A fluid and solid inclusion study of the Sukulu carbonatite complex, Uganda
The thesis consists of 8 chapters. The first Chapter gives an introduction to carbonatites and outlines the general aims of this study. The general geology of Sukulu, the methodology used in this research, and previous work are presented in Chapter 2. Detailed descriptions and analytical results on the principal minerals are given in Chapter 3. Chapters 4 and 5 focus on aqueous and solid inclusions in apatite, and detailed descriptions, microthermometric results and interpretations are presented. Determination of oxygen and carbon stable isotope compositions and their interpretations are covered in Chapter 6. Chapter 7 describes geothermometric and geobarometric investigations and the calculation of oxygen fugacities during the deposition of apatite and carbonate. The final chapter discusses evolution of the fluids in the Sukulu carbonatite complex and presents a petrogenetic model. Aqueous inclusions in apatite from the Sukulu carbonatite consist essentially of three types: CO[sub]2-bearing, H[sub]2 0-rich and CH[sub]4-bearing. The CO[sub]2- and CH[sub]4-bearing inclusions, in general, are not present together in individual apatite crystals. It is considered that these compositionally discrete inclusions represent different fluids trapped during different stages of apatite crystallisation. The CO[sub]2-bearing fluid probably formed from an originally H[sub]2 0-rich fluid containing significant CO[sub]2 by immiscible separation under high pressure and temperature. This precursor H[sub]2 0-CO[sub]2 fluid was probably derived from a carbonatite melt, also by a possible process of liquid immiscibility. The CH[sub]4-bearing inclusions were probably formed by late stage hydrothermal processes under different P-T conditions. Many solid inclusions occur in apatite of the Sukulu carbonatite, of which the most abundant are carbonate. They can be classified into Mg-calcite. (primary) and calcite (secondary) inclusions based on their morphology, texture and chemical composition. Although such carbonate inclusions are ubiquitous in carbonatite apatite and have been described by many other workers, this study provides new insight into their genesis and petrogenetic significance. Carbon and oxygen stable isotopic composition from fluid inclusions, in both apatite and matrix carbonate, suggest that the CO[sub]2-bearing fluid was equilibrated with carbonate fluids at an early stage, but it evolved along a different path. The CO[sub]2-bearing fluids which has a stable isotopic composition close to upper-mantle values, evolved in a closed-system after being trapped by apatite, but the carbonate fluid evolved in an open-system and its isotopic composition was elevated by assimilation and contamination during ascent. The results also reveal that post-magmatic processes played an important role in the development of the Sukulu carbonatite. P-T-X isochores calculated for each type of fluid indicate that their evolution was probably from a CO[sub]2-bearing fluid, through a moderate to highly saline one, to a CH[sub]4-bearing one, and took place under temperatures and pressures varying from >1000°C and >7.4kb, through >560°C and >5kb, to about 500°C and <3 kb. This trend represents evolution of the carbonatite from a deep magmatic (carbonate melt) environment towards a shallow level hydrothermal system. This study confirms that both apatite and carbonate-can be precipitated over a wide range of temperatures and melt fluid compositions. The present findings indicate that the compositions of the fluids associated with the Sukulu carbonatite complex appear to have evolved chemically from a Mg-bearing calcite melt, through aqueous CO[sub]2-bearing and bicarbonate-rich melts (NaHC0[sub]3 daughters) to a final aqueous CH[sub]4-bearing fluid.