Seamount formation and associated caldera complex and hydrothermal mineralization in ancient oceanic crust, Troodos ophiolite (Cyprus)

Abstract

The inter-graben zone between the Solea and Mitsero grabens in the northern part of the Troodos ophiolite (Cyprus) displays igneous, tectonic, and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers. In this zone the 200-m-high Alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late-stage, off-axis shallow-level gabbroic pluton that was intrusive into young oceanic crust. Concentric cone sheet intrusions and associated faults and shear zones in and around the Alestos hill seamount collectively form a seafloor caldera complex, which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton. The ENE-trending elongated massive sulfide ore deposit formation in the Alestos–Memi area represents a focused upflow zone for hydrothermal fluids driven by the same off-axis gabbroic intrusion responsible for formation of the Alestos hill seamount. The ENE-trending and up to 150-m-wide Alestos–Memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism. The geometry of this fault zone and its kinematic relations with a N–S-striking Alestos Forest normal fault in the west are suggestive of its transfer fault origin, reminiscent of mineralized transfer faults found at the Mid-Atlantic Ridge. The active TAG hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off-axis and along a transfer fault in the Trans-Atlantic Geotraverse (TAG) area at 26°N provides a modern analogue for the Alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the Troodos ophiolite. These findings indicate that off-axis magmatism played a major role in crustal accretion, hydrothermal alteration, and ore mineralization during the late-stage evolution of the Troodos ophiolite.