THE ROOT ZONES OF OCEANIC HYDROTHERMAL SYSTEMS - CONSTRAINTS FROM THE SAMAIL OPHIOLITE (OMAN)

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 along-strike 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 along-strike 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.