Trace Element and Sulfur Isotope Signatures of Volcanogenic Massive Sulfide (VMS) Mineralization: A Case Study from the Sunnhordland Area in SW Norway

Abstract

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, mostly of volcanogenic massive sulfide (VMS) and orogeny Au types. The VMS mineralization is hosted by plutonic, volcanic and sedimentary lithologies of the Lower Ordovician ophiolitic complexes. This study presents new trace element and δ³⁴S data from VMS deposits hosted by gabbro and basalt of the Lykling Ophiolite Complex and organic-rich sediments of the Langevåg Group. The Alsvågen gabbro-hosted VMS mineralization exhibits a significant Cu content (1.2 to >10 wt.%), with chalcopyrite and cubanite being the main Cu-bearing minerals. The enrichment of pyrite in Co, Se, and Te and the high Se/As and Se/Tl ratios indicate elevated formation temperatures, while the high Se/S ratio indicates a contribution of magmatic volatiles. The δ³⁴S values of the sulfide phases also support a substantial influx of magmatic sulfur. Chalcopyrite from the Alsvågen VMS mineralization shows significant enrichment in Se, Ag, Zn, Cd and In, while pyrrhotite concentrates Ni and Co. The Lindøya basalt-hosted VMS mineralization consists mainly of pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V, and Tl. The δ³⁴S values of sulfides and the Se/S ratio in pyrite suggest that sulfur was predominantly sourced from the host basalt. The Litlabø sediment-hosted VMS mineralization is also dominated by pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V and Tl. The δ³⁴S values, which range from −19.7 to −15.7 ‰ VCDT, point to the bacterial reduction of marine sulfate as the main source of sulfur. Trace element characteristics of pyrite, especially the Tl, Sb, Se, As, Co and Ni concentrations, together with their mutual ratios, provide a solid basis for distinguishing gabbro-hosted VMS mineralization from basalt- and sediment-hosted types of VMS mineralization in the study area. The distinctive trace element features of pyrite, in conjunction with its sulfur isotope signature, have been identified as a robust tool for the discrimination of gabbro-, basalt- and sediment-hosted VMS mineralization.

1. Introduction

Volcanogenic massive sulfide (VMS) deposits have traditionally been mined for Cu, Zn and eventually Pb, with Au and Ag as common by-products. Recently, this type of mineralization has gained attention due to its potential enrichment in numerous technology-critical elements such as Co, Ni, Mn, Cd, In, Sn, Se, Bi, Te, Ga, Ge and Sb (e.g., [1,2,3]). The VMS deposits represent a product of convective hydrothermal circulation within the seafloor, driven by a combination of extensional tectonics and an elevated geothermal gradient (e.g., [4]). This type of mineralization has been found in various ancient and recent geological settings, including mid-ocean ridges, back-arc basins, intra-oceanic arc systems and continental arc rifts (e.g., [5,6,7]). The mineralization is dominated by pyrite and pyrrhotite and has a stratiform character. Depending on its geological setting, VMS mineralization can be hosted by hydrothermally altered ultramafic, mafic or felsic magmatic rocks or by siliciclastic sedimentary sequences (e.g., [5,8,9]).

Mining of VMS deposits in Norway has a tradition of over 450 years. During this time, more than 100 Mt of ore have been mined in 10 major mining districts located along the Upper Allochthon of the Scandinavian Caledonides, an orogenic belt that extends for about 1500 km from the Stavanger region in southern Norway to the Barents Sea region in northern Norway (e.g., [10,11]; Figure 1). The VMSVMS mineralization is predominantly associated with two generations of ophiolite terranes of the Iapetus Ocean realm. The Early Ordovician ophiolites exhibit a range of compositions from MORB, IAT, boninites and calc-alkaline to alkaline basalts, whereas the Late Ordovician ophiolite complexes are characterized by N- to E-MORB compositions [11,12,13].

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, most of them of the VMS and orogeny Au types (Figure 2; [10,15,16]). The VMS mineralization in this area is associated with the Early Ordovician ophiolitic terranes. Depending on their immediate host rock, the deposits can be categorized into three main groups: (1) gabbro-hosted VMS deposits, (2) basalt-hosted VMS deposits, and (3) sediment-hosted VMS deposits. Despite the abundance and diversity of the VMS occurrences in the Sunnhordland region, their mineralogical and geochemical characteristics, as well as the genetic models, have been poorly constrained [10].

This paper presents new data on ore mineralogy, multi-element LA-ICP-MS analyses and sulfur isotope (δ³⁴S) obtained from gabbro-, basalt- and sediment-hosted VMS deposits in the Sunnhordland area, SW Norway (Figure 1). The main objective of the study is to reveal the trace element and stable isotope characteristics of sulfide minerals associated with VMS mineralization hosted by diverse lithologies and thus contribute to the advancement of geochemical exploration methodology.

2. Geological Setting

2.1. Regional Geology

Scandinavian Caledonides (Figure 1), an orogenic belt underlaid by the Fennoscandian shield, document an approximately 900 Ma year-long geological history that began with the breakup of Rodinia [17], which was followed by the opening of the Iapetus Ocean (e.g., [10,11,18]). The closure of the Iapetus Ocean in the Late Cambrian cumulated with the subduction of the Baltica terrains beneath Laurentia in the late Silurian to Early Devonian. During an oblique collision episode, the allochthonous tectonic units were accreted onto the Baltic continent [14,19,20]. The Devonian period is characterized by post-collisional processes associated with extensional tectonics, the formation of basins and extensive sedimentation [14].

The Sunnhordland region is composed of Precambrian basement rocks overlain by the Caledonian allochthonous units. The contact between the basement and the Caledonian lithologies is marked by the low-angle Hardangerfjord Shear Zone [21]. The VMS deposits are exclusively hosted within the Upper Allochthon ([10]; Figure 1), a tectono-stratigraphic unit composed of ophiolites, igneous and island arc complexes and several adjacent sedimentary basins (Figure 2). Based on their age and genetic features, the ophiolite complexes of the Upper Allochthon were divided into two generations: (1) Early Ordovician ophiolites (i.e., the Leka, Løkken, Gullfjellet, Lykling and Karmøy ophiolite complexes, as shown in Figure 2) that formed in an intra-oceanic arc environment, and (2) Late Ordovician ophiolites (i.e., the Solund-Stavfjord and Sulitjelma ophiolite complexes, as shown in Figure 2) that developed in a back-arc setting before the Laurentia-Baltica collision event [22,23,24,25].

Figure 2. Geological map of the Sunnhordland region showing the locations of the investigated VMS deposits (Alsvågen gabbro-hosted VMS mineralization, Lindøya basalt-hosted VMS mineralization and Litlabø sediment-hosted VMS mineralization). The map is based on the 1:250,000 geological map of the Norwegian Geological Survey [26].

Figure 2. Geological map of the Sunnhordland region showing the locations of the investigated VMS deposits (Alsvågen gabbro-hosted VMS mineralization, Lindøya basalt-hosted VMS mineralization and Litlabø sediment-hosted VMS mineralization). The map is based on the 1:250,000 geological map of the Norwegian Geological Survey [26].

2.2. Geology of the Study Area

Stratigraphically, the lowermost part of the Bømlo island consists of the Lykling Ophiolite Complex, which is unconformably overlain by immature island arc lithologies of the Geitung Unit (Figure 2; [27]). The Lykling Ophiolite Complex records almost complete oceanic crust succession, including chromite-bearing serpentinites in its deepest part, overlain by layered and isotropic gabbro sequences, sheeted dyke complexes and basaltic pillow lavas [22,27,28]. The geochemical characteristics of its mafic lithologies suggest that the Lykling Ophiolite Complex was formed in a supra-subduction zone [29]. The Geitung Unit is made of dacites, basaltic pillow lavas, hyaloclastites and interbedded sediments [27]. Both the Lykling Ophiolite Complex and the overlying Geitung Unit were intruded by tonalite, trondhjemite and dacitic dykes [28].

The Langevåg Group is exposed along the southern parts of the islands of Bømlo and Stord (Figure 2). This volcano–sedimentary complex consists of six formations with a total thickness of ~1250 m, reflecting the progressive deepening of a marine back-arc basin [28]. The lowermost Krekjebær Formation records subaerial mafic volcanic activity. It is overlain by radiolarian cherts, turbidites, tuffs and possible submarine pyroclastic flows of the Kyrkjetuft Formation. The Vorland Formation and Stavaneset Formation mostly consist of green phyllites and sandstones sporadically intercalated with vesicular greenstones and radiolarian cherts [27,28]. These two units are stratigraphically separated with an over 100 m thick chert-bearing Hesthaugen Formation. The uppermost portion of the Langevåg Group is represented by the Vespestad Formation, which mainly consists of dark shales and quartz-rich turbidites [28].

The Siggjo Complex unconformably overlies the Lykling Ophiolite Complex and the Geitung Unit (Figure 2; [27]). It predominantly consists of basalts and basaltic andesites with a calc-alkaline character [28]. The U/Pb dating of the andesites yielded 473 ± 2 Ma [29].

The northern parts of the islands of Bømlo and Stord are intruded by the Sunnhordland Batholith, an intrusive magmatic body with an area of about 1.000 km² (Figure 2; [30]). The batholith is composed of a range of mafic to felsic lithologies, including gabbros, diorites, granodiorites and granites. U/Pb dating of zircons indicates the gabbro emplacement at 472 ± 2 Ma [29], while the granites have been dated at 468 ± 3 Ma [31].

2.3. The VMS Mineralization in the Study Area

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, including VMS deposits hosted by plutonic and volcanic lithologies as well as sediments (e.g., [10,15]). The Alsvågen and Lindøya VMS deposits are the largest known VMS occurrences associated with the Lykling Ophiolite Complex (Figure 2). The Alsvågen mineralization is hosted by the gabbroic sequence, while the Lindøya mineralization is hosted by overlying basalts. The Litlabø VMS deposit, on the other hand, is hosted by sedimentary rocks of the Langevåg Group. With over 9 Mt of ore historically mined, Litlabø is known as the largest VMS deposit in the entire Sunnhordland region (Figure 2).

The southern part of the Alsvågen area is dominated by barren layered and isotropic gabbro of the Lykling Ophiolite Complex. Mineralization occurs in the northern part of the Alsvågen area, where the layered gabbro becomes scarcer and is replaced by altering coarse- and micro-gabbroic lithologies. The contact between the coarse- and micro-gabbro is often sharp, indicating a high-level gabbroic zone where micro-gabbroic dykes have intruded coarser-grained isotropic gabbro. A younger generation of east–west orientated basaltic dykes crosscut the Alsvågen area. The entire area was affected by the greenschist facies regional metamorphism during the Caledonian orogeny [32,33]. The Alsvågen region contains four mineralized shear zones trending NW–SE (Figure 3A). While two of these shear zones are partially visible at the surface (Figure 4A), historical mining activities have removed a significant part of the mineralization they host. Mineralization along the shear zones has a massive appearance, mainly characterized by chalcopyrite and pyrrhotite as the predominant sulfide minerals. Away from the shear zones, the sulfide mineralization takes the form of disseminations and veinlets hosted by both coarse- and micro-gabbroic lithologies (Figure 4B; [33]).

The Lindøya area consists mainly of trondhjemite that has intruded into the Geitung Unit and the Lykling Ophiolite Complex (Figure 3). Micro-gabbro and basalt are exposed along the central part of the area, but their extent is still unknown. Younger basaltic dykes crosscut trondhjemite and possibly the micro-gabbro/basaltic lithologies. The main ore body is hosted by basalts. The mineralization, consisting predominantly of pyrite and minor amounts of chalcopyrite, occurs in the form of massive lenses and disseminations (Figure 4C; [33]).

The Litlabø mineralization is hosted by the volcano–sedimentary complex of the Langevåg Group (Figure 3), which documents the deepening of the back-arc basin [28]. The mineralization is embedded in organic-rich black shales (Figure 4D,E). It mostly occurs in the form of bedding-parallel massive sulfide layers predominantly composed of pyrite and pyrrhotite. Locally, pyrite-rich veinlets that crosscut the primary bedding were found (Figure 4F; [33]).

3. Samples and Methods

3.1. Samples

The sampling campaign in the Sunnhordland area (Figure 2) was carried out in the summer of 2020 as a part of TF’s Master project. In total, over 40 samples of barren and mineralized host rocks and sulfide mineralization were collected, but the results presented in this paper are from 14 samples of sulfide mineralization (Figure 3). The representative samples were prepared as polished thick sections for petrography and in situ trace element analyses. The samples selected for ore grade analyses were pulverized in an agate mill. The mineral grains selected for sulfur isotope analyses were micro-drilled.

3.2. Analytical Methods

Petrographic studies were carried out on polished thick sections using reflected light microscopy. Textural features and semi-quantitative analyses of the mineralized samples were studied on carbon-coated polished sections using a Zeiss supra 55VP Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-ray (EDX) spectrometer at the University of Bergen. EDX analyses were performed with a Thermo Noran detector at a working distance of 8.5 mm, an accelerating voltage of 20 kV and an aperture of 60 µm.

The ore grade was determined at Bureau Veritas Minerals, Canada. The multi-acid digestion and ICP-ES method (code MA370) was combined with the Fire Assay Au method (code FA330-Au). A total of 24 elements were analyzed. Their detection limits are listed in

Table 1. The ore grade analysis of selected mineralized samples from the Alsvågen, Lindøya and Litlabø VMS deposits, SW Norway.

	Method	FA330	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370	MA370
	Element	Au	Ag	Mo	Cu	Pb	Zn	Ni	Co	Mn	Fe	As	Sr	Cd	Sb	Bi	Ca	P	Cr	Mg	Al	Na	K	W	S
	Unit	ppb	ppm	wt.%
Sample	d.l. *	2	2	0.001	0.001	0.02	0.01	0.001	0.001	0.01	0.01	0.02	0.01	0.001	0.01	0.01	0.01	0.01	0.001	0.01	0.01	0.01	0.01	0.01	0.05
The Alsvågen gabbro-hosted VMS deposit
20ALS7S1		40	3	<0.001	3339	<0.02	0.01	<0.001	0.004	0.09	9.04	<0.02	0.02	<0.001	<0.01	<0.01	2.18	0.18	<0.001	1.55	8.50	4.39	0.21	<0.01	2.90
20ALS7S3		85	12	<0.001	>10.000	<0.02	0.04	0.006	0.029	0.02	15.02	<0.02	<0.01	<0.001	<0.01	<0.01	1.00	0.04	<0.001	0.90	1.58	0.17	0.03	<0.01	10.18
20ALS6D1		70	5	<0.001	5427	<0.02	0.03	0.010	0.036	0.04	19.47	<0.02	<0.01	<0.001	<0.01	<0.01	0.71	0.01	0.001	1.37	2.13	0.69	0.02	<0.01	7.86
20ALS3S1		45	3	<0.001	1196	<0.02	0.03	0.005	0.011	0.02	9.63	<0.02	<0.01	<0.001	<0.01	<0.01	0.13	<0.01	0.012	0.38	0.57	0.11	0.02	<0.01	5.76
The Lindøya basalt-hosted VMS deposit
20LIND4B		67	<2	0.004	0.008	<0.02	0.02	0.003	<0.001	0.03	30.61	0.03	<0.01	<0.001	<0.01	<0.01	0.16	<0.01	0.001	0.60	1.14	0.13	0.30	<0.01	>30.00
20LINDH		14	<2	<0.001	0.006	<0.02	0.01	0.001	<0.001	0.09	7.28	<0.02	<0.01	<0.001	<0.01	<0.01	0.96	0.07	<0.001	3.04	9.12	4.83	0.11	<0.01	4.19
The Litlabø sediment-hosted VMS deposit
19LIT3		201	<2	0.002	0.056	0.07	0.12	0.021	0.005	0.15	51.84	0.04	<0.01	<0.001	<0.01	<0.01	0.15	0.07	<0.001	0.17	0.36	0.03	0.12	<0.01	24.87
19LIT2		39	2	<0.001	0.035	<0.02	0.04	0.006	<0.001	0.07	38.40	0.05	<0.01	<0.001	<0.01	<0.01	0.70	<0.01	<0.001	0.17	0.69	<0.01	0.23	<0.01	>30.00

* d.l.—detection limit.

.

Laser ablation single collector ICP-MS analyses of sulfide minerals were performed at the Geological Survey of Finland (GTK), using a Nu AttoM SC-ICPMS (Nu Instruments Ltd., Wrexham, UK) and Excite193nm ArF laser-ablation system (Photon Machines, San Diego, USA). The laser was operated at a pulse frequency of 5 Hz and a pulse energy of 5 mJ at 40% attenuation with a spot size of 40 µm. Each analysis began with a 20 s baseline measurement followed by switching on the laser for 40 s for signal acquisition. Analyses were performed using time-resolved analysis (TRA) with continuous data acquisition for each set of points (generally following the scheme of primary standard, quality control standard, 10–15 unknowns). Measurements were performed at low resolution (M/∆M = 300) in fast-scanning mode. The synthetic pressed nanopellet sulfide standard UQAC FeS-1 was used for external standardization, and the nanopellet sulfide standards FeS-5 and FeS-6 were used as quality controls. Data reduction was performed using GLITTER TM software [34], which allows for baseline subtraction, the integration of the signal over a selected time resolve area and quantification using known concentrations of the external and internal standards. The isotope ⁵⁷Fe was used as the internal standard, assuming a stoichiometric composition.

Sulfur isotope analyses were carried out at the University of Lausanne, Switzerland, using elemental analysis and isotope ratio mass spectrometry (EA/IRMS). The EA/IRMS system consisted of a Carlo Erba 1108 elemental analyzer coupled with a continuous helium flow interface to a Thermo Fisher Delta V isotope ratio mass spectrometer [35]. The samples were combusted at 1030 °C in a single oxidation–reduction quartz tube filled with oxidizing (tungsten trioxide) and reducing (elemental copper) agents. The measured values of sulfur isotopes are given in the δ-notation and relative to the Vienna Cañon Diablo Troilite (VCDT) standard. The laboratory assured that the reproducibility with replicate analysis with given standards (natural pyrite –6.72 ‰, synthetic mercury sulfide, +15.82 ‰, barium sulfate, +12.73 ‰ δ³⁴S; [35]) is better than 0.2‰.

4. Results

4.1. Petrography of Host Rock and Ore Mineralization

4.1.1. The Alsvågen Gabbro-Hosted VMS Mineralization

VMS mineralization at Alsvågen is hosted by the gabbro of the Lykling Ophiolite Complex (Figure 3A). Barren layered gabbro consists predominantly of amphiboles, saussurite and chlorites (Figure 5A,B), indicating a regional metamorphic overprint. The mineral composition of slightly mineralized isotropic gabbro is also characterized by abundant amphiboles and chlorites, whereas plagioclase shows only weak saussuritization (Figure 5C,D). Minor amounts of epidote were also observed (Figure 6A). The abundant occurrence of quartz indicates that the mineralization event was associated with extensive silicification. The gabbro in the immediate vicinity of the mineralized shear zones is composed of amphiboles, chlorites, weakly saussuritized plagioclase and quartz and locally records epidotization (Figure 5E,F).

Massive sulfide mineralization hosted by the shear zones consists mainly of pyrrhotite and chalcopyrite in association with variable amounts of pyrite and cubanite and minor amounts of sphalerite (Figure 6B,C). The textural features indicate post-ore deformation processes that led to brittle fragmentation of pyrite and ductile deformation of other sulfide phases (Figure 6B,C). Outside the shear zones, the mineralization occurs in the form of veinlets and dissemination. In both cases, pyrrhotite and chalcopyrite predominate, often accompanied by minor amounts of titanite (Figure 6D).

4.1.2. The Lindøya Basalt-Hosted VMS Mineralization

The VMS mineralization at Lindøya is hosted by basalts of the Lykling Ophiolite Complex (Figure 3B). Both barren and mineralized basalts consist of amphiboles, chlorites and weakly saussuritized plagioclases (Figure 7A). Lenses of massive sulfide mineralization show an intercalation of sulfide and silica layers. The sulfide layers are predominantly composed of pyrite and pyrrhotite, associated with minor amounts of chalcopyrite, magnetite and arsenopyrite (Figure 7B,C). Locally, younger generations of sulfides, mostly pyrite, crosscut the sulfide-silica layers. Disseminated mineralization is characterized by pyrite, chalcopyrite and minor amounts of sphalerite, often accompanied by extensive chloritization (Figure 7D).

4.1.3. The Litlabø Sediment-Hosted VMS Mineralization

The VMS mineralization at Litlabø is associated with a volcano–sedimentary complex composed of organic-rich black shales, layered siliciclastic sedimentary rocks, cherts and greenstones (Figure 3C). The intermediate host rock is finely laminated organic-rich black shale (Figure 4D,E).

Massive sulfide mineralization occurs in the form of bedding-parallel layers and can be subdivided into pyrite-rich mineralization and pyrrhotite-rich mineralization based on its mineral composition. The pyrite-rich sulfide mineralization consists of intercalated rhythmically altering layers composed of fine-grained porous pyrite (Py1) with layers composed of massive pyrite (Py2; Figure 8A). Only traces of pyrrhotite, chalcopyrite and sphalerite were observed. The mineralized layers are often crosscut by quartz veinlets a few millimeters thick. The pyrrhotite-rich sulfide mineralization consists of at least two generations of pyrrhotite. Early pyrrhotite is often associated with minor amounts of arsenopyrite and occurs in the form of fine-grained lamellas. These lamellas were subjected to extensive brecciation and afterwards cemented with the late generation of pyrrhotite (Figure 8B).

4.2. Ore Grade Analysis

The results of the ore grade analysis of representative massive sulfide samples collected from the Alsvågen, Lindøya and Litlabø VMS deposits as part of this study are shown in Table 1. Ore grade data from previous studies in the area are summarized in

Table 2. Previously published ore grade analysis from the Alsvågen, Lindøya and Litlabø VMS deposits, SW Norway *.

Sample	Element	Cu	Zn	Fe	Mn	S	Pb	Mo	As	Ni	Se	Tl	Ag	Au
	Unit	wt.%					ppm							ppb
The Alsvågen gabbro-hosted VMS deposit
Alsvåg 01		3.77	0.04	20.10		8.52	2	7.72	3	74	118	<d.l. **	3.3	35
Alsvåg 02		2.94	0.02	11.56		3.57	4	4.00	3	13	45	<d.l.	2.6	46
The Lindøya basalt-hosted VMS deposit
Lindøya 01		0.02	0.02	39.5		-	12	41.3	233	76	16	1	0.5	9
Lindøya 02		0.02	0.02	31.8		-	15	26.3	42	52	6	<d.l.	0.5	16
Lindøya 03		0.02	0.02	33.9		-	5	43.5	12	60	13	2	0.5	4
Lindøya 04		0.01	0.03	30.9		-	24	30.6	506	35	9	<d.l.	0.5	25
The Litlabø sediment-hosted VMS deposit
HO0065.01		0.11	0.04	47.83	0.24	34.14	371	22	1412	341	-	-	0.8	284
HO0065.02		0.05	0.04	31.18	0.88	34.39	203	9	1027	142	-	-	1.2	160
HO0065.03		0.06	0.04	33.03	0.77	27.63	78	13	470	103	-	-	0.3	27
HO0065.04		0.02	0.04	27.73	0.61	35.21	40	5	571	74	-	-	0.3	22
HO0065.05		0.02	0.03	24.71	1.76	11.19	25	10	430	41	-	-	1.4	7
HO0065.06		0.06	0.06	47.11	0.08	33.72	130	24	729	129	-	-	<d.l.	25
HO0065.07		0.02	0.03	27.83	0.10	40.75	62	4	684	81	-	-	0.4	26
HO0065.08		0.04	0.05	44.11	0.08	34.70	152	18	744	130	-	-	<d.l.	39
HO0065.09		0.03	0.03	26.39	0.40	33.89	76	9	843	110	-	-	0.3	32

*—data from the Norwegian Geological Survey mineral resource database; ** < d.l.—below detection limit.

[37].

The Alsvågen gabbro-hosted mineralization is characterized by a high Cu content, ranging from 1.2 to >10 wt.%. The mineralization is depleted in all other economically important elements. The Cu/(Zn+Pb) and Fe/base metal ratios range between ~40 and 300 and ~1.5 and 8, respectively (Table 1 and Table 2; Figure 9). In contrast, the Lindøya basalt-hosted mineralization contains only 0.006 to 0.02 wt.% Cu. All other economically important elements are also depleted. The Cu/(Zn+Pb) ratio is <1, while the Fe/base metal ratio varies between ~400 and 1000 (Table 1 and Table 2; Figure 9). The Litlabø-sediment hosted VMS mineralization contains between 0.02 and 0.11 wt.% Cu. The pyrrhotite-rich sample is enriched in Au, Ni and Co compared to the pyrite-rich sample. In general, the Litlabø mineralization is characterized by a low Cu/(Zn+Pb) ratio (<1.4) and a high Fe/base metal ratio (~200 to 500; Table 1 and Table 2; Figure 9).

4.3. Trace Element Composition of Sulfide Minerals

The trace element composition of selected sulfide phases from the Alsvågen, Lindøya and Litlabø VMS deposits is listed in Appendix A and illustrated in Figure 10.

Sulfide minerals (pyrite, pyrrhotite and chalcopyrite) from the gabbro-hosted Alsvågen VMS mineralization are enriched in Se and Te and depleted in Ti, V, Mn, As, Sb, Tl and Pb compared to pyrite from the basalt-hosted Lindøya and the sediment-hosted Litlabø VMS deposits (Figure 10). Sulfides of the Alsvågen VMS mineralization are also characterized by high Se/As, Se/Tl and Se/S ratios compared to sulfides from the latter two deposits (Appendix A). The Alsvågen pyrrhotite is enriched in Co (8.5–4651.0 ppm, with a mean value of 1251.5 ppm) and Ni (0.9–680.9 ppm, with a mean value of 375.7 ppm). In contrast, chalcopyrite from Alsvågen is enriched in Zn (442.5–1062.2 ppm, with a mean value of 643.6 ppm), Ag (7.0–31.0 ppm, with a mean value of 13.7 ppm), Cd (3.3–15.9 ppm, with a mean value of 8.3 ppm) and In (2.4–23.2 ppm, with a mean value of 8.2 ppm).

The comparison of the trace element composition of the pyrite from the investigated VMS deposits revealed that the gabbro-hosted pyrite is depleted in most of the analyzed elements, with the exception of Co, Se, Te, In and Bi (Figure 10). The gabbro-hosted pyrite is characterized by Se/As and Se/Tl ratios in the ranges between ~0.6 and 285 (a mean value of 82) and >5*10⁶, respectively. Its Se/S*10⁶ value spans between 131 and 1650 (a mean value of 672; Appendix A). The basalt-hosted pyrite exhibits elevated contents of As (450.2–1907.7 ppm, with a mean value of 827.9 ppm), Cu (0.5–257.9 ppm, with a mean value of 123.1 ppm), Mn (0.5–170.5 ppm; 73.6 ppm), Pb (0.1–135.0 ppm, with a mean value of 61.8 ppm), Ti (5.4–200.9 ppm, with a mean value of 61.4 ppm), Sb (<56.9 ppm, with a mean value of 24.3 ppm), Hg (<14.7 ppm, with a mean value of 6.9 ppm) and Tl (<3.8 ppm, with a mean value of 1.6 ppm). The sediment-hosted pyrite shows an enrichment in As (Py1: 423.2–1104.2 ppm, with a mean value of 769.9 ppm; Py2: 553.0–919.0 ppm, with a mean value of 713.0 ppm), Pb (Py1: 3.5–64.3 ppm, with a mean value of 27.3 ppm; Py2: 4.8–29.3 ppm, with a mean value of 14.3 ppm); Mo (Py1: 1.6–30.6 ppm, with a mean value of 8.6 ppm; Py2: 1.3–17.6 ppm, with a mean value of 7.9 ppm), Ge (Py1: 0.8–1.7 ppm, with a mean value of 1.1 ppm; Py2: 1.1–1.6 ppm, with a mean value of 1.4 ppm) and Tl (Py1: 0.1–1.0 ppm, with a mean value of 0.5 ppm; Py2: 0.1–0.9 ppm, with a mean value of 0.4 ppm).

4.4. Sulfur Isotopes

The sulfur isotope composition of sulfide phases from the VMS deposits Alsvågen, Lindøya and Litlabø is shown in

Table 3. Sulfur isotope (δ³⁴S) composition of sulfide minerals from the Alsvågen, Lindøya and Litlabø VMS deposits, SW Norway.

Sample	Mineralogy	δ34S (VCDT, ‰)
The Alsvågen gabbro-hosted VMS deposit
20ALS3S1	Pyrite	0.5
20ALS3S2-po1	Pyrite	3.9
20ALS3S2-po2	Pyrite	0.4
20ALS4E	Pyrite	1.8
20ALS4F	Pyrite	3.0
20ALS7S1-po2	Pyrite	2.4
20ALS7S3-po1	Pyrite	1.4
20ALS7S3-po2	Pyrite	1.6
20ALS6D1-po1	Pyrite	1.5
20ALS6D1-po2	Pyrite	0.6
20ALS6D2	Pyrite	1.8
20ALS10B	Chalcopyrite	0.9
The Lindøya basalt-hosted VMS deposit
20LIND-H	Pyrite	4.9
20LIND4B-po1	Pyrite	5.9
20LIND4B-po2	Pyrite	5.7
20LIND4B-po3	Pyrrhotite	5.9
20LIND4B-po4	Pyrrhotite	5.7
The Litlabø sediment-hosted VMS deposit
19LIT2-po1	Pyrite	−19.3
19LIT2-po2	Pyrite	−19.7
19LIT3-po1	Pyrrhotite	−15.8
19LIT3-po2	Pyrrhotite	−15.7
19LIT3-po2	Pyrrhotite	−15.7

and graphically presented in Figure 11. Sulfide minerals from the Alsvågen gabbro-hosted deposit show a range of δ³⁴S values between 0.4 and 3.9 ‰ VCDT (n = 12, with a mean value of 1.6 ‰, as shown in Table 3). The sulfides from the Lindøya basalt-hosted are characterized by δ³⁴S values between 4.9 and 5.9 ‰ VCDT (n = 5, with a mean value of 5.4 ‰, as shown in Table 3). The Litlabø sediment-hosted sulfides exhibit a significant depletion of ³⁴S and show δ³⁴S values between −19.7 and −15.7 ‰ VCDT (n = 4; mean = −17.6 ‰; Table 3).

5. Discussion

Although all of the VMS deposits indicate that they are products of hydrothermal activity within the seafloor, they can be a source of different commodities (e.g., Cu-Zn-rich, Cu-Zn-Pb-rich, Cu-Zn-Au-rich, Cu-Zn-Co-rich, etc.; [5]) and may have variable economic potential depending on their initial geological setting and the local physicochemical conditions during their formation as well as during the tectonic transport and obduction of the seafloor onto the continental crust (e.g., [38]). In addition, deeper parts of ophiolite sequences may host massive sulfides related to primary magmatic processes [39,40,41,42].

Recent developments in microbeam analytical methods, including the in situ LA-ICP-MS technique, enable the determination of trace element distributions in individual mineral phases, including sulfides (e.g., [43,44,45,46]). The trace element content of pyrite, the most common sulfide mineral in various types of mineral deposits, is considered to be an important source of information on the depositional mechanisms of sulfide in both magmatic and hydrothermal deposits (e.g., [47,48,49,50]). Several studies on VMS deposits suggest that the trace element content of pyrite can also be used as a vector in the exploration of this type of deposit (e.g., [51,52,53]).

In this study, we selected three massive sulfide deposits from the Sunnhordland region in SW Norway hosted by different lithologies to test the effectiveness of trace element and δ³⁴S signatures of sulfide phases in distinguishing VMS deposits in different geological settings. The Alsvågen gabbro-hosted VMS deposit is characterized by a high Cu grade (1.2 to >10 wt.%) and an abundance of chalcopyrite and cubanite associated with pyrrhotite, pyrite and sphalerite (Figure 6; Table 1 and Table 2). In contrast, the Lindøya basalt-hosted VMS deposit is characterized by a high Fe/base metal ratio and a Cu grade ranging between 0.006 and 0.02 wt.% (Figure 7; Table 1 and Table 2). The main sulfide phases are pyrite and pyrrhotite. The Litlabø sediment-hosted VMS deposit also has a low Cu grade (0.02–0.11 wt.%), with pyrite and pyrrhotite being the main sulfide minerals. The pyrrhotite-rich mineralization is slightly enriched in Au, Ni and Co (Figure 8; Table 1 and Table 2).

Pyrite from the Alsvågen gabbro-hosted VMS deposit shows a considerable enrichment in Co, Se, Te, In and Bi compared to pyrite from the Lindøya basalt-hosted and Litlabø sediment-hosted VMS deposits. In contrast, pyrite from the latter two deposits is enriched in V, Mn, As, Sb, Tl and Pb (Appendix A; Figure 10). Previous studies on the trace element characteristics of pyrite from VMS deposits indicated a significant negative correlation between As and Se in hydrothermal pyrite, presumably controlled by their competition for S site and fluid temperature [51]. The Se/Tl ratio has also been proposed as an indicator of fluid temperature and can be used directly to distinguish between mineralized and barren zones in VMS prospects [51,54]. The high Se/As and Se/Tl ratios recorded from the gabbro-hosted pyrite indicate a relatively high formation temperature for the Alsvågen mineralization and clearly distinguish the high-grade gabbro-hosted mineralization from the low-grade basalt-hosted and sediment-hosted types of VMS mineralization. The high Se/S ratio in the Alsvågen pyrite (Se/S*10⁶ = 131–1650, with a the mean value of 672) indicates an influx of magmatic volatiles (e.g., [55,56,57]). The Co/Sb vs. Se/As ratios support the magmatic affinity of the Alsvågen mineralization (Figure 12; [45,58]).

Pyrite from the Lindøya basalt-hosted and Litlabø sediment-hosted VMS deposits overlap considerably in terms of trace element composition (Figure 10); therefore, the binary trace element plots cannot be used for their discrimination (Figure 13). In contrast to pyrite from the gabbro-hosted Alsvågen mineralization, pyrite from Lindøya and Litlabø reflects a hydrothermal affinity in the Co/Sb vs. Se/As diagram (Figure 12A; [45,58]).

Chalcopyrite and pyrrhotite from the Alsvågen VMS deposit were also analyzed for their trace element content. The recorded enrichments of chalcopyrite in Se, Ag, Zn, Cd and In and of pyrrhotite in Ni and Co (Appendix A; Figure 10) are probably controlled by the partition of trace elements between coprecipitating sulfide phases (e.g., [59,60,61,62]). The Zn vs. Cd content of chalcopyrite reflects that the Alsvågen mineralization has been subjected to post-formation recrystallization processes at elevated temperatures (Figure 12B; [45]).

Sulfide mineralization from the studied VMS deposits in the Sunnhordland region, SW Norway, shows a distinct δ³⁴S signature (Table 3, Figure 11). The Alsvågen gabbro-hosted mineralization is characterized by δ³⁴S values between 0.4 and 3.9 ‰ VCDT (n = 12, with a mean value of 1.6 ‰, as shown in Table 3), indicating a magmatic contribution of sulfur (e.g., [63]).

The Lindøya basalt-hosted mineralization shows δ³⁴S values ranging between 4.9 and 5.9 ‰ VCDT (n = 5, with a mean value of 5.4 ‰, as shown in Table 3). These values are similar to the sulfur isotope composition of sulfide minerals from the basalt-hosted VMS deposits elsewhere (e.g., [64,65,66]). Pyrite with Se/S*10⁶ values of <11.5 reflects a negligible contribution from magmatic volatiles, additionally supporting the notion that sulfur at Lindøya was predominantly leached from the host basalt.

The Litlabø sediment-hosted mineralization with δ³⁴S values between −19.7 and −15.7 ‰ VCDT (n = 4; mean = −17.6 ‰; Table 3) reflects the bacterial reduction of marine sulfate as the main source of sulfur in this deposit (e.g., [67,68]). The low Se/S*10⁶ values measured in pyrite (<14.9) are consistent with a non-magmatic source of sulfur.

The distinctive sulfur isotope signature of pyrite from the three examined types of VMS deposits can be used to distinguish gabbro-, basalt- and sediment-hosted VMS deposits. As illustrated in Figure 14, the combination of characteristic δ³⁴S ranges with trace element attributes provides a robust tool for the differentiation of pyrite from VMS deposits hosted by different lithologies. Therefore, the proposed binary δ³⁴S trace element plots have a strong potential for use in discriminating provenances of individual pyrite grains extracted from stream sediments, soils or till material.

6. Conclusions

Despite their abundance and diversity, the VMS deposits of the Sunnhordland region, SW Norway, have not been subjected to previous mineralogical, geochemical or stable isotope investigations. This study brings an overview of mineral characteristics of the VMS deposits hosted by different lithologies, including gabbro and basalt of the Lykling Ophiolite Complex, as well as organic-rich sediments of the Langevåg Group, coupled with new trace element and δ³⁴S data obtained sulfide mineral phases.

The Alsvågen gabbro-hosted VMS mineralization (1.2 to >10 wt.% of Cu) consists of chalcopyrite and cubanite in association with pyrrhotite, pyrite and sphalerite. Pyrite has a significant enrichment of Co (7–4867 ppm, with an average value of 1105 ppm), Se (80–723 ppm, with an average value of 345 ppm) and Te (<36 ppm, with an average value of 12 ppm). The high Se/As and Se/Tl ratios reflect a relatively high formation temperature for Alsvågen mineralization, while the high Se/S ratio (Se/S*10⁶ = 131–1650, with a mean value of 672) suggests a contribution from magmatic volatiles. The δ³⁴S values of sulfide phases that range within the interval from 0.4 to 3.9 ‰ VCDT (n = 12, with a mean value of 1.6 ‰) additionally support a significant influx of magmatic sulfur. Chalcopyrite from the Alsvågen VMS shows enrichment of Se, Ag, Zn, Cd and In, while pyrrhotite concentrates Ni and Co.

The Lindøya basalt-hosted VMS mineralization (0.006 to 0.02 wt.% of Cu) consists predominantly of pyrite and pyrrhotite. Pyrite is enriched in As (450–1908 ppm, with an average value of 828 ppm), Mn (0.03–171 ppm, with an average value of 74 ppm), Pb (0.07–135 ppm, with an average value of 62 ppm), Sb (0.03–57 ppm, with an average value of 24 ppm), V (0.5–48 ppm, with an average value of 16 ppm) and Tl (0.02–4 ppm, with an average value of 2 ppm). The δ³⁴S values of sulfides range between 4.9 and 5.9 ‰ VCDT (n = 5, with a mean value of 5.4 ‰). These values, together with the Se/S ratio in pyrite (Se/S*10⁶ values <11.5), suggest that the sulfur at Lindøya was predominantly leached from the host basalt.

The Litlabø sediment-hosted VMS mineralization (0.02–0.11 wt.% of Cu) also consists predominantly of pyrite and pyrrhotite. Pyrite is enriched in the same suite of elements as pyrite from Lindøya (As (423–1104 ppm, with an average value of 739 ppm), Mn (5–82 ppm, with an average value of 24 ppm), Pb (4–64 ppm, with an average value of 20 ppm), V (1–52 ppm, with an average value of 14 ppm), Sb (0.7–19 ppm, with an average value of 6 ppm) and Tl (0.1–1 ppm, with an average value og 0.5 ppm). The Litlabø sediment-hosted mineralization has δ³⁴S values between –19.7 and –15.7‰ VCDT, indicating that the bacterial reduction of marine sulfate is the main source of sulfur in this deposit. The low Se/S*10⁶ values measured in pyrite (<14.9) support a non-magmatic source of sulfur.

Trace element characteristics, in particular Tl, Sb, Se, As, Co and Ni concentrations and their mutual ratios have been identified as an efficient tool to distinguish the gabbro-hosted VMS mineralization from the basalt- and sediment-hosted types of VMS mineralization. The trace element features of pyrite, coupled with its distinctive sulfur isotope signature, represent a novel and robust tool for distinguishing gabbro-, basalt- and sediment-hosted VMS mineralization.