Alteration mapping with short wavelength infrared (SWIR) spectroscopy on Xiaokelehe porphyry Cu-Mo deposit in the Great Xing’an Range, NE China: Metallogenic and exploration implications

Abstract

The newly discovered Xiaokelehe porphyry Cu-Mo deposit (PCD) is situated in northern part of the Great Xing’an metallogenic belt, northeastern China. The syn-mineralization granodiorite porphyry was intruded by multiple phases of post-ore barren monzonite, diorite porphyry and granite porphyry, leading to complex and overprinting hydrothermal alteration patterns. This paper present new data on the alteration mineralogy, vein paragenesis, sulfide assemblages and SWIR (short wavelength infrared) spectroscopy at Xiaokelehe, and discuss how SWIR spectral data can help to understand hydrothermal fluid evolution and assist PCD exploration.At Xiaokelehe, oligoclase-epidote alteration is distributed at the deeper levels of the granodiorite porphyry but played little role in mineralization. The following potassic alteration (hydrothermal biotite and K-feldspar) was developed in the granodiorite porphyry with the main Cu-Mo mineralization. Drill core logging suggests that the subsequent chlorite-illite alteration affected almost the entire granodiorite porphyry, and is spatially and genetically associated with minor Cu mineralization. The youngest quartz-muscovite-pyrite alteration dominated in the upper level and outer parts of the PCD, and overprints the earlier alteration phases with only very minor mineralization.The short wavelength infrared (SWIR) analysis results indicate that chlorite and white micas (illite and muscovite) are the most abundant alteration minerals at Xiaokelehe. The wavelengths of Fe-OH band (Pos2250) of chlorite show a decreasing trend away from the granodiorite porphyry into the surrounding sedimentary wall rocks, probably reflecting a drop in temperature and Fe contents of the hydrothermal fluids. The Illite crystallinity (IC) values of white micas show a reverse trend, different from many PCDs, which is likely caused by the distribution pattern of illite and muscovite formed in the ore-proximal chlorite-illite and ore-distal quartz-muscovite-pyrite alteration, respectively.At Xiaokelehe, Cu and Mo mineralization is mainly distributed in quartz veins or disseminated within the potassic alteration zones, resembling typical PCDs. Meanwhile, chlorite-illite alteration is also associated with minor Cu mineralization. Thus, areas with intensive chlorite-illite alteration overlapping on potassic alteration likely represent the best exploration targets for high grade Cu-Mo mineralization. These target areas coincide spatially with areas of lower IC (<0.8) and higher Pos2250 (>2245 nm) values, suggesting the latter two are effective vectoring tools for PCD exploration at Xiaokelehe.