Alteration characteristics and short wavelength infrared (SWIR) spectroscopy of the Huangtupo Cu-Zn VMS deposit, NW China: Implications for ore genesis and exploration

The Huangtupo Cu-Zn deposit is a typical Early-Paleozoic volcanogenic massive sulfide (VMS) deposit situated in Eastern Tianshan, within the Central Asia Orogenic Belt. Based on the paragenetic sequence reflected by mineral assemblages and their textural characteristics, the alteration and mineralization processes at Huangtupo can be categorized into two major periods comprising eight distinct stages: the VMS period (including the premineralization stage S1, stockwork ore stage S2, massive sulfide ore stage S3, and barite stage S4), and the late hydrothermal overprinting period (encompassing the albite stage S5, silicification stage S6, chlorite + epidote stage S7, and carbonate stage S8). By correlating petrographic investigation and short-wave infrared (SWIR) spectral analysis, five alteration/mineralization zones have been distinguished along the vertical profile, characterized by: Zone I (albite-white mica-chlorite zone), Zone II (albite-white mica-chlorite-epidote zone), Zone III (chlorite-pyrite ± white mica zone), Zone IV (massive sulfide zone), and Zone V (pyrite-quartz-white mica zone). Alteration and mineralization features indicate that Huangtupo is a highly overprinted sub-seafloor replacement VMS deposit.

SWIR spectrometry shows that chlorite and white mica are the primary alteration minerals in Huangtupo. For white mica, the position of Al-OH absorption feature (Pos2200) exhibits a spectral shift between 2198 and 2214 nm which is interpreted to reflect a compositional trend spanning from paragonite towards phengite. The illite crystallinity (IC) values of white mica vary from 0.79 to 3.11. For chlorite, the position of Fe-OH absorption feature (Pos2250) ranges from 2241 to 2253 nm, indicating Mg-rich to Mg-Fe chlorite compositions. Proximal to massive sulfide mineralization, white mica exhibits shorter Pos2200 (wavelengths below 2202 nm) and higher IC values (1.4–2.1). Additionally, chlorite near the ore body is typically predominantly Mg-rich (wavelengths below 2250 nm), though no significant spatial variation pattern is observed. These findings suggest that white mica formed under high-temperature, acidic conditions near massive sulfides, while chlorite formation was likely influenced by Mg-rich seawater infiltration. The alteration mineral assemblage in the hanging wall may reflect subsequent overprinting by porphyry-like hydrothermal fluids. This study highlights the coexistence of VMS and porphyry-type mineralization potential at Huangtupo, providing a valuable case for SWIR applications in ancient sub-seafloor replacement VMS systems.