Transition from Fe-skarn to carbonate-replacement deposits: Evidence for structural control in the world-class Lavrion Mining District, Greece.

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.