Rapid generation of reaction permeability in the roots of black smoker systems, Troodos ophiolite, Cyprus

Abstract

The deep levels of former black smoker hydrothermal systems are widespread in the Troodos ophiolite in Cyprus. They are marked by zones of hydrothermal reaction in the sheeted dyke unit close to the underlying gabbros. These zones are characterized by the presence of epidosite (epidote-quartz rock). In the reaction zones, the dykes are altered to a range of greenschist facies mineral assemblages, from a low degree of alteration with a five to seven phase metabasaltic assemblage to a high degree of alteration with a two to three phase epidosite assemblage. Individual dykes may contain the full range, with the epidosites forming yellow-green stripes within a darker background, often extending for more than several metres, parallel to the dyke margins. Field relations show that the alteration took place on a dyke-by-dyke basis and was not a regional process. SEM petrography reveals that the epidosites contain millimetre scale pores. The minerals surrounding the pores show euhedral overgrowths into the free pore space, indicating a former transient porosity of up to 20%. We conclude that the epidosites formed by reaction between newly intruded basaltic dykes and actively circulating black smoker fluid leading to extensive dissolution of primary dyke minerals. This reaction generated the porosity in the stripes and transiently led to a much increased permeability, allowing the rapid penetration of the black smoker fluid into the dykes and flow along them in fingers. As the system evolved, the same flow regime allowed mineral precipitation and partial infilling of the porosity. This mechanism allows rapid recrystallization of the rock with release of metals and other components into the fluid. This explains the depletion of these components in epidosites and their enrichment in black smoker vent fluids and the relatively constant composition of vent fluids as fresh rock is continually mined.