Metallogeny of Serpentinite‐hosted Magnetite Deposits: Hydrothermal Overgrowth on Chromite or Metamorphic Transformation of Chromite?

Abstract

AbstractPeculiar and rare occurrences of serpentinite‐hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece (Skyros), Iran (Nain and Sabzevar) and Oman (Aniba). These deposits have diverse thickness (from a few centimeters up to 50 m) and length (2 to >500 m). Magnetite ores show variable textures, including massive, nodular and banded ores, veins, net and fine‐grained disseminations in serpentinites. Intriguingly, the investigated magnetite deposits can be mistaken for chromitite pods. Serpentinite‐hosted magnetite deposits show three modes of occurrences including: (i) boulders strewn across the serpentinites (i.e. Skyros Island); (ii) ore bodies along the nonconformity contacts between serpentinites and limestones (i.e. Aniba); (iii) irregular and discontinuous trails of massive and semi‐massive ore bodies within highly sheared serpentinite masses (i.e. Nain; Sabzevar). In all of these magnetite ore bodies, relicts of chromian spinel grains are occasionally enclosed in magnetite crystals. The chemistry of Cr‐spinel relics found in these magnetite bodies are comparable to those of accessory Cr‐spinels in the surrounding serpentinized peridotites. BSE images and elemental mapping revealed that magnetite occurs as a nucleation on chromian spinels but not being involved in reaction either with chromite or ferritchromite. Low‐grade metamorphic transformation of chromite into Fe‐chromite is documented along the cracks and fractures of a few chromite grains. Generally, magnetite has typical hydrothermal compositions, characterized by low Cr, V and Ti and high Mg and Mn. It is crucial to note that a few magnetite grains with metamorphic origin are characterized by high Cr and low Ti and Ni. The potential source of iron is essentially the Fe‐rich olivine. We believe that multi‐episodic serpentinization of peridotite systems at high fluid‐rock ratios is the main process responsible for precipitation of magnetite at ore levels whereas low‐grade metamorphic transformation of chromite to magnetite has minor contribution. Cumulative factors in generation of these deposits are modal volume of mantle olivine, peridotite composition, fluid chemistry, fluid‐rock ratio, mechanisms of transportation and precipitation, structural controls such as cracks and shear zones.