Zoned tungstenoan molybdenite from a fenitized megaxenolith in the Khibinaalkaline complex, Kola Peninsula, Russia

Zoned tungstenoan molybdenite occurs in a fenite (similar to 80 modal % nep heline) within a strongly heterogeneous fenitized megaxenolith (similar to0 .2 km) enclosed by nepheline syenite, in the Khibina alkaline complex, Kola Peninsula, Russia. Molybdenite forms euhedral crystals (similar to 50-60 m um in length), enclosed by pyrrholite, and abundant veinlets among silicate s. The crystals are zoned with respect to tungsten, which is strongly enric hed in the central zone. This area is 5-6 mum across and contains up to 5.8 5 wt.% W (0.05 atoms per formula unit). Tungsten-rich zones of the molybden ite veinlets contain the same concentration of W as the central zone of the euhedral crystals, implying similar conditions of formation. There is thus a substantial solid-solution between molybdenite and tungstenite. We attri bute the outward zoning to W-poor molybdenite in terms of increasing fugaci ty of oxygen and decreasing fugacity of sulfur, apparently in the presence of a reduced H2S-CH4-rich fluid of subalkaline character. The existence of locally reducing conditions in the fenite(s) at Khibina also is implied by the adjacent occurrences of titanium and niobium-rich sulfides and a graphi te-like material.