The rootzone of an ancient hydrothermal system exposed in the Troodos ophiolite, Cyprus

A well-exposed, oceanic hydrothermal rootzone in the Troodos ophiolite, Cyprus, is composed of the basal sheeted dikes, the upper 50-100 m of the gabbroic sequence, and a contact aureole, interpreted as a fossil conductive boundary layer. The basal sheeted dikes and upper gabbros were pervasively altered at high temperatures (400 degrees -775 degreesC) and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex. Fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350 degrees -400 degreesC, fairly oxidizing, and relatively H2S rich, with a range in salinity (1.5-7.8 wt % NaCl equivalent). Field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again, perhaps for several cycles. These migrations and P-T oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreading center. The hydrothermal rootzone did not migrate to significant depths in the plutonic sequence until the waning stages of magmatism.