Structural Architecture of Mineralized Veins in a Cenozoic Orogenic Gold Deposit Along the North Cycladic Detachment System, Greece

Abstract

The Kallianos Au-Ag-Te deposit, a Cenozoic post-orogenic ore deposit on Evia island (NW Aegean Sea), is hosted in the high-pressure Cycladic Blueschist Unit (CBU). Whereas Cycladic ore deposits are broadly related to late Miocene granitoids, magmatism of this age is undocumented on southeastern Evia. Integrated field observations and isotopic data illustrate a connection between the structural architecture that hosts mineralization in the deposit and the regional stress field associated with a crustal-scale detachment, the North Cycladic Detachment System (NCDS), implying a first-order paragenetic relationship to post-orogenic extension. Mineralized and unmineralized veins occur alongside parallel faults and joints in two distinct orientations, each locally defining conjugate or en-echelon sets. Two younger sub-vertical tension gash sets (V₂, V₃) cross-cut older transposed and boudinaged mineralized veins (V₁) parallel to the metamorphic foliation. Sulfide mineralization is hosted in steeply-dipping, cm-scale qz ± cal ± ab V₃ veins. Structural relationships show clear cross-cutting of V₂ (NW-striking) by V₃ (NNW-striking). Veins of the deposit opened orthogonal to the NE-trending stretching lineations, synchronous with brittle-ductile shear bands that indicate top-NE kinematics, further demonstrating their relationship to crustal extension accommodated by the detachment system. Vein arrays developed within the brittle-ductile zone below the exposed detachment plane are parallel to those observed deeper in the footwall (within the deposit) and were likely induced by a coaxial strain component with maximum shortening perpendicular to the detachment plane. C-O isotope data from vein calcite (δ¹³C: -3.31 - 0.40‰ and δ¹⁸O: 14.43 - 20.78‰) reflect fluid mixing with the graphite-bearing CBU wall rock. White mica ⁴⁰Ar/³⁹Ar and in-situ ⁸⁷Rb/⁸⁷Sr geochronology suggests host rock greenschist facies deformation between c. 20-31 Ma, whereas vein halo hydrothermal mica reveals slightly younger fluid flow at c. 21-26 Ma, indicating vein formation occurred during the early Miocene. Zircon (U-Th)/He dates provide the timing of host rock cooling to <200°C at c. 11-14 Ma, corresponding to the exhumation of the deposit into the brittle crust. The comprehensive study of this younger, mostly undeformed post-orogenic deposit can help elucidate the structural controls of older polydeformed orogenic gold deposits.