ABSTRACT

Sulfide precipitation took place within pillow lavas in the Agrokipia ‘B’ deposit of the Troodos ophiolite, Cyprus. Such a deposit would have been originally expressed on the seafloor by the emergence of warm fluids in a situation comparable to that described at the Galapagos Spreading Center. Holes CY2A and CY4 by the International Crustal Research Drilling Group (ICRDG) provide an opportunity to study water-rock interaction in pillow lavas and the sheeted dike complex below the ancient seafloor. Sr-Rb isotope analysis as well as heat energy constraints yield an age of about 90 Ma. for the formation of the Agrokipia ‘B’ deposit and requires that the related hydrothermal alteration occurred on or close to a spreading axis. A narrow range of the ⁸⁷Sr/⁸⁶Sr ratios (0.70614-0.70660, av. 0.70633) of the hydrothermal solutions in the stockwork zone and underlying basement indicates that off-axis seafloor metamorphism and/or any alteration during obduction of the ophiolite were not accompanied by extensive metasomatism. The seawater/rock ratio of the completely hydrothermally altered rocks and the related hydrothermal solution in the stockwork zone and underlying basement was 12, which is unusually high compared with water/rock ratios from present seafloor hot springs (1 to 5). On the other hand, the end-member hydrothermal solution which entered the stockwork zone had an ⁸⁷Sr/⁸⁶Sr ratio of 0.70541 and a water/rock ratio of 5. Thus local, secondary circulation systems are required to explain the strontium isotopes and water/rock data, and probably are necessary for the formation of the subseafloor ore deposits.