The mineralogy and geochemistry of the carbonatites, syenites and fenites of North West Frontier Province Pakistan.

Tertiary carbonatites occur along thrust planes at Loe Shilman in Kyber Agency and at Silai Patti in Malakand Agency, NW Pakistan. The Loe Shilman carbonatite sheet complex comprises an amphibole sovite which is intruded by a biotite sovite and an ankeritic dolomite carbonatite. These carbonatites have produced zoned fenites as a result of Na, K and Na+Mg+Fe fenitizing-fluids emanating from the amphibole sovite, biotite sovite and ankeritic dolomite carbonatite respectively. The fenites grading into unfenitized Palaeozoic thinly bedded slates and phyllites. The variation in the whole-rock and mineral chemistries of the fenites correlates with the distances from the carbonatite contact. These gradual variations in chemistry are attributed to the low permeability of the slates and phyllites. A path of fractionation from calcite-rich to dolomite-rich carbonatites can be distinguished, while the final-stages which return to the calcite-rich carbonate phase are recorded only in the carbonatite veins The Silai Patti carbonatite sheet complex comprises a biotite sovite which is intruded by an amphibole sovite. These sovites have induced K-and Na-fenitizations respectively in granite-gneisses, amphibolites, dolerites and quartzites. The chemical variations in the rocks and minerals of the fenites correspond with the degree of intensity of fenitization. The chemistry of amphiboles, micas and pyroxenes which are in equilibrium with the carbonatitic fenitizing fluids are typically magnesio-arfvedsonite, phlogopite and aegirine-augite respectively. It is proposed that the Na-rich carbonatite magmas are derived from the magma produced by liquid immiscibi1ity from phonolite, but the K-rich magma has evolved from the primitive Na-rich carbonatite magma by crystal fractionation.