Hydrogen and oxygen isotope evidence for sea-water-hydrothermal alteration and ore deposition, Troodos complex, Cyprus

Abstract

Synopsis A model is presented for the genesis of the Cyprus massive cupriferous sulphide deposits, their associated stockwork mineralization and the metamorphism of the Troodos ophiolite complex based on D/H, 18 O/ 16 O and 13 C/ 12 C ratio analyses, where applicable, of 94 whole rocks and minerals, chemical, mineralogical and sulphur iso tope data. The upper 3–5 km of the hot Troodos crust — pillow lavas, Sheeted Intrusive Complex, trondhjemites and upper gabbros — were metamorphosed to zeolite facies, temperatures of about 0 to 250° C, and greenschist-amphibolite facies, temperatures of about 250 to 450–550° C, during interaction with a deep circulating sea-water geothermal system. Minimum isotopic temperatures of 320–400°C are given for the epidote-bearing vein-halo assemblages in the upper gabbros, trondhjemites and Sheeted Intrusive Complex. If temperatures at the base of the Sheeted Intrusive Complex were higher than ∼ 500° C, either a magmatic or metamorphic water component must have been present in the hydrothermal fluids. The magmatic water could be of sea-water origin, being derived from the melting or assimilation of sea-water-hydrothermally altered roof rocks overlying the gabbroic magma chambers. Minimum water/rock ratios during pervasive alteration decrease from > 1 by weight in zeolite facies pillow lavas to ∼ 0.1 in the underlying green-schist facies rocks, but were an order of magnitude larger in the veins and fractures. In the stockwork zones, which underlie the sea-floor sites of massive sulphide orebodies, isotopic temperatures declined from initial values greater than 300° C (with water/rock ratios > 1–5, by weight) to less than 220° C during post-sulphide later-stage vein quartz formation; these temperatures were higher than temperatures in the predominantly zeolite facies country rocks. The stockwork zones are interpreted to represent the discharge zones of the seawater-hydrothermal fluids responsible for the regional-scale mass transfer and the mineralization. At least some of the sulphide sulphur in the stockworks and orebodies is of seawater origin. Formation of massive sulphide deposits requires the coincidence of several factors, including the penetration of a brine to deep levels, where temperatures were greater than 300° C in an active spreading centre, leaching of ore metals, and discharge of the hot springs into basins where both sulphide deposition and localization could occur.