Gahnite in rare-metal metasomatites of the Verkhneurmiysky ore cluster (Amur river region)
Alekseev V. I., Marin Yu. B., Gembitsksaya I. M.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 2. P. 100-112
The new finding of zinc-bearing spinel—gahnite in biotite metasomatites of the Verkhneurmiysky copper-tungsten-tin ore cluster has allowed to study composition of this mineral, its paragenesis and position in evolution of hydrothermal processes in the district. Gahnite of rare-metal ore occurrences contains significant isomorphous admixtures of iron (24.3 %) and manganese (0.65 %), it belongs to the hercynite—gahnite—franclinite series. Gahnite occurs typically in biotite-microcline metasomatites of the lower («sub-ore») stage of greisen metasomatic column located in the proximate exo-contact of rare-metal granite massif. On the base of obtained results and published data, there is displayed hypothesis of appearance of zinc in the early post-magmatic solutions of some rare-metal ore-magmatic systems. This means the conclusion than gahnite may be considered as the mineral-indicator for rare-metal-metasomatic processes.
Key words: gahnite, zinc-bearing hercynite, siderophyllite, lithium-fluoric granite, greisen, zwitter, tungsten-tin ore deposits, Far East, Amur River region.