EXTENSION AND MAGMATISM IN THE MIRDITA OPHIOLITE, ALBANIA: IMPLICATIONS FOR THE MUNELLA MASSIVE SULFIDES DEPOSIT

The Mirdita ophiolite represents an obducted Jurassic oceanic lithosphere in Albania. Peridotite mantle rocks occupy the western and eastern periphery of the ophiolite whereas the central part is mainly composed of crustal rocks. The ultramafic mantle rocks are divided into Western- and Eastern-type massifs, based on the nature of mantle peridotites and geochemical affinities of the overlying extrusive crust. The boundary between the two types of mantle massifs is a low-angle detachment interpreted as an oceanic core complex, with Western-type massifs in the footwall and Eastern-type massifs in the hanging-wall of the detachment. From west to east, and stratigraphically upwards, the mafic lavas of the Mirdita ophiolite vary from MORB-like to Island Arc Tholeiites (IAT) with late boninite dykes and lavas.

The Munella Cu-massive sulfides deposit formed in the crustal sequence overlying the Eastern-type massif, a few km to the East (in the hanging-wall) of the main detachment fault. Sulfides mainly occur at the base of the basaltic sequence, may be associated with rhyolitic (keratophyric) layers, and are everywhere concordant with the hosting volcanic sequence. A thin horizon of red chert commonly directly overlies the massive sulfides. We mapped a 10 km-long section through the crust along the access road to the Munella mine. There, isotropic gabbro with abundant plagiogranite injections/bodies occur just above the detachment fault, followed by sheeted dykes, and two volcanic sequences, a lower MORB/IAT and an upper boninitic series. Stockwerks of albite-epidote veins/veinlets are abundant in the gabbro-sheeted dyke sequence and lower lavas, indicating intense sub-seafloor hydrothermal circulation during seafloor-spreading. The upper lavas, which host the Munella VMS, are almost devoid of hydrothermal alteration, however. The MORB/IAT-to-boninite sequence of the Munella road section suggests an early stage of suprasubduction seafloor-spreading, followed by an amagmatic hiatus, with late boninitic magmatism, possibly during pre-obduction extensional tectonism that led to the exhumation of the Western-type ultramafic massifs.