Short wavelength infrared (SWIR) spectral and geochemical characteristics of white micas in the Xiaonangou gold deposit, East Qinling: As a hyperspectral tool in exploration

The Xiaoqinling-Xiong’ershan region, a prominent component of the East Qinling Belt, is renowned worldwide for its molybdenum and gold ore fields. Moreover, with gold reserves of at least 1300 tons, it is also China’s major gold mining province, second only to Jiaodong. Seated in eastern center of the Xiong’ershan area, the Xiaonangou gold deposit has an estimated gold reserve of over 60 tons and is characterized by disseminated gold in altered rocks. Based on detailed petrographic investigation, coupled with an analysis of crosscutting relationships, there are three stages of hydrothermal alteration and mineralization: pre-ore K-feldspar + quartz (stage Ⅰ), ore quartz + ankerite + disseminated pyrite + polymetallic sulfide (stage Ⅱ), and post-ore quartz + carbonate (stage Ⅲ). As revealed by a systematic analysis of short wavelength infrared (SWIR) spectroscopy, four main types of alteration minerals exist, including white micas, clay minerals, chlorite, and carbonate minerals. In the ore body, white micas mainly developed in stage Ⅱ, and slightly occurred in stage Ⅰ. The clay minerals and chlorite usually developed in the banded alteration zones outside the orebody. Most of the carbonate minerals, including calcite and dolomite, mainly developed in stage Ⅲ, but the ankerite was associated with disseminated pyrite, quartz and polymetallic sulfides in stage Ⅱ. Of these, white micas were recognized as the most abundant hydrothermal alteration minerals, which were observed to be widely distributed within alteration zones in the Xiaonangou gold deposit. The SWIR parameters and its spatial variation of white mica indicate that the Al-OH absorption positions (Pos2200) exhibit a drifting tendency towards longer (>2205 nm) wavelengths in the ore body vicinity. Additionally, there is a discernible increase in illite crystallinity (IC values) in the same region. The temperature, redox, and pH conditions for associated fluids can be reflected through characterization of these parameters. Furthermore, it demonstrates that the ore-proximal zones are situated in a relatively oxidized, alkaline, and high-temperature environment in the Xiaonangou gold deposit. According to the electron probe microanalysis (EPMA) results, two distinct end members exist in white mica: a Si-poor, Al-rich muscovite and an Al-poor, Si-, Fe-, and Mg-rich phengite. The Si, ^ivAl, ^viAl, Mg, Fe, and Ti elements exhibit a linear correlation with the wavelength of Pos2200, demonstrating that the shift in wavelength of white mica is predominantly controlled by Tschermak substitution (^ivSi^vi(Mg，Fe) ↔^ivAl^viAl). The application of spatial variation in the SWIR spectral parameters along with geochemical properties of white mica facilitates the effective guidance of mineral exploration. In comparison to other deposits, it can be concluded that the longer (>2205 nm) wavelength of the Pos2200 along with the higher IC values (>1.4) of white mica are applicable as valid vectors towards gold mineralization in the Xiaonangou gold deposit.