Sotirios Kokkalas

The Lavrion metallogenetic district, located in the Attico-Cycladic Massif, Greece, hosts an evolving proximal calcic Fe-skarn followed by distal sulfide-rich skarn, transitioning into the world-class Pb-Zn-Ag carbonate-replacement deposit. This study investigates the structural and microstructural controls on skarn metasomatism and its transition to carbonate-replacement mineralization, with emphasis on the role of extensional tectonics and detachment faulting.The study integrates detailed structural mapping with microstructural and mineralogical analyses of thin sections. In detail, the study included microtectonic analysis, fluid inclusion microthermometry, EBSD, SEM-EDS, stable and radiogenic isotopes, and geochronological constraints (U-Pb, Re-Os, Pb-Pb). These datasets allow us to reconstruct deformation conditions, fluid evolution, and the timing of fault-related mineralization.The Lavrion skarn system occurred within the footwall of the Lower Tectonic Unit, where NW–SE-trending brittle to ductile-brittle faults and the West Cycladic Detachment System (WCDS) generated an extensive damage zone characterized by intense fracturing, brecciation, and enhanced permeability. Skarns, skarnoids, and associated oxide and sulfide ores are spatially localized along these fault-related structures, which acted as infiltration paths for magmatic-hydrothermal fluids.Microstructures in the different skarn zones, i.e., garnet-clinopyroxene and garnet-epidote, and associated ores, i.e., magnetite, pyrrhotite, and chalcopyrite, including oscillatory and sector zoning, sigma-type tails, replacement fronts, crack-seal textures, and mineralized breccias record episodic fluid flow, fluctuating redox conditions, and syn-tectonic mineral growth. Prograde Fe-skarn assemblages formed at ~560–530 °C and ~0.2 GPa under relatively oxidizing conditions, leading to widespread magnetite ores. Subsequent cooling to ~460–380 °C, combined with variations in fO₂ and fS₂, acid and saline fluids, promoted extensive retrograde replacement of magnetite by sulfide ores, i.e., pyrrhotite, galena, sphalerite and chalcopyrite.Fluid inclusion microthermometry and stable and radiogenic isotopes indicate that the skarn-forming fluids were primarily magmatic in origin and were sourced from the Miocene Plaka and Villia granitoids. The ore fluids were significantly modified through wallrock-fluid interaction with the metasedimentary host rocks within the detachment damage zone.The WCDS, not only controlled skarn formation but also exerted a first-order influence on the temporal and spatial development of the overlying Pb-Zn-Ag carbonate-replacement deposits. The Lavrion district represents a structurally controlled calcic Fe-skarn transitioning toward the carbonate-replacement deposit, where extensional faulting, folding, detachment-related damage zones, and microstructural evolution governed fluid pathways and ore deposition. Our results highlight the importance of integrating microstructural analysis with tectonic architecture in exploration modeling for skarn-oxide-related and carbonate-replacement-sulfide related deposits in extensional or post-collisional settings.

Aquifer structure plays a critical role in controlling groundwater flow through both saturated and unsaturated zones. In remote and rugged terrains, GIS and remote sensing tools are essential for delineating geological structures. This study integrates stereoscopic aerial photograph analysis and GIS-based hill-shading techniques to enhance lineament detection within the Ziria karst system in northern Peloponnese, Greece. By combining these techniques, 1,214 lineaments were identified, revealing structural and erosional features that were not detected using a single approach. Field validation confirmed that lineaments identified by aerial photography corresponded to faults or fracture systems, with a predominant NNW–SSE orientation. Fracture network analysis was subsequently applied to assess structural feature connectivity. The results indicated a low degree of connectivity within the lineament dataset, with isolated (I) features being more prevalent than branching (Y) and intersection (X) nodes. Additionally, detailed karst landform mapping identified 325 karst depressions, where approximately 62.5% of the depressions were in direct association with the identified lineaments. The majority of the remaining karst depressions, while not in direct contact, were predominantly located within zones characterized by high lineament density. The mapped lineaments were ultimately used to refine the boundaries of the karst aquifer, defined by crystalline metamorphic bedrock, and to update the area’s groundwater vulnerability and recharge regimes. The findings emphasize the role of structural control in shaping hydrological systems, offering a robust framework for assessing various hydrogeological aspects and guiding effective groundwater management in tectonically active karst regions.

This study provides a comprehensive characterization of the low-to-intermediate sulfidation (LS-to-IS) epithermal Molai Zn-Pb±(Ag,Ge) ore (Vigla-Mesovouni orebody) in Laconia, Greece, and provides insights on how such data may be employed in beneficiation flow-sheet design. Detailed mineralogical, chemical, textural, and physicochemical characterization defines a systematic transition from early refractory Ge-rich to late-stage refractory Ag-rich mineralization, including sulfides and fahlores. Germanium, although present in all sphalerite varieties (Sp-I, Sp-II, and Sp-III), is predominantly enriched in early sphalerite (Sp-I, up to 1891.60 ppm). Interestingly, Ge is also enriched in early Py-I pyrite, with content reaching up to 383 ppm. Silver is mainly concentrated in late-stage tetrahedrite Ttr-II (up to 3.60%), galena (Ga-II), and, to a lesser extent, late sphalerite (Sp-III). Liberation studies reveal effective liberation of Py-I and Sp-I, major Ge carriers, in the coarser fractions (+0.150 mm) and near complete liberation of all ore phases below 0.036 mm. Combined beneficiation via Wilfley pre-concentration and differential flotation produced up to ~35% Pb and ~65% Zn at >85% recovery for the smallest fractions (−0.036 mm). Ore characterization revealed that secondary circuits may be developed to further enhance the economic value of Molai ore (Ge from Py-I, and Ag±[Sb,As] from Ttr-II and Ag-bearing sulfosalts), which are dismissed as wastes in Pb and Zn flotation circuits. The results of our study establish a robust foundation for the design of tailored, multi-stage metallurgical flow-sheets aimed at maximizing the economic value of the Molai epithermal resource.

As part of the energy transition needed to mitigate global warming, the study and sustainable exploitation of geothermal resources—a largely underutilized form of energy and heat production—is crucial. The availability of subsurface data acquired for oil and gas exploration purposes provides an opportunity to reconsider these data to enhance the use of geothermal potential. This is the case of a fractured carbonate reservoir in the Southern Apennines (Italy). All available subsurface data were gathered, homogenized, and reinterpreted to build a 3D geological model of the study area, where a positive thermal anomaly is known, yet the mechanisms and pathways of heat transport were previously unclear. By integrating subsurface, temperature, and literature data, a geological model is proposed that explains how high temperatures and heat propagation are closely linked to specific geological features. By cross-referencing and weighing the relevance of data for geothermal purposes, an attempt is made to rank the geothermal potential of existing wells in the area. This study demonstrates how a well-constrained geological model and the joint analysis of multidisciplinary data can provide the necessary knowledge base for conducting further technical, engineering, and economic analyses to assess the commercial viability of the identified geothermal resource.

Triassic calc‐alkaline volcanics that are mainly exposed in the Hellenides—Dinarides orogen, as well as eastwards in the Afyan Zone, are considered to constrain the petrogenetic mechanisms that led to their formation, and to unravel the geodynamic processes that occurred in the western Tethys, related to the NW Gondwana (Apulia) promontory and the northwards transposing Pelagonian microcontinent. Lavas from Triassic key localities were examined from the broader northern Peloponnese, which include basaltic andesites, andesites and rhyodacites, and are classified as medium‐K calc‐alkaline volcanics. Partial melting of ∼10%–12% of a fertile peridotitic mantle source contributed to primitive magma formation. Whole‐rock geochemical data and Sr‐Nd isotopic compositions point to upper crustal assimilation processes (AFC) for the formation of the andesites and rhyodacites, with variable fractionation degrees (∼20%–50%), coupled by mixing with ∼2%–4% of an upper crustal component. To elucidate the co‐existence of forearc volcanics with the Triassic rift‐related E‐MORB and OIB basalts, we propose two scenarios for the geotectonic setting of the associated magmatic suites. Scenario‐1 considers intra‐oceanic induced subduction initiation with southwestern dipping of the oceanic slab beneath the Pelagonian margin, whereas scenario‐2 considers fast‐spreading rift oceanic accretion, leading to northeastern subduction of the oceanic lithosphere towards the Pelagonian microcontinent. We envisage the potential for the two proposed scenarios to have acted in conjunction with slab break‐off, facilitating the subduction polarity‐reversal process. This can successfully account for the occurrence of calc‐alkaline volcanics as adjacent outcrops along with IAT‐type volcanics or even solely as calc‐alkaline pyroclastic tuff formations.

This study focuses on the 3D seismic investigation of high-amplitude elliptical reflections (HAER) within Miocene stratigraphic interval, in Western Patras Gulf, in a sedimentary basin that is affected by salt tectonics.Miocene basins across Western Greece have been attributed to the formation of foreland and piggy-back basins of a westward advancing fold-and-thrust belt. The base and top of the Miocene basin in the study area are marked by two regional unconformities. The lower unconformity has formed during Burdigalian, following uplift related to an early compressional phase. The upper unconformity is related to the sea-level fall during the Messinian Salinity Crisis (MSC). A salt diapiric wall of NW-SE orientation along the eastern side of the basin is interpreted of Triassic age. The different deformation style between the underlying Miocene and the overlying Pliocene – Quaternary strata indicates that the salt wall went through at least two stages of re-activation, one during late Miocene and another one during Pleistocene. Seismic stratigraphy and neighboring outcrop data onshore Kephalonia Island, reveal a basin infill ranging from fluvial to lagoonal and progradational deposits to more hemipelagic mud-dominated deposits towards the top.HAER are structures of circular to elliptical shape, that appear as patches of high amplitude anomalies at the upper Miocene stratigraphic level. Due to their seismic signal, indicative of hard lithologies, they are interpreted as methane-derived authigenic carbonates (MDAC), precipitated on top of paleo-pockmarks. Our interpretation infers that those paleo-pockmarks develop through the gas escape along a fault network associated with a late Miocene diapiric re-activation. The presence of those paleo-pockmarks, combined with the underlying Mesozoic sequence, raises two major questions: 1) the origin of the paleo-pockmarks is thermogenic or biogenic, and 2) is it possible for the Miocene subsidence to result in thermal maturation of Mesozoic source rocks in the area?A preliminary thermal maturity modeling indicates that there is a late kick during Neogene, and thus, a thermogenic origin for the paleo-pockmarks seems reasonable. This is also supported by multiple present-day oil seeps and gas-escape structures along Western Greece. The absence of paleo-pockmarks within the Pliocene – Quaternary section is attributed to the extensive erosion during MSC and the reduction of Pliocene - Quaternary sedimentation rates.

Seabed fluid flows (SFF) refer to the movement of fluids (gases and liquids) from sediments to seawater. SFF has broad implications for (i) human activity in the ocean, which is often associated with geohazards, (ii) global climate, and (iii) benthic ecology. BLUEL project aimed to long-term monitor the submarine active pockmark field in the Gulf of Patras, Greece, and investigate its relationship to seismic activity, examining the occurrence of changes in their fluid flow behavior during local earthquakes towards evaluating their potential for use as earthquake precursors. The pockmark field extends to an area of 2.4 km2, in water depths of 17 to 45 meters and consists of 115 pockmarks of which 92 are visible and 23 are buried under the infrastructure of the recently constructed South Port of Patras. The formation and activity of the field appears to be controlled by tectonics (faults) while methane fluxes and fluid escapes into the water column were recorded in the past, increased after strong earthquakes.A high-resolution mapping and monitoring of the Patras Gulf pockmark filed (PGPF) was carried out through high resolution acoustic mapping techniques, including swath bathymetry, sidescan sonar backscatter and sub-bottom profiling, revealing pockmarks morphological evolution through time and assessing the spatial patterns of bubble flares after major seismic events. Results showed that the main mechanisms for the development of the field are local tectonism and internal characteristics of gas-charged sedimentary layers. Sediment and water samples were collected and in-situ measurements of CH4 concentration were performed using a methane sensor. The chemical composition and origin of the fluids in the seawater and the sediments were assessed and implications about the volume of greenhouse gases escaping to the atmosphere were made. The geochemical analysis showed that heavy metal concentrations are always higher in sediments collected inside the pockmarks than those collected outside the sites. Isotopic analysis also revealed that CH4 of microbial origin is the dominant component of the released gas. The annual emissions of methane from the pockmark field wider area to the atmosphere have been also estimated between 7.6 to 8.4 tons per year.A 200m long submarine optical fiber was installed inside a selected active pockmark to measure the water temperature through a Distributed Temperature Sensing (DTS) system, acquiring data over 1.5 years. Spectral analysis methods were applied to fill missing data, reconstruct the temperature time series along the cable length and reveal any underlying periodicities or anomalous events. The above measurements were supported by meteorological and tidal data collected in the area, as well as by a microseismic network to record the seismic activity over the corresponding period. Comparisons were performed between the above datasets, revealing significant relationships between anomalous thermal events and local seismicity.

The concealed Molai Zn-Pb±(Ag,Ge) stratiform deposit in southeastern Peloponnese is hosted in Triassic intermediate tuffs, ignimbrites and subaerial andesitic flows. The host rocks display trace element signatures of a Supra-Subduction Zone (SSZ) setting. Three ore-forming stages are recognized, with stages I and II related to formation of the epigenetic, stratiform, massive-to-semi-massive ore and a late stage III associated with vein-type mineralization. The O and D isotope geochemistry of gangue chlorite and epidote reveal mixing with fresh meteoric water during the weaning stages of the hydrothermal activity of the late stage II due to uplifting of the hydrothermal system. Sphalerite is the major ore phase, with three different varieties formed during stages I (Sp-I) and II (Sp-II and Sp-III). All sphalerite varieties coexist, depicting gradual change in the chemistry of the ore-forming fluids. Molai ores are characterized by elevated Ag and Ge contents. Tetrahedrite is the major Ag carrier, while among the three sphalerite varieties, early Sp-I comprises the highest Ge contents. The Molai Zn-Pb±(Ag,Ge) deposit is characterized by intermediate features between bimodal felsic massive sulfides and subaerial epithermal systems based on the shallow formation depth, the presence of hydraulic breccias associated with phase separation, the ore formation along high-angle faults, the relatively low ore-forming temperatures below 250 °C obtained from geothermometry, and the absence of the typical structure of bimodal felsic type ores.

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field.