1. Introduction
Garnet and biotite are common silicates in igneous, metamorphic and sedimentary rocks, as well as hydrothermal ore deposits. Both minerals have been used as exploration vectors to various ore deposit types, including skarn (e.g. Yu et al. Reference Yu, Shu, Niu, Xing, Li, Lentz, Zeng and Yang2020; Xie et al. Reference Xie, Yang, He and Gao2022), Sn-W-Mo bearing granite (e.g. Azadbakht et al. Reference Azadbakht, Lentz, McFarlane and Whalen2020; Mohammadi et al. Reference Mohammadi, Lentz, McFarlane and Yang2021), orogenic Au–Cu–Pb–Zn (Hu et al. Reference Hu, Zheng, Yu, Wu and Wang2021), orogenic Au (Kuikka, Reference Kuikka2018) and porphyry Cu-Mo deposits (Yu et al. Reference Yu, Li, Zhao, Evans, Li, Jiang, Zou, Qin and Guo2022). Studies of the trace element composition of garnet and biotite in regionally metamorphosed massive sulphide deposits are limited in number but include those of Pollock et al. (Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018), Tott et al. (Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019) and Lisboa et al. (Reference Lisboa, Ferreira Filho, Monteiro and Mansur2023) who evaluated their compositions in metamorphosed sedimentary exhalative (SEDEX) deposits, while Lottermoser (Reference Lottermoser1988, Reference Lottermoser1989), Schwandt et al. (Reference Schwandt, Papike, Shearer and Brearley1993), Spry et al. (Reference Spry, Heimann, Messerly and Houk2007) and Heimann et al. (Reference Heimann, Spry, Teale, Conor and Pearson2011) evaluated the REE composition of garnet spatially associated with Broken Hill-type (BHT) mineralisation in the Curnamona Province, Australia and their application as potential exploration guides. A similar REE study was conducted by Stalder and Rozendaal (Reference Stalder, Rozendaal, Mao and Bierlein2005) on calderitic garnets associated with the giant Gamsberg SEDEX Zn deposit, Aggeneys ore district, South Africa. A recent study of garnet composition was completed on the King volcanogenic massive sulphide deposit, Western Australia by Dana et al. (Reference Dana, Hollis, Spry, Rodgers, James, Podmore and Azri2025).
The Stollberg ore field, Bergslagen district, Sweden, consists of a group of stratabound, volcanic-associated, limestone-skarn Zn-Pb-Ag-(Cu-Au) deposits (or stratabound volcanic-associated, limestone skarn Zn-Pb-Ag-(Cu-Au) deposits (SVALS)-type using the terminology of Allen et al. Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996). These include the Baklängen, Dammberget, Gränsgruvan, Lustigkulla, Norrgruvan and Tvistbo deposits. Several mineralogical, geochemical and geological studies have been conducted in the Stollberg area including those of Geijer (Reference Geijer1917), Selinus (Reference Selinus1983), Ripa (Reference Ripa1988, Reference Ripa1994, Reference Ripa2012), Beetsma (Reference Beetsma1992), Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013), Raat et al. (Reference Raat, Jansson and Lundstam2013), Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019, Reference Frank, Spry, O’Brien, Koenig, Allen and Jansson2022) and Spry et al. (Reference Spry, Jansson and Allen2024). Models of ore formation can be grouped into pre-metamorphic, syngenetic and exhalative (e.g. Arvanitidis & Rickard, Reference Arvanitidis and Rickard1981; Frietsch, Reference Frietsch1982), post-peak metamorphism (Beetsma, Reference Beetsma1992) and epigenetic (Tegengren, Reference Tegengren1924; Geijer & Magnusson, Reference Geijer and Magnusson1944; Magnusson, Reference Magnusson1970), with the most recent model of Ripa (Reference Ripa1988, Reference Ripa2012), Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013) and Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) suggesting that sulphide mineralisation was a synvolcanic, sea floor hydrothermal system that was partly exhalative to produce iron oxides and partly carbonate replacive to form sulphides and additional iron oxides below the sea floor.
Major element compositions of garnet and biotite in metamorphosed altered rocks in the Stollberg ore field were obtained by Ripa (Reference Ripa1994, Reference Ripa2012) and Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) but only preliminary studies of the trace element composition of these minerals, which were presented in abstract form, were given by O’Brien et al. (Reference O’Brien, Spry, Raat, Allen, Jansson and Frank2014) and Spry et al. (Reference Spry, O’Brien, Frank, Teale, Koenig, Jansson, Allen and Raat2015). No detailed study of the trace element compositions of garnet and biotite has yet been conducted. The compositions of amphiboles, pyroxene and magnetite appear in companion papers by us (Frank et al. Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019, Reference Frank, Spry, O’Brien, Koenig, Allen and Jansson2022) and are not duplicated here. Given the potential of mineral chemistry as a guide to ore, the aims of the present study are to 1. Determine whether or not the trace element chemistry of garnet and biotite in metamorphosed hydrothermal alteration zones spatially associated with the Baklängen, Dammberget, Gränsgruvan, Lustigkulla, Norrgruvan and Tvistbo deposits in the Stollberg ore field can be used as vectors to Pb-Zn-Ag-(Cu-Au) to mineralisation and 2. How the trace element compositions of garnet and biotite compare with those previously obtained from metamorphosed massive sulphide deposits (i.e. SEDEX and BHT deposits) elsewhere and other ore types.
2. Regional geology
Details of the regional geological setting, geochronology and mineral deposits in the Paleoproterozoic Bergslagen district on the Fennoscandian Shield in south-central Sweden are given in Allen et al. (Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996, Reference Allen, Bull, Ripa and Jonsson2003, Reference Allen, Ripa and Jansson2008) and Stephens and Jansson (Reference Stephens and Jansson2020), and is only summarised here (Fig. 1). The district hosts thousands of mineral deposits including banded iron formation, Fe oxide-calc-silicate skarn, polymetallic Zn-Pb-Ag-Cu-Au sulphide and iron oxide deposits, Fe oxide apatite and Mn oxide deposits that occur in supracrustal metavolcanic (predominantly calc-alkaline rhyolite)-metasedimentary rocks (∼1.91–1.86 Ga), along with less abundant granite-pegmatite-hosted molybdenite and W skarn deposits associated with younger granites (∼1.82–1.75 Ga) (Allen et al. Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996; Stephens & Jansson, Reference Stephens and Jansson2020). The Stollberg Zn-Pb-Ag-(Cu-Au) ore field is one of the several polymetallic base metal deposits/districts in the Bergslagen district that include the Garpenberg, Falun, Lovisa, Zinkgruvan and Sala deposits (e.g. Hedström et al. Reference Hedström, Simeonov and Malström1989; Reference Jansson and AllenJansson & Allen, 2011b ; Kampmann et al. Reference Kampmann, Jansson, Stephens, Majka and Lasskogen2017; Jansson et al. Reference Jansson, Zetterqvist, Malmström and Allen2017, Reference Jansson, Sädbom, Allen, Billström and Spry2018, Reference Jansson, Allen, Skogsmo and Turner2022; Tiu et al. Reference Tiu, Jansson, Wanhainen, Ghorbani and Lilja2021). Allen et al. (Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996) suggested the Bergslagen district represents an extensional, back-arc region that formed on the edge of a convergent, continental plate margin. Allen et al. (Reference Allen, Bull, Ripa and Jonsson2003) later showed that felsic pyroclastic rocks and volcaniclastics were derived from several caldera volcanoes and deposited in a shallow marine basin, and that stromatolite and microbial reef growth developed carbonate horizons during periods of relative volcanic inactivity. The polymetallic Zn-Pb-Ag-(Cu)-(Au) sulphide deposits generally occur in hydrothermally altered metavolcanic rocks and associated metacarbonates (Allen et al. Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996).
Figure 1. Geologic map of Bergslagen region modified after Allen et al. (Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996), Stephens et al. (Reference Stephens, Ripa, Lundström, Persson, Bergman, Ahl, Wahlgren, Persson and Wickström2009) and Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013) showing location of the Stollberg area (S). Also shown are locations of the Falun (F), Garpenberg (G) and Zinkgruvan (Z) deposits. Various ore deposits are shown with different symbols and sizes; all from the database of the Geological Survey of Sweden. Inset shows simplified geologic setting of the Bergslagen region (BR) in northern Europe.
The Bergslagen district was metamorphosed to the amphibolite facies, with local areas of granulite (including anatexis) and greenschist facies during the Svecokarelian orogeny (1.89–1.75 Ga). Although geochronological studies have not specifically been conducted on rocks in the Stollberg district, it is assumed that the age is ∼1.9–1.8 Ga, consistent with the ages determined for various metamorphic rocks and metamorphosed ore deposits elsewhere in the Bergslagen district (e.g. Allen et al. Reference Allen, Lundström, Ripa, Simeonov and Christofferson1996; Jansson & Allen, Reference Jansson and Allen2011a ; Stephens & Jansson, Reference Stephens and Jansson2020). Four main phases of deformation in the district include tight, isoclinal and open folds (at least three fold episodes), rifts and the development of shear-zones (Stephens et al. Reference Stephens, Ripa, Lundström, Persson, Bergman, Ahl, Wahlgren, Persson and Wickström2009; Beunk & Kuipers, Reference Beunk and Kuipers2012), with regional Na or K alteration predating the first deformation (Frietsch, Reference Frietsch1982; Lagerblad & Gorbatschev, Reference Lagerblad and Gorbatschev1985).
3. Local geology
The Stollberg ore field consists of more than a dozen base metal and magnetite deposits that are spatially associated with the regional N-S trending, steeply E-dipping Stollberg syncline (Fig. 2). Most of the base metal deposits occur on the eastern limb of the syncline with Norrgruvan and Tvistbo located near the nose of the syncline. Gränsgruvan is the only deposit on the western limb of the syncline. The local geology is described in detail by Ripa (Reference Ripa1988, Reference Ripa1994, Reference Ripa2012) and Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013) and summarised in Table 1 along with grade and tonnage information. A total of 6.65 Mt ore was mined from medieval times until 1982 (Jansson et al. Reference Jansson, Erismann, Lundstam and Allen2013). Lithogeochemical studies of the host rocks, including the metamorphosed altered rocks, are given in Selinus (Reference Selinus1982, Reference Selinus1983), Ripa (Reference Ripa1988, Reference Ripa1994, Reference Ripa2012), Björklund (Reference Björklund2011), Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013) and Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019).
Figure 2. Geologic map of the Stollberg area, showing the location of mines, mineral occurrences and drill cores. 1 = Gränsgruvan, 2 = Norrgruvan, 3 = Tvistbo, 4 = Lustigkulla-Marnäs, 5 = Cedercreutz, 6 = Baklängan, 7 = Dammberget, 8 = Stollmalmen, 9 = Brusgruvan, 10 = Grönkullan. Drill cores from which samples were taken are shown. Grid is Swedish National Grid RT90, and inset map shows location of Stollberg in Sweden. Key provided on following page. Modified after Raat et al. (Reference Raat, Jansson and Lundstam2013).
Table 1. General geological characteristics of ore deposits in the Stollberg ore field *
Abbreviations: Au native Au, Ab albite, Act actinolite, Alm almandine, Amp amphibole, Apy arsenopyrite, Ath anthophyllite, Bt biotite, Cal calcite, Cam clinoamphibole, Chl chlorite, Ccp chalcopyrite, Crd cordierite, Cpx clinopyroxene, Czo clinozoisite, Dsp diaspore, Ep epidote, Fl fluorite, Gn galena, Ged gedrite, Ghn gahnite, Grs grossular, Grt garnet, Gru grunerite, Hbl hornblende, Hd hedenbergite, Hu-group humite group, Kfs K-feldspar, Kne knebelite, Lo löllingite, Mag magnetite, Mc microcline, Mn-Ilm Mn ilmenite, Mn-Gru Mn grunerite, Ms muscovite, Phl phlogopite, Pl plagioclase, Po pyrrhotite, Py pyrite, Pxn pyroxenoid, Qz quartz, Ser sericite, Sp sphalerite, Sps spessartine, St staurolite, Tlc talc, Tr tremolite. Most abbreviations from Warr (Reference Warr2021).
* Modified after Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013), Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019, Reference Frank, Spry, O’Brien, Koenig, Allen and Jansson2022).
Ore in the Stollberg district is hosted in marble, skarn and planar-bedded rhyolitic ash silt-sandstone and spatially associated with a variety of metamorphosed hydrothermally altered rocks that include quartz-garnet-pyroxene (clinopyroxene), garnet-biotite, cordierite-biotite, quartz-sericite, gedrite-albite and garnet-magnetite rocks. The sericite-altered rocks are particularly prominent on the western side of the Stollberg syncline in the Gränsgruvan deposit. The ore, which consists of varying proportions of sphalerite, galena and pyrrhotite with lesser amounts of magnetite, pyrite, chalcopyrite and arsenopyrite, was metamorphosed to the amphibolite facies (560–600oC, 2–3.5kb; Beetsma, Reference Beetsma1992; Björklund, Reference Björklund2011; Jansson et al. Reference Jansson, Erismann, Lundstam and Allen2013). Ripa (Reference Ripa2012) noted the following gangue assemblages in ore on the eastern side of the Stollberg syncline: biotite-garnet±sillimanite, amphibole-garnet±andalusite±staurolite±cordierite±biotite±gahnite and olivine±pyroxene±garnet±carbonate. The ore and surrounding rocks were subject to two deformation events (D1–2) with the first event consisting of a prominent S1 schistosity that parallels bedding. A second schistosity (S2) is associated with N-S trending F2 folds (including the Stollberg syncline), a subvertical stretching lineation, and shear zones that parallel the F2 fold axes (Jansson et al. Reference Jansson, Erismann, Lundstam and Allen2013).
On the eastern side of the syncline, the stratigraphic upper Stollberg succession is separated from a lower stratigraphic succession known as the Staren succession by a rhyolitic intrusion (Fig. 3). All of these rocks are affected, in part, by Na-alteration (albite-gedrite assemblage). The Staren succession consists of a metamorphosed sequence of quartz-feldspar rhyolitic sandstone interbedded with rhyolitic siltstone that is overlain by limestone, which consists of coarse-grained pure marble and skarn. A package of rhyolitic and calcareous sandstones, conglomerates and breccias overlies the Staren limestone. The base of the metamorphosed Stollberg succession consists of hydrothermally altered feldspar-phyric rhyolitic pumice breccia-sandstone, which grades upwards into the base-metal hosting Stollberg limestone (Table 1). The limestone consists of dolomitic and Mn-bearing marble and is locally altered to skarn. More than 700 m of metamorphosed variably hydrothermally altered planar-bedded rhyolitic ash silt-sandstone overlies the limestone (e.g. Jansson et al. Reference Jansson, Erismann, Lundstam and Allen2013; Ripa, Reference Ripa1988, Reference Ripa1994, Reference Ripa2012). Other rock types in the Stollberg ore field include amphibolite sills within the Stollberg succession (Björklund, Reference Björklund2011), diabase sills and dykes, rhyolitic-dacitic intrusions and undifferentiated granites.
Figure 3. Geologic cross section of the northern Staren area (profile 3 – along 6676775 – on Figure 2). Interpreted pre-metamorphic protoliths are given in brackets. Grid is Swedish National Grid RT90. Modified after Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013).
4. Sampling and analytical methods
Garnet- and biotite-bearing samples were collected from drill core from the Gränsgruvan (1), Norrgruvan (2), Tvistbo (3), Lustigkulla-Marnas (4), Baklängan (6) and Dammberget (7) deposits (Fig. 2, see map legend for numbers tied to deposit name). A total of 340 polished thin sections were examined with an Olympus BX-60 dual reflected-transmitted light microscope. Garnet and biotite in garnet-biotite rock, skarn, gedrite-albite rock, ash silt-sandstone, garnet-magnetite rock, quartz-garnet-pyroxene rock, sericite altered rock, sulphide mineralisation and marble were analysed.
Major element compositions of garnet and biotite were obtained using a JEOL 8900 electron probe microanalyser at the University of Minnesota. Operating conditions included an accelerating voltage of 15 kV and a beam current of 20 nA. Mineral standards included gahnite (Zn, Al), pyrope (Si, Mg), hornblende (Si, Al, Ca, Mg, Ti), almandine (Si, Al, Fe), ilmenite (Fe, Ti), spessartine (Al, Mn), albite (Al, Na) and K-feldspar (K). Garnet end-members considered the Fe2+ and Fe3+ contents, which were determined using the procedure of Frieberg (Reference Frieberg1989) and Deer et al. (Reference Deer, Howie and Zussman1992). The composition of garnet is given in terms of the almandine (Alm), andradite (Adr), grossular (Grs), pyrope (Pyr) and spessartine (Sps) components using the following format: Alm35–50Pyr2–10Grs8–10Sps10–20Adr2–10. The mineral abbreviations of Warr (Reference Warr2021) are used throughout the paper.
Trace element concentrations were measured at the U.S. Geological Survey in Denver, Colorado, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), comprising a Photon Machines Analyte LA system (excimer 193 nm) coupled to a PerkinElmer DRC-e ICP-MS. Analytical details are given in Supplementary Table S1 and summarised here. Depending on the grain size and presence of inclusions, minerals were ablated using spot sizes of 30, 40, 65, 85, 110 and 135 μm. Analyses were externally calibrated using the basalt glass microbeam reference standard GSD-1G (Guillong et al. Reference Guillong, Hametner, Reusser, Wilson and Gunther2005). The electron microprobe data were required for the trace element study of garnet and biotite since values of Al served as the internal standard. GSD-1G was analysed three times during a 10-hour day to correct for instrument sensitivity drift. Concentrations of elements were determined off-line using GeoPro (CETAC Technologies software), following the procedures of Longerich et al. (Reference Longerich, Jackson and Gunther1996). Signals were screened for microinclusions before further data processing (Nadoll & Koenig, Reference Nadoll and Koenig2011). Fifty-two elements were analysed: 75As, 137Ba, 44Ca, 111Cd, 140Ce, 59Co, 52Cr, 133Cs, 63Cu, 163Dy, 166Er, 153Eu, 57Fe, 71Ga, 157Gd, 72Ge, 180Hf, 165Ho, 115In, 39K, 139La, 175Lu, 7Li, 23Na, 60Ni, 24Mg, 25Mg, 55Mn, 98Mo, 93Nb, 146Nd, 31P, 208Pb, 141Pr, 85Rb, 45Sc, 29Si, 147Sm, 118Sn, 88Sr, 181Ta, 159Tb, 232Th, 47Ti, 205Tl, 169Tm, 51V, 238U, 89Y, 172Yb, 182W, 66Zn and 90Zr. Of these elements, 47Ti, 49Sc 51V, 52Cr, 55Mn, 60Ni, 66Zn, 71Ga, 74Ge, 85Rb, 90Zr, 93Nb and 118Sn were typically above detection limits for garnet whereas 7Li, 23Na, 31P, 44Ca, 45Sc, 47Ti, 51V, 55Mn, 59Co, 66Zn, 71Ga, 85Rb, 88Sr, 93Nb, 133Cs, 137Ba, 181Ta, 182W and 205Tl exceed the detection limits in biotite (Supplementary Table S2). The REEs were above detection limits in garnet but mostly below them in biotite. Although LA-ICP-MS analyses were not standardised for the major elements Ca, Fe, Mg and Mn, they exhibit percent-level concentrations as expected.
5. Mineralogy of garnet- and biotite-bearing altered rocks and sulphide mineralisation
5.a. Garnet-biotite rock
Garnet-biotite rocks occur in the hanging wall and footwall of sulphide mineralisation commonly in contact with or adjacent to skarn (Fig. 2). They are up to 70 m wide and well exposed at surface adjacent to Stollmalmen where it occurs as layers of coarse garnets (up to 2 cm in diameter) intergrown with biotite, gahnite, quartz, fibrolite, muscovite, cordierite and quartz alternating with layers of less altered rhyolitic ash silt-sandstone (Fig. 4a, Table 2). It occurs on both sides of the limestone-hosted mineralisation here. On the hanging wall side, it also contains prominent radiating fans and sheaths of anthophyllite. At Dammberget, which occurs along strike to the north of Stollmalmen, fluorite, gahnite and grunerite also occur in minor amounts with trace monazite. Amphiboles are a common accessory in garnet-biotite rocks in the Tvistbo (grunerite and ferro-hornblende), Dammberget (hornblende, grunerite and gedrite) and Gränsgruvan deposits (grunerite and ferro-tschermakite) (Frank et al. Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) as are sulphides (pyrrhotite, pyrite, chalcopyrite, sphalerite, galena). Garnet contains inclusions of gahnite (Fig. 4b) and quartz (Fig. 4c), the latter forming trails that parallel the S1 schistosity. Garnet is commonly stretched and fractured, likely during D1 (Fig. 4d, e). In places, garnets were rotated with a later-formed overgrowth suggesting garnet growth after S1 also (Fig. 4f), a feature previously recognised by Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013).
Figure 4. Photographs of outcrops and photomicrographs of polished-thin sections of garnet and biotite-bearing altered rocks in the Stollberg syncline. (a) Laminated garnet (Grt)-biotite rock alternating with ash silt-sandstone layers. The garnet-biotite rock also contains gahnite, cordierite, gedrite and quartz in the hanging wall of Stollmalmen deposit. (b) Gahnite (Ghn) inclusions in garnet (Grt) in garnet-biotite altered rock in the hanging wall of Stollmalmen deposit (plane-polarised transmitted light). (c) Garnet-biotite alteration spatially associated with the Dammberget deposit. Note the quartz inclusion trails in the largest garnet approximately parallel to the S1 schististy developed in biotite. Garnet formed during S2 is generally more inclusion free but where present the inclusions are coarser (see Qz inclusions). The dark spots in biotite are monazite crystals. (d) Elongate and fractured garnets in garnet-biotite altered rock from the Gränsgruvan deposit (micro X-ray fluorescence image). The bright spots in the fractured parts of some garnets are pyrite grains. Note the elongation direction parallels S1. (e) Elongate garnet parallel to S1 in garnet-biotite altered rock from the Gränsgruvan deposit. f. Elongate and fractured garnet in garnet-biotite altered rock from the Gränsgruvan deposit (micro X-ray fluorescence image). The zoning in the large garnet is due to an enrichment of Mn in the core of the garnet. Note also the rim of the garnet (S2) overgrowing the S1 schistosity developed in the biotite suggesting that garnet growth outlasted the development of the schistosity (likely S1).
Table 2. Biotite- and/or garnet-bearing assemblages in altered rock types in the Stollberg ore field
* present in trace amounts,
** secondary mineral, minerals listed in approximate order of abundance, abbreviations after Warr (Reference Warr2021).
5.b. Quartz-garnet-pyroxene rock and garnet-pyroxene skarn
In a previous study of the Stollberg ore district, Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) reported that the main difference between quartz-garnet-pyroxene rocks (Fig. 5a, b) and garnet-pyroxene skarn (hereafter referred to as skarn) (Fig. 5c) was the presence of quartz in the former and only minor, if any, quartz in the latter. The proportion of calcite in these rocks is variable with calcite generally being proportionally higher in skarns. At Gränsgruvan, where quartz-garnet-pyroxene rocks are at least 120 m wide in contact with sulphide mineralisation, the calcite content is relatively minor along with biotite, microcline and tremolite in a rock mainly composed of quartz, garnet and clinopyroxene (diopside/hedenbergite). A variety of amphiboles (actinolite (Fig. 5b), ferro- and magnesio-hornblende, ferro-tschermakite, ferro-edenite, ferro-pargasite, pargasite) are associated with quartz-garnet-pyroxene rocks. The same varieties can occur in skarns along with anthophyllite and tremolite in some locations. Other minerals found in skarns include chlorite, knebelite, gahnite, ilmenite, fluorite (Fig. 5d), talc, clinozoisite, cordierite, staurolite and magnetite, along with sulphides (mainly pyrite, pyrrhotite, chalcopyrite, sphalerite, galena) and rare monazite. Many of these minerals are also found in other rock types. Garnet is fractured, corroded and commonly shows a poikilitic texture in both skarns and quartz-garnet-pyroxene rocks.
Figure 5. Photographs of drill core, outcrop and thin-section photomicrographs. (a) Quartz (Qz)-garnet (Grt)-hedenbergite (Hd) altered rock from the Gränsgruvan deposit. (b) Actinolite (Act), garnet and plagioclase (Pl) in quartz-garnet-pyroxene (hedenbergite) altered rock from the Gränsgruvan deposit. (c) Actinolite, garnet and biotite in sulphide-bearing skarn from the Gränsgruvan deposit. (d) Massive garnet in skarn with fluorite (Flr) from the Norrgruvan deposit. (e) Photomicrograph of garnet in gedrite (Ged)-plagioclase rock in the footwall of the Dammberget deposit (plane-polarised light). (f) Outcrop of coarse garnet-magnetite (Mag) rock spatially associated with the Baklängan deposit.
5.c. Sericite altered rock
Sericite altered rocks are most common at Gränsgruvan and Tvistbo and less common on the eastern limb of the Stollberg syncline (Fig. 2). They are dominated by quartz, microcline and sericite with minor poikilitic garnet and, along with silica-altered rock and skarn, are host to sulphide mineralisation at Gränsgruvan (Table 2). Sericite altered rocks can also contain biotite, coarse muscovite, hornblende, actinolite, cordierite, calcite and ferro-tschermakite.
5.d. Gedrite-albite rock
Gedrite-albite rock occurs in the stratigraphic footwall on the eastern side of the Stollberg syncline from the Cedercreutz deposit to just south of Brusgruvan and locally in the hanging wall of sulphide mineralisation. It consists of quartz, ferrogedrite, albite, along with lesser garnet and biotite, and trace amounts of cordierite, magnetite, pyrite, titanite and epidote (Fig. 5e).
5.e. Rhyolitic ash siltstone-sandstone
Rhyolitic ash siltstone-sandstone is the most abundant rock type in the Stollberg succession and is composed of quartz, feldspar, biotite and lesser muscovite with local accumulations of chlorite, feldspar, amphibole, garnet, epidote, cordierite, monazite, pyrrhotite and pyrite in the more altered equivalents.
5.f. Garnet-magnetite rock
Garnet-magnetite rock is not common in the Stollberg area but is locally spatially associated with the Baklängen deposit where it consists of garnet, magnetite, quartz, biotite and rare monazite (Fig. 5f).
5.g. Sulphide mineralisation
Sulphide mineralisation occurs as massive ore bodies or disseminations with the metallic minerals consisting mainly of sphalerite, galena, chalcopyrite, pyrrhotite, pyrite, arsenopyrite and magnetite. Gangue minerals are varied, depending on the deposit, but include quartz, garnet, gahnite, orth- and clino-amphiboles, clinopyroxene, calcite, olivine, biotite, muscovite, chlorite, fluorite and staurolite (Table 2).
5.h. Marble
Marble is located on both limbs of the Stollberg syncline where it occurs as coarse-grained massive, recrystallised calcite, or as dolomitic or Mn-rich marble. Although relatively pure, it contains minor amounts of garnet, epidote, clinopyroxene, actinolite and hornblende, particularly where it locally grades into skarn, along with minor amounts of pyrrhotite, sphalerite, chalcopyrite, galena and magnetite.
5.i. Mafic rocks
Sills and dykes of post-Svecokarelian age occur in altered rocks in the Stollberg area. Two amphibolite sills likely of basaltic compositions, above (40 m wide) and below (80 m wide) the sulphide mineralisation at Gränsgruvan, contain plagioclase, hornblende, actinolite and magnetite or ilmenite in addition to minor amounts of garnet (Fig. 2). In addition, there is a cross-cutting mafic dyke at Gränsgruvan with up to centimetre size plagioclase crystals in a fine-grained matrix. Phlogopite is common in the lower mafic sill, especially at its margins. Mafic dykes also occur on the eastern side of the Stollberg syncline cross-cutting the stratigraphic sequence in the Staren area including the Staren marble.
6. Results
6.a. Major-element compositions of garnet and biotite
The locations and mineral assemblages of garnet-bearing samples garnet in the various rock types and mean garnet end-member compositions are listed in Tables 2 and 3, respectively, while individual compositions for each sample are in Supplementary Table S3. Major element compositions of garnet are in Supplementary Table S4, while representative compositions of garnet are given in Table 4 and plot in Figure 6. Major element compositions reported by Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) as garnet end-members are also listed in Table 3 for comparative purposes. Included in these analyses are both Fe2+ and Fe3+ measurements for garnet, which show that the Fe3+ contents are generally low by comparison to the amount of Fe2+.
Table 3. Mean garnet end-member compositions in various rock types in the Stollberg ore field
Alm = almandine, And = andradite, Grs = grossular, Pyr = pyrope, Sps = spessartine.
Mineral abbreviations after Warr (Reference Warr2021).
Table 4. Representative major (wt.%) and trace element (μg/g) compositions of garnet
1 DBH82007 153.5 Tvistbo Qz-Grt-Px rock, 2 GGR125 260.9 Gränsgruvan mafic dyke, 3 GGR125 269.1 Gränsgruvan skarn, 4 GGR125 310.9 Grt-Bt rock, 5 SSF26 465.4 Ged-Alb rock, 6 SSF21 380.0 Baklängan Grt-Bt rock, 7 SSF21 541.5 Baklängan skarn, 8 SSF21 583 Baklängan Grt-Mag rock, 9 StolStop1e Lustigkulla Grt-Bt rock, 10 DBH86001 44.6 Norrgruvan Grt-Bt rock, b.d. below detection limit.
Figure 6. Ternary plot of garnet compositions of least altered (rhyolitic ash-siltstone) and altered rocks from the Stollberg ore field. Abbreviations are after Warr (Reference Warr2021): Alm = almandine, And = andradite, Grs = grossular, Pyr = pyrope, Sps = spessartine.
Garnet shows a very broad range of compositions, with the almandine end-member dominating (i.e. generally > 50%) in most samples of garnet-biotite, gedrite-albite and sericite-altered rocks along with garnet in sulphide mineralisation. In contrast, the spessartine end-member is enriched in garnet in skarn, magnetite-garnet altered rock (Lustigkulla-Marnäs deposit), some garnet-biotite altered rocks and single samples of marble and mafic dyke from the Gränsgruvan deposit. For these last two samples, garnet is also enriched in grossular. It should be noted that roughly equal proportions of the grossular, spessartine and almandine end-members occur in garnet in garnet-pyroxene altered rock from the Gränsgruvan and Tvistbo deposits (Table 3, Supplementary Table S3).
Although most biotite compositions fall in the range Fe/(Fe+Mg) = 0.4–0.6 reflecting intermediate compositions, they range from phlogopite to siderophyllite (Fe/(Fe+Mg) = 0.29–0.87 and total Al per formula unit = 2.0–4.5) regardless of host rock lithology (Fig. 7). The end-member compositional extremes are of biotite in garnet-biotite altered rocks from the Dammberget and Baklängan deposits. Representative major-element compositions of biotite are in Table 5 while mean compositions of all garnets analysed here are given in Supplementary Table S5.
Figure 7. Major-element compositions of biotite as a function of total Al (a.p.f.u.) vs Fe/(Fe+Mg). Also shown are clusters of data for biotite in garnet-biotite (Grt-Bt) altered rocks from Dammberget, Lustigkulla-Marnäs, Tvistbo and Baklängen showing the broad range of compositions for biotite in garnet-biotite rocks in the Stollberg ore field.
Table 5. Representative major (wt.%) and trace element (μg/g) compositions of biotite
1 GGR138 607.6 Gränsgruvan Grt-Bt rock, 2. Gränsgruvan GGR142 397.2 Grt-Bt rock, 3. Baklangen SSF21 541.5 Skarn, 4. Gränsgruvan SSF30 641 Grt-Bt rock, 5. Gränsgruvan GGR142 408.4 Grt-Bt rock, 6. Dammberget SSF28 847.5, 7. Lustigkulla STOL STP-1e Grt-Bt rock, 8. Norrgruvan DBH86004 175.2 Rhyolitic ash silt-sandstone, b.d. below detection limits.
6.b. Trace-element compositions of garnet
It has long been known that garnet can contain a large number of trace elements including B, Be, Cr, Cu, F, Ga, Ge, Na, Nb, Pb, Li, Sn, Sr, Ti and V, in addition to the REEs (e.g. Jaffe, Reference Jaffe1951). Garnet compositions (n = 1001) were obtained from 58 samples of gedrite-albite, garnet-biotite, garnet-magnetite, sericite and garnet-pyroxene altered rocks, as well as marble, sulphide mineralisation, rhyolitic ash silt-sandstone and a garnet-bearing mafic dyke. More than half the samples (n = 32) were of garnet in garnet-biotite altered rocks. Mean trace elements concentrations are given in Supplementary Table S6 while representative trace element compositions are given in Table 4. Unless otherwise stated, values reported here for a given sample are mean concentrations. Box and whisker plots of the concentrations of Sc (Fig. 8a), Zn (Fig. 8b), Ga (Fig. 8c) and Ti (Fig. 8d) in garnet are shown.
Figure 8. Trace-element compositions (µg/g) of garnet in various alteration types and host rocks. (a) Sc; (b) Zn; (c) Ga; and (d) Ti. Location abbreviations are: B = Baklängan, D = Dammberget, G = Gränsgruvan, L = Lustigkulla-Marnäs, N = Norrgruvan, T = Tvistbo.
Given that the Stollberg district is host to base and precious metals (Pb-Zn-Ag-(Cu-Au)), preliminary LA-ICP-MS analyses of garnet were conducted but showed concentrations of Ag and Au below detection limits, while routine analyses for Cu showed it to be almost entirely below detection limits with the remainder (>90%) below 2 µg/g. Five outliers (out of > 900 analyses) were higher than 80 µg/g but are likely due to the laser beam incorporating inclusions of chalcopyrite. Of the nine deposits studied here, six of the eight possess concentrations of Zn (as sphalerite) >Pb (as galena) with Brusgruvan and Baklängan being Pb-rich deposits (see Ripa, Reference Ripa2012; Spry et al. Reference Spry, Jansson and Allen2024). The metal grades for Norrgruvan are unknown. The concentration of Pb in garnet, like Cu, is close to background values with a few outliers that are likely related to inclusions of galena. In contrast to Cu and Pb, Zn is enriched in garnet with mean concentrations of between 8 and 666 µg/g Zn (Supplementary Table S6). More than half the analyses are of garnet from garnet-biotite altered rock adjacent to sulphide mineralisation. It contains up to 239 µg/g Zn with values of Zn occurring in garnet in rhyolitic ash silt-sandstone (up to 352 µg/g), quartz-garnet-pyroxene altered rock (up to 426 µg/g) and skarn (up to 33 µg/g). Sulphide mineralisation contains garnet with the highest Zn content (up to 666 µg/g) (Supplementary Table S6, Fig. 8b).
Although there is variability in the Zn content of garnet in the various lithologies, the Sc content is, by comparison, less variable, ranging from 0.6 to 103 µg/g Sc (Supplementary Table S6, Fig. 8a). Values of > 50 µg/g Sc occur in garnet in garnet-biotite altered rock (50.4–103.2 µg/g, n = 4), sericite altered rock (53.1 µg/g) from Gränsgruvan, and gedrite-albite altered rock from Dammberget (70 µg/g).
The highest concentrations of Ga in garnet occur in sulphide mineralisation (49 µg/g) and quartz-garnet-pyroxene rock from Gränsgruvan (26 µg/g). These values are higher than the highest values of 19 µg/g Ge, which occur in garnet-bearing sericite-altered rock (Gränsgruvan) (Fig. 8c, Supplementary Table S6).
Concentrations of Ti in garnet for the various rock types are high relative to other metallic elements and range from 11 µg/g in garnet-biotite rock from Norrgruvan to 1138 µg/g Ti, a mafic dyke at Gränsgruvan. The mafic dyke along with marble from Gränsgruvan, the latter of which has the second highest Ti concentration (870 µg/g) in garnet, also contains the highest concentrations of V (122 µg/g) and Na (277 µg/g) (Supplementary Table S6, Fig. 8d).
Pyralspite garnet generally incorporates a higher concentration of heavy REEs than light REEs in its lattice (see Lottermoser, Reference Lottermoser1988, Reference Lottermoser1989; Schwandt et al. Reference Schwandt, Papike, Shearer and Brearley1993, Heimann et al. Reference Heimann, Spry, Teale, Conor and Pearson2011). This is also the case for pyralspite garnet in all lithologies in the Stollberg ore district, where the mean concentration of HREEs (Eu is included in the HREEs) commonly exceeds 100 µg/g (Fig. 9a) compared to the LREEs, which are generally less than 10 µg/g (Fig. 9b). The sample with the highest total REEs is sample GGR 137 733.2, a sericite-altered rock from Gränsgruvan. It contains 26 µg/g total LREEs, while the total HREEs are 196 µg/g. Garnet in garnet-biotite rocks generally contains the highest HREEs regardless of location. Samples with the five highest concentrations of Er (up to 224 µg/g), Tm (up to 38 µg/g), Yb (up to 303 µg/g) and Lu (50 µg/g) are in garnet from garnet-biotite rocks from Norrgruvan, Gränsgruvan, Dammberget and Tvistbo. The highest concentrations of Tb (15 µg/g), Dy (189 µg/g), Lu (60 µg/g), Nd (23 µg/g), Pr (5 µg/g), Ce (49 µg/g) and La (25 µg/g) are also from garnet-biotite rocks with the last four elements being highest in sample GGR 142 448.8 in garnet-biotite rock from the Gränsgruvan deposit. Four of the seven highest concentrations of Eu (9–20 µg/g) are in garnet from sulphide mineralisation in the Dammberget deposit, while a quartz-garnet-pyroxene rock, sample GGR 137 459.5 from Gränsgruvan, contains garnet with the second highest concentrations of Eu (15 µg/g) and Zn (423 µg/g) and the highest concentration of Sm (10 µg/g). The Eu contents of garnet relative to chondrite values of McDonough and Sun (Reference McDonough and Sun1995) are ∼0.25 for garnet from garnet-pyroxene rock from Gränsgruvan and sulphide mineralisation from Dammberget. Values for other rocks in the district are < 0.1 (Fig. 9c).
Figure 9. Rare earth element (REE) contents of garnet. (a) Total REE content (µg/g); (b) Total light (LREE) (µg/g) and (c) the ratio of Eu in garnet/chondrite. Location abbreviations are: B = Baklängan, D = Dammberget, G = Gränsgruvan, L = Lustigkulla-Marnäs, N = Norrgruvan, T = Tvistbo.
Although there is an overlap in trace element compositions, as shown in the element-element and element-ratio plots in Figure 10a–d, some lithologies can be distinguished, for example, on the basis of Zn concentration in garnet versus Ga (Fig. 10a) and Zn versus Eu content of garnet relative to chondrite (Fig. 10c). In these plots, garnet in rhyolitic ash silt-sandstone, sulphide mineralisation and garnet-biotite altered rock from Dammberget contrasts with those for other rock types. Similarly, plots of Zn versus Ge (Fig. 10b) and Y versus Eu (Fig. 10d) content of garnet relative to chondrite show overlapping data for all lithologies regardless of the spatially associated deposit.
Figure 10. Element-element and element-ratio plots of garnet composition (µg/g) of garnet in various alteration types and host rocks. (a) Zn vs Ga; (b) Zn vs Ge; (c) Zn vs Eu garnet/chondrite and (d) Y vs Eu garnet/chondrite.
Down-hole plots of the Eu anomaly in garnet in various lithologies in diamond drill holes SSF 26 (Fig. 11) from Dammberget and GGR 125 and GGR 137 from Gränsgruvan (Fig. 12) are shown in geological cross-sections and in down-hole block sections along with the concentrations of Zn (µg/g), MnO (wt. %), Sc (µg/g), Ga (µg/g), Ge (µg/g) and Pb (µg/g) (Fig. 13a–f). Europium anomalies for garnet in all samples are listed in Supplementary Table S7 and show that in the uppermost samples of GGR 137 (0.81, depth 340.2 m) garnet in a garnet-pyroxene altered rock and in the three lower most samples of garnet-biotite altered rock (0.26, depth 682.8 m; 0.50, depth 699.0 m and 0.40, depth 733.2 m) exhibit Eu anomalies <1. Similarly, garnet in gedrite-albite rocks, which are the uppermost samples analysed in SSF 26, are <1 as is the lowermost sample (garnet-biotite altered rock) (Figs. 11 and 13). Garnet in all other samples in both drill hole has Eu anomalies of between 1.90 and 15.34 (Figs. 11–13, Supplementary Table S7).
Figure 11. Cross section of the Dammberget deposit from drill hole SSF 26 that intersects sulphide mineralisation, which is associated with skarn and garnet-biotite alteration. Chondrite-normalised rare earth element (REE) plots of garnet show down-hole variability in garnet chemistry. Garnet is generally enriched in heavy REEs and depleted in light REEs. Garnet near sulphide mineralisation generally possess a strong positive Eu anomaly, whereas garnet in gedrite-albite alteration and sulphide-barren unaltered ash silt- sandstone possess no Eu anomaly.
Figure 12. Cross section of the Gränsgruvan deposit from drill holes GGR 137 and GGR 125. Garnet intergrown with phlogopite, in the footwall below sulphide mineralisation, possesses a positive Eu anomaly. Garnets in garnet-biotite alteration (601.8 m) in the hanging wall (not associated with sulphides) possess no Eu anomaly. Garnet-pyroxene alteration (537.0 m) associated with a narrow intersection of sulphides (GGR 125) has a positive Eu anomaly.
Figure 13. Down-hole variation of the mean value of Zn, Sc, Ga, Ge and Pb (in µg/g), MnO (wt. %) and the Eu anomaly (Eu/Eu*) in garnet from drill cores GGR 137 (a and d), GGR 125 (b and e) from the Gränsgruvan deposit and SSF 26 (c and f) from the Dammberget deposit. The element Zn is plotted in both images as a basis for comparison with other elements.
6.c. Trace-element compositions of biotite
Biotite has the general formula XY3Z4O10(W)2, where X site can be occupied by K, Na, Ba, Ca, Cs, Ag, Cu+, H3O+, NH4+; Y site by Al, Mg, Fe2+, Fe3+, Ti, Li, Cr, Mn, V, Zn, Co, Cu2+, Ni, Pb2+; Z site by Si, Al, Be; and W site by (OH)−, F, Cl−, O2 − (Rieder et al. Reference Rieder, Cavazzini, D’yakonov, Frank-Kamenetskii, Gottardi, Guggenheim, Koval, Müller, Neiva, Radoslovich, Robert, Sassi, Takeda, Weiss and Wones1999; Li et al. Reference Li, Fan, Zhang, Ding, Yue, Xie, Cai, Quan and Sein2020; Baidya et al. Reference Baidya, Maiti, Mondal and Upadhyay2024).
Of the three common base metals, Cu, Pb and Zn, like garnet, biotite in the Stollberg ore district contains elevated mean concentrations of Zn in rhyolitic ash silt-sandstone (396–2072 µg/g), garnet-biotite altered rock (95–2788 µg/g), sulphide mineralisation (927–1066 µg/g) and skarn (1460 µg/g) (Supplementary Table S8). Concentrations of Cu and Pb in biotite are commonly below the limits of detection. However, values > 110 and up to 596 µg/g Pb occur in garnet-biotite altered rock from the Dammberget and Baklängan deposits on the east side of the Stollberg syncline. The highest concentrations of Nb (138 µg/g) and Ga (121 µg/g) occur in garnet-biotite rock and sulphide mineralisation, while the highest concentrations of Li (152 µg/g) and Sn (57 µg/g) are present in biotite in rhyolitic ash silt-sandstone from Norrgruvan and Dammberget, respectively. Biotite in drill core SSF26 in rhyolitic ash silt-sandstone, sulphide mineralisation and garnet-biotite rock contains the highest concentrations of Tl (68–330 µg/g) from the Dammberget deposit.
In general, high concentrations of Ba, Na, Mn, Rb and Ti are common in biotite (e.g. Ibhi & Nachit, Reference Ibhi and Nachit2000; Tischendorf et al. Reference Tischendorf, Förster and Gottesmann2001). In the rocks analysed here, Ba in biotite ranges from 164 to 2748 µg/g, with biotite in garnet-biotite rock containing the highest concentration. This rock type also contains biotite in single samples with the highest mean concentrations of Na (7146 µg/g), Mn (8801 µg/g), Rb (906 µg/g) and Ti (10714 µg/g) (Supplementary Table S8). In the other rock types analysed here, the following mean concentrations are observed for these elements: Na (1221–6083 µg/g), Mn (91–1809 µg/g), Rb (128–472 µg/g) and Ti (1177–5092 µg/g).
Given that most biotite analyses (n = 297) were from garnet-biotite rocks, scatter plots are presented here for Mn vs Zn (Fig. 14a), Ga vs Zn (Fig. 14b), Pb vs Zn (Fig. 14c) and Sn vs Zn (Fig. 14d). The mean concentrations of Mn in biotite from Dammberget (48–1809 µg/g) and Baklängan (137–1260 µg/g) show a smaller range compared to those from Norrgruvan (247–8801 µg/g) and Gränsgruvan (153–7144 µg/g) (Fig. 14a). The high concentrations of Ga (47 and 107 µg/g) and Zn (2192 and 2788 µg/g), occur in biotite in garnet-biotite altered rocks in samples adjacent to each other (GGR 138 591.8 and GGR 138 593.3) from the Gränsgruvan deposit (Fig. 14b).
Figure 14. Scatter plots of trace element concentrations of Mn, Ga, Pb and Sn vs Zn in biotite. (a) Mn vs Zn. (b) Ga vs Zn. (c) Pb vs Zn. (d) Sn vs Zn for various rocks spatially associated with mineralisation.
7. Discussion
7.a. Compositions of garnet
Garnet in garnet-biotite, sericite and gedrite-albite altered rocks as well as in rhyolitic ash silt-sandstone and sulphide mineralisation is generally Fe-rich and contains >50 mole % of the almandine end-member. However, garnet in garnet-biotite altered rock from Tvistbo differs from other locations in that it contains up to 59 mole % spessartine. A similar composition (Alm24–34Pyr0–2Grs7–16Sps50–68And0–1) was reported previously by Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) for garnet in the garnet-biotite rock in the hanging wall of the Tvistbo deposit. The composition of most garnets in the aforementioned rock types contrasts to those in skarn, quartz-garnet-pyroxene rock and marble, which mostly contain < 50 mole% of the almandine end-member and higher proportions of the grossular and spessartine end-members. For example, garnet in quartz-garnet-pyroxene rock contains up to 43 and 28 mole % of grossular and spessartine, respectively, while garnet in skarn from Tvistbo contains up to 60 mole % spessartine. Garnet in quartz-garnet-pyroxene rocks from the Gränsgruvan contains up to 65 and 39 mole % grossular and spessartine, respectively. The major element compositions of garnet in the rocks reported here are very similar to those reported previously by Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) for each of the same rock types (Table 3). Although not analysed by Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019), a single sample of garnet in marble from Gränsgruvan contains 39 and 31 mole % grossular and spessartine, respectively. Similarly, garnet in a metamorphosed mafic dyke contains 19 and 31 mole % grossular and spessartine, respectively.
In evaluating the composition of garnet from nine samples of altered rock in the Stollberg ore field, Ripa (Reference Ripa2012) showed garnet in biotite-rich samples (i.e. rhyolitic ash silt-sandstone, garnet-biotite altered rocks) was enriched in almandine (Alm72–87Pyr5–11Grs1–8Sps1–16And0–1, n = 6), to an intermediate composition between almandine and spessartine (Alm40Pyr2Grs18Sps40And0, n = 1). Garnet in olivine-pyroxene parageneses was more enriched in the spessartine and grossular end-members (Alm32–43Pyr3–8Grs12–16Sps40–43And3, n = 2) relative to those in biotite-rich samples. The data collected here and by Frank et al. (Reference Frank, Spry, Raat, Allen, Jansson and Ripa2019) reinforce the proposal of Ripa (Reference Ripa2012) that the major element composition of garnet in the Stollberg ore field is, in large part, dictated by the bulk composition of the host rock. However, it should be noted that garnet compositions can also be affected by several other parameters including temperature, rate of crystal growth, diffusion rates and kinetics (e.g. Yogi et al. Reference Yogi, Gaidies, Heldwein and Rice2025).
Although an investigation of the trace element composition of garnet in SVALS-type deposits is restricted to the current contribution, trace element studies of garnet in other types of metallic mineral deposits have largely focused on rocks spatially related to contact metamorphosed skarn (Au, Cu, Fe, Mo, Sn, W, Zn) deposits (e.g. Gaspar et al. Reference Gaspar, Knaack, Meinert and Moretti2008; Xu et al. Reference Xu, Ciobanu, Cook, Zheng, Sun and Wade2016; Zhang et al. Reference Zhang, Shao, Wu and Chen2017; Xie et al. Reference Xie, Yang, He and Gao2022; He et al. Reference He, Liang, Wang, Zhao, Liu, Gao and Cen2023). In contrast, trace element studies of garnet in regionally metamorphosed massive sulphide deposits are restricted to SEDEX (Stalder and Rozendaal, Reference Stalder, Rozendaal, Mao and Bierlein2005; Pollock et al. Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018, Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019) and BHT deposits (Lottermoser, Reference Lottermoser1988, Reference Lottermoser1989; Schwandt et al. Reference Schwandt, Papike, Shearer and Brearley1993; Spry et al. Reference Spry, Heimann, Messerly and Houk2007; Heimann et al. Reference Heimann, Spry, Teale, Conor and Pearson2011). The REE pattern of garnet in metamorphic rocks is dependent on several variables including bulk-rock composition, relative contribution of hydrothermal and detrital components, the partitioning of elements among coexisting minerals, fO2, crystal chemistry, aqueous speciation of REE in the metamorphic fluid phase in equilibrium with garnet, and P-T conditions (e.g. Sverjensky, Reference Sverjensky1984; Bau, Reference Bau1991; Smith et al. Reference Smith, Henderson, Jeffries, Long and Williams2004; Gaspar et al. Reference Gaspar, Knaack, Meinert and Moretti2008). As pointed out by Gaspar et al. (Reference Gaspar, Knaack, Meinert and Moretti2008), Al-rich garnets (almandine, pyrope, spessartine and grossular) in metamorphic and magmatic rocks generally show chondrite-normalised patterns that are LREE-depleted, HREE-enriched and show weak negative or positive Eu anomalies. They also showed that garnets with a high proportion of the andradite end-member commonly as andradite-grossular (i.e. grandite) solid solutions in skarn deposits, exhibit LREE-enriched and HREE-depleted patterns with pronounced positive Eu anomalies. The latter patterns are observed in grandite garnets with the andradite end-member being dominant, for example, from the Sangdong and Weondong W deposits, South Korea (Park et al. Reference Park, Choi, Kim, Park, Kang, Lee and Song2017), the Xinqiao Cu-S-Fe-Au deposit, China (Zhang et al. Reference Zhang, Shao, Wu and Chen2017) and the Yangla Cu deposit, China (Xie et al. Reference Xie, Yang, He and Gao2022).
Garnet in altered rocks from the Stollberg ore field are Al-rich with only a small andradite component. As such, LREE-depleted and HREE-enriched patterns are not surprising. Of note, however, is that the Eu anomaly is positive in sulphide mineralisation and spatially related altered rocks, with negative Eu anomalies in more weakly altered rocks distal to mineralisation (Figs. 10–13, Supplementary Table S7). Jansson et al. (Reference Jansson, Erismann, Lundstam and Allen2013) showed that whole-rock lithogeochemical samples in and near the Stollberg limestone, which were enriched in base metals, and Mn, Fe and Ca, showed positive Eu anomalies. They ascribed these anomalies to the pre-metamorphic ore-forming fluids being reduced, high-temperature (> 250 oC) fluids that leached Eu-bearing feldspar in sediments and/or volcanic rocks. Such physicochemical conditions of the ore-forming fluid are consistent with those determined by Spry et al. (Reference Spry, Jansson and Allen2024) based on stable isotope (S, C and O) compositions of sulphides and carbonates in the ore-forming sequence. The most likely reason for the positive Eu anomalies in garnet at Stollberg is also related to the relatively reduced nature of the pre-metamorphic ore-forming fluid and to its high temperature. The low proportion of andradite in Stollberg garnets, which is also an indirect evidence for the reduced state of the ore fluid, suggests that the cause of the positive Eu anomaly is not dictated by crystal considerations, which is typically ascribed to explaining the positive anomalies observed in garnets in skarn deposits (e.g. Gaspar et al. Reference Gaspar, Knaack, Meinert and Moretti2008). The reduced state of the ore zone is likely, notwithstanding the presence of Fe3+ in biotite, amphibole and clinopyroxene. Minor gahnite (ZnAl3 + 2O4) occurs in the ore rather than franklinite (ZnFe3+ 2O4) further supporting the supposition that the ore zone is reduced rather than oxidized (O’Brien et al. Reference O’Brien, Spry, Teale, Jackson and Koenig2015). An increase in fO2 conditions and a decrease in the amount of hydrothermal components and ore fluid temperature are the likely reasons for the presence of negative Eu anomalies in garnet in rhyolitic ash silt-sandstone distal to sulphide mineralisation (Fig. 12). However, it should be noted that a detailed assessment of garnet compositions distal to ore was not undertaken here. The presence of magnetite, an Fe3+-bearing mineral, is common in alkaline reducing environments.
Aside from the REEs, garnet in the various altered rock types in the Stollberg ore field (excluding post-metamorphic mafic dykes) contains the following elemental mean values: 8–666 µg/g Zn, 1–103 µg/g Sc, 2–49 µg/g Ga, 0.1–231 µg/g Pb, below detection limits to 363 µg/g Cu, 2–21 µg/g Ge, below detection limits to 65 µg/g V, below detection limits to 517 µg/g Cr, 0.4–22 µg/g Sn, 8–1440 µg/g Y, 11–870 µg/g Ti and 2–362 µg/g Zr (Table 6, Supplementary Table S6). In comparison, garnet in rocks spatially related to SEDEX deposits in the Kanmantoo Group, South Australia, which, like Stollberg, were metamorphosed to the amphibolite facies (Pollock et al. Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018; Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019), and to garnet in quartz garnetite and garnetite spatially associated with BHT deposits in the Curnamona province, South Australia and New South Wales, Australia (Heimann et al. Reference Heimann, Spry, Teale, Conor and Leyh2009), show similar concentrations of Cu (excluding one outlier 363 µg/g, which is likely due to inclusions of chalcopyrite in the lattice) and Zn but lower concentrations of Sn. Concentrations of Pb in garnet from the South Australian deposits have mean concentrations of 1–4 µg/g and are low, like most analyses of garnet in the Stollberg area, compared to the Broken Hill deposit, which has mean concentrations of 111 µg/g (Table 6). Concentrations of Ga, Zn and Zr are generally similar in garnet from the Stollberg, SEDEX and BHT deposits in South Australia while Co (491 µg/g), Cr (106 µg/g), Ge (42 µg/g), Pb (111 µg/g), Zn (324 µg/g) and Zr (609 µg/g) concentrations are highest in garnet in BHT deposits in New South Wales in the Broken Hill area, which were metamorphosed to the granulite facies (Table 6). Concentrations of Cr, Ge and Sc in garnet are generally lower than those in the SEDEX and BHT deposits reported here (Supplementary Table S6).
Table 6. Comparison of selected trace element compositions in garnet from Stollberg and SEDEX and BHT deposits (mean composition, μg/g)
1 Stollberg ore field range of means of various rock types (see Suplementary Table S6), 2. Kanmantoo Cu deposit, South Australia (Pollock et al. Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018), 3 Angas, Wheal Ellen, Scott’s Creek Pb-Zn-Ag deposits, South Australia (Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019), 4. Garnet in garnetite and quartz garnetite in upper greenschist-lower amphibolite BHT-type mineralisation, Curnamona Province, South Australia (Heimann et al. Reference Heimann, Spry, Teale, Conor and Leyh2009), 5. Garnet in garnetite and quartz garnetite associated with BHT mineralisation in the Broken Hill area, granulite facies (Heimann et al. Reference Heimann, Spry, Teale, Conor and Leyh2009), b.d. below limits of detection, n.d. not determined.
The concentration of certain trace elements in garnet from skarn deposits contrasts markedly to those in garnet in the Stollberg, BHT and SEDEX deposits in that they generally contain considerably higher concentrations of Cu, Mo, Sn, In, W and U. For example, grandite garnet in the Zhibula Cu skarn deposit, Tibet, contains mean values of 580 µg/g W, 170 µg/g Mo and up to 15 µg/g U, 200 µg/g Sn and 20 µg/g In (Xu et al. Reference Xu, Ciobanu, Cook, Zheng, Sun and Wade2016). Similarly, garnet from the Sangdong W skarn deposit, South Korea, contains up to 1447 µg/g W, 8050 µg/g Sn, 1196 µg/g Zn and 1296 µg/g Cu but low concentrations of Mo (up to 12 µg/g but mostly < 1µg/g) and U (up to 4 µg/g) (Park et al. Reference Park, Choi, Kim, Park, Kang, Lee and Song2017), whereas those from the polymetallic Sn and Zn-Cu skarn Changpo-Tongkeng deposit, China contains elevated mean values for Sn (up to 2821 µg/g), V (up to 278 µg/g), Zn (up to 9 µg/g), In (up to 23 µg/g) and U (up to 0.4 µg/g) (He et al. Reference He, Liang, Wang, Zhao, Liu, Gao and Cen2023).
7.b. Compositions of biotite
In a study of pelitic rocks in the Kanmantoo Group, South Australia, Hammerli et al. (Reference Hammerli, Spandler and Oliver2016) showed that the Zn content of biotite in pelitic rocks progressively decreased from greenschist (348–896 µg/g) through upper amphibolite facies (48–52 µg/g). Such values in rocks metamorphosed to the amphibolite facies are low by comparison to biotite in altered rocks in the Stollberg ore field (mean values = 959–2788 µg/g Zn). Note that the concentrations of Zn in biotite in Cu and Pb-Zn-Ag deposits in the Kanmantoo Group possess mean values of 595 and 926 µg/g, respectively (Table 7). Mean values of Zn in biotite are typically >100 µg/g in various types of ore deposits shown in Table 7, which include the orogenic Pampalo Au deposit, Finland (398 µg/g, Kuikka, Reference Kuikka2018) and skarn-porphyry (141–318 µg/g), polymetallic Au (174–280 µg/g), Au-Bi (482 µg/g), porphyry Mo (134 µg/g) and rare metal deposits (437–811 µg/g) within the Shakhtama intrusive complex, Russia (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023). Of the deposits reported here, only IOCG mineralisation in the Khetri Cu belt, India (Baidya et al. Reference Baidya, Saha, Pal and Upadhyay2023) contains biotite with mean Zn values of < 100 µg/g (i.e. 39 µg/g).
Table 7. Comparison of selected trace element compositions in biotite from Stollberg and various ore deposit types (mean composition, μg/g)
1. Stollberg ore field range of means of various rock types (see Supplementary Table S8), 2. Kanmantoo Cu deposit, South Australia (Pollock et al. Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018), 3. Angas, Wheal Ellen, Scott’s Creek Pb-Zn-Ag deposits, South Australia (Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019), 4. Types 3, 4 and 5 biotite in altered rocks associated with IOCG mineralisation in the Khetri iron oxide-copper-gold (IOCG) in the Khetri Copper Belt, India (Baidya et al. Reference Baidya, Saha, Pal and Upadhyay2023), 5. Archean lode gold mineralisation, Pampalo Au deposit, Finland (Kuikka, Reference Kuikka2018), 6. Lukogan, Kultuma, Bystrinsky Au-Cu-Fe skarn and skarn-porphyry deposits, Shakhtama Complex, Russia (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023), 7. Antiinskoe, Lugiinskoe, Notsuyskoe Au polymetallic deposits, Shakhtama Shakhtama Complex (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023), 8. Sredne-Golgotain Au-Bi deposit, Shakthama Complex, (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023), 9. Shakhtama porphyry Mo deposit, Shakhtama Complex (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023), 10. Belukhinskoe, Antonovpgorshoe rare metal deposits, Shakhtama Complex (Redin et al. Reference Redin, Redina, Malyuitina, Dultsev, Kalinin, Abramov and Borisenko2023), b.d. below the limits of detection.
Biotite can contain several percent Cu in its structure although it is uncertain as to the form it takes and where it resides in the structure. For example, Hiroi et al. (Reference Hiroi, Harada-Kondo and Ogo1992) reported up to 5.5 wt. % CuO in manganoan phlogopite from oxidized schists from Kamogawa, Japan and proposed that Cu+ substitutes for Mg2+ in octahedral sites, whereas Ilton and Veblen (Reference Ilton and Veblen1988) proposed that particles of native Cu are incorporated between sheets of biotite. Alternatively, Wang et al. (Reference Wang, Yang, Liu, Tong and Auwala2019) suggested that Cu was incorporated in Fe oxides in the structure of biotite. Up to 2.38 and 2.43 wt. % Cu also occurs in biotite in the Kanmantoo Cu (Pollock et al. Reference Pollock, Spry, Tott, Koenig, Both and Ogierman2018) and St. Ives Pb-Zn-Ag deposits (Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019), respectively. Such high concentrations of Cu are in contrast to the very low concentrations of Cu in biotite in the Stollberg deposits (primarily below the limits of detection), IOCG deposits in the Khetri Cu belt (6 µg/g) and the Pampalo Au deposit (0.5 µg/g). Low concentrations of Pb in biotite also occur in the last two deposits (1 and 2 µg/g Pb, respectively). Biotite in altered rocks associated with the Stollberg deposits contains up to 596 µg/g Pb, which is higher than the mean value for biotite from the Kanmantoo Cu deposit (46 µg/g Pb) but lower compared to that from Pb-Zn-Ag deposits in the Kanmantoo Group (889 µg/g Pb).
The Ba, Ga and Sn concentrations of biotite in various types of ore deposits range from 32 to 78 µg/g Ga and 1 to 9 µg/g Sn and are within the range of those in the Stollberg ore field (20–121 µg/g Ga and 3–57 µg/g Sn), with the exception being for Sn in biotite from rare metal deposits, which have considerably higher concentrations (60–262 µg/g Sn) (Table 7). Concentrations of Ba and Mn in biotite from Stollberg (122–2748 µg/g Ba and 48–8801 µg/g Mn) overlap with those from other deposits listed in Table 7 (711–917 µg/g Ba and 309–3817 Mn).
7.c. Garnet and biotite composition as exploration tools
A down-hole plot of biotite compositions in drill hole SSF 26 (Dammberget), shows they are highest in Ga, Mn, Pb, Sn and Zn in and adjacent to sulphide mineralisation and constitute potential exploration guides to ore (Fig. 15). Moreover, garnet shows LREE-depleted and HREE-enriched chondrite-normalised patterns with positive Eu anomalies in and adjacent to ore and negative Eu anomalies in more distal altered rocks (Figs. 11 and 12). In drill holes GGR 137 (Gränsgruvan) and SSF 26, positive Eu anomalies in garnet are also associated with the highest concentrations of Mn, Zn, Ga, Ge and Pb (Fig. 13). Such positive Eu anomalies in garnet from Stollberg resemble those in pyralspite garnets in quartz-garnetites proximal to the giant Broken Hill-Pb-Zn-Ag deposit, Australia (Spry et al. Reference Spry, Heimann, Messerly and Houk2007). In contrast, pyralspite garnet in metamorphosed SEDEX deposits in the Kanmantoo Group primarily show negative Eu anomalies and LREE-depleted and HREE-enriched chondrite-normalised patterns, reflecting the high detrital component in pelitic rocks enveloping the deposits and ore-forming temperatures of < 250 oC (Tott et al. Reference Tott, Spry, Pollock, Koenig, Both and Ogierman2019). However, it should be noted that garnet spatially associated with the Angas deposit, which is the largest Pb-Zn deposit in the Kanmantoo Group, shows chondrite-normalised Eu anomalies of up to 2.8 reflecting a higher hydrothermal component to the ore fluid. Reduced, pre-metamorphic ore-bearing fluids at Stollberg were likely > 250 oC and account for the positive Eu anomalies. Altogether, the results of this study show that positive europium anomalies in garnet can be used as a proximity indicator to metamorphosed massive sulphide mineralisation as has been previously proposed by Rozendaal & Stalder (Reference Rozendaal, Stalder and Piestrzyñski2001) and Stalder & Rozendaal (Reference Stalder, Rozendaal, Mao and Bierlein2005) in evaluating the REE composition of garnet in ore and rocks spatially associated with the Gamsberg SEDEX Zn deposit, South Africa.
Figure 15. Down-hole plot of trace element compositions (µg/g) for Mn, Zn, Ga, Y, Sn, Cs and Ti biotite in various altered rocks and unaltered host rocks in drill core SSF 26 from the Dammberget deposit.
8. Conclusions
Studies of the major and trace element compositions of garnet and biotite in the Stollberg ore field yield the following conclusions:
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(1) Garnet is generally Fe-rich in metamorphosed altered rocks and is more enriched in Ca (as grossular) and Mn in marbles and skarn near ore and contains measureable concentrations of Cr, Ga, Ge, Mn, Nb, Ni, Rb, Sc, Sn, Ti, V, Zn, Zr and REEs, whereas biotite shows variable Fe/(Fe+Mg) ratios and is enriched in Ba, Ca, Co, Cs, Ga, Li, Mn, Na, Nb, P, Pb, Rb, Sc, Sn, Sr, Ta, Ti, Tl, V, W and Zn and depleted in REEs.
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(2) Chondrite-normalised REE patterns of garnet in all altered rock types are depleted in light REEs and enriched in heavy REEs. They also show positive Eu anomalies in and adjacent to ore and negative Eu anomalies more distal to ore zones.
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(3) Positive Eu anomalies in garnet and associated elevated concentrations of Mn, Zn, Ga, Pb and Ge and high concentrations of Ga, Mn, Pb, Zn, Ga and Sn in biotite constitute potential exploration guides to ore. These elements can be used in conjunction with elevated concentrations of Cu, Mn, Mo, Pb, Sb and Zn in magnetite and light C and O isotope values in carbonates in the ore field that were proposed recently by Frank et al. (Reference Frank, Spry, O’Brien, Koenig, Allen and Jansson2022) and Spry et al. (Reference Spry, Jansson and Allen2024), respectively. Such elemental and isotopic compositions comprise potential vectors to metamorphosed massive sulphide deposits elsewhere.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756825100228.
Acknowledgements
Boliden Mineral AB is thanked for financially supporting this project. Discussions with Magnus Ripa (Geological Survey of Sweden) about the geology of the Stollberg ore field are greatly appreciated. The comments of Simon Schorn (Associate Editor), Alexandre Peillod and an anonymous reviewer are also greatly appreciated and improved the quality of this contribution.
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