Downhole lithogeochemical patterns relating to chemostratigraphy and igneous fractionation processes in the Golden Mile dolerite, Western Australia

Abstract

The Fimiston Superpit, Kalgoorlie, Western Australia, is the largest gold mine in Australia. The main host to mineralization, the differentiated Golden Mile dolerite (GMD) sill, has undergone several episodes of deformation, metamorphism and intense deuteric and hydrothermal alteration. This has replaced the primary silicate minerals but at least partially preserved the original igneous textures in the intrusion, upon which its ten horizontally stratified units are defined. An examination of petrologic and lithogeochemical data from drill-core through the GMD sill was undertaken to identify mineralogical and geochemical parameters that confirm stratigraphic position within the igneous stratigraphy. These parameters facilitate construction of palinspastic reconstructions of cross-sections through the GMD sill, and will thus enable identification and testing of deep exploration targets. Results also provide more detailed information about the history of the intrusion. Conserved element ratio analysis indicates that the intrusion formed from a single cooling magma body. Plagioclase and pyroxene fractionation occurred throughout during cooling, but magnetite fractionation occurred only after c. 50% crystallization. Calculations reveal unbalanced enrichments and depletions of magnetite-borne Ti and V that are possibly caused by tapping of the magma chamber after magnetite saturation, or by lateral variations in the magma chamber, possibly due to turbulence. Magnetite saturation also triggered saturation of immiscible sulphide droplets, which gravitationally settled from the magma and fractionated Cu and Ni. In summary, textural and compositional variations within the GMD can be explained by simple igneous processes that affected the magma chamber during cooling. The parameters that track these processes allow a stratigraphic re-assessment of the original ten stratigraphic units. Only seven units bounded by sharp and consistent contacts defined by obvious mineralogical and lithogeochemical breaks can be confidently discerned.