Structural architecture of brittle-ductile mineralized veins in a Cenozoic orogenic gold deposit along the North Cycladic Detachment System, Greece

The Kallianos Au-Ag-Te deposit, located on southern Evia island in the NW Aegean Sea, is a Cenozoic orogenic gold deposit hosted in carbonate-epidote-phlogopite schists and phlogopite marbles of the Cycladic Blueschist Unit (CBU). Fluids that generated the deposit were channelized by a crustal scale post-orogenic extensional structure, the North Cycladic Detachment System (NCDS), which facilitated Miocene exhumation of the CBU into the brittle crust. Whereas ore deposits in the Cyclades have been broadly related to late Miocene granitic intrusions, magmatism of this age is notably undocumented on Evia. Field observations illustrate the connection between the structural architecture that host mineralization and deformation associated with the post-orogenic structures, refining the paragenetic model for Cenozoic gold deposits in the Cyclades. Mineralized veins, alongside unmineralized tension gashes, faults, conjugate faults, and joints, occur in parallel sets that locally define en-echelon arrays. Younger sub-vertical tension gashes cross-cut older boudinage mineralized veins. The vein orientations of the Kallianos deposit strike NW-SE and NNW-SSE, which is generally orthogonal to the sub-horizontal ~NE stretching lineations related to crustal extension and thinning accommodated by the NCDS. Brittle-ductile kinematic indicators such as shear bands exhibit top-NE displacement, consistent with footwall deformation related to the NCDS documented elsewhere along strike of the detachment. The two populations of vein orientations are not evident based on structural data alone, but field observations show clear cross-cutting of the earlier NW-striking vein set by later NNW-striking veins. The mineralization is hosted in subvertical mm- to m-scale veins composed of quartz, calcite, albite, with minor titanite and epidote and notable sulfide mineralization including pyrite, galena, chalcopyrite, bornite and hematite concentrated in cm-scale veins. Obvious native Au and Ag are not observed in the veins. The NNW-striking vein sets contain significantly more albite and mineralization than the NW-striking veins and generally exhibit greater evidence of strain, with an abundance of sutured and bulging grain boundaries preserved in the quartz. Vein arrays developed within the cataclastic deformation zone below the exposed NCDS detachment plane are parallel to those observed deeper in its footwall. Our structural data strongly imply a connection between mineralized veins of the Kallianos Au-Ag-Te deposit and the regional strain field imposed by displacement along the NCDS. Despite structural evidence linking the architecture of this Cenozoic gold deposit to the crustal scale NCDS, an origin for the mineralizing fluids remains equivocal due to the local absence of magmatism, and the distribution of brittle-ductile strain to significant depths in the footwall may implicate devolatilization reactions coinciding with exhumation through the brittle-ductile transition as an important fluid source.