no geological work has been conducted in the area, albeit the interest of international mining companies in exploration at Jõhvi has been high for decades.Although iron may yet not be of the highest economic interest, the sulphide mineralizationrelated base metals (Zn, Pb, Cu, Ag and others) are needed on the market and may thus be important for the Estonian economics in the future.
As the Jõhvi geological zone bears several geo chemical differences from the rest of the Estonian crystalline basement, the geology of the area has been revisited.During late 2019-early 2020, two new inclined holes were drilled in the vicinity of drill hole J1, unfortunately, the new material is not available yet.The main focus of this paper is to provide data on general geochemistry, mineral chemistry, ore microscopy and assess possible pressure-temperature (P-T) conditions of the Jõhvi iron ore formation.

GEOLOGICAL OVERVIEW OF THE ESTONIAN BASEMENT ROCKS AND THE POSITION OF THE JÕHVI ZONE
The Estonian Precambrian basement can be considered as a southern continuation of the Fennoscandian Shield rock complexes of the East European Craton.This basement comprises two major units: amphibolite facies rocks of northern Estonia, which are similar to the rocks of south ern Finland and mostly granulite facies rocks of southern Estonia.
Based on geophysical and petrological studies, six structuralgeological zones can be distinguished within these major units: the Tallinn, Alutaguse and Jõhvi zones, located in northern Estonia and the West Estonian, Tapa and South Estonian zones (Fig. 1; Puura et al. 1983;Soesoo et al. 2004Soesoo et al. , 2020)).The rocks with granulite meta morphic assemblages are found in several zones within the Estonian Precambrian basement.In the South Estonian and Jõhvi zones, the granulite facies mineral assemblages are still preserved, while in the Tapa and West Estonian zones the rocks were mostly retrogressed under amphibolite facies conditions (Fig. 1).
The Tallinn Zone is characterized by negative free air gravity and magnetic fields.The zone is bordered by a regional deformation zone in the southwest, which separates it from the West Estonian Zone (Fig. 1).Another, less prominent tectonic zone separates the Tallinn Zone from the Tapa Zone in the east.The rocks of the Tallinn Zone vary from mafic amphibolite facies metavolcanites to metasediments represented by amphi bole gneisses, biotiteplagioclase gneisses, quartzfeldspar gneisses, mica gneisses, and minor sulphidegraphite gneisses and MagQ.
The Alutaguse Zone is characterized by nearzero freeair gravity and slightly negative magnetic anomalies.Local positive anomalies are possibly associated with sulphidegraphite gneisses (black schists), quartzites, skarn carbonaceous rocks and pyroxene gneisses.The main rock types of the Alutaguse Zone are aluminarich gneisses (with biotite, cordierite, garnet, sillimanite) and biotiteplagioclase gneisses, less abundant are amphibole gneisses, amphibolites and quartzfeldspar gneisses.The rocks of the Alutaguse Zone have metamorphosed under the conditions of amphibolite facies.
The Tapa Zone is bordered by tectonic contacts from the Tallinn Zone in the west and the Alutaguse Zone in the east, while the southern contact with the West Estonian Zone is not so clearly defined (Fig. 1).The zone is char acterized by slightly positive gravity and frizzy magnetic anomalies.The main rock types comprise a sequence of alternating Fe and Sirich garnet-pyroxenebearing quartzites, highAl garnetcordieritesillimanite gneisses, and Carich and Capoor pyroxene, amphibole and biotite gneisses.Mineralogical assemblages indicate amphibolite and granulite facies metamorphism.
The West Estonian Zone is bounded by the NW trending tectonic zone and the E-Wstriking Middle Estonian Saaremaa tectonic zone.The zone is domi nated by metasedimentary rocks in the amphibolite to granulite facies, resembling somewhat the rocks in the southern part of the Bergslagen area in Sweden (Bogdanova et al. 2015).The main rock types in the West Estonian Zone are medium to finegrained amphibolites, biotiteplagioclase gneisses, and quartzfeldspar gneisses with minor pyroxene gneisses.The mineral assemblage of these rocks points to the hightemperature amphi bolite facies and in many places to the granulite facies metamorphism (Puura et al. 1983(Puura et al. , 2004;;Soesoo et al. 2020).
The South Estonian Zone (Fig. 1) comprises pre dominantly metaigneous rocks and minor metasediment ary rocks.This zone is characterized by a band of intensive gravity and magnetic anomalies embracing the southern part of Estonia and northern Latvia.The magnetic field is strongly differentiated, the anomalies are generally linear and trend E-W or N-S.The gravity field anomalies are coarsely mosaic.The boundary between the South and West Estonian zones generally follows the tectonic zones.The South Estonian Zone contains both mafic and felsic granulitic components -amphibole pyroxene and biotitehypersthene gneisses, and quartz feldspar gneisses.
The Jõhvi Zone is a narrow 20-30 km wide and 100 km long zone complex in NE Estonia, separated from the South Estonian Zone by the Alutaguse Zone amphibolite grade gneisses (Fig. 1).The zone is characterized by extremely strong (up to 19 300 nT in the Jõhvi magnetic anomaly) E-Wtrending magnetic anomalies and slight positive gravity anomalies (Plado et al. 2020).The complex consists of pyroxene gneisses, quartzfeldspar gneisses, biotiteplagioclase gneisses, amphibole gneisses, garnetcordierite gneisses (Vaivara complex) and MagQ (the cause of the Jõhvi magnetic anomaly).Migmatization is widespread, resulting in the formation of leucosomes and small granitoid veins and bodies with charnockitic and enderbitic compositions (Soesoo et al. 2006(Soesoo et al. , 2020)).Generally, these rocks have formed under the conditions of granulite facies metamorphism.Some of the rocks of this zone may have high ore potential.
The Jõhvi MagQ occur as subvertical beds with complicated internal structures in garnetcordierite or pyroxene gneisses.Seven out of historically drilled 17 drill holes in the area of the Jõhvi magnetic anomaly contain magnetiterich gneisses.Historical, as well as the present study, shows that the complex of MagQ may be up to 100 m thick.Due to complicated structural features (probably several folding epochs) and unorientated drill cores the real thickness of the ore zone needs to be established during future drillings and research.The section of drill core J1 is shown in Fig. 2 as an example of the Jõhvi iron formation.

MATERIAL AND METHODS
The descriptions of major rock types in the drill cores of the study area are based on about 500 thin sections and 11 hand specimens that had been selected and prepared as a part of the crystalline bedrock mapping during 1960-1980(Erisalu et al. 1969;;Puura et al. 1983).The present study includes samples from the following drill cores:  University of Technology, using the Bruker S8 XRF spectrometer on lithium metaborate fused glass disks and Thermo Xseries II quad rupole ICPMS on liquid samples prepared by lithium metaborate fusion and subsequent dissolution in dilute nitric acid.The ICPMS instrument at Tallinn University of Technology was calibrated using multielement standard solutions from Inorganic Ventures.For quality control, GeoPT certified proficiency testing materials GRI1 (granite) and TLM1 (tonalite) were used (www.geoanalyst.org/geopt).The two preserved drill cores (J1 and J2) out of 17 that were drilled in the area of the magnetic anomaly were reviewed and sampled.Bulk chemical composition was analysed in 56 samples of various rock types in Bureau Veritas Minerals, Canada, using fouracid digestion or lithium borate fusion together with inductively coupled plasma mass spectrometry (ICPMS) and inductively coupled plasma emission spectroscopy (ICPES).
Ore microscopy studies were carried out at the Department of Geology, Tallinn University of Technology on selected 40 polished thick sections from drill cores J1, J2, 315, F1, F13, F5 and F9, sampled and prepared in the 1960s during the crystalline bedrock mapping.The sections were repolished and examined using reflected light microscopy in planepolarized and crosspolarized incident light.As polished thin sections were not available for the particular samples, the study was focused on reflective ore minerals only.
Mineral chemistry was studied on samples from drill cores J1, J2, F1 and F12 using a scanning electron microscope with energy dispersive and wavelength dispersive spectrometry and laser ablation coupled ICPMS methods at the University of Tartu and Tallinn University of Technology in order to (1) establish compositional spectra of rockforming min erals; (2) investigate rock unit formation conditions; (3) compare the magnetite trace element fingerprint with other magnetiteore deposit types.
Altogether, 38 analysis from 5 magnetite grains were performed from the polished section of MagQ in drill core J1 (312.1 m) by laser ablation inductively coupled plasma mass spectrometry (LAICPMS) using Agilent 8800 quadrupole ICPMS coupled to a Cetac LSX 213 G2+ laser at the University of Tartu.The GSE2G certified reference material was used as an external calibration standard, and BCR2G was used as a quality control standard.
Pressure-temperature conditions that reflect the metamorphism environment at the time of equilibration were described from a set of selected samples.
The distribution of Fe 2+ and Mg was analysed in coexisting phases controlled by the following exchange reactions between garnet-biotite (a) and garnetclinopyroxene (b) end members:

ROCK DESCRIPTIONS
The studied samples can be divided into three major rock groups: (1) gneisses and ironrich gneisses, (2) MagQ and (3) granitoids.Gneisses, ironrich gneisses and MagQ can be divided according to the content of primary and accessory (lowcontent) minerals and magnetite into garnetpyroxeneamphibole gneisses, pyroxenegarnet, amphibolegarnetfeldsparpyroxene, ironrich gneisses and MagQ.The mineralogical composition of gneisses is quite variable, spanning from heavily migmatized biotite gneisses to garnetcordieriteandalusite gneisses.The content of ironrich minerals, primarily magnetite, is also quite variable in both MagQ and magnetiterich surround ing gneisses.

Aluminiumrich gneisses
Aluminiumrich gneisses are widely spread in the drill cores.They are alternating with other types of gneisses and have often smooth transitional boundaries.Granitic veins cutting all types of gneisses can also be seen.

Magnetite quartzites
Magnetite quartzites are found in seven drill cores.

Granites, pegmatites and migmatites
The gneisses are cut by medium to largegrained granite veins in all drill cores, however, pegmatitic veins and large granitic and pegmatitic bodies (several tens of metres in size) are also common.The granites and pegmatites are microcline and plagiomicrocline in their mineral com position.Plagioclase is often replaced by microcline, which may suggest potassium metasomatism.Migmatiza tion is observed in all drill cores.Migmatized and non migmatized sections can be easily distinguished visually.

ORE MINERAL DESCRIPTIONS
The key optical features specific to ore minerals in reflected light are reflectivity (brightness), apparent colour, bireflection (change in reflectivity and apparent colour on rotation in planepolarized light), anisotropy (change in the apparent colour on rotation in cross polarized light) and internal reflections of (semi)trans parent ore minerals (Craig & Vaughan 1994).In the Jõhvi samples, optically isotropic magnetite occurs as anhedral grains elongated along rock fabric, interstitially filling the space between other minerals by forming patches of up to several millimetres in size or embedded in silicate minerals as rounded inclusions with the size of 20-100 µm (Fig. 4A).Magnetite can also be present as tiny platelets along grain edges and along cleavage planes of amphibole and biotite.The patches of magnetite are often oriented as distinct bands alternating with silicate minerals dominated layers.In some sections magnetite is altered into anisotropic and bireflective hematite along grain edges, cracks and cleavage planes (Fig. 4B).In places, hematite also appears as colloform filling of 5 mm wide veins cutting subparallel the main rock fabric and cogenetic intensive alteration of magnetite in close proximity to the veins (Fig. 4C, D), suggesting the precipitation from metamorphic fluids or brines.
Sulphides are present mostly as isotropic pyrite, which is occasionally exhibiting weak anisotropy.At some levels anisotropic pyrrhotite occurs, being far less abundant than pyrite.Pyrite appears as sub to euhedral crystals or interstitial patches between other grains, commonly closely related to magnetite and garnet.Together with magnetite they form ore mineralrich bands, which are often more pronounced in close proximity to or at the contact with subparallel quartz or granitic veins.At the vein tips, brittle fracturing has occurred with pyrite and magnetite filling the space (Fig. 4E).Sulphides also appear as fillings in later thin veins crosscutting the main banding (Fig. 4F).Occasional chalcopyrite, galena and sphalerite grains were found in magnetiterich drill core sections of F1 and F5.
Different generations of iron oxides appearing as magnetite or hematite, magnetite to hematite alteration, sulphide precipitation or alteration as a pyrite/pyrrhotite association and minor chalcopyrite, galena and sphalerite, as well as coprecipitation of sulphides and magnetite indicate a complex redox history (e.g.Hall 1986;Lagoeiro 2004;Slotznick et al. 2018) of ironrich rocks of the Jõhvi area.The succession of general geological processes in time and their relations to specific oreforming processes are not yet clear and need definitely future studies.

Major element composition
The results show that the magnetite quartzites provide a wide range of major element compositions (Table 1; Fig. 5;   In other gneisses that surround the MagQ, SiO 2 ranges from 52 to 76 wt%, Al 2 O 3 from 9.3 to 19.1 wt%, Fe 2 O 3 from 4.7 to 15 wt% and MgO from 1.52 to 9.95 wt%.The Na 2 O and K 2 O contents have slightly higher variance than in MagQ, ranging from 1 to 3.14 wt% and from 1.06 to 5.93 wt% respectively.The TiO 2 content varies between 0.40 and 1.26 wt%; P 2 O 5 is up to 1.09 wt% and has also a positive correlation with CaO that ranges up to 11.4 wt%.Contrary to the high content of MnO in MagQ, the surrounding gneisses contain less than 1.23 wt% MnO.
In granitic and pegmatitic rocks the content of SiO 2 ranges from 61 to 72 wt%, Al 2 O 3 from 11.62 to 17.92 wt%, Fe 2 O 3 from 0.3 to 8.3 wt%, MgO from 0.09 to 2.58 wt%

Biotite
Biotite occurs in a greater variety of geological environments than any of other micas.In metamorphic rocks it is formed under a wide range of P-T conditions, and it occurs abundantly in many contact and regionally metamorphosed sediments.The most common occurrence of biotites and phlogopites is by the metamorphism of pelites, basic and ultrabasic rocks and siliceous limestones and dolomites.Biotite in MagQ is commonly Ferich.The X Mg (Mg/Fe + Mg + Ti + Al VI ) is 0.46 to 0.55 (Table 3).
In sample F12910 the X Mg is the lowest, 0.35 as in garnets from the same sample.

Pyroxene
The studied pyroxenes are mostly clinopyroxenes that belong to the solid solution series between diopside and hedenbergite (or johannsenite).Minerals have approxi mately equal amounts of manganese and iron; those with a composition closer to johannsenite than hedenbergite are described as ferroan johannsenite, those closer to hedenbergite as manganoan hedenbergite.Diopside and hedenbergite are typical minerals of many meta morphic rocks, being particularly characteristic of contact metamorphosed calcium/ironrich sediments (Deer et al. 1992).There are also orthopyroxenes present.Unfortunately only few were analysed, which were compositionally enstatites -En 66 Fs 30 Wo 4 (Table 4).

Garnet
Garnets in drill core F12 are Almandine 0.6-0.74Pyrope 0.20-0.26Grossular 0.04-0.07Spessartine 0.03-0.1 .The variations in com position from core to rim are small.The X Mg (Mg/Fe + Mn + Mg + Ca) is 0.20-0.25 with the lowest value 0.11 in sample F12910 (Table 5).Garnets, in which spessartine is the dominant mol ecule, are usually found in some skarn deposits.They frequently occur in manganeserich assemblages with rhodonite, pyroxmangite, tephroite, etc. of metasomatic origin associated either with adjacent igneous intrusions or with a more widespread regional metamorphism (Deer et al. 1992).
In the studied samples, three distinctly different garnet solid solutions can be distinguished by microprobe mineral chemistry, possibly indicating different protolith: 1. Spess occur in the mineral assemblage of garnet-pyroxenemagnetite-quartzite (samples F122788, F123369).

Magnetite
According to 60 thin sections of gneisses from drill core J1, the MagQ may contain up to 45 vol% magnetite.The average magnetite abundance is around 20 vol%.Large, up to 1 mm, stretched, highly irregular and oriented grains form magnetiterich bands of different width.Also, abundant rounded and disseminated magnetite grains 5-100 μm in size occur as inclusions in the other rockforming minerals.Such variance in grain properties refers to the possibility of multiple generations of mag netite.Multigeneration of magnetite is also supported by ore microscopy studies (see above).Trace element concen trations analysed in different magnetite grains show no table variation for several elements (Table 6).This suggests either the impurity of analysed grains or the origin of multiple generations.

Ilmenite
Ilmenite has been found in the earlier study (unpublished data by M. Voolma andA. Soesoo, 2010-2013) in samples F122788, F123369, F13090 and F122547.It forms (FeTiO 3 )-pyrophanite (MnTiO 3 ) solid solution.Ilmenite from samples F122788 and F123369 contains about 30-35 wt% MnO, 40-45 wt% TiO 2 and 10-20 wt% FeO and in sample F13090 about 5-6 wt% MnO.In ilmenite and magnetite coexisting in igneous and metamorphic rocks, Mn is always preferentially found in the ilmenite (Deer et al. 1992).Manganoan ilmenite is found in granitic rocks and also in some carbonatites.In solid solution between ilmenite and pyrophanite molecule (MnTiO 3 ) the pyrophanite molecule may become important in ilmenites 85 in differentiated acid rocks and carbonatites.However, in the case of the Jõhvi rocks, especially MagQ, also mag netite is rich in manganese constituting in some magnetite gains more than 4000 ppm (Table 6).

DISCUSSION
Historically, the term 'magnetite quartzites' has been extensively used for the magnetiterich variety of Jõhvi gneisses.In the case where magnetite and quartz with a minor amount of other minerals (garnet, micas, feldspars) form microlayered rock associations, this term is well characterizing the rock type.However, in a classical meaning quartzite is a nonfoliated metamorphic rock which was originally pure quartz sandstone.It may contain magnetite, but usually not in economic quantities.The British Geological Survey defines quartzite as psammites containing more than 80% quartz (Robertson 1999), while the US Geological Survey describes it as a rock containing >75% quartz (North American Geologic Map Data Model Science Language Technical Team 2004).The term 'quartzite' is also used for quartz cemented quartz arenites.In the case of Jõhvi rocks, the term 'magnetite quartzite' is somewhat misleading if applying to the whole section of ironrich gneisses, since the magnetiterich rocks are gneisses containing plagio clase (up to 35%), other feldspars, pyroxenes (up to 20%), garnet and sulphide ore minerals along with magnetite and hematite.However, there are beds from several tens of centimetres to several metres thick where quartz prevails in the mineral composition of rock and which are enriched in magnetite (small amount of other minerals).Those beds, restricted in size, can be named as magnetite quartzites proper, but in a large variety of magnetite bearing gneisses the content of quartz is less than 50% in volume.Therefore, in the future, after new drilling material becomes available, a more detailed study of the mineralogical and geochemical variation of the Jõhvi ore formation should be carried out in order to define rock varieties.It also needs to be noted that the boundary between the magnetiterich varieties and the surrounding gneisses which may also contain some magnetite (and hematite) is gradual.

A question of manganese and other base metals
The new studies on major and trace element geochemistry revealed high manganese contents (1-6 wt%) in most of the MagQ samples and in rare occasions in the surrounding gneisses.Manganeserich MagQ are also known in the Jägala rock complex of the Tallinn Zone (Luha 1946;Erisalu et al. 1969;Petersell 1976).In most cases, ironmineralization spreads in garnetcordierite, amphibole and pyroxene gneisses.Some amount of fine grained magnetite along with hematite, pyrrhotite and pyrite may occur in smallgrained cordieritesillimanite biotite metasedimentary aluminiumrich gneisses through out Estonia.In gabbroic intrusions, ilmenite-magnetite and magnetite-apatite mineralization has also been noted (Petersell et al. 1991).Various hypotheses of ore genesis have been proposed for the Jõhvi MagQ.Linari (1940) suggests that the ore is skarnlike, while Vaganova & Kadyrova (1948) propose metasomatic origin.Tikhomirov (1966) has described these rocks as alternating layers of skarn, cummingtonite, biotitesillimanite and other hornfels that are cut by granite veins.However, the most widely recognized is the concept of metamorphic origin after the rocks of volcano genicsedimentary origin (Puura & Kuuspalu 1966;Erisalu et al. 1969).
The type of ironore mineralization (e.g.deposit type) can be followed, for instance, by magnetite chemical composition.According to Dupuis & Beaudoin (2011), magnetite trace element composition can be used for drawing petrogenetic discriminant diagrams that show trace element fingerprint of magnetite.Based on 111 samples, a plot for the characteristic zones of main iron deposit types has been constructed by Dupuis & Beaudoin (2011).They recommended to use these diagrams together and in the suggested order in order to classify the deposit type.
The Si + Mg vs Ni + Cr diagram (Fig. 8A) discards a Ni-Cu type of deposit for the Jõhvi rocks as the analysed grains are not enriched in Ni or Cr, unlike the magnetites in typical Ni-Cu and Cr deposits (Table 6).The second diagram, Cu/(Si + Ca) vs Al/(Zn + Ca), differentiates volcanic massive sulphate (VMS) deposit type from the other types of deposits.The Jõhvi magnetites plot far away from the VMS zone (Fig. 8B).The third, Ti + V vs Ni/(Cr + Mn) diagram shows clear clusters of various deposit types aside from the skarn type of deposit where magnetites have generally lower Ti + V concentrations but variable Ni/(Cr + Mn) ratios (Fig. 8C).The analysed Jõhvi magnetites form a group outside the plotted types of deposits, suggesting that Jõhvi magnetites do not represent clearly any of the compared deposit types (Fig. 8C).As the skarn field is not distinguished by the lower Ni/(Cr + Mn) border in the diagram by Dupuis & Beaudoin (2011), it may be plausible that the Jõhvi magnetites bear similarities with the skarn type of de posits, albeit having lower nickel and higher manganese concentrations.The analysed magnetites also have a high Ti + V concentration which is not common for skarn deposits (Dupuis & Beaudoin 2011) and therefore plot on the Ti + V vs Ca + Al + Mn diagram in the porphyrytype field, close to the skarn and iron oxide-copper-gold type of ores (Fig. 8D).
The magnetite grains analysed from drill core J1 show that classifying Jõhvi magnetites into a certain deposit type is not unambiguous.On the Ti + V vs Ni/(Cr + Mn) diagram, the analysed samples plot quite away from the other types possibly because of the relatively high content of manganese in the magnetite.Duo to a small number of analyses from only a single drill core, the question of the origin of the Jõhvi ironore remains open, as well as the source for the elevated manganese concentrations in the rock and magnetite.
Two minerals carrying manganese can be envisagedgarnets and Mnilmenites.In some parts, Mnrich pyroxenes can probably also contribute to the overall manganese budget of the ore formation.abundant in the Jõhvi gneisses, however, its overall content is neither high nor consistent with the location of Mnrich MagQ (Fig. 4).Ilmenite may have 5-35 wt% Mn, but it is not widely spread in the rock and cannot account for high wholerock manganese oxide contents (1-6 wt%).Still, the actual protolith of the gneisses and the source of mineralization of the iron formation could only be hy pothesized as the available rock material was too limited.The geological setting, banded texture of proper ironrich MagQ and abundance of Alrich garnets support the volcanogenicsedimentary origin.Therefore, the initial enrichment of iron could be explained by iron deposition similar to the genesis of the Algomatype banded iron formations.However, metasomatic processes are responsible for an additional concentration of some metals, i.e. iron and manganese.In this respect, the origin of the Jõhvi iron formation may well be a complex process.Hopefully, the future studies on the new rock material from the Jõhvi area will help to answer these questions.
The modern geologicalgeochemical correlation hints at geological similarities between the Bergslagen area in Sweden and the Jõhvi area (Voolma et al. 2010;Bogdanova et al. 2015, fig. 9;Nirgi & Soesoo 2019).By far the most common type of metallic mineral deposits in Bergslagen consists of iron oxide with variable amounts of Mn in associated skarn and metacarbonate rocks (Allen et al. 2003(Allen et al. , 2008;;Voolma et al. 2010).More than 2000 deposits are known in the Bergslagen area, most of which are small and, since the middle of the 19th century, without any economic significance.The economically important iron oxide deposits in Bergslagen may be subdivided into the following categories: (1) iron oxide deposits in Mnpoor and Mnrich skarn and car bonate rocks, (2) quartzrich iron oxide deposits including banded iron formations and (3) apatitebearing iron oxide deposits.In addition, iron oxide deposits associated with high contents of sulphides of base metals are present.The abovementioned iron oxide types may be considered as end members, and characteristics corresponding to more than one type are locally found in different places of, or along strike, in the same deposit.For example, some iron oxide skarn deposits in Bergslagen appear to grade into quartzrich iron deposits.Most of the skarns associated with Fe oxide ores, base metal sulphide ores and those scattered through the volcanic succession, are currently interpreted as a result of regional metamorphism of interbedded sedimentary and volcanic rocks and/or hydrothermal sediments, or of regional metamorphism of earlier hydrothermally altered rocks (Allen et al. 2003(Allen et al. , 2008)).Most of the skarns are not spatially as sociated with particular intrusions.This may or may not be the case for the Jõhvi iron ore and associated metals as well.New geological material and geochemical studies are needed to prove or disprove this possible geological similarity.

Evaluation of metamorphic P-T conditions
The rocks of the Jõhvi Zone are usually regarded as rocks of granulite metamorphic facies (Puura et al. 1983).However, very little research has been done on the geo thermobarometry of these rocks.To understand the geo logical history, and also ore genesis, some knowledge about the metamorphic history is necessary.Therefore, an attempt was made to get some indication for tem perature and pressure estimates on the Jõhvi MagQ and surround ing gneisses.
In order to acquire rock formation temperature con ditions, the major element composition was measured in garnet and biotite.Metamorphism temperatures were calculated using the following calibration methods: Thompson (1976), Holdaway & Lee (1977), Perchuk & Lavrent´eva (1983), Dasgupta et al. (1991), Bhattacharya et al. (1992) (for results refer to Table 7).As the biotite grains in analysed samples were chemically similar (stan dard deviation of Mg and Fe concentrations below 0.06), the average values of Mg and Fe concentrations of biotite were used.The obtained results (Table 7) show that according to the Fe/Mg ratio of garnets and biotite, the metamorphism temperatures that reflect the chemical variance of garnets form three recognizable clusters.The majority of calculated temperatures were between 650 and 750 ºC, which is also comparable with previous studies that suggest the temperature values of 650 ± 50 ºC (Voolma et al. 2010).
There are nearly 30 versions of the garnet-biotite thermometers at present.Among these the Holdaway (2000) version yields the smallest absolute error (±25 ºC) in reproducing the experimental temperatures of Ferry & Spear (1978) and Perchuk & Lavrent´eva (1983), in the wide temperature range of 550-950 ºC (Wu et al. 2004; Table 7).Six thin sections from two drill cores (F1 and F12) were analysed.In sample F12910 the garnet-biotite temperature is 649 ºC according to the Holdaway (2000) thermometer.In sample F122517 seven garnet-biotite pairs were analysed.Temperature ranges from 658 to 755 ºC, with an average of 708 ºC according to Holdaway (2000).In sample F122547 the average temperature was 647 ºC in three garnet-biotite pairs.The lowest tem perature was 629 ºC and the highest was 674 ºC.The garnet-biotite pair in sample F123276 gave the tem perature of 669 ºC.In sample F13792 five garnetclinopyroxene pairs were analysed.According to the Nakamura (2009) geothermometer, the calculated tem perature was 605-629 ºC, with a mean value of 615 ºC.In sample F13473 garnet-clinopyroxene pairs gave two different results.The manganeserich pyroxene (johannsenite) temperature was 942 and 971 ºC according to the Nakamura (2009) geothermometer and manganese poor pyroxene gave lower temperatures around 630 ºC.The large difference in temperatures may infer towards prolonged metasomatic processes during which some pyroxenes may have been reequilibrated (showing the lower temperature).
Alumosilicate minerals are not always present.In this case the widely used garnet-aluminosilicate-plagioclasequartz barometer cannot be applied.Wu et al. (2004) calibrated the garnet-biotite-plagioclase-quartz barometer, so that it may be applied to metapelites, especially when aluminosilicate is absent.Metamorphic pressures for the studied rocks were calculated by the garnet-biotiteplagioclase-quartz (Wu et al. 2004) geobarometer.The cal culated pressure was 4.3 kbar in sample F12910, 4.9 kbar in sample F122517, 2.9 kbar in sample F122547 and 5.6 kbar in sample F123276.
The metamorphic temperature was varying between 570 and 670 ºC in the samples from drill cores J1 and J2, which is similar to the results of an earlier study by Voolma et al. (2010) giving an average of 650 ± 50 ºC.It is noted that sample J132843294 gives systematically lower temperatures -between 508 ºC (calculated after Ferry &Spear 1978) and717 ºC (after Dasgupta et al. 1991); still, the majority of calculated temperatures are between 530 to 570 ºC.However, two different sections of sample J259505960 (drill core J2) gave a much smaller spread of calculated temperatures with an average of 645 ºC (Table 7).The preliminary conclusion is that drill core J1 shows a lower metamorphic temperature than core J2.However, due to the small number of analyses, this difference cannot be confirmed yet.Having in mind that the entire Jõhvi ore complex may be a result of repeated metasomatic events, for example throughout the metasomatic processes, there is likely no simple way for estimating P-T conditions.Instead, the surrounding rocks, which have not gone through metasomatic processes, should be targeted for pressure and temperature estimation purposes in the future.

CONCLUSIONS
The Jõhvi magnetite quartzites occur as subvertical beds with complicated structural elements in garnetcordierite and pyroxene gneisses.Drill core study shows that the complex of magnetiterich rocks may be up to 100 m thick.The studied samples can be divided into three major rock groups: (1) gneisses and ironrich gneisses, (2) magnetite quartzites and (3) granitoids.Magnetite quartzites provide a wide range of major element com position while SiO 2 ranges between 40.3 and 60.1 wt%, Al 2 O 3 between 1.7 and 19.7 wt% and Fe 2 O 3 between 15 and 45.2 wt%.The present study revealed unusually high manganese contents (1-6 wt%).The majority of the magnetite quartzites follow a similar REE pattern to the surrounding gneisses.Granitoids form two entirely different patterns of REE.
Magnetite occurs as anhedral grains elongated along rock fabric, as rounded inclusions in other minerals or as tiny platelets along grain edges and along cleavage planes of amphibole and biotite.Sulphides are present as pyrite, pyrrhotite and other minor sulphide minerals (chal copyrite, galena and sphalerite).
The calculated metamorphic temperatures (garnetbiotite geothermometer) fall between 650 and 750 ºC.The garnet-biotite-plagioclase-quartz geobarometer yielded the pressure range of 2.9 to 4.9 kbar.However, having in mind that the entire Jõhvi ore complex may be a result of repeated metasomatic events, there is likely no simple way for estimating P-T conditions.
The magnetite grains analysed from drill core J1 show that classifying Jõhvi magnetites into a certain deposit type is not unambiguous.The modern geological geochemical correlation hints at geological similarities between the Bergslagen area in Sweden and the Jõhvi area.

Fig. 1 .
Fig. 1.The main geological outline of the Estonian Precambrian basement and the location of the Jõhvi Zone (A).Magnetic (B) and gravimetric (C) fields of the Jõhvi magnetic anomaly with location of historical drill holes.

Fig. 2 .
Fig. 2. Simplified cross section of drill core J1 of the Jõhvi area.The cross section is based on the original logging in 1940 (Report No. 8, https://fond.egt.ee/fond/egf/8).Average iron contents (Fe%) are shown for ore sections (vertical values) and for sites of the present analyses (horizontal values).

Table 1 .
Representative wholerock major (in wt%) and trace (in ppm) element analyses of the Jõhvi magnetite quartzites (MagQ) and surrounding granitoids and gneisses; * indicates samples that have been analysed for major elements by XRF at the laboratory of the Department of Geology, Tallinn University of Technology.Full set of analyses at https://doi.org/10.23679/506

Table 1 .
Continued 2 O and K 2 O contents can be as high as 1.71 and 7.22 wt%, respectively.The amount of TiO 2 ranges from 0.09 to 1.16 wt%, P 2 O 5 can be as high as 0.46 wt% and manganese content is 0.17-5.88wt%.
Geochemical variation plot for selected major oxides in the Jõhvi Zone rock types (in wt%).Selected geochemical analyses in Table1; full set of analyses at https://doi.org/10.23679/506.and CaO from 0.29 to 4.34 wt%.The Na 2 O and K 2 O values can be as high as 3.63 and 11.45 wt%, respect ively.The TiO 2 content is between 0.02 and 0.76 wt%, P 2 O 5 ranges from 0.02 to 0.48 wt% and MnO is below 0.46 wt%.

Table 2 .
Selected ICPMS rare earth element analyses in the Jõhvi magnetite quartzites (MagQ) and surrounding granitoids and gneisses.Full set of analyses at https://doi.org/10.23679/506 (Anders & Grevesse 1989tion plot for selected trace elements in the Jõhvi Zone rock types (in ppm).Selected geochemical analyses in Table1; full set of analyses at https://doi.org/10.23679/506.The concentrations of rare earth elements (REE) were obtained from ICPES/MS analysis and plotted on the chondritenormalized(Anders & Grevesse 1989) spider diagram (Table2; Fig.7; geochemistry of all analysed samples is available online at https://doi.org/10.23679/506).Despite a few exceptions, the majority of the MagQ follow a similar pattern to the surrounding gneisses, but have somewhat lower REE, especially of light REE contents.Few high REE values obtained from the surrounding gneisses, which result in the wide band of the REE pattern, were produced by samples collected near the contact zones of gneisses and granitoids.This gives a reason to believe that fluids passing the rock have been mobilizing a notable amount of at least REE, but probably other trace elements as well.

Table 5 .
Electron microprobe analyses of garnet of the Jõhvi magnetite quartzites; bdl, below detection limit; nc,

Table 6 .
Garnet is 87 A.Soesoo et al.:Geochemistry and P-T conditions of Jõhvi magnetite gneisses LAICPMS analyses of magnetites from the Jõhvi magnetite quartzites.Altogether 38 analyses were performed on five magnetite grains (Mt1-Mt5) from the polished section of drill core J1 (depth 312.1 m).Average compositions of these grains are provided in the table