Geological controls on gold mineralization in the Kukuluma Terrain, Geita Greenstone Belt, NW Tanzania

Abstract

[Extract] This thesis applies a multidisciplinary approach to better understand the geology of the Kukuluma Terrain and the geological controls on gold mineralization. I used a combination of field, computer and laboratory based techniques to help me constrain a variety of factors influencing the gold mineralization. This thesis is subdivided into 6 chapters with appendices as follows: Chapter 1 is the introduction to the thesis study topic and the geological setting of the Tanzania craton and Geita Greenstone Belt. It also provides a brief history of the Geita Gold Mine. Chapter 2 presents the methodologies followed in compiling the data sets presented in chapters 3, 4 and 5. I report on data collected from internal mine reports, regional and pit scale geological mapping, drill core logging, cross-section interpretations, and 3D modelling using Leapfrog (software version 3.1.1), and provide an overview of the methodologies used in the structural (chapter 3), geochemical (chapters 4 and 5) and geochronological (chapter 5) studies. Chapter 3 presents the first detailed structural evolution history of the Kukuluma Terrain and discusses the structural controls on gold mineralization. The structural evolution of the area was developed based on overprinting relationships observed from detailed regional and deposit-scale geological mapping, drill core logging and the review of historical datasets such as maps and geophysical images, the details of which are included in chapter 2. I use the relationships between the distribution of gold mineralization and different structures to place spatial and temporal correlation between the structural evolution and the mineralizing event. I also constrained the relative timing of dykes and intrusive rocks emplacement in relation to deformation and mineralization. This chapter has been prepared for publication. Chapter 4 deals with the petrogenesis of the dykes and intrusive units within the Kukuluma Terrain. I present detailed petrographic descriptions of the igneous rocks and high quality major and trace element geochemical analyses. The igneous rocks that intruded the Kukuluma Terrain form three suites: a monzonite suite, a diorite suite, and a granodiorite suite. Because the three suites overlap in space and partly in time I grouped them into the Kukuluma Intrusive Complex. The monzonite and the diorite suites form a ~ northwestsoutheast oriented body that runs through the middle of the Kukuluma Terrain and they are virtually undistinguishable in the field while the granodiorite suite occurs mostly as dykes with various orientations and locally form small intrusive bodies. The petrogenetic evolution of the igneous rocks is of particular importance, because of their close spatial relationship with the gold mineralization. It is also possible that fluids related to the emplacement of the igneous rocks may have played a role in gold mineralization. For example, the timing of emplacement and the geochemical characteristics of sanukitoid-type igneous rocks have been linked to gold mineralization in many Archaean gold deposits (Mikkola et al., 2014). This chapter has been submitted for publication. Chapter 5 presents the first zircon ages and Hf isotope data from igneous rocks and sediments from the Kukuluma Terrain. The geochronological results are used to constrain the absolute timing of emplacement and deformation where possible. The results of zircon dating made it possible to place a maximum mineralization age in the area. The detrital age data is used to understand the timing of sedimentation in the area, but also set up the timing on the beginning of deformation in the area. I used the Hf isotope data to better constrain the sources and to understand the processes of crustal growth in this part of the Tanzania Craton. This chapter has been submitted for publication. Chapter 6 contains the conclusions of the thesis and provides recommendations for further work in the area.