Applying the fault-carbonate intersection model- From near-mine discoveries at Olympias and Stratoni to new targets in the Vina Corridor Nikolaos Arvanitidis, Dr Economic Geologist

 

Recently, Hellas Gold announced the discovery of new deposits in the vicinity of the Olympias and Stratoni mines. Something that shows the high-level skills of the geologists who make up the exploration team, but also the conviction of many colleagues who have studied the area over time about the strong ore potential of the area.

In my book, “Halkidiki Forged from Land and Sea”, which describes the economic geology of Kassandra mines (Olympias, Stratoni), in a simplified way, it is written:

“Ancient artisanal mining and metallogenetic modelling of exposed mineralised structures guided mineral exploration to prospect further for new potential targets in the brownfield area of the Olympias mining field in Halkidiki, in Northern Greece. Generating successful new projects is mainly based on the ability of exploration geologists to identify new mineralisation features overlooked in the past and to explore in areas of known mineralisation based on geological knowledge gained from previous prospecting and mining activities. The construction of ore deposit models in exploration is of great value and contributes to a better understanding of why mineral deposits occur where they do, and where they are expected to be found. Mineral exploration conducted in NE Halkidiki resulted in two major ore deposit types, namely (i) the polymetallic massive sulphide replacement mantos found in Olympias and Stratoni (Madem Lakkos, Mavres Petres), and (ii) the porphyry copper-gold ore located in Skouries (Fig. 1). The Olympias deposit is a polymetallic (Pb, Zn, Ag, Au) massive sulphide replacement ore body hosted by the so-called Kerdylia marbles (Fig. 2). The mineralisation is emplaced in deep-seated fault zones and their intersections with marble beds, giving place to replacement and formation of lateral stratabound mantos, very often deposited at the contact zone between marbles and adjacent gneisses (Fig. 3).  The Olympias deposit formed during the Late Oligocene to Early Miocene, coincident with magmatism in the region and related hydrothermal fluids. The depositional mechanism and structural setting of the ore bodies are genetically related to fault-controlled mineralising fluids ascending and interacting with reactive carbonate rocks interbedded within the crystalline basement of the Kerdylia formation. The respective mineralising fault-related fracture zone extends over 1,500 m, strikes to the NNΕ and dips towards the SW down to a depth of more than 300 m, with an average thickness of 12 m. The ore bodies are stratabound and develop normally at the contact zone between marble and gneiss, making respectively the footwall and hanging wall rocks. The contacts between the ore and host marble are sharp, concordant or discordant to the foliation. Sulphide mineralisation comprises pyrite, arsenopyrite, sphalerite, galena, tetrahedrite — tennantite, boulangerite, and chalcopyrite. Gold values are associated almost exclusively with arsenopyrite and pyrite. The 3D model created shows the structure of the deposit and the fault-controlled setting of the ore bodies, as well as the stratabound replacement mantos at the expense of marbles. A deeper-seated extension of the west ore body is indicated and makes a potential target for further underground surveys and exploration drilling.” 

Fig. 1: Schematic model referring to the metallogenetic interconnection of porphyry, epithermal and carbonate replacement mineralisations, giving relevant examples from Greek deposits.

Fig. 2: Simplified geological map of Halkidiki.

Fig. 3: Schematic metallogenetic model showing the main geological characteristics of manto and porphyry ore deposits in the area of NE Halkidiki. Mantos are massive sulphide ores created through the stratabound replacement of carbonate rocks.

Focusing on the intersection zones between faults and carbonate layers, a targeted exploration strategy is being developed for the Olympias and Stratoni areas. The key geological and structural criteria for identifying these high-potential sites are drawn on the latest discoveries, and deposit models could be drafted as follows.

Exploration target criteria

Fault system control

·        Target feature: Major fault intersections & splays, particularly where they cross lithological contacts.

·        Geological rationale & Exploration evidence: Mineralising fluids use major faults as conduits; deposits form where secondary structures create fluid traps. At Olympias, the high-grade NW Zone was found within 200m of mine infrastructure, associated with the controlling fault system.

Host rock & Structural geometry

·        Target feature: Favourable reactive host rocks (e.g., marble) at specific structural positions (fold hinges, dilatant jogs).

·        Geological rationale & Exploration evidence: Reactive lithology (like marble) is essential for replacement-style mineralization. Olympias mineralization replaces marble layers within gneiss (Fig. 4); at Stratoni, a new skarn was discovered adjacent to historic mines.

Geochemical & Alteration signature

·        Target feature: Diagnostic geochemical halos and mineral zonation (e.g., specific element enrichments, skarn alteration)

·        Geological rationale & Exploration evidence: Proximal vs. distal fluid-rock interaction creates predictable geochemical patterns. Skarn deposits are defined by high-temperature geochemical halos (e.g., Mo, W, Mn). Olympias West Flats shows massive sulfides with exceptional grades.

Spatial relationship to known mineralisation

·        Target feature: Areas within 200-500m of known ore zones or existing mine infrastructure.

·        Geological rationale & exploration Evidence: Mineralising systems cluster; near-mine exploration offers high discovery potential and capital efficiency. Both the Olympias NW Zone and Stratoni Skarn are near-mine discoveries.

Integrated exploration methodology

The discovery of new high-grade zones demonstrates the effectiveness of a focused, near-mine approach. To systematically locate intersection sites, a multi-stage workflow is recommended:

·        Stage 1: Structural-Lithological targeting
Use 3D geological modelling to integrate drill data, underground mapping, and geophysics. The goal is to map the 3D architecture of faults and the geometry of carbonate layers (e.g., marble units within the Kerdylia Formation). This helps identify intersections, favourable structural traps (such as fold hinges), and potential fluid pathways.

·        Stage 2: Geophysical & Geochemical vectoring
Deploy ground gravity surveys to detect dense sulfide bodies at shallow depth and use Induced Polarization (IP) surveys to map disseminated sulfide halos around ore zones. In tandem, conduct systematic geochemical sampling of soils, rocks, and possibly vegetation to look for pathfinder element anomalies (e.g., Pb, Zn, As, Ag, Au for Olympias; Cu, Au for Stratoni).

·        Stage 3: High-Resolution drilling & Validation
Prioritize directional diamond drilling from underground stations to test targets with precision and minimize meterage. Core logging must document detailed lithology, structure, vein types, and alteration mineralogy (e.g., garnet, diopside for skarns).

Fig. 4: Vein-shaped relatively coarse-grained galena mineralisation in Olympias mine with obvious the stratabound metasomatic control in the presence of interbedded marble horizons.

Practical implementation priorities

Given the recent exploration success, here are actionable next steps:

·        Expand the 3D Model: Integrate all new drill data from the Olympias NW Zone and West Flats to improve the fault network controlling these zones. The same should be done for the Stratoni Fault, where a new gold-copper skarn system was discovered.

·        Test the "Stratoni Skarn" model: The Stratoni discovery (42.75m at 0.83 g/t Au and 0.49% Cu) confirms skarn potential along the Stratoni Fault. Follow-up should map the contact between the intrusive body and carbonate host rock to define the full skarn system extent.

·        Apply predictive analytics: Employ the data-driven workflow described in a relevant case study ("Lasso–RFECV → XGBoost → SHAP") to distinguish geochemical signatures of different deposit styles (e.g., skarn vs. vein-hosted) and refine target ranking.

·        Leverage existing infrastructure: Prioritize targets like the Olympias NW Zone that are within 200 meters of existing mine infrastructure. This allows for rapid, low-cost evaluation and potential development.

The Vina exploration challenge

In local and more regional scale, a major exploration target is the area of Vina. situated between Stratoni in the south and Olympias in the north. The epithermal quartz-pyrite mineralised faults mapped are expected to intersect deeper-seated carbonate rocks to potentially form polymetallic replacement deposits.  In this respect, a classic and high-potential exploration concept for the Vina area could be applied, using shallow, mapped epithermal features as vectoring tools to a deeper, blind carbonate replacement deposit (CRD) or skarn system.

This "leakage" or telescoping model is a powerful strategy in mineral exploration. The presence of epithermal quartz-pyrite faults at the surface is a strong indicator of a past, large-scale hydrothermal system, with the potential for more valuable base metal mineralization where those fluids met carbonate rocks at depth.

The integration of all available data into a robust 3D model is the most critical step. The goal is to move from mapping 2D surface faults to predicting 3D fluid pathways.

Stage 1: 3D Structural & Lithological modelling

This stage builds the predictive framework.

·        Objective: Create an integrated 3D geological model of the Vina corridor.

·        Key data integration:

o   Surface mapping: Precisely digitize the trace, dip, and kinematics (movement sense) of the mapped epithermal quartz-pyrite faults.

o   Structural analysis: Determine if these faults are part of a larger, regionally significant fault system (like the Stratoni Fault system) or local splays. Understanding their relationship to the Olympias and Stratoni controlling structures is crucial.

o   Subsurface geometry: Use all available regional geophysical data (gravity, magnetics), historical drill logs, and geologic cross-sections to model the likely depth and geometry of the prospective carbonate horizon(s) (e.g., marbles within the Kerdylia Formation).

·        Critical output: A 3D model showing the hypothesized intersection points between the downward projection of the epithermal fault zones and the top of the modelled carbonate unit. These intersections are your primary targets.

Fig. 5: Geological map showing the main mining areas of Stratoni and Olympias, and outlining the metallogenetic potential of the Vina area, in terms of the prospect of locating carbonate replacement deposits (CRD).

Stage 2: Multi-Scale Geophysical & Geochemical surveys

This stage tests the model and seeks direct vectors to mineralization.

·        Objective: Detect geophysical and geochemical signatures of the hydrothermal system and any associated sulfide mineralization.

·        Recommended techniques

o   Regional Scale (Airborne):

§  Airborne magnetics: To map lithological contacts, intrusive bodies (potential heat/fluid sources), and deep-seated faults that may not be exposed.

§  Airborne electromagnetics (EM): Potentially useful if massive sulfides are present at reachable depths, though challenging in conductive cover.

o   Local/Target scale (Ground):

§  Ground gravity survey: The highest-priority technique. It can detect subsurface density contrasts caused by silicification (quartz veins) or, more importantly, by massive sulfide replacement bodies, which are significantly denser than host rocks.

§  Induced polarization (IP) survey: Essential for mapping disseminated sulfide halos that typically surround a CRD/skarn core. An IP chargeability anomaly coincident with a gravity high is a very compelling drill target.

§  Strategic geochemistry: Soil and rock sampling along faults for a broad suite of pathfinder elements. Look for zonation patterns: distal elements like As, Sb, Hg near epithermal faults, transitioning to Pb, Zn, Ag anomalies, potentially with Cu, Bi, Mo as vectors closer to a potential intrusive heat source.

Stage 3: Targeted drilling to test the hypothesis

·        Objective: Validate the existence of the carbonate horizon and test for mineralization at the predicted intersection.

·        Drilling Strategy: Initial drill holes should be deep and strategically located.

o   They must be designed to first intersect the projected fault zone and then continue to penetrate the target carbonate unit at the calculated intersection depth.

o   Core logging must meticulously document lithology, alteration (especially marble recrystallization, skarn minerals like garnet/diopside), veining, and sulfide mineralogy.

Key geological risks & mitigations

·        Risk 1: The carbonate unit is absent or non-reactive at depth.

o   Mitigation: Use seismic refraction or detailed gravity modelling to better constrain the basement architecture and lithology before major drilling.

·        Risk 2: The epithermal faults are disconnected from the deeper system.

o   Mitigation: Perform fluid inclusion and stable isotope (O, H) studies on the epithermal quartz to determine the temperature and source of the fluids, checking for affinity with known CRD fluids.

·        Risk 3: The system is barren or only weakly mineralized.

o   Mitigation: This is inherent to exploration. Rigorous target ranking using all geophysical and geochemical data is key to prioritizing the best intersection to test first.

Practical next steps for Vina

·        Immediate action: Compile all existing data (maps, drill logs, old reports, geophysics) for Vina into a single GIS project. Digitise the epithermal fault traces.

·        Priority survey: Commission a detailed ground gravity survey over the Vina corridor, with lines oriented to cross the fault structures. This is likely the fastest way to identify density anomalies worthy of follow-up.

·        Model & Predict: Build the initial 3D intersection model. The priority drill target is the location where the largest/most continuous epithermal fault intersects the shallowest projected depth to carbonate.

The Vina area, situated between two known world-class deposits, represents a classic "gap" target. The surface expression of epithermal veins makes it particularly compelling. The exploration strategy should be to treat the surface veins as the "smoke" and systematically search for the "fire" of a replacement body at their roots.

Relevant references

Arvanitidis, N. (2025). Halkidiki Forged from Land and Sea. Zenodo. https://doi.org/10.5281/zenodo.17379019

Arvanitidis N.D., Theodoroudis A.C., Hellingwerf R., Kougoulis C. and Dimou E. - (1992) - Preliminary ore mineralogy and gold Geo-Chemistry of the Zepkos – Papades – Stratoniki mineralized fault zone - Forth progress report in IGME-EC 1992, p 13

Arvanitidis, N.D. - (1993) - Regional ore geologic studies: Setting controls and distribution of metallic deposit types in the Serbo- Macedonian and Western Rhodope zone. - EEC project MA2M-CT90-0015 "Carbonate-hosted precious and base metal mineralization in Greece. Development of new exploration strategies", IGME, Thessaloniki, 99 pp

Hellingwerf R., Arvanitidis N. - (1993) - Ore geology exploration tools and new targets for non-outcropping ore deposits in Chalkidiki, N. Greece. - Final report in : IGME-EEC project: MAZM-0015: Carbonate –hosted precious and base metal mineralization in Greece: development new exploration strategies

Thedoroudis A. C., Arvanitidιs N., Dimou E. - (1999) - Geology and ore mineralogy in the Vina area, NE Chalkidiki - I.G.M.E. Thessaloniki. Internal report (In Greek) p.34, 1999

ARANITIS S., 1977: The basic faults on the Northern Aegean and their importance for the understading of the geological structure and development of the region VI Colloquim on the Geology of the Aegean Region, ATHENS.

ARVANITIDIS N.D., THEODOROUDIS A.C., HELLINGWERF R., KOUGOULIS C. and DIMOU E., 1992: Preliminary ore mineralogy and gold Geochemistry of the Zepκos - Papades-Stratoniki mineralized fault zone. Fourth progress report in IGME-EC project: MA2M-0015.

HELLINGWERF R., ARVANITIDIS N. D., KOUGOULIS C., THEODOROUDIS A.C. and VERANIS N. (1993): mineralogy and geochemistry of marbles in the Olympias - Stratoni - Varvara - Rendina area, Chalkidiki, N. Greece. Final report in IGME-EEC project MA2M-0015: "Carbonate - hosted precious and base metal mineralization in Greece:  development of new exploration strategies".

HELLINGWERF R., ARVANITIDIS N. D. et al. (1993): Ore geology, exploration tools and new targets for non-outcropping ore deposits in Chalkidiki N. Greece final report in: IGME-EEC project: MA2M-00015:"Carbonate-hosted precious and base metal mineralization in Greece: development new exploration strategies.

ΓΑΛΑΝΟΠΟΥΛΟΣ Β, ΘΕΟΔΩΡΟΥΔΗΣ Α., (1994) : Ο χρυσός , άργυρος και τα βασικά μέταλλα στην μαγγανιούχο παραγένεση των κοιτασμάτων της Β.Α. Χαλκιδικής. Δελτίο  Ε.Γ.Ε. τομ. ΧΧΧ/1, σ. 507-518.

GILG A. (1993): Geochronology (K-Ar), fluid inclusion, and stable isotope (C, H, O) studies of skarn, porphyry copper, and carbonate-hosted Pb-Zn (Ag, Au) replacement deposits in the Kassandra mining district (Eastern Chalkidiki, Greece) (Ph degree) Zurich 153p.

ΔΗΜΗΤΡΙΑΔΗΣ Σ. (1974): Πετρολογική μελέτη των  μιγματιτικών γνευσίων και αμφιβολιτών των περιοχών Ρεντίνας - Ασπροβάλτας - Σταυρού - Ολυμπιάδας. Διδακτ. Διατρ. Πανεπιστημίου Θεσσαλονίκης 278 σελ.

ΔΙΑΚΑΚΗΣ  Μ. (1975): Υπολογισμοί αποθεμάτων περιοχών Φακής - Παλιοχώρας (αδημ. Έκθεση ΓΕΜΕΕ).

ΔΙΑΚΑΚΗΣ Ε. (1994) :  ΄Εκθεση  τριετούς προγραμματισμού. Έργο: Επιθερμικός  χρυσός περιοχής Ζέπκου - Ολυμπιάδος. Αδημ. Εκθεση ΙΓΜΕ.

ΘΕΟΔΩΡΟΥΔΗΣ Α., ΓΑΛΑΝΟΠΟΥΛΟΣ Β. (1991) : Η μαγγανιούχος μεταλλοφορία του Δημόσιου Μεταλλείου Βαρβάρας (τ. ΟΠ-13) του Ν. Χαλκιδικής (Αδημ. Εκθ. ΙΓΜΕ).

ΘΕΟΔΩΡΟΥΔΗΣ Α., ΓΑΛΑΝΟΠΟΥΛΟΣ Β. (1993) : Κοιτασματολογική μελέτη της μεταλλοφορίας Au, Ag  στα  μαγγανιούχα κοιτάσματα της ΒΑ Χαλκιδικής. Αδημ. Εκθ. ΙΓΜΕ.

ΚΑΛΟΓΕΡΟΠΟΥΛΟΣ Σ., ΘΕΟΔΩΡΟΥΔΗΣ Α. (1989) : Γεωχημεία ισοτόπων Κ/Ar   και  Rb/Sr  στα  ορυκτά βιοτίτη, μοσχοβίτη και αμφίβολο, πετρωμάτων της Σερβομακεδονικής μάζας, Β. Ελλάδα (αδημ. Εκθ. ΙΓΜΕ).

KALOGEROPOULOS S., KILIAS S., BITZIOS D., NIKOLAOU M. BOTH R.  (1989): Genesis of the Olympias Carbonate - Hosted Pb-Zn (Au, Ag) sulfide ore deposit Chalkidiki Peninsula Northern Greece. Economic Geology vol. 84 p.p. 1210-1234.

ΚΑΛΟΓΕΡΟΠΟΥΛΟΣ Σ., ΜΠΙΤΖΙΟΣ Δ., ΒΕΡΑΝΗΣ Ν., ΓΕΡΟΥΚΗ Φ. (και ομάδα γεωλόγων ΙΓΜΕ), (1989). Πρόδρομος μεταλλογενετικός χάρτης ΣΜΖ  (περιοχή Χαλκιδικής - Βόλβης. 9 τόμοι. ΙΓΜΕ.

COSTANTINIDOU E. (1993): Fluid inclusions data from Madem Lakos Pianitsa, Stratoniki, Zepkos and Varvara mineralisations, Kerdylia Formation. Chalkidiki N. Greece. IGME -EEC PROJECT, MA2M - CT90-0015.

ΜΑΡΑΤΟΣ Ε. (1955) : Γεωλογική  και κοιτασματολογική αναγνώρισης  της μεταλλοφόρου ΒΑ Χαλκιδικής. Περιοχή Βαρβάρας - Ολυμπιάδας - Στρατονίκης. Εκθεσις αρ. 22. ΙΓΕΥ.

ΚΑΡΜΗΣ Π. (1987) : Αερογεωφυσική έρευνα ΒΑ Χαλκιδικής (Αδημ. Εκθ. ΙΓΜΕ).

ΚΑΡΜΗΣ Π., ΤΣΟΜΠΟΣ Π., (1989) : Η ρηξιγενής τεκτονική της βορειοανατολικής Χαλκιδικής. Μια συγκριτική μελέτη φωτογεωλογικών  και αερογεωφυσικών στοιχείων. Δελτίο Ελληνικής Γεωλογικής Εταιρίας τόμος  ΧΧΙΙΙ τεύχος  1, σελ. 89-100.

KOCKEL F., MOLLATH.  and WALTER H.W. (1977): Erauterunen zur Geologischen Karte der Chalkidiki und angrenrender Gebiete 1:100.000 (Nord - Griecheland) Bundesanstall fur Geowisseschafter und Rohstoff Hannover 119p.

ΜΟΥΝΤΡΑΚΗΣ  Δ. (1985): Γεωλογία της Ελλάδος. University Studio Press 207 σελ. Θεσσαλονίκη.

 NEBEL M., L. HUTCHINSON R.W., ZARTMAN R.E. (1991): Metamorphism and polygenesis of the Madem Lakkos polymetallic sulphide deposit, Chalkidiki Greece.