The root zones of oceanic hydrothermal systems: Constraints from the Samail ophiolite (Oman)

Abstract

The Cretaceous Samail ophiolite in Oman exposes an almost complete, 500‐km‐long, along‐axis section of oceanic crust, providing a unique opportunity to study the geometry, physical conditions, and effects of the hydrothermal circulation that fed the volcanic‐hosted massive‐sulfide deposits. These fossil discharge zones are rooted in the sheeted‐dike complex, down to the transition zone with the plutonic sequence. The sheeted‐dike complex as a whole was affected by greenschist‐facies metamorphism (albite, actinolite, chlorite, quartz, sphene). Diabase dikes are commonly altered into three major alteration end‐members termed spilite, mineralized spilite, and epidosite. These alteration patterns usually follow dikes, thus resulting in a well‐marked alongstrike vertical anisotropy. Their vertical distribution is also anisotropic: epidosites occur mainly in the basal sheeted‐dike complex, and mineralized spilites are usually limited to its top and to the transition zone with the volcanic extrusives. The alongstrike distribution of the different alteration patterns also shows a well‐marked correlation with the occurrence of massive‐sulfide deposits. The sheeted‐dike complex underlying these deposits is characterized by a sharp increase in the volume of epidosite and mineralized spilite. The stratigraphic position of the epidosite zones just above the magma chamber and in the lower sheeted‐dike complex, their attitude parallel to the margins of the dikes, the recorded high trapping temperatures, the high water/rock ratios, their textural reconstitution and base metal depletion suggest that they formed in a focused upflow portion of a vertical, along‐strike convecting, high‐temperature (subcritical) hydrothermal system a short time after emplacement of the dikes. The common occurrence of epidosite bands devoid of veins, and the absence of major fracturing or listric faulting, indicates that formation of epidosite does not first require tectonic extension.

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

“…It was recognized (Richardson et al 1987) that the extreme phase reduction of the epidosites might be the product of intense metasomatic alteration of the dykes under open-system conditions as envisaged by Korzhinskii (1959Korzhinskii ( , 1965. After their discovery in Troodos, epidosites were identified and described in several other ophiolites, the Josephine ophiolite in California/Oregon (Harper et al 1988 and related papers), the Semail ophiolite in Oman (Nehlig & Juteau 1988;Nehlig et al 1994) the Pindos ophiolite in Greece (Valsami & Cann 1992) and the Solund ophiolite in Norway (Fonneland-Jorgensen et al 2005). Epidosites have only rarely been found in the modern oceans, but have been dredged from the fore arc of the Tonga Arc (Banerjee et al 2000;Banerjee & Gillis 2001) and have been reported in rocks dredged from the Mid-Atlantic Ridge at 30°N (Shand 1949;Quon & Ehlers 1963).…”

“…The major element chemistry of rocks from epidosite zones shows the wide range in composition expected from the changing mineralogy (Richardson et al 1987;Harper et al 1988;Nehlig et al 1994;Bettison-Varga et al 1995). All of the rocks within epidosite zones, including the whole range from diabase to epidosite, are strongly depleted in Cu, Zn and Mn by between 50% and 90%, with Cu the most depleted and Mn the least (Richardson et al 1987;Harper et al 1988;Nehlig et al 1994;Bettison-Varga et al 1995). Jowitt et al (2012) (Bickle & Teagle 1992;Bickle et al 1998).…”

“…Recent alteration of diabase in ODP Hole 504B is much less intense than that seen in Troodos . Fluid inclusions within the epidosite zones, including all rock types, show homogenization temperatures broadly within the range 250-400°C, and salinities typically ranging from sea water to twice sea water concentrations (Richardson et al 1987;Nehlig et al 1994;Bettison-Varga et al 1995;Juteau et al 2000). Oxygen isotopes give evidence of extensive water-rock reaction at similar temperatures (Harper et al 1988;Schiffman & Smith 1988;Nehlig et al 1994;FonnelandJorgensen et al 2005).…”

“…Fluid inclusions within the epidosite zones, including all rock types, show homogenization temperatures broadly within the range 250-400°C, and salinities typically ranging from sea water to twice sea water concentrations (Richardson et al 1987;Nehlig et al 1994;Bettison-Varga et al 1995;Juteau et al 2000). Oxygen isotopes give evidence of extensive water-rock reaction at similar temperatures (Harper et al 1988;Schiffman & Smith 1988;Nehlig et al 1994;FonnelandJorgensen et al 2005). All previous studies conclude from this evidence that epidosite zones have been the sites of intense hydrothermal interaction between black smoker fluids and dyke rock, with the true epidosites within them representing the places where fluid-rock interaction has been most intense.…”

“…One is that permeability has been related to generation of fractures in the rocks by tectonic processes at the spreading centre (Bettison-Varga et al 1992;Nehlig et al 1994). The other is that the replacement of lower density silicates by dense epidote has generated porosity within the rocks and hence higher permeability.…”