Pegmatites at the Okatjimukuju farm, Karibib, Namibia Part I: Phosphate mineral associations of the Clementine II pegmatite
Keller, Paul; Knorring, Olegvon
European Journal of Mineralogy Volume 1 Number 4 (1989), p. 567 - 594
published: Aug 31, 1989
manuscript accepted: Mar 24, 1989
manuscript received: Jan 9, 1989
ArtNo. ESP147050104007, Price: 29.00 €
Abstract The pegmatite Clementine II is a steeply dipping dyke of medium size which has been intruded into dolomitic marbles belonging to the Karibib Formation of the Upper Precambrian Damara sequence in Namibia. The phosphate-bearing pegmatite is moderately fractionated with a moderate Li-stage. The pegmatite body is well zoned and small replacement units are exposed. A most remarkable feature is the development of a zone, intercalated between the outer and the inner intermediate zones, on both sides of the nearly symmetrical pegmatite body. The composition and the emplacement of this "intercalated zone" is interpreted as an "early space filling". The relation is discussed between this intercalated zone and some unusual aspects of the ore and phosphate mineralization. Six different associations of phosphate minerals can be distinguished. Each is characterized by a specific primary phosphate mineral, viz. members of the triphylite-lithiophilite series with six distinct Fe/(Fe+Mn) ratios, triplite/ zwieselite, and amblygonite/montebrasite. The Fe/(Fe+Mn) ratios of the primary triphylite-lithiophilite vary over an unusually wide range. A microprobe study indicates values of0.45 < Fe/(Fe+Mn) < 0.79. In accordance with the hypothesis by Ginsburg (I960), the Fe/(Fe+Mn) ratio decreases generally from the intercalated zone to the inner intermediate zone. Partitioning of iron and manganese between the vapour phase and the remaining silicate liquid is also discussed. Triphylite-lithiophilite displays the usual properties and alteration sequences as reported from many other granitic pegmatites, but dendrites and skeletal crystals have also been seen. Unique or rarely reported features of the phosphate mineralization are: (i) an assemblage consisting oftriplite/zwieselite, magnetite, and triphylite; (ii) arrojadite noted as a rather common metasomatic mineral (however, it should be emphasized that Na-metasomatism has not produced alluaudite as reported in several other phosphate-bearing pegmatites); (iii) oxidized eosphorite and kryzhanovskite occur in unusually high amounts. Clementine II has been mined for beryl, lithium mica, and columbite. Beryl and columbite occur in the intercalated zone, whereas lithium mica has been mined from the replacement units. Small reserves of all three minerals may occur in the unexposed parts of the pegmatite body.