THE ORIGIN OF HYDROTHERMAL METALLIFEROUS SEDIMENTS ASSOCIATED WITH THE EARLY MESOZOIC OTHRIS AND PINDOS OPHIOLITES, MAINLAND GREECE

Metalliferous sediments and sulphides are associated with the Jurassic Othris and Pindos ophiolites, central and northern Greece. These ophiolites formed at an Early Mesozoic spreading ridge within a small, Pindos ocean basin created by rifting of the northern margin of Gondwanaland. Ubiquitous terrigenous silt was derived from adjacent continental margins, mainly the Pelagonian microcontinent to the east. In the Othris area, disseminated, massive and vein sulphides occur within and above MORB-type pillow lavas and volcaniclastic sediments. Sulphide was precipitated in quartz veins within fault-controlled stockworks. Epidote (with local epidosite) is locally very abundant in extrusives immediately underlying thick Si-Fe-rich hydrothermal sediments. Massive sulphides were precipitated as small fault-controlled bodies, that are directly overlain by Fe-rich mudstones and Fe-rich cherts, strongly depleted in Mn. Mudstones, relatively enriched in hydrothermal Fe, Mn, Cu, Zn, Pb, REE and Ba, were ponded into small fault-controlled hollows around the fringes of the sulphide-precipitating field. Elsewhere (e.g. northern Othris), manganese-rich layers in ribbon radiolarites are strongly enriched in Mn and trace-elements. Metal-enriched deposits are also found within slices of mid-ocean ridge-type lavas associated with melange (Avdella Melange) structurally underlying the Pindos ophiolite, NW Greece. Sulphides there are restricted to tiny massive deposits overlying pillow lava and sulphide veins within ophiolitic extrusives. Pillow lavas are overlain by volcanic talus breccias, shed from submarine fault scarps. Overlying metalliferous mudstones are enriched in Fe and other hydrothermal constituents, probably derived from nearby sulphide precipitating zones and mixed with lithogenous sediments. The metalliferous deposits preserve a variety of hydrothermal deposits within an Early Mesozoic small ocean basin. The Pindos sediments reflect mixing of ferruginous hydrothermal constituents with terrigenous sediment, possibly during the earlier stages of basin opening. The Othris sulphide and ferruginous oxide-sediments record evidence of a small, high-temperature hydrothermal field, formed near the spreading axis when the ocean neared its maximum width. Manganese-rich cherts in the northern Othris area reflect widespread accumulation of hydrothermal constituents on the abyssal plain, derived from axial high-temperature hydrothermal fields and/or related to low-temperature (off-axis?) discharge.