Alteration mineralogy of the Chaganbulagen Ag-Pb-Zn deposit (Inner Mongolia): insights from SWIR spectroscopy

The Chaganbulagen Ag–Pb–Zn deposit, located in Xin Barag Youqi, Inner Mongolia, is a large epithermal system discovered in the 1990s. To better constrain the spatial distribution of alteration minerals and the zoning of mineral assemblages, we carried out short-wave infrared (SWIR) spectroscopy using a portable full-range near-infrared mineral analyzer, integrated with systematic drill-core logging and field-based alteration mapping. The SWIR measurements allow recognition of 12 alteration-related minerals, illite, muscovite, chlorite, montmorillonite, epidote, kaolinite, calcite, dolomite, ankerite, kutnohorite, siderite, and rhodochrosite. Their distributions define four alteration zones: sericite–quartz, argillic, propylitic, and surface oxidation. High sericite crystallinity (index > 1.5) and disseminated mineralized veinlets hosted by quartz diorite porphyry and quartz porphyry together suggest that borehole ZK155601 is likely close to the deeper magmatic–hydrothermal center of the deposit.