Electron Probe Microanalyses of Chlorites from Low Grade Metamorphic Rocks of Tennant Creek Area Central Australia

Electron probe microanalyses of 47 chlorites from the Lower Proterozoic rocks, , Porphyroidal' rocks and from magnetite lode structures of Tennant Creek field are presented. Recrystallisation of chlorite, progressive in Warramunga rocks and retrogressive in 'Porphyroidal' rocks was contemporaneous and took place under lower greenschist facies conditions during regional deformation. Chlorite in the magnetite lode structures had formed ahead of the sulphide mineralisation in the Warramunga rocks subsequent to regional metamorphism and was due to metasomatic alteration of the wall rocks. Chlorite occurs with muscovite in all the three rock groups studied.

Structurally, the majority of the analysed chlorites have an octahedral cation total short of the stipulated 6.0 per half unit cell. This deficiency is systematically related to the difference between Al^vI and AI^IV; the former is half the latter and confirms Foster's (1962) conclusion. The generalised relationship is (R³⁺ - Al^IV) : (6.0 - Al^IV- R²⁺) : : 2 : 3. Most of the analysed chlorites have Al^vI greater than Al^IV and the value of 1.5 reported here for (Al^vI - Al^IV) may be the limit in natural chlorites. The majority of the analysed chlorites fall on the composition field of ripidolite or brunsvigite or diabantite. The occurrence of diabantite variety in the Tennant Creek field is the first report of its kind from any low grade metamorphic terrain.