Potassic alkaline lamprophyres with affinities to lamproites from the Karinya Syncline, South Australia

Abstract

Most newly discovered unmetamorphosed lamprophyres from the Karinya Syncline, South Australia, occur as dykes with alkaline geochemistry (TiO 2 up to 2.36 wt.%; Zr up to 1135 ppm), high fluorine (F up to 5800 ppm) and high potassium contents (K 2O up to 9.81 wt.%). The majority of samples are ultrapotassic. Two samples, one of which occurs in a diatreme, have the very low SiO 2 contents (39 and 40 wt.%) and high MgO contents (> 16 wt.%), which are more typical of olivine lamproites. Dyke emplacement occurred during the Ordovician after the Delamerian orogeny and was controlled by deep-seated crustal lineaments which are detectable by satellite imagery. The lamprophyres have porphyritic textures, with phlogopite phenocrysts and apatite microphenocrysts within a groundmass of orthoclase, leucite, plagioclase and minor quartz. Two petrographic subgroups can be distinguished: (I) phlogopite-phyric lamprophyres and (II) apatite-phlogopite-phyric lamprophyres. Type (I) lamprophyres have KAr ages of 458±2 Ma, while those from petrographic type (II) have older KAr ages of 480±3 Ma. Some phlogopite phenocrysts are zoned, with pale yellow Mg-rich cores and brown Fe-rich biotitic rims. Both mica phenocrysts, which are characterized by high F and BaO concentrations (up to 3.79 wt.% and 0.65 wt.%, respectively), and groundmass feldspars have a mineral chemistry transitional between lamprophyres and lamproites. Apatite microphenocrysts contain very high halogen concentrations (up to 3.04 wt.% F and 0.13 wt.% Cl). One lamprophyre contains alkali-amphibole (riebeckite). Whole-rock geochemistry, especially the very high HFSE concentrations of the lamprophyres (e.g. Zr, Nb and Hf up to 1135 ppm, 70 ppm and 28 ppm, respectively) are typical for potassic magmatic rocks intruded in a continental within-plate tectonic setting. This is consistent with tectonic interpretations of the area and the fact, that no subduction-indicators, such as blueschists, ophiolites and melanges have been recorded in the Karinya Syncline. Several samples have elevated precious metal contents. Gold concentrations up to 23 ppb, Pt up to 19 ppb and Pd up to 49 ppb may be attributed to late-magmatic hydrothermal fluids. The highest precious metal concentrations are present in a sample affected by secondary carbonate replacement.