Application of geological and short wavelength infrared (SWIR) spectroscopy mapping in the Mailong gold deposit, East Kunlun: Implications for exploration targeting

Abstract

Widespread regolith cover makes it difficult to identify the anomalous size of their mineralogical or geochemical footprint at the surface in gold exploration. Short wavelength infrared (SWIR) spectroscopy with effective identification of clays and other hydrous minerals shows potential application in gold exploration. The detailed geological and short wavelength infrared (SWIR) spectroscopy mapping was conducted in the newly-discovered orogenic-type Mailong gold deposit (13.06 t @ 6.29 g/t Au), East Kunlun, to determine alteration stage and mineral assemblages and discuss the potential values of SWIR spectral in gold exploration.In the Mailong area, magmatic rocks consist of granodiorite, biotite granodiorite, plagiogranite and mafic dikes, and the gold mineralization shows a close spatial and temporal relationship with mafic dikes. The earliest chloritization (chlorite-quartz) is widely distributed in granodiorite, biotite granodiorite and plagiogranite. The following potassic alteration (hydrothermal K-feldspar and quartz) is developed along high-angle fault/fracture, which cut through granodiorite, biotite granodiorite and plagiogranite. The subsequent sulfide alteration (chlorite-muscovite-sulfide-carbonate) overprints the former potassic and chloritization alteration, and shows spatial association with mafic dikes. Native gold occurs in fracture arsenopyrite or associated with minerals such as pyrite, and chalcopyrite during the early sulfide stage. Supergene alteration is characterized by limonite and malachite. At Mailong, the SWIR data indicate that the most abundant alteration minerals (chlorite, white micas and ankerite) mainly occurred in the northern area. White mica Al-OH absorption occurs at longer wavelengths (Pos2200 = 2202.5 ∼ 2203 nm) and higher Illite crystallinity (IC = 1.2 ∼ 3.1) probably reflecting a higher temperature and pH conduit at the bottom of ZK0001 and ZK0801. The sharp shift of IC values near the bottom reflects the control of fault. This area is also situated at the V-shaped intersections of the three granitic intrusions and structural transection, indicating more of a possible hydrothermal fluid channel than a mineralization center. Low-grade Au mineralization (Au = 0.1 ∼ 0.5 g/t) is mainly distributed within hydrothermal channels with higher Pos2200 (mainly 2022.5 ∼ 2203 nm) and IC values (mainly 1.2 ∼ 3.1), while high-grade Au endowment associated with mafic dikes has low Pos2200 (mainly 2201 ∼ 2202.5 nm) and IC values (mainly 0.43 ∼ 1.2). Meanwhile, an equation of distance = -15*IC + 31 (R2 = 0.47) can be summarized for low-grade orebodies vectoring. Integrating geological mapping and spectral data, target areas for high-grade ore bodies can be outlined using the criteria of white mica, shorter wavelength of Al-OH absorption position (Pos2200 = 2201 ∼ 2202.5 nm), lower IC values (IC = 0.43 ∼ 1.2), and chlorite Fe-OH absorption position Pos2250 (>2247 nm) along with occurrences of mafic dikes. This study demonstrates that utilizing the detailed SWIR identification in conjunction with accurate geological mapping has great potential for greenfield exploration of orogenic gold deposits.