Deep flow paths in VMS systems: Porosity and permeability of epidosite alteration in the Semail ophiolite, Oman

The classic model of hydrothermal alteration of the upper oceanic crust identifies epidosites (basalts altered to epidote + quartz + titanite ± Fe-oxides) as markers of discharging hydrothermal fluid. Epidosites are depleted in metals and therefore they have been proposed as sources for the metals in volcanogenic massive sulfide deposits. However, this model is challenged by recent observations in ophiolites. One way to test the model is by reactive-transport simulations of the alteration. In preparation for this we report new observations of flow paths and of the porosity and permeability of epidosites and their precursor rocks in the Semail ophiolite Oman. We have found that discharge through the layer of pillow lavas is not controlled by fractures or faults but rather occurs via pervasive rock-matrix flow. Interpillow hyaloclastite is first altered to epidosite, followed by the cores of pillows and then by the rims of pillows. Our measurements show that the metasomatism leading to epidotisation of pillow lavas increases their connected porosity by 4–10 vol.% and it increases their permeability by 1–3 orders of magnitude. Thus, enhancement of permeability by epidotisation enables self-propagation of the discharging fluid through the extrusive oceanic crust.