Hydrothermal alteration and shortwave infrared (SWIR) spectroscopy exploration of the Jinwozi gold deposit in the Beishan Orogenic Belt, NW China

Abstract

The Jinwozi orogenic gold deposit, located in the western Beishan mineralization belt of the Central Asian Orogenic Belt (CAOB), comprises three mining areas: Jinwozi (biotite granite-hosted), Shuangxin (altered sandstone-hosted), and 210 (carbonaceous mylonite-hosted). The paragenesis sequence of mineralization and alteration in three mining areas can be broadly divided into the metamorphic deformation stage, hydrothermal mineralization stage and supergene stage. In the Jinwozi mining area, the hydrothermal stage is further divided into the early silicification-chloritization-pyritization, main mineralization with intensive silicification-pyritization-sericitization, and late carbonation. In contrast, the Shuangxin mining area is characterized by silicification and chloritization as the early phase, while the 210 mining area is dominated by silicification and pyritization. The main mineralization and late phase are similar across all three areas.

By combining petrography and shortwave infrared (SWIR) spectroscopy, alteration zonation in the Jinwozi gold deposit can be delineated as quartz-muscovite ± chlorite zone, quartz-muscovite-chlorite ± epidote zone, and quartz-muscovite-pyrite zone, with the latter being most closely related to mineralization. In all three mining areas, spectral analysis shows that the Al-OH absorption peak position (Pos2200) of white micas (muscovite, illite, phengite and paragonite) proximal to ore bodies are shorter (Pos2200 < 2202.5 nm), whereas the spatial distribution of illite crystallinity (IC) values exhibits inconsistencies. Specifically, the Jinwozi (0.3–1.2) and Shuangxin (0.5–1.3) mining areas both exhibit lower IC values proximal to ore bodies, contrasting with the 210 mining area where higher values (1.3–2.1) occur within proximity. Meanwhile, the IC values show an increasing trend from the Jinwozi (0.3–1.2) to Shuangxin (0.5–1.3), and the 210 mining area (1.3–2.1), suggesting that the 210 mining area could be the hydrothermal center, and the 210 structural fracture zone may serve as the primary hydrothermal channels. Therefore, Pos2200 < 2202.5 nm, Jinwozi mining area IC: 0.3–1.2, Shuangxin mining area IC: 0.5–1.3, 210 mining area IC: 1.3–2.1, can be considered as new exploration indicators for the Jinwozi gold deposit. This study provides practical support for the application of SWIR technology in orogenic gold exploration, offering new scientific evidence and technical guidance for the exploration of the Jinwozi gold deposit and similar deposits in the Beishan Orogenic Belt.