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PROSPECTING CRITERIA FOR OFF-SET APATITE-NEPHELINE DEPOSITS IN THE KHIBINY MASSIF AREA

Dmitry Zhirov

Abstract

Phosphates are one of strategic mineral resources that provide the food safety of any country. The specific feature of the raw-material base in the Kola region is the endogenic type of apatite deposits. All industrial reserves and resources of apatite-nepheline ores (ANO) are confined to PZ alkaline-ultrabasic carbonatite complexes and, mainly, to the Khibiny massif (KM). 15 unique apatite-nepheline deposits and occurrences are located here. Some of them are the world-class scale. ANO of the KM are highly eco-friendly compared to sedimentary sources of phosphate raw materials, because they contain far less As, Hg, Cd, U, Th and other harmful and dangerous impurities. All deposits of the KM occur within an ijolite-urtite arc (IUA) - cone-like intrusions of foidolites and the biggest deposits are clustered in the southern part of the arc. Unfortunately, the remaining reserves after mining for 50-85 years are localized exclusively on the flanks and deep horizons (more than 1000 m below the surface). Tectonic-physical and structural surveys of the Khibiny pluton allowed justifying the tectonic background for prospecting new-type deposits, i.e. off-set ANO outside the traditional ore-control structure of an IUA. There are some criteria that were not considered in previous prospecting models: 1) intensive visco-elastic and plicative tectonics of foidolites and, first of all, ANO; 2) formation of a ?cap?, a sub-horizontal swell structure of foidolites and ores, in the upper by-contact part of the Khibiny pluton because of the flowable extruding and transfer; 3) significant posterior protrusive uplift of the massif, presumably at the ?Paleogene-Neogene? stage, with destruction of marginal parts and forming ?pseudo-faults?. In result of this criteria combination, the most fertile by-contact swell (?cap?) of ore-control foidolites collapsed in some marginal parts of the Khibiny pluton and then preserved at the primary bedding site. The central part of the massif was exhumed, eroded and weathered. The areas that collapsed and preserved at the primary bedding site became off-set regarding the contemporary geological structure. The eastern, south-eastern and north-eastern parts of the massif are the most promising for discovery of off-set ores. The supposed depth of occurrence is estimated at 250-400 m below the surface. Forecast resources of off-set ores of just the eastern area (Mt. Suoluayv) are 75-95 million tons of the ore with P2O5 average contents of 14-16%.

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