Hydrothermal alteration and short wavelength infrared (SWIR) characteristics of the Tongshankou porphyry-skarn Cu-Mo deposit, Yangtze craton, Eastern China

Abstract

The Early Cretaceous Tongshankou Cu-Mo deposit is located in the eastern Yangtze craton and comprises both porphyry and skarn mineralization. The porphyry ore is hosted in a granodiorite porphyry with the skarn mineralization found along the contacts with the carbonate host rocks. Alteration in the porphyry mineralization can be divided into three stages: potassic alteration, phyllic alteration and a carbonate stage, with phyllic alteration associated with the main porphyry mineralization. No propylitic alteration is present at Tongshankou. The skarn-type alteration comprises five stages: early skarn stage, late skarn stage, oxide stage, quartz-sulfide stage and late vein stage. Short wave-length infrared (SWIR) analysis identified 15 hydrothermal minerals in the Tongshankou deposit, including illite, dickite, halloysite, phengite, talc, muscovite, saponite, gypsum, chlorite, prehnite, montmorillonite, serpentine, phlogopite, actinolite, kaolinite, with montmorillonite, illite and chlorite being the most common. SWIR parameters and the electron microprobe results of chlorites show that chlorites close to the mineralization center tend to be iron-rich and have high FeOH absorption position (Pos 2250 > 2251 nm). Consequently, chlorite may be a useful indicator mineral for mineralization in the Tongshankou deposit. Pos 2250 is not correlated to temperature as calculated using a chlorite geothermometer but does correlate with the iron contents of the chlorites. Our results show that the ore-forming fluids are Fe-rich in the proximity to the mineralization but Mg-rich towards the distal area. Unlike in other porphyry deposits the white mica show no consistent variation in the Tongshankou deposit and cannot be used as vectors towards mineralization. The Fe-Mg-Al poor carbonate host rocks of the Tongshankou deposit restrict the development of a propylitic alteration zone. The Fe-Mg-Al host rocks also prevent Tschermak exchange in white mica ((Siiv(Mg,Fe)vi ↔ AlivAlvi)), making white mica Pos 2200 and Illite crystallinity (IC) randomly distributed. Our results show that SWIR spectroscopy of chlorite may be an applicable exploration tool in skarn-related hydrothermal systems.