Giant carbonatite-nepheline syenite concentric massifs with the largest REE-Nb-P deposits
published: Jan 1, 1998
ArtNo. ESP136000603006, Price: 19.00 €
This paper presents data that support the concept that giant carbonatite-nepheline syenite massifs should be regarded as a distinct group which contains, as a rule, the largest REE-Nb-P ore deposits. Separation into a separate group is based on the following observations: (1) the size of the areas they occupy; (2) the high degrees of source magma fractionation and crystallization over extended periods of time and (3) the pattern of their localization relative to deep structures of convecting mantle. The giant massifs are sharply distinguished from similar massifs of lesser size. The giant massifs compose a distinct group (200-1350 km2) on a histogram of massif areas (n = 100) with a gap between giant and smaller massifs (less than 80 km2). To a first approximation, alkaline ultramafic massifs consist of five rock subseries: ultramafic, melilite-bearing, jacupirangite-urtite, syenite, and carbonatite. Their relationships were previously explained in different ways. Rass and I have established that alkaline ultramafic massifs are the product of two series: (1) meimechite-peridotite-pyroxenite -ijolite-urtite-alkaline and nepheline syenites, and (2) melilite-bearing series (kugdite, uncompahgrite, turjaite, okaite) (Kravchenko & Rass, 1985; Rass, 1986). The latter rock series has a kimberlite-like source magma. Rare elements-bearing carbonatite is derived in some occurrences from kimberlite-like source magmas. Comparative analysis of the Tomtor and Khibina massifs reveals significant similarities (Kravchenko & Pokrovsky, 1995). They both have two types of nepheline syenites: (1) potassic-type with gradual transition to the ijolite-carbonatite rock series, and (2) predominantly sodic-type. A specific feature of the Tomtor massif is the world's largest carbonatitic stock, up to 40 km2 total area (with 12 km2 of real carbonatite). The agpaitic coefficients of the Tomtor and Khibina nepheline syenites are near 1. The appearance of rock-forming eudialite in the nepheline syenite is in general a good indicator that a massif is a giant. Most massifs with rock-forming eudialite are giant: Pilansberg -660 km2, Lovozero -650 km2, Illimausaq -400 km2. The giant size of the alkaline massifs is the main factor which determines their metallogenic specialization.