Comprehensive application of multispectral data for alteration mapping in the volcanogenic massive sulphide deposit: a case study in Baiyin area, China

The Baiyin area of Gansu Province (GSBY), located in the eastern North Qilian Orogenic Belt, exhibits active tectonic-magmatic features such as Early Palaeozoic submarine volcanic rocks and ductile shear zones, which provide favourable conditions for the formation of a volcanogenic massive sulphide (VMS) deposit. Several large-scale VMS copper-polymetallic deposits have been discovered in this region through exploration. However, systematic remote-sensing-based mapping of VMS-related hydrothermal alteration in the Baiyin district remains limited. To accurately delineate the potential alteration anomalies associated with VMS deposits, this study integrates multi-source remote sensing data from ASTER, Landsat 8, and Sentinel-2A, based on the diagnostic spectral absorption characteristics of altered minerals. Techniques including false colour composite (FCC), band ratios (BR), and principal component analysis (PCA) were jointly applied to delineate hydrothermal alteration information. Four main alteration types were identified, including carbonate alteration (CO32-) (characterized by calcite and dolomite), ferric iron (Fe3+) alteration (represented by limonite and jarosite), phyllic alteration (Al-OH) (dominated by muscovite), and propylitic alteration (Mg-OH) (typified by chlorite). Furthermore, lineaments were extracted from a 12.5 m resolution ALOS PALSAR digital elevation model (DEM). By integrating alteration information with structural features through weighted overlay analysis (WOA), structurally controlled zones of intense alteration were delineated. Validation using petrographic thin-section analysis, spectral measurements of representative samples, field verification points, and geological profiles yielded a validation accuracy of approximately 80%, confirming the effectiveness of the proposed approach. This study presents a practical workflow for mapping VMS-related hydrothermal alteration in complex geological settings by integrating multisource remote sensing data and analytical techniques, and provides a useful reference for similar mineralized areas.