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Article

Deciphering the Evolution of Adjacent Volcanogenic Massive Sulfide (VMS) Systems Based on Radiogenic and Stable Isotopes, the Case of Ermioni, Argolis Peninsula, Ne Peloponnese, Greece

by
Stavros Savvas Triantafyllidis
1,* and
Stylianos Fotios Tombros
2
1
School of Mining and Metallurgical Engineering, National Technical University of Athens, Iroon Polytechneiou 9, 157 80 Athens, Greece
2
Department of Materials Science, University of Patras, 26504 Rio Patras, Greece
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(4), 474; https://doi.org/10.3390/min13040474
Submission received: 22 February 2023 / Revised: 22 March 2023 / Accepted: 24 March 2023 / Published: 27 March 2023
(This article belongs to the Special Issue Advances in Computational Modeling and Isotope Analyses in Volcanogenic Massive Sulfide (VMS) Deposits)
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Abstract

:
The study follows previous work on Ermioni VMS and addresses in detail the formation and evolution of two adjacent VMS systems, Karakasi and Roro. It is based on a stable and radiogenic isotopic composition of sulfides and ganguefrom stringer (Karakasi) and massive (Roro) VMS ore. The isotopic geochemistry of Pb and noble gases (Ar-He) of pyrite from both sites indicates the development of a deep and evolved heat and possibly metal source attributed to subduction of radiogenic material (Pindos oceanic crust). The differences in the stable (Fe, S) and radiogenic (Sr, Ar) isotopic compositions between the two sites depict variation in the geologic environment of VMS formation, and in particular the effect of seawater. The higher δ57Fe and δ34S values of Roro massive pyrite are attributed to direct interaction of hot, ascending metal-bearing hydrothermal fluids with cold seawater. Karakasi stringer oreis characterized by higher 87Sr/86Sr ratios and radiogenic Ar values (as 40Ar/36Ar), indicating interaction of ore-bearing, hydrothermal fluids with crustal material (hanging-wall turbidites). During the approximate 0.5 Ma period separating the two systems, the hydrothermal system migrated from east to west, and at the same time evolved from free discharge on the seafloor (Roro—easterly), resembling contemporary seafloor style and mound-shaped massive sulfides, to a sediment-confined, subseafloor system (Karakasi—westerly).

1. Introduction

Massive sulfide deposits were among the earliest metallic ore deposits exploited by mankind, namely because of their high grade, texture (massive ore with limited requirements for pre-enrichment), strong contrast with country rocks (especially when surface-exposed outcrops have suffered supergene oxidation), and their relatively simple mining and extraction [1]. The term volcanogenic massive sulfide (VMS) has been in use for nearly 50 years [2] and embraces the temporal and spatial association of sulfide mineralizations with submarine volcanic processes [3]. The generalized structure of VMS deposits includes a “mushroom-shaped”, stratabound, polymetallic, massive sulfide lens (>40 vol. % sulfides) underlain by silica stockworks, known as stringer zones, which function as the feeder zones for the overlying massive ore bodies. Distinctive alteration zones envelope the feeder zones and the massive ore body both vertically and laterally [4]. Most commonly, VMSs are hosted in submarine volcanic and volcanosedimentary successions located at or close to divergent margins [5,6]. According to Barrie et al. [7], they are formed in marine environments after discharge of high-temperature (T ≤ 350 °C), metal-enriched hydrothermal fluids onto the seafloor during contact with cold seawater (≈4 °C).
Detailed research on VMS deposits has shown that they are characterized by large diversities in ore mineralogy and geochemistry (e.g., base, precious, and strategic metal and metalloid contents), isotopic signature, and host rock lithologies (e.g., [4,8,9,10,11,12,13,14,15,16]. Subsequently, the most common characteristics shared between the various VMS types are the massive texture of the sulfide ore (regardless of ore mineralogy and geochemistry) and the genetic relation to submarine volcanism.
Although hydrothermal circulation and convection are prerequisites for VMS formation, it is very common for them not to be accompanied by distinctive volcanism at the seafloor, and in many cases the host volcanic and volcaniclastic rocks act only as sources of metals through depletion during hydrothermal convection [17]. Therefore, the geochronological ages calculated by conventional radiogenic isotopes (e.g., zircon U-Th dating, K-Ar on K-bearing silicates) may only provide answers on the age of the hosting volcanic rocks and indirect information on the actual age of ore formation (see [18,19,20]). This issue has been addressed by recent advances in radiogenic isotope geochronology, and the development of more sophisticated dating methods, including Re-Os, and Pb geochronological dating in sulfides [21,22]. They are considered the most reliable methods for geochronological dating of sulfide ores and have been successfully implemented in VMS deposits (see [19,20,23,24]).
Another major challenge during VMS studies is the identification of the pulses of hydrothermal circulation, as in many cases the mode and the characteristics of VMS formation may change through time, as in the case of the Zn-Pb-Ag deposits of Australia [25], the VMS deposits in the Jinshajiang orogenic belt, southwestern China [26], and the Rudny Altai VMS deposits in Siberia [19]. This is a crucial parameter since the precise definition of the ore-forming system (or systems) not only provides valuable information on other possible deposit types developed in the area under investigation ([3] and references therein)but may also define future exploration strategies.
The objective of the paper is to continue the work of Triantafyllidis et al. [27] regarding the formation of Roro and Karakasi VMS sites from Ermioni area (Figure 1). It is based on previously published data by Triantafyllidis et al. [27] and Tombros et al. [28], and we will show how focused examination of exotic isotopes (both radiogenic and stable) in case-specific phases may provide solid answers on deciphering the origin, development, and evolution of two adjacent VMS systems. The study is relevant for the field of massive sulfide deposits, as the obtained results can be used in the exploration of the marine field to identify new accumulations of useful mineral substances of this type. Moreover, the atypical upper Cretaceous age of the Ermioni VMS relative to neighboring VMS provinces in the Alpine Orogen (e.g.,northern Italy [29], and Balkans [30,31]) provides beneficial information on future exploration strategies involving fossilized marine settings of similar age, as in the case of the Kuroko-type VMS deposit of Cerattepe-Artvin in Turkey which is dated to 62±3 Ma ([32,33]). It is worth mentioning that although VMS deposits have been extensively studied, there is very limited work regarding the examination of stable and radiogenic isotopes in specific phases of smaller VMS systems developed within a broader area (see Tornos et al., 2015 [34]). The Ermioni VMS (Figure 1) is a perfect example of such an investigation, as despite its small size and simple mineralogy and geochemistry, there has been a long debate on the actual VMS type this deposit belongs to, and whether it comprises one or more hydrothermal centers. There area large number of parameters and ore features related to this question, one of them being post-ore deformation and tectonism of the southeast Argolis Peninsula from the Paleocene–Oligocene, that have greatly affected the primary structure of the VMS systems, resulting in dismembered ore bodies scattered within the fine-grained upper Cretaceous–Paleocene turbidites.

2. Case Study: The Ermioni VMS

2.1. Argolis Peninsula Regional Geology

The Argolis Peninsula (Figure 1) is an area characterized by complex geology and geodynamic evolution, and several geotectonic and stratigraphic models have been proposed since the late 19th century for the area largely based on conventional geological data [35,36,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. According to the latest views [35], the Argolis Peninsula comprises a metamorphosed Paleozoic basement overlain by five Triassic-to-Paleocene–Eocene nappes [34,52,53,54,55,56] (Figure 1A) including: (a) The lowermost “Trapezona Unit” composed of middle Triassic-to-upper Jurassic neritic carbonates, deep-water limestones, and ophiolite olistostromes, (b) the “Dhimaina Ophiolitic Unit” comprising MORBs covered by Cretaceous limestones and Paleocene–Eocene flysch, (c) the “Adheres Mélange Unit” (also referred as the “Ermioni Complex” by Robertson et al. [50]) composed of ophiolite blocks, volcanic rocks, and late Cretaceous to Paleocene turbidites and carbonates, (d) the “Iliokastron Mélange Unit” consisting of an ophiolite mélange, and (e) the uppermost “Faniskos Unit” composed of upper Cretaceous neritic carbonates.
The difficulty in distinguishing the geotectonic setting and the geodynamic evolution of the southeast Argolis Peninsula is partly linked to the diversity of volcanic rocks that have been identified, including boninites, island arc tholeiites (IAT), and transition (T-MOR) and normal (N-MOR) basalts to name a few [55,56,57,58,59,60,61]. In particular, the “Adheres Mélange Unit”, which hosts the Ermioni VMS deposit, comprises ophiolite blocks, ultramafic rocks, calc-alkaline volcanic rocks, boninites, and IAT [48,53]. According to Robertson et al. [50], the boninites and IAT of the Argolis Peninsula depict fore-arc volcanism (subduction initiation) at a Tethys Ocean remnant (Vardar-Axios Ocean) during the late Jurassic–early Cretaceous (Eohellenic phase). During the Cretaceous, the geotectonic setting changed from fore-arc to back-arc, as evident by the calc-alkaline volcaniclastic rocks of the footwall [27].

2.2. Ermioni VMS Deposit Geology and Historical Background

Despite the diverse metallogeny of Greece, only a very small number of VMS mineralizations have been identified, including the Kuroko-type Skra deposit in northern Greece [62,63,64], and the Molaoi and Ermioni VMS deposits in southern Greece (Peloponnese). The Molaoi VMS is considered a small Kuroko-type mineralization [65], whereas the Ermioni VMS is the only known deposit of this type exploited during the 20th century due to its high pyrite content used for sulfuric acid production for fertilizers. The nearly monomineralic character of the ore with abundant pyrite (>95% modal in the massive ore) and the very low content in As and other volatile and toxic metals made it ideal for such applications [36].
The genetic relation between the massive ore and the host volcanic rocks was supported from very early studies [36,37], and [66,67,68] have described the Ermioni mineralization as VMS based on early-developed models [2]. Later, Varnavas et al. [69] and Robertson et al. [50] moved one step further and classified it as Cyprus type, mainly based on the ore mineralogy, geochemistry, and the close proximity of the ore to ultramafic lithologies associated with Vardar-Axios Ocean Jurassic ophiolites [54,59]. Nearly 20 years later, Tombros and Seymour [70] and Tombros et al. [71] focused their research on stable isotope geochemistry and the mineral chemistry of sulfides and proposed formation of the Ermioni VMS in a submarine environment analogous to contemporary ocean-ridge black smokers. Recently, Triantafyllidis et al. [27] and Tombros et al. [28] performed a detailed study on the Ermioni VMS, focusing on the most important mine sites, Karakasi and Roro (Figure 1B,C). Their results discarded the previous consensus of the Cyprus-type character of the ore and showed that the Ermioni VMS is actually an atypical “mafic–pelitic” VMS with geochemistry resembling Cyprus-type deposits (Figure 2). The Ermioni VMS was formed due to hydrothermal circulation in a depression of the “Adheres Mélange Unit” (henceforth “Ermioni basin”) in the upper Cretaceous, with abundant pyrite, minor chalcopyrite, and traces of sphalerite ([27,36,37]).

2.3. Karakasi and Roro VMS Geochronology

The Ermioni VMS proved to be very challenging when the age of the ore formation was in question. The mineralogical investigation of the footwall lithologies (mafic–intermediate volcaniclastic rocks) revealed that zircon crystals, K-bearing silicates, or other phases that could be used for geochronological dating are absent, making the exact dating of the footwall lithologies problematic. Considering these issues and taking into account the previous consensus of the Ermioni VMS, Triantafyllidis et al. [27] focused on radiogenic isotope geochronology of ore-specific cases, and in particular Re-Os dating on pyrite crystals from the massive and stringer ore, and Rb-Sr dating in silicic inclusions in pyrite from both sites(for details, refer to Triantafyllidis et al. [27]). Their Re-Os dating showed that the Ermioni VMS is of upper Cretaceous age. More interestingly, they calculated variations in the ages between the Roro massive (65.58 ± 0.9 to 66.02 ± 0.1 Ma; 2σ = ± 0.98, MSWD = 0.98, 187Os(i) = 2.26 ± 0.03 and 187Os/188Os(i) = 2.18 ± 0.06) and the Karakasi stringer ore (64.96 ± 0.9 to 65.12 ± 0.1 Ma; 2σ = ± 1.0, MSWD = 1.01, 187Os(i) = 1.83 ± 0.05 and 187Os/188Os(i) = 1.82 ± 0.18), indicating that the Karakasi VMS postdates the Roro VMS by approximately 0.5 Ma. Considering that in a typical VMS structure, the stringer ore predates the overlying massive ore, in the Ermioni area there were at least two different localities where hydrothermal activity was taking place during the upper Cretaceous [27].

3. Analytical Data

The data presented in this study come from the work of Triantafyllidis et al. [27] and come from focused investigation of the stable and radiogenic isotope geochemistry of gangue phases and sulfides from the two most important mine sites, Karakasi and Roro (Figure 1, Table 1 and Table 2). Pyrite separates from massive (Roro) and stringer (Karakasi) ore were used for Re-Os, Pb, Rb-Sr (silicic inclusions), He-Ar, Fe, and S isotopic analyses. Details on the analytical techniques, statistical analyses, and software employed can be found in [27].

4. Results and Discussion

4.1. Origin of Hydrothermal Fluids

In cases where several hydrothermal venting centers function in different time periods, the radiogenic isotopic signatures of sulfides may be employed in defining the source (or sources) of metals. For Karakasi stringer and Roro massive pyrite, the Pb isotopic values are similar and the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios range between 18.06 and 18.10, 15.60 and 15.63, and 38.81 and 38.83, respectively (Table 1), indicating a single and common source of metals (Figure 3A). The Pb radiogenic values for both VMS systems present many similarities to the Pb isotopic signatures of fossilized VMS deposits (e.g., Australian VMS systems [25,74]; Rudny Altai VMS systems [19]; Iberian Pyrite Belt VMS [75]; and modern day black smokers (e.g., Endeavour Segment, Juan de Fuca Ridge [76]; Pito Seamount, Easter Microplate [77]) and are very different to the adjacent and younger in age Cycladic Massif (Aegean Sea) mineralizations related to continental intermediate–felsic magmatism (Figure 3B). The noble gases’ isotopic composition of Karakasi and Roro pyrites is typical for VMS sulfides (e.g., Rodriguez Triple Junction, Central Indian Ridge [78]; Southwest Indian Ridge [79]; East Pacific Rise [80]) with R/RA values ranging between 5.24 and 6.55 (Table 1, Figure 3C), depicting a deep magmatic source affecting the ore-forming hydrothermal fluids [81].
A closer investigation in the Pb and noble gases’ isotope geochemistry shows that the Karakasi and Roro pyrites are more radiogenic than enriched mantle (Figure 3A), with 4He/3He values ranging between 108×103 and 137 × 103 (Table 1), which in turn are more radiogenic than average MORB (≈90 × 103 [82,83]), indicating that a possible primitive source with large mantle input is rather improbable [25], especially when considering the geotectonic setting of the Ermioni basin during the Cretaceous. According to Kurz et al. [84], subducting oceanic crust in convergent margins is a possible radiogenic He source. Mougel et al. [80] and Wang et al. [79] state that degassing of oceanic crust takes place during both formation along ocean ridges and subduction due to generation of high (U+ Th)/3He material. Therefore, the Pb and noble gases’ isotopic signature of massive and stringer pyrites could indicate an evolved heat source related to partial melting of subducting radiogenic oceanic crust (Pindos Ocean). This setting is also supported by the trace element geochemistry of the footwall, calc-alkaline volcaniclastic rocks revealing volcanism in a supra-subduction zone setting (SSZ) related to highly evolved magmatism [27]. Both footwall volcaniclastic rocks and the VMS systems under investigation were formed in a similar geotectonic setting including calc-alkaline volcanism in a depression of the Ermioni basin. The textural characteristics of the footwall volcaniclastic rocks (e.g., angular albite fragments, mafic clasts; for details refer to [27,85]) confirm that magmatic activity predated ore formation. Therefore, the Pb and noble gases’ isotopic geochemistry of stringer (Karakasi) and massive ore (Roro) pyrite could show depletion of evolved lithologies (footwall volcaniclastic rocks) during hydrothermal convection. Still, we cannot disregard the possibility that magmatic fluids from this deep source (subducting Pindos Ocean) also contributed to the hydrothermal system, as the Pb and noble gases’ isotope geochemistry could point to this too.
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Figure 3. Radiogenic isotope binary diagrams for massive and stringer pyrite from Roro and Karakasi VMS, respecitvely. (A) 206Pb/204Pb vs. 207Pb/204Pb binary diagram of Ermioni VMS pyrite (data from [20,86,87,88,89]). (B) 207Pb/206Pb vs. 204Pb/206Pb binary diagram of Ermioni VMS pyrite (data from [75,89] and references therein). (C) 4He vs. R/RA binary diagram of Ermioni pyrites. The light orange shaded area depicts the range of R/RA and 4He values of seafloor massive sulfides (data from [79]).(D) 206Pb/204Pb vs. 87Sr/86Sr binary diagram of Ermioni pyrites and silicic inclusions (fields from [87]).
Figure 3. Radiogenic isotope binary diagrams for massive and stringer pyrite from Roro and Karakasi VMS, respecitvely. (A) 206Pb/204Pb vs. 207Pb/204Pb binary diagram of Ermioni VMS pyrite (data from [20,86,87,88,89]). (B) 207Pb/206Pb vs. 204Pb/206Pb binary diagram of Ermioni VMS pyrite (data from [75,89] and references therein). (C) 4He vs. R/RA binary diagram of Ermioni pyrites. The light orange shaded area depicts the range of R/RA and 4He values of seafloor massive sulfides (data from [79]).(D) 206Pb/204Pb vs. 87Sr/86Sr binary diagram of Ermioni pyrites and silicic inclusions (fields from [87]).
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4.2. Geologic Setting of Karakasi and Roro VMS Systems

Despite the common heat source of hydrothermal fluids, there are small, yet distinct differences in the radiogenic (Sr and noble gases) and stable (Fe and S) isotopic compositions between Roro and Karakasi VMS. The 87Sr/86Sr values of silicic inclusions in Ermioni pyrites (massive and stringer ore) range between 0.7019 and 0.7109, with inclusions from the Karakasi stringer ore having slightly higher 87Sr/86Sr values relative to Roro massive ore (Table 1, Figure 3D). For both sites, the δ18O and δ30Si values (Table 2) are higher than typical MORB, mantle-derived, and differentiation-derived magmas ([90,91] and references therein), proving the incorporation of seawater in the hydrothermal and ore-forming system. Yet, the δ18O and δ30Si values of hydrothermal quartz from the footwall volcaniclastic rocks at Roro are higher than similar facies at Karakasi VMS (Figure 4A), indicating a higher degree of seawater input in the hydrothermal and ore-forming system. The range of δ34S values of sulfides from Karakasi and Roro sites is typical for VMS mineralizations (see [19,32]) (Table 2). For the Karakasi VMS site, the disseminated and stringer pyrite crystals show fluctuations in the S isotopic values, whereas in the Roro massive ore the δ34S values for pyrite fall in a narrow range (4.82 to 6.21 per mil) (Figure 4B). Interestingly, sphalerite from both sites (Table 2) is characterized by a lighter δ34S isotopic signature (still positive) relative to pyrite. Although the behavior of S isotopes in hydrothermal systems is complex, involving many different factors (e.g., bacterial activity, sulfate reduction during convection), the lower δ34S values of late-stage sphalerite could be attributed to a lower seawater contribution, or fractionation of S species during sulfide deposition. According to Tombros et al. [28], the predominance of pyrite in the Karakasi and Roro VMS is attributed to massive input of Fe in the hydrothermal system, a statement supported by the lithologies leached during hydrothermal convection (mafic and ultramafic rocks). The high Fe content of the hydrothermal fluids provokes deposition of pyrite incorporating heavier S isotopes, whereas fractionation during the late stage affects the S isotopic composition of sphalerite (Table 2). The Fe isotope geochemistry of pyrite from Karakasi and Roro VMS also falls in the typical range of values for sulfides from VMS deposits (see [92]). Yet, massive pyrite from Roro VMS shows slightly higher δ57Fe values relative to stringer pyrite from Karakasi VMS (−0.56 and −0.77, and −0.87 and −0.89, respectively) (Table 2). Comparing the pyrite Fe and S isotopic geochemistry from both sites, the Roro massive pyrite is characterized by slightly higher δ34S and δ57Fe values relative to Karakasi stringer pyrite (Figure 4B), which is attributed to the interaction of cold seawater with ascending hydrothermal fluids. The degree of interaction differs between the two sites, and is higher in Roro massive and lower in Karakasi stringer ore.
The early Roro VMS was developed most probably along high-angle normal fault(s) of the eastern part of the Ermioni depression and within the footwall volcaniclastic rocks. The fault(s) acted as feeder zone leading to the formation of the lower stringer ore. Hydrothermal, metal-bearing fluids were fed through these zones and were discharged on the seafloor, causing rapid sulfide deposition after interaction with cold seawater (Figure 5). The aforementioned model of Roro VMS formation is supported by ore texture and stable and radiogenic isotope geochemistry. As stated by Galley et al. [4], the clastic–sandy and fine-grained texture of the Roro massive ore depicts ore deposition on the seafloor, and in particular cementation of loosely held massive ore fragments, later cemented by quartz and calcite (Figure 5). The R/RA values of Roro massive pyrite also support direct discharge of hydrothermal fluids on the seafloor without prior interaction with crustal material (e.g., turbidites) (Figure 3C). Moreover, the positive δ34S values are also related to direct interaction between hot ascending ore-bearing hydrothermal fluids with cold seawater, thus provoking incorporation of heavier S species in massive pyrite deposited on the seafloor. The higher δ57Fe values of massive pyrite from Roro are also attributed to a higher degree of mixing between cold seawater and hot ascending hydrothermal fluids, resulting in increased δ57Fe in the late-stage ore-forming system (stage II of [27,28]).
The later (by approx. 0.5 Ma) Karakasi VMS has a different texture relative to Roro VMS. The size of pyrite crystals in the semi-massive to massive ore is greater (>1 mm) (Figure 6) and the hydraulic modeling of [28] showed that hydrothermal convection and ore deposition was taking place within the footwall and hanging-wall lithologies (volcaniclastic rocks, arkoses, and turbidites) [27] (Figure 6A–D).The hydraulic pressure of the system was greater relative to Roro VMS [28], due to the fine-grained character of the hanging-wall turbidites acting as an effective barrier, thus enabling the formation of larger, euhedral pyrite [93,94]. The stable and radiogenic isotope geochemistry of sulfides also supports the views of Tombros et al. [28]. The Sr isotopic composition of silicic inclusions in Karakasi stringer pyrite implies a higher degree of mixing between a low-87Sr/86Sr source (e.g., mantle ≈ 0.700) and a high-87Sr/86Sr source (oceanic crust/sediments ≈ 0.725), depicting incorporation of crustal material in the ore-forming hydrothermal fluids, attributed to circulation/convection within the unconsolidated fine-grained turbidites of the hangingwall [28] (Figure 6A–D).The noble gases’ geochemistry also supports this hypothesis; Karakasi stringer pyrite shows significantly higher Ar values (as 40Ar/36Ar) relative to massive pyrite from Roro VMS (Table 1), indicating more radiogenic Ar incorporation in sulfides (hanging-wall turbidites) [79] (Figure 3C).When compared to Roro massive ore, the lower 34S and 57Fe isotope composition of Karakasi stringer pyrite corresponds to either a lower degree of interaction between penetrating seawater and ascending ore-forming fluids, or formation at higher temperatures [92], which is supported by the ore-forming temperatures (between 330 and 430 °C) calculated by Tombros et al. [28].
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Figure 5. Schematic depiction of the geotectonic evolution of Roro VMS (right side (A–E)). The Roro VMS was formed easterly (relative to Karakasi) on the seafloor prior turbidite deposition resembling the mode of formation of contemporary mound-shaped massive sulfides. (1–2) Photomicrographs of Roro VMS massive ore and their corresponding position in the Roro VMS. (1) SEM-EDS image of massive ore from Roro VMS, with abundant pyrite (Py) and minor chalcopyrite (Ccp). (2) Reflected light optical microscopy image (plain light) of Roro massive ore with typical clastic–sandy texture depicting formation on the seafloor. The angular massive ore fragments with predominant pyrite (Py) and minor chalcopyrite (Ccp) are cemented by later quartz±calcite. (Abbreviations after [95]).
Figure 5. Schematic depiction of the geotectonic evolution of Roro VMS (right side (A–E)). The Roro VMS was formed easterly (relative to Karakasi) on the seafloor prior turbidite deposition resembling the mode of formation of contemporary mound-shaped massive sulfides. (1–2) Photomicrographs of Roro VMS massive ore and their corresponding position in the Roro VMS. (1) SEM-EDS image of massive ore from Roro VMS, with abundant pyrite (Py) and minor chalcopyrite (Ccp). (2) Reflected light optical microscopy image (plain light) of Roro massive ore with typical clastic–sandy texture depicting formation on the seafloor. The angular massive ore fragments with predominant pyrite (Py) and minor chalcopyrite (Ccp) are cemented by later quartz±calcite. (Abbreviations after [95]).
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During the approximately 0.5 Ma period separating the two VMS systems (Figure 1D), the same, deep-heated hydrothermal system evolved from free discharge on the seafloor (Roro), forming fine-grained massive sulfides, to sediment-confined, subseafloor semi-massive-to-massive pyrite rich masses within unconsolidated turbidites and arkoses. For both locations, seawater was the major component of both hydrothermal systems, yet the seawater effect on the sulfide ore is more prominent in Roro VMS. The latter was developed easterly (based on contemporary coordinates) along normal, high-angle faults, and the metal-bearing fluids were discharged on a seafloor comprising mafic–intermediate volcaniclastic rocks, forming fine-grained massive sulfides(Figure 5). The brittle massive ore suffered limited scale transportation, leading to the development of clastic–sandy texture, in a similar manner to the seafloor style of formation of contemporary mound-shaped massive sulfides. Within the following 0.5 Ma, the VMS-hosting depression in the Ermioni basin was subjected to fine-grained clastic sedimentation (turbidites), that covered the already formed Roro VMS, the volcaniclastic rocks, and the overlying arkoses (based on foraminifera and microfossil species identified in [37,42,54,96]). Contemporaneously to turbidite deposition, the hydrothermal system migrated westwards, finding new pathways through the high-angle normal faults of the western margin of the Ermioni basin and ascended towards the seabed (Figure 6A–D). Convection occurred within the footwall volcaniclastic rocks, arkoses, and the unconsolidated hanging-wall turbidites, leading to the formation of the Karakasi VMS system within the host lithologies. From the Paleocene–Eocene, deformation and tectonism, related to accretion of the Ermioni basin on the Pelagonian continental margin [53], resulted in dismemberment of the Karakasi and Roro VMS systems (Figure 5 and Figure 6). Tectonic separation occurred along discrete ore zones, including the lower stringer zone, and the overlying semi-massive and massive ore, forming small lenses and irregular ore bodies concordant with the hanging-wall turbidites.
The results of this study could be employed in future exploration strategies regarding fossilized upper Cretaceous marine environments in the Alpine Orogen. To this day, the vast majority of VMS mineralizations in the circum-Mediterranean region are considered earlier than the Ermioni VMS (e.g.,northern Italy, Balkans, northern Turkey), yet detailed radiogenic studies (Re-Os geochronology) of Kuroko-type VMS in Pontides (northern Turkey, [33]) have shown that the previously accepted ore formation time setting (90–82 Ma) is under question, and upper Cretaceous ages are now accepted. This proves the importance of detailed investigation of upper Cretaceous marine environments, with beneficial results for the metallogenic potential of the eastern Mediterranean.

5. Conclusions

The Ermioni VMS is a perfect example of the mode of evolution of VMS-related hydrothermal systems in geologically active areas, and how focused examination of stable and radiogenic isotopes in specific cases may be employed in providing answers regarding genesis and evolution of hydrothermal systems, their mixing with various sources, the geologic environment of hydrothermal circulation/convection, and VMS formation. Examination of the Karakasi and Roro VMS systems in the Ermioni area revealed the following:
  • The Pb and noble gases’ (Ar-He) isotopic geochemistry of massive (Roro) and stringer (Karakasi) pyrite indicates a deep and evolved heat source, which probably also acted as a source of metals, attributed to subduction and partial melting of a radiogenic He source (depleted Pindos Oceanic crust).
  • Despite the unknown age of footwall volcaniclastic rocks, the radiogenic isotope composition of massive and stringer pyrite indicates a similar geotectonic setting for both footwall volcaniclastic rocks and the VMS systems.
  • This study shows how stable (Fe, S) and radiogenic (Pb, Sr, Ar) isotopic examination focused on specific phases may be used in the identification of both the source of metals and the setting of VMS ore formation. In the case of Ermioni VMS, the stable and radiogenic isotope compositions of VMS pyrite from Karakasi (stringer ore) and Roro (massive ore) point to differences in the geologic environment of VMS formations. Roro massive pyrite shows higher δ57Fe and δ34S values relative to Karakasi stringer pyrite, attributed to direct interaction of ascending metal-bearing hydrothermal fluids with cold seawater. Karakasi stringer pyrite shows higher 87Sr/86Sr ratios and radiogenic Ar values (as 40Ar/36Ar) depicting the interaction of hydrothermal fluids with crustal material (hanging-wall turbidites).
  • During the approximately 0.5 Ma period separating the two systems, the same hydrothermal and ore-forming system evolved from free discharge on the seafloor (Roro—easterly) to a sediment-confined, subseafloor system (Karakasi—westerly).The Roro massive ore resembles a seafloor style of formation of contemporary mound-shaped massive sulfides, whereas the later Karakasi VMS hosted in the footwall and hanging-wall lithologies points to hydrothermal circulation within the Ermioni basin floor lithologies.
  • The results of this study can be used during exploration of fossilized upper Cretaceous marine environments of the Alpine Orogen, with corresponding benefits in the metallogenic potential of the eastern Mediterranean.

Author Contributions

Conceptualization, S.S.T.; methodology, S.S.T. and S.F.T.; software, S.F.T.; validation, S.S.T. and S.F.T.; formal analysis, S.S.T. and S.F.T.; investigation, S.S.T. and S.F.T.; resources, S.S.T. and S.F.T.; data curation, S.S.T. and S.F.T.; writing—original draft preparation, S.S.T.; writing—review and editing, S.S.T.; visualization, S.S.T. and S.F.T.; supervision, S.S.T. and S.F.T.; project administration, S.S.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank the anonymous reviewers for their insightful and valuable comments during the development of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Simplified geologic map of Argolis Peninsula with the locations of the major Ermioni VMS sites (with modifications after [27,35]), (B,C) geology of Karakasi and Roro mining sites (with modifications after [28,36,37]), (D) Simplified geologic map of Ermioni basin in upper Cretaceous during Roro and Karakasi VMS formation (with modifications after [27]).
Figure 1. (A) Simplified geologic map of Argolis Peninsula with the locations of the major Ermioni VMS sites (with modifications after [27,35]), (B,C) geology of Karakasi and Roro mining sites (with modifications after [28,36,37]), (D) Simplified geologic map of Ermioni basin in upper Cretaceous during Roro and Karakasi VMS formation (with modifications after [27]).
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Figure 2. Cu-Pb-Zn ternary diagram of the base-metal composition of various VMS deposit types (data after [4,19]). Red circles depict the massive ore geochemistry of Ermioni mafic–pelitic VMS, plotted in the field of “Cyprus-type” deposits.
Figure 2. Cu-Pb-Zn ternary diagram of the base-metal composition of various VMS deposit types (data after [4,19]). Red circles depict the massive ore geochemistry of Ermioni mafic–pelitic VMS, plotted in the field of “Cyprus-type” deposits.
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Figure 4. Stable isotope binary diagrams of pyrite and gangue phases from Roro and Karakasi VMS. (A) δ18O–δ30Si binary diagram of gangue quartz (Qz) and calcite (Cal) from the footwall volcaniclastic rocks from Roro and Karakasi VMS. (B) δ34S–δ57Fe binary diagram of massive (Roro) and stringer (Karakasi) pyrite from Ermioni VMS (with modifications after [27]).
Figure 4. Stable isotope binary diagrams of pyrite and gangue phases from Roro and Karakasi VMS. (A) δ18O–δ30Si binary diagram of gangue quartz (Qz) and calcite (Cal) from the footwall volcaniclastic rocks from Roro and Karakasi VMS. (B) δ34S–δ57Fe binary diagram of massive (Roro) and stringer (Karakasi) pyrite from Ermioni VMS (with modifications after [27]).
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Figure 6. Schematic depiction of the geotectonic evolution of Karakasi VMS (right side (A–D), legend as in Figure 5). The Karakasi VMS was formed later and further west (based on contemporary coordinates) relative to Roro VMS, contemporaneously to arkose and turbidite deposition during upper Cretaceous. (1–4) Photomicrographs of Karakasi VMS ore and corresponding position in the Roro VMS. (1) Disseminated, coarse-grained, euhedral pyrite (Py) and gangue quartz (Qz) from Karakasi stringer ore. (2) Semi-massive pyritic ore (Py) from the transition zone between the lower stringer ore and the upper massive sulfide orebody. Quartz (Qz) is the predominant gangue phase. (3) Semi-massive, coarse-grained, euhedral pyrite (Py) with gangue calcite (Cal), chlorite (Chl), and quartz (Qz). The mineralization is hosted in the hanging-wall turbidites. (4) Massive sulfide ore from Karakasi VMS with abundant pyrite (Py) and gangue calcite (Cal) and quartz (Qz). (Abbreviations after [95]).
Figure 6. Schematic depiction of the geotectonic evolution of Karakasi VMS (right side (A–D), legend as in Figure 5). The Karakasi VMS was formed later and further west (based on contemporary coordinates) relative to Roro VMS, contemporaneously to arkose and turbidite deposition during upper Cretaceous. (1–4) Photomicrographs of Karakasi VMS ore and corresponding position in the Roro VMS. (1) Disseminated, coarse-grained, euhedral pyrite (Py) and gangue quartz (Qz) from Karakasi stringer ore. (2) Semi-massive pyritic ore (Py) from the transition zone between the lower stringer ore and the upper massive sulfide orebody. Quartz (Qz) is the predominant gangue phase. (3) Semi-massive, coarse-grained, euhedral pyrite (Py) with gangue calcite (Cal), chlorite (Chl), and quartz (Qz). The mineralization is hosted in the hanging-wall turbidites. (4) Massive sulfide ore from Karakasi VMS with abundant pyrite (Py) and gangue calcite (Cal) and quartz (Qz). (Abbreviations after [95]).
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Table
Table 1. Radiogenic isotope data (Pb, He-Ar, and Rb-Sr) from Ermioni VMS (EK—Karakasi VMS, ER—Roro VMS) (radiogenic isotope data taken from [27]).
Table 1. Radiogenic isotope data (Pb, He-Ar, and Rb-Sr) from Ermioni VMS (EK—Karakasi VMS, ER—Roro VMS) (radiogenic isotope data taken from [27]).
Sample206Pb/204Pb207Pb/204Pb208Pb/204Pb4HeA3HeB40Ar/36ArC
ER118.0815.638.814.1937.05301.2
ER218.0615.6338.822.522.92315.9
ER318.115.6138.825.7249.51358.1
ER418.0615.6138.833.8833.05314.2
ER5---2.4922.32331.3
ER12---1.4312.99328.8
1σ± 0.019±0.008±0.012±1.53±12.94±19.57
2σ±0.038±0.016±0.025±3.07±25.88±39.14
St. Error0.010.0040.0060.6265.287.99
EK1118.115.6338.831.7812.98410.2
EK1218.115.6338.831.5511.48453.7
1σ---±0.163±1.06±30.75
2σ---±0.326±2.12±61.51
St. Error---0.1150.7521.75
SampleR/RADRb (ppm)Sr (ppm)87Rb/86Sr87Sr/86Sr
ER16.370.935.980.44360.7091
ER26.61.675.360.89540.7092
ER36.220.843.780.76460.7092
ER46.131.934.120.78960.7091
ER56.431.368.040.54770.7099
ER126.551.013.560.65490.7092
1σ±0.183±0.44±1.71±0.1671±0.00074
2σ±0.367±0.88±3.41±0.3341±0.00155
St. Error0.0740.1790.6960.06830.00032
EK115.241.4310.450.55660.7103
EK125.341.599.640.59870.7109
1σ±0.071±0.11±0.57±0.0298±0.00042
2σ±0.141±0.23±1.15±0.0595±0.00085
St. Error0.050.080.410.02110.0003
A10−7; B10−13 cm3 STP/g; C10−3; D RA is the 3He/4He ratio of atmosphere (3He/4He)Air = 1.4.10−6 and (4He/36Ar)Air = 0.1655.Typical R/RA values for MORB are ~ 8.2 ±0.7 [72], whereas for crust the R/RA value is ~ 1 [73].
Table
Table 2. Stable isotope data from Ermioni VMS; oxygen and silicon isotopic data were obtained from gangue phases (quartz—Qz, albite—Alb, and calcite—Cal), S and Fe isotopic data were obtained from sulfide separates from massive and stringer ore (EK—Karakasi VMS, ER—Roro VMS, Py—pyrite, Sp—sphalerite) (stable isotope data were taken from [27]).
Table 2. Stable isotope data from Ermioni VMS; oxygen and silicon isotopic data were obtained from gangue phases (quartz—Qz, albite—Alb, and calcite—Cal), S and Fe isotopic data were obtained from sulfide separates from massive and stringer ore (EK—Karakasi VMS, ER—Roro VMS, Py—pyrite, Sp—sphalerite) (stable isotope data were taken from [27]).
SampleLithotypeMineralδ18OV-SMOWδ30Si
EK1VolcaniclasticQz+13.11+1.11
EK2VolcaniclasticAlb+11.42−0.73
EK3VolcaniclasticQz+13.84−0.98
EK4VolcaniclasticQz+11.22−0.68
EK5VolcaniclasticQz+11.95−1.03
ΕK6VolcaniclasticQz + cal+5.06+0.85
ΕK7Stringer oreCal+15.98−0.47
EK10Stringer oreCal+17.59−0.59
1σ ±2.42±0.82
2σ ±4.83±1.65
St. Error 0.910.29
ER8VolcaniclasticQz14.12−0.47
ER10VolcaniclasticQz+14.43−0.54
1σ ±0.22±0.05
2σ ±0.44±0.1
St. Error 0.160.04
SampleLithotypeMineralδ34S VCDT57Fe
EK1VolcaniclasticPy+0.68-
EK2VolcaniclasticPy+1.85-
EK3VolcaniclasticPy+0.54-
EK4VolcaniclasticPy+4.71-
EK5VolcaniclasticPy+3.17-
EK10Stringer oreSp+1.96-
EK11Stringer orePy+5.32−0.89
EK12Stringer orePy+4.82−0.87
1σ ±1.87±0.01
2σ ±3.74±0.03
St. Error 0.590.01
ER1Massive orePy+6.21−0.56
ER2Massive orePy+5.32−0.62
ER3Massive orePy+6.12−0.67
ER4Massive orePy+5.93−0.62
ER5Massive orePy+5.79−0.70
ER8Massive oreSp+3.96-
ER10Massive oreSp+2.17-
ER12Massive orePy+5.53−0.77
1σ ±1.39±0.07
2σ ±2.78±0.15
St. Error 0.490.03
Analytical precision was better than ± 0.1 per mil for δ18O and δ30Si, ± 0.2 per mil for δ34S, and ± 0.03 per mil for δ57Fe.
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Triantafyllidis, S.S.; Tombros, S.F. Deciphering the Evolution of Adjacent Volcanogenic Massive Sulfide (VMS) Systems Based on Radiogenic and Stable Isotopes, the Case of Ermioni, Argolis Peninsula, Ne Peloponnese, Greece. Minerals 2023, 13, 474. https://doi.org/10.3390/min13040474

AMA Style

Triantafyllidis SS, Tombros SF. Deciphering the Evolution of Adjacent Volcanogenic Massive Sulfide (VMS) Systems Based on Radiogenic and Stable Isotopes, the Case of Ermioni, Argolis Peninsula, Ne Peloponnese, Greece. Minerals. 2023; 13(4):474. https://doi.org/10.3390/min13040474

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Triantafyllidis, Stavros Savvas, and Stylianos Fotios Tombros. 2023. "Deciphering the Evolution of Adjacent Volcanogenic Massive Sulfide (VMS) Systems Based on Radiogenic and Stable Isotopes, the Case of Ermioni, Argolis Peninsula, Ne Peloponnese, Greece" Minerals 13, no. 4: 474. https://doi.org/10.3390/min13040474

APA Style

Triantafyllidis, S. S., & Tombros, S. F. (2023). Deciphering the Evolution of Adjacent Volcanogenic Massive Sulfide (VMS) Systems Based on Radiogenic and Stable Isotopes, the Case of Ermioni, Argolis Peninsula, Ne Peloponnese, Greece. Minerals, 13(4), 474. https://doi.org/10.3390/min13040474

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