The geochemical chararateristics of the marble deposits east of Federal Capital Territory (FCT), Nigeria.

The geology, petrography and geochemistry of the Federal Capital Territory (FCT), marbles were investigated with the view to establishing marble occurrences and their geochemical characteristics. Crystalline rocks of the Nigerian Basement Complex (migmatite – gneiss complex) underlie the area. Ten marble bodies were mapped. The marbles are metasedimentary in origin and were deposited in a structurally controlled basin. They maintain a consistent NNE/SSW (N26° E – N35° E) strike similar to strikes of major regional structures. The marbles are interbedded with mica schist, which is suggestive of periodic variation during their depositional history. The marbles have fine (0.2 – 1.0mm) to medium (1.5 – 4.0mm) calcite and dolomite grains, and exhibit porphyroblastic texture. The calcite and dolomite grains are uniformly distributed and constitute 97.00 – 99.78% of the marbles. The carbonate grains are idiomorphic to subhedral with triple junctions at angles of 110° – 130°, which is suggestive of an equilibrium and stable environment of crystallization. The mineral paragenesis (hornblende – plagioclase – garnet – wollastonite – tremolite – silimanite – zircon – orthoclase – apatite) is suggestive of amphibolite facies of metamorphism to which the marble host rocks have been subjected. It is here suggested that the organic matter of the FCT marbles were destroyed during metamorphism resulting in the production of garnet, biotite and hornblende. The marbles are dolomitic (MgO, 16.05 – 21.65%) and have average Mg/Ca ratio of 2:3. The Na2O and K2O contents are low (0.053% and 0.007% respectively), which is suggestive of a shallow marine depositional environment for the initial limestone material. The environment was rich in Mg and Ca and the Mg co-precipitated with Ca due to poisoning effect of Mg on calcite. The initial environment of limestone precipitation was of moderate temperature, hypersaline with respect to Ca and Mg and precipitation was influenced by microbial activity. The associated MgO was, therefore, precipitatory in origin and not replacement during metamorphism.KEY WORDS: marble, dolomitic, geochemistry, precipitatory and Abuj