Mapping hydrothermal alteration zones with short wavelength infrared (SWIR) spectra and magnetic susceptibility at the Pulang porphyry Cu-Au deposit, Yunnan, SW China

Abstract

Delineation of hydrothermal alteration zoning is important for exploration vectoring toward mineralization centers in porphyry systems, and shortwave infrared (SWIR) spectroscopy is widely used to map hydrothermal minerals distribution for porphyry Cu exploration. However, the SWIR method cannot effectively detect anhydrous alteration minerals (e.g., K-feldspar) in the potassic zone. Magnetite can be formed by potassic alteration and destroyed by phyllic (quartz-sericite-pyrite) alteration. The relative intensity of these two alteration types can be quantified by magnetic susceptibility. Here, we integrate the SWIR and magnetic susceptibility measurements to map hydrothermal alteration zones at the Pulang porphyry Cu-Au deposit in northwestern Yunnan, one of the largest porphyry deposits in the SW China-mainland SE Asia region. White mica, chlorite, and montmorillonite + kaolinite were identified in ~ 60%, ~ 30%, and ~ 15% of the analyzed samples from the Pulang deposit, respectively. Volumetric bulk magnetic susceptibility (K_bulk) values are high in the potassic-altered rocks, but low in phyllic-altered rocks. Using white mica as a proxy for sericite alteration, white mica-chlorite assemblage for chlorite-sericite alteration, chlorite-epidote-actinolite assemblage for propylitic alteration, montmorillonite-kaolinite-dickite assemblage for argillic alteration, and K_bulk (> 0.5 × 10^–3 SI) for potassic alteration, we delineate the alteration zoning at Pulang. From the causative porphyry center outward, four alteration zones are delineated (potassic → chlorite-sericite → sericite → argillic). The ore-distal propylitic alteration was developed both in the shallow and deeper levels of the hydrothermal system, resembling typical porphyry-style alteration zoning patterns. Our work shows that high K_bulk value is a useful vector toward Cu mineralization at Pulang, whereas illite crystallinity (SWIR-IC), white mica Al–OH spectral absorption peak, and chlorite Fe-OH spectral absorption peak are less effective. We highlight that magnetic susceptibility measurement is an effective alteration-mapping method when mineralization is developed in the potassic zone (with largely aspectral minerals such as quartz, K-feldspar, and magnetite), while SWIR scalars are more useful when mineralization is developed in the phyllic and/or propylitic zones.