Mineralogy and geochemistry of a base metal sulfide-bearing magnetitite body from the Eretria mine, East Othris massif, Greece: insights into an ancient seafloor hydrothermal system

Magnetite deposits comprise a very small volume of a serpentinized peridotite body that constitutes the Eretria chrome mine of the East Othris ophiolite in central Greece. Magnetite deposits have been examined to aid determination of their mode of occurrence, mineralogy and origin. Research attention has been focused on an ore body that consists of a ~30 cm-thick pod of semi-massive to massive magnetite (~75%–85% modal) hosted in a serpentinite shear zone. The silicate matrix of magnetitite (~10%–15% modal) is composed of unstrained serpentine, chlorite and subordinate amounts of andradite and actinolite. Base metal sulfides (BMS; ~5%–10% modal) and phosphates are dispersed between the magnetite grains. Variably altered Cr-spinel crystals are poikilitically enclosed within magnetite. The investigated ore samples show great variations in the Cu (890–9530 ppm), Ni (190–3260 ppm), Co (120–2180 ppm) and Zn (20–1240 ppm) contents. The δ18ΟSMOW value for magnetite from a magnetitite sample is 2.2‰. The undeformed nature of the silicate minerals in magnetitite points toward crystal growth from a postmagmatic fluid. The low Ni, Mn, Mg and high Si contents in magnetite suggest that it was not derived from fluids produced concurrent to serpentinization. The presence of phosphates, BMS and hydrosilicates suggest that the ore forming fluids were acidic and halogen-bearing. Our preferred hypothesis for the formation of magnetitite is that mixing of upwelling, metal-rich brines along the dilational segments of the host shear zone caused saturation of the ensuing fluids with Fe, which eventually prompted magnetite precipitation. The T estimates deduced from mineralogy, with an uppermost limit of ~460 ◦C required, and cessation of mineralization at ~150–100 ◦C, are consistent with the ambient T present within a cooling oceanic lithospheric slab. In analogy with some modern serpentinite-hosted BMS deposits at rifted settings, the BMS-bearing magnetitite of Eretria may represent part of an ancient seafloor hydrothermal system. Our study provides important insights into the initiation and episodicity of hydrothermal activity in oceanic settings.