Variations in White Mica Chemistry at the E26 Porphyry Cu-Au Deposit, NSW, Australia

Abstract Spatial variations in white mica chemistry can be a valuable tool for exploration in the proximal porphyry environment. However, the successful application of white mica as a vectoring tool requires a detailed paragenetic framework. At the E26 alkalic porphyry Cu-Au deposit in New South Wales, Australia, six substages of phyllic alteration have been identified, each with complex overprinting patterns. The six substages of phyllic alteration are transitional stages T1 (albite-white mica-chlorite + bornite) and T2 (hematite-white mica-chlorite), late stages L1 (quartz-white mica-pyrite) and L2 (quartz-white mica-chalcopyrite ± bornite), and two fault-related substages—L3 (white mica-anhydrite-chlorite + chalcopyrite) and a postmineralization stage Pm1 (quartz-white mica-pyrite). Some individual phyllic substages are mineralized and are prevalent at depth (T1, L2, and L3), whereas others are barren and shallow (T2, L1, Pm1). Copper-gold mineralization occurred during early, transitional, and late stages associated with potassic and phyllic substages. Variations in white mica chemistry between paragenetic substages were identified using short-wave infrared spectroscopy, electron microprobe analysis, and laser ablation-inductively coupled plasma-mass spectrometry. White micas from mineralized paragenetic substages are more phengitic, have higher 2,200-nm feature positions, a higher concentration of La, Ce, Zn, and Cu, and lower Zr and Tl concentrations than weakly mineralized or barren phyllic substages. Despite compositional variations in white mica between paragenetic substages, spatial variations in white mica chemistry through a vertical profile of 1,000 m were identified. Zirconium and Tl decrease, and Li, Mg, and Zn increase, with increasing depth. Ratios between these elements (Zr/Li, Zr/Mg, and Zr/Zn), which vary from four to five orders of magnitude with elevation, can be applied as potential vectoring indicators in the proximal porphyry environment. Variations in white mica chemistry from deeper to shallower parts of E26 reflect a decrease in temperature and increase in acidity, reflective of variations between the fluids that formed phyllic alteration. Vectoring indicators from white mica chemistry coupled with the recognition of multiple stages of Cu mineralization have important implications in constraining the timing of Cu ore formation and enhances our current knowledge of proximal porphyry environments that can be applied to porphyry exploration models worldwide.