The Zinkgruvan deposit is the largest stratiform Zn-Pb-Ag mineralization in Sweden. The most recent genetic model attributes ore formation to the discharge of oxidized, near-neutral pH, metalliferous brines into a reduced basin, forming laterally extensive, stratiform sulfide mineralization on the seafloor. It has a known strike extent of 5 km and is underlain by a regionally extensive zone of K-altered metavolcanic rock and dolomitic marble, the latter hosting Cu-(Co-Ni) replacement mineralization near the inferred hydrothermal vent to the stratiform sulfides. The deposit is stratigraphically overlain by migmatized,  pyrrhotite- and graphite-rich pelite that is in turn overlain by a banded almandine-biotite-quartz-ferrosilite-bearing unit at the base of an regionally extensive metasedimentary succession. These laterally continuous units are interpreted as metamorphosed organic-rich sulphidic mudstone and silicate-dominated Fe formation, respectively.

The favorable stratigraphic interval contains anomalously high Zn, Pb, Ag, Cu, K₂O/(K₂O+Na₂O), Mn, Co, Tl, Ba and B relative to adjacent metatuffite. However, only Zn, Pb, Ag, K₂O/(K₂O+Na₂O) and Mn are significantly enriched relative to adjacent strata beyond the known lateral extent of the ore. Elevated copper, Co and Tl only occur in the vent-proximal part of the deposit, whereas anomalous enrichments of Ba and B are sporadic and occur mainly in the stratigraphic footwall. Many elements such as Si, Fe, Mg, Ca and Cs are of limited use in vectoring due to low enrichment factors relative to inferred background compositions and/or strong lithological controls on their distribution.

Although ore metal (Zn, Pb and Ag) enrichments are the best quantitative and qualitative guides to ore, K, Mn and Co enrichments also provide corroborative support. The most useful elements for vectoring have been synthesized into exploration indices. The Modified Sedex Metal Index (MSMI; Zn+3Pb+100Ag) is a vector towards stratiform Zn-Pb-Ag mineralization, whereas MSMI2 [Zn+3Pb+10(Cu+Co)] alsoallows targeting of proximal Cu mineralization.

The banded iron formation and the pyrrhotite- and graphite-rich pelite of the stratigraphic hangingwall are consistently enriched in base metals (e.g. 500-1000 ppm Zn), total S and Mn throughout the entire Zinkgruvan area. However, these units are not known to grade laterally along strata into economic base metal sulfide mineralization, and they are not obviously products of the same hydrothermal system which formed the stratiform Zn-Pb-Ag deposit.

In a vent-distal setting, the somewhat spurious metal anomalies of the hangingwall units can be difficult to distinguish from those of the favorable interval. The favorable stratigraphic interval can, however, be recognized by also taking into account that positive Zn anomalies are mainly coincident with positive anomalies in both K and Mn only in the favorable interval. Furthermore, samples from the favorable interval generally have Co/Ni > 1 and displays a positive Co/Ni vs. Zn trend, whereas samples of the pyrrhotite- and graphite-rich pelite have Co/Ni < 1 and define a negative Co/Ni vs. Zn trend. Thus, the index (Co/Ni)*Zn allows easy detection of weak Zn anomalies associated with the stratiform Zn-Pb-Ag mineralization.

Medium- to high-grade metamorphosed, 1.9 Ga, stratiform, syngenetic Zn-Pb±Ag sulfide deposits comprise an economically important type of ore deposit in the Bergslagen lithotectonic unit of the Fennoscandian shield. The Lovisa Zn-Pb deposit occurs in a metamorphosed succession of rhyolitic ash-siltstone, rhyolitic mass flow deposits, limestone and iron formation, deposited at a stage of waning volcanism in Bergslagen.

Accessory graphite, absence of Ce anomalies in shale-normalized rare-earth element (REE) data, and absence of hematite in Mn-rich iron formations stratigraphically below the Lovisa Zn-Pb deposit indicate a suboxic-anoxic depositional environment. The uppermost Mn-rich iron formation contains disseminated, inferred syngenetic Pb-Ag mineralization with mainly negative δ³⁴S values in sphalerite and galena (-6.1 to -1.9‰).

Deposition of this iron formation terminated during a pulse of explosive felsic volcanism. The Lovisa Zn-Pb deposit is interpreted to have formed in an alkali-rich brine pool developed immediately after this volcanic event, based on lithogeochemical and stratigraphic evidence. The first stage of mineralization deposited stratiform sphalerite mineralization with mainly positive δ³⁴S values (-0.9 to +4.7‰). This was succeeded by deposition of more sphalerite-galena stratiform mineralization with δ³⁴S values close to 0‰ (-2.1 to +1.5‰). The more galena-rich mineralization partitioned strain and was partly remobilized during later ductile deformation.

The stratigraphic context, sulfide mineralogy, sulfur isotopes and alteration geochemistry suggest that the metalliferous fluids and the depositional environment were H₂S-deficient (S-poor or SO₄^2--dominant). The source of sulfur is interpreted to have been a mixture of H₂S derived from bacterial and thermochemical seawater sulfate reduction, and sulfur derived from leaching of volcanic rocks, with the latter becoming more important over time.

Lovisa formed in a setting where basin subsidence was periodically punctuated by the deposition of thick, syn-eruptive felsic volcaniclastic massflow deposits. Coeval volcanism was likely important for driving hydrothermal activity and supplying a reservoir of metals and sulfur. However, the high rate of deposition of volcaniclastic sediment in Bergslagen also precluded the establishment of long-lived, deep and anoxic environments favorable for accumulation of organic matter and H₂S. This stratigraphic pattern is common in Bergslagen and may explain why large stratiform Zn-Pb deposits are uncommon in the region and restricted to the uppermost part of the metavolcanic succession, directly stratigraphically beneath post-volcanic pelitic rocks.

The Zinkgruvan deposit has been included in compilations of exceptionally Ga- and Ge-endowed deposits in Sweden. Available published data sets do however not support a substantial enrichment in Ga and Ge. In this contribution, we investigate the Ga- and Ge-endowment based on a whole-rock lithogeochemical data and ore grade analyses from the deposit. Based on our results, we find it highly unlikely that a Ga-endowment exists in the ore. A Ge-endowment may exist, but we find no evidence of Ge grades at the 1,000 ppm level that have been reported previously.

Zinkgruvan, a major stratiform Zn-Pb-Ag deposit in the Paleoproterozoic Bergslagen region, south-central Sweden, was overprinted by polyphase ductile deformation and high-grade metamorphism (including partial melting of the host succession) during the 1.9-1.8 Ga Svecokarelian orogeny. This complex history of post-ore modification has made classification of the deposit difficult. General consensus exists on a syngenetic-exhalative origin, yet the deposit has been variably classified as a volcanogenic massive sulfide (VMS) deposit, a sediment-hosted Zn (SEDEX) deposit, and a Broken Hill-type (BHT) deposit. Since 2010, stratabound, cobaltiferous and nickeliferous Cu ore, comprising schlieren and impregnations of Cu, Co and Ni sulfide minerals in dolomitic marble, is mined from the stratigraphic footwall to the stratiform Zn-Pb-Ag ore. This ore type has not been fully integrated into any of the existing genetic models. Based on a combination of 1) widespread hematite-staining and oxidizing conditions (Fe₂O₃>FeO) in the stratigraphic footwall, 2) presence of graphite and reducing conditions (Fe₂O₃<FeO) in the ore horizon and hangingwall and 3) intense K-feldspar alteration and lack of feldspar-destructive alteration in the stratigraphic footwall, we suggest that both the stratiform Zn-Pb-Ag and the dolomite-hosted Cu ore can be attributed to the ascent and discharge of an oxidized, saline brine at near neutral pH. Interaction of this brine with organic matter below the seafloor, especially within limestone, formed stratabound, disseminated Cu ore, and exhalation of the brine into a reduced environment on the sea floor produced a brine pool from which the regionally extensive (> 5 km) Zn-Pb-Ag ore was precipitated.

Both ore types are characterized by significant spread in δ³⁴S, with the sulphur in the Cu ore and associate marble-hosted Zn mineralization on average being somewhat heavier (δ³⁴S = -4.7 to +10.5 ‰, average 3.9 ‰) than that in the stratiform Zn-Pb-Ag ore (δ34S = -6 to +17 ‰, average 2.0 ‰). The ranges in δ³⁴S are significantly larger than those observed in syn-volcanic massive sulphide deposits in Bergslagen, for which simple magmatic/volcanic sulphur sources have been invoked. Mixing of magmatic-volcanic sulfur leached from underlying volcanic rocks and sulfur sourced from abiotic or bacterial sulfate reduction in a mixing zone at the seafloor could explain the range observed at Zinkgruvan.

A distinct discontinuity in the stratigraphy, at which key stratigraphic units stop abruptly, is interpreted as a syn-sedimentary fault. Metal zonation in the stratiform ore (decreasing Zn/Pb from distal to proximal) and the spatial distribution of Cu mineralization in underlying dolomitic marble suggest that this fault was a major feeder to the mineralization. Our interpretation of ore-forming fluid composition and a dominant redox trap rather than a pH and/or temperature trap differs from most VMS models, with Selwyn-type SEDEX models, and most BHT models. Zinkgruvan has similarities to both McArthur-type SEDEX deposits and sediment-hosted Cu deposits in terms of the inferred ore fluid chemistry, yet the basinal setting has more similarities to BHT and felsic-bimodal VMS districts. We speculate that besides an oxidized footwall stratigraphy, regionally extensive banded iron formations and limestone horizons in the Bergslagen stratigraphy may have aided in buffering ore-forming brines to oxidized, near-neutral conditions. In terms of fluid chemistry, Zinkgruvan could comprise one of the oldest known manifestations of Zn and Cu ore-forming systems involving oxidized near-neutral brines following oxygenation of the Earth’s atmosphere.

Volcanogenic massive sulphide (VMS) deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity. These deposits form as sulphide mounds, stratiform exhalative and/or replacive bodies and commonly have stockwork/vein mineralization in their immediate footwall. These various “styles” are essentially facies of mineralization, each one being the product of a particular set of conditions that control the ore-forming processes and the consequent geometry and architecture (style) of the deposits. These controls include the physical and/or chemical nature of the host rocks, the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment.The style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor. Hydrothermal fluids with salinities less than twice that of seawater that vented into open, oxic oceanic environments, typically formed small mound and chimney complexes, unless they were rapidly covered by sediments or volcanic rocks. The massive sulphides were rapidly oxidized and partly dissolved by seawater. In contrast, stratiform sheet-like deposits are typically formed in anoxic bottom waters. Anoxic marine conditions were periodically of global extent, – particularly prior to 2.4 Ga – or of a regional nature. Local anoxic conditions can also be self-induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second- or third-order basins to form a brine pool. These exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self-induced anoxic conditions. Deposits formed in anoxic environments can be significantly larger than those in oxic settings, and this is attributed to several factors that include longer-lived hydrothermal circulation, more efficient sulphide precipitation and reduced or inhibited oxidation thereof.Replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization. However, successions with abundant porous, permeable and/or reactive rocks such as glassy and/or pumiceous volcaniclastic rocks, and in some cases limestone, favoured the development of large replacive deposits, that may have had little surficial expression on the sea floor.VMS deposits at spreading centres within oceanic crust formed almost exclusively as mounds. Most of them have not been preserved, likely due to oxidation of the sulphides in the prevailing oxic environment and/or destruction of oceanic crust during subsequent subduction. Intra-continental rifts, arc rifts and back-arc rifts commonly have more complexity in their structure, and facies architecture and environments and can host all styles of VMS mineralization. In these settings, early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters, whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments. Formation of replacive deposits was permissible in all settings with porous or reactive subsea-floor strata. Replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor.The various styles of VMS mineralization can rarely be distinguished using a single criterion; in most cases multiple criteria are required.Mound style mineralization is distinguished by: (a) mound- or lens-shaped morphology; (b) presence of chimney fragments; (c) presence of abundant sedimented sulphide breccias; (d) location on a stratigraphic boundary (ore horizon); and, (e) association with a thin horizon or thicker stratigraphic interval of fine-grained clastic rocks (e.g., shale, mudstone) that accumulated at slow sedimentation rates.Stratiform exhalative mineralization is distinguished by: (a) sheet-like morphology prior to deformation; (b) presence of fine-grained clastic host rocks that accumulated at relatively slow rates (e.g., mudstone); (c) presence of local or extensive planar stratification.Replacive mineralization is characterized by: (a) irregular geometry and distribution of sulphide bodies; (b) gradation from massive sulphides to semi-massive sulphides and disseminated mineralization with relict textures of the host rock; and, (c) originally pumiceous, glassy or reactive host rocks emplaced at high depositional rates (mass flow deposits, lavas, carbonate-altered mass flow deposits) or limestone.One deposit or district may comprise two or more of these main styles of mineralization. In many cases the main styles of VMS mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession. Recognition of the style(s) of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for VMS deposits.

The Skellefte mining district in northern Sweden contains over 85 pyritic Zn-Cu-Au-Ag massive sulphide deposits. The Renstrom area, one of the most intensely mineralized parts of the Skellefte district, contains five zinc-and gold-rich deposits, three of which are confined to a specific continuous stratigraphic unit, the "Renstrom ore host unit". The great structural complexity of the area made it difficult to locate and follow the ore horizon to generate new exploration targets. A new study in the Kyrkvagen area, based on stratigraphic correlations, structural interpretations and lithogeochemical and geophysical data interpretation, revealed several NW-trending faults which separate five structural blocks. The rocks of the area could be characterized in terms of geochemistry, stratigraphy and their position in the hanging or footwall with respect to the ore horizon. Moreover, alteration patterns allowed predictions of possible extensions of the ore horizon. This increased knowledge of the Kyrkvagen area led to the identification of five new drilling targets for further exploration in one of Boliden's most important mining areas.

Numerous magnetite skarn deposits and marble- and skarn-hosted base metal sulphide deposits occur in polydeformed and metamorphosed, felsic-dominated metavolcanic inliers in the Palaeoproterozoic Bergslagen region of south-central Sweden, including the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit, approximately 2.5 km SW of the large Garpenberg Zn-Pb-Ag-(Cu-Au) deposit. The Ryllshyttan deposit, from which approximately 1 Mt of Zn-rich massive sulphide ore and 0.2 Mt of semi-massive magnetite were extracted, is located near a transition between magnesian skarn and dolomitic marble. The host unit consists of a 10-20 m-thick former calcitic limestone of likely stromatolitic origin that is commonly pervasively altered to skarn, locally hosting magnetite skarn deposits. The ore-bearing unit is one of several mineralised marble units within a more than 1 km-thick, felsic-dominated metavolcanic succession that includes a metamorphosed, large caldera-fill pyroclastic deposit, 800 m stratigraphically above the Ryllshyttan host succession. The Garpenberg stratabound Zn-Pb-Ag-(Cu)-(Au) deposit is located higher in the stratigraphy, just below the caldera fill deposits. The metavolcanic succession is bounded to the NW by a large granitoid batholith and intruded by a microgranodiorite pluton less than a 100 m from the Ryllshyttan deposit. Magnetite laminae in bedded skarns and metavolcanic rocks in the hanging wall of Ryllshyttan indicate an early (syngenetic) accumulation of Fe-rich exhalites. In contrast, the sulphide mineralisation consists of stratabound replacement-style ore associated with dolomitisation of the host and with discordant K-Mg-Fe±Si alteration of volcanic rocks and early porphyritic intrusions in the footwall and hanging wall. The microgranodiorite that intrudes the host succession crosscuts the K-Mg-Fe±Si alteration envelope and is overprinted by Na-Ca alteration (diopside and plagioclase-bearing mineral associations) that also overprints K-Mg-Fe±Si-altered rocks. The Na-Ca alteration is interpreted to be associated with the formation of calcic and magnesian iron skarn deposits semi-regionally at a similar stratigraphic position. Despite superimposed amphibolite facies regional metamorphism and substantial syn-D2-D3 remobilisation of sulphides concurrent with retrograde alteration of skarn assemblages, cross-cutting field relationships indicate that the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit results from protracted VMS-style hydrothermal activity including early seafloor mineralisation (Fe-rich exhalites), closely followed by sub-seafloor carbonate-replacement-style mineralisation (base metal-bearing massive sulphides). Both mineralisation styles were overprinted by contact metasomatism associated with the formation of abundant magnetite skarn deposits during the emplacement of granitoid intrusions. As for other deposits in the Bergslagen region, the ore-forming system at Ryllshyttan thus has similarities to both metamorphosed VMS deposits and metasomatic Fe and Zn skarn deposits. Our results suggest that the sequence of volcanic, intrusive and hydrothermal events in this region is compatible with prograde heating of a long-lived hydrothermal system, wherein a shift from a convective seawater-dominated system to a contact metamorphic and/or metasomatic environment occurred during the early stage of the 1.9-1.8 Ga Svecokarelian orogeny. This model partly resolves the controversy regarding genesis of the iron oxide and base metal sulphide deposits in Bergslagen, as we recognise that these deposits have a complex history of alteration, metamorphism, deformation and (re)mobilisation, and no unique established genetic model can account for all their features.

Zinkgruvan is an elusive Palaeoproterozoic stratiform Zn-Pb-Ag deposit which has been discussed in the context of sediment-hosted Zn-Pb (SEDEX), volcanic-hosted massive sulphide (VHMS) and Broken Hill-type (BHT) deposits. In this contribution, we address the chemistry of the ore-forming fluid, the nature of the depositional environment and the controls on ore formation based on a review of previous work complemented with new geological data from a stratigraphically underlying dolomite-hosted, zinciferous, cobaltiferous and nickeliferous Cu ore. We conclude that both deposit types can be explained as the product of a saline, oxidizing metalliferous brine which formed Cu mineralization by interaction with reduced pore waters, prior to exhalation into an anoxic brine pool, forming the stratiform Zn-Pb-Ag deposit. Our inference of fluid composition differs from many inferences on the chemistry of hydrothermal fluids involved in the formation of typical VHMS and BHT deposits, but is similar to that inferred for Proterozoic sediment-hosted Zn-Pb deposits in the McArthur basin, Australia.