Native iron in the dolerites of the Aikhal sill (the first discovery in Yakutia)

A massive manifestation of native iron nodules weighing up to four hundred kilograms was established in a trap intrusive near Aikhal settlement. In addition to large segregations, native iron occurs as the drops in the rock-forming minerals of dolerites rimming the iron nodules. In terms of petrochemical composition, the dolerites of the sill belong to typical tholeiites of the traps of the Siberian platform, and correspond to low-Ti basites (TiO₂ ~ 1 %) with increased magnesium content (Mg# = 56–63) that have passed through the deep (pre-chamber) stage of melt crystallization.

The dominant mineral in the nodules is native iron, with subordinate cohenite (Fe₃C), troilite (FeS) and magnetite (Fe₃O₄). X-ray phase analysis revealed that the native iron has the a-Fe structure with the unit cell parameter a = 0.2860 nm. The Brinell hardness is in the range of 110–117 HB units. X-ray spectral analysis showed that native iron belongs to a high-carbon variety (2.14–4.02 wt.% C). The high carbon content suggests that the iron is a Fe – C alloy. With regard to low Ni content (< 1 %) the native iron corresponds to the mineral species ferrite. It is assumed that the main condition for the presence of macro amounts of native iron in traps is fractionation of basaltic melt in the deep-seated (about 40 km from the paleosurface) intermediate chamber. In this situation, the interaction of the basaltic melt with a high-temperature intratelluric fluid, characterized by highly reducing properties, led to the dispersion of initially homogeneous basaltic liquid into liquates of silicate and metal composition. It is the finely dispersed state of the fluid-magmatic system that favored the appearance of the native phase, above all of iron.

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