The Koga feldspathoidal syenite, Northwestern Pakistan: Mineralogy and industrial applications.

The Koga feldspathoidal syenite complex is a part of the alkaline igneous province of Northwest Pakistan lying between Loe Shilman and Tarbela. It consists of sodalite - cancrinite rich pegmatites, foyaites, feldspathoidal syenites, pulaskitic and garnet bearing feldspathoidal syenites, alkali syenite, lamprophyres, carbonatites and fenites. The Koga complex rocks have SiO2 concentrations from 55 - 62 % with Agpaitic Index (Na2O + K2O)/(Al2O3 mol. prop) for feldspathoidal syenites range from 0.98 to 1.12 indicating both miaskitic and agpaitic character, showing considerable variation in the concentration and distribution of major oxides. Na2O > K2O except in one rock (alkali syenite). The mafics occur as aggregates, almost without amphibole except in one sample. There are two distinct trends of silica undersaturation. The foyaites and foyaitic feldspathoidal syenites display both a phonolitic trend (eventual decrease in silica with differentiation) and a trachytic trend towards oversaturation of silica. The Koga feldspathoidal syenite rocks are depleted in TiO2, MgO, CaO, Sr, Ba and heavy rare earth elements. Ba and Sr show a strong geochemical coherence. K/Rb ratios are different in different groups ranging from 274 to 758 and are higher than the main trend of igneous suites. Rocks are rich in light rare earths and depleted in heavy rare earths with Ce negative anomaly. The composition of the nepheline is mostly restricted to the Morozewicz - Buerger convergence field in the system Ne - Ks - Qz, which is a characteristic of subsolvus nepheline syenites. The recrystallization process at lower temperatures might have given rise to subsolvus types from a single high temperature feldspar hypersolvus assemblage. The composition of all the groups of the Koga feldspathoidal syenites suggest a pressure > 1 and < 5 Kb, except sodalite rich foyaites. The geochemical and mineralogical evidence suggests a magmatic progression from Babaji soda granite → Babaji nordmarkite → alkali syenite → garnet bearing feldspathoidal syenite → foyaitic feldspathoidal syenite → feldspathoidal syenite → foyaitic feldspathoidal syenite → feldspathoidal foyaites, miaskitic → feldspathoidal foyaites, agpaitic → sodalite - cancrinite rich foyaites and finally sodalite rich foyaites. The processing of the raw material is more effective on -250 +125 ? size fraction and total alkalis increase relative to the starting material and F2O3 contents decrease to 0.13%. Following chemical analyses of sieved and magnetically separated fractions of a number of representative rocks, it is clear that the Koga feldspathoidal syenite could be benificiated to make commercially accepted material.