Hydrothermal Alteration Architecture of the Pefka-Loutros High- to Intermediate-Sulfidation Epithermal Area, NE Greece: Constraints from Integrated pVNIR-SWIR Spectroscopy and XRD

Hydrothermal alteration assemblages provide valuable information on epithermal mineralization, but superimposed events may obscure their relationships with ore formation. This study characterizes the gangue and alteration architecture of the Pefka-Loutros high- to intermediate-sulfidation (HS-IS) epithermal district, NE Greece, and evaluates which assemblages and spectral features distinguish higher-grade mineralization from alteration in the surrounding volcanic rocks. Particularly, the enrichment of the Pefka deposit in critical and precious metals, including In, Te, Se, Au, and Ag, provides an opportunity to assess alteration-based exploration vectors in a complex polymetallic system. We integrate 764 portable visible-near-infrared and shortwave-infrared (pVNIR-SWIR) measurements and X-ray diffraction (XRD) analyses of 60 samples with petrography, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and bulk-rock geochemistry of 136 samples. At Pefka, adularia-bearing volcanic rocks surrounding the mineralized veins contain Mg-bearing muscovite with relatively long Al-OH absorption wavelengths, commonly around 2210 nm. In contrast, the white mica in the immediate vein wall rocks and ore-bearing assemblages is predominantly muscovitic, while higher-grade samples tend to exhibit shorter Al-OH wavelengths, commonly around 2200 nm. The HS quartz-barite veins contain white mica and aluminum phosphate-sulfate minerals, whereas later IS veins are associated with carbonate gangue and white mica-carbonate wall-rock alteration. Principal component analysis (PCA) of centered log-ratio-transformed bulk-rock geochemical data identifies a mineralization-related association involving Cu, In, As, Te, Au, and Ag toward negative PC1 scores, mainly related to samples containing white mica and/or carbonate. In contrast quartz-“adularia” veinlets and associated altered rocks show positive PC1 scores and represent a comparatively lower-grade mineralized stage, and the replacement of “adularia” by white mica near the HS and IS veins supports alteration overprinting. In the southern sector of the Pefka-Loutros area, alunite-bearing gangue and wall-rock alteration occur along silicified faults and hydrothermal breccias, whereas hilltop opaline silica blankets and associated kaolinite-alunite alteration indicate a shallow steam-heated environment. These results define a multistage alteration architecture in which mineral assemblages and spectral variations help distinguish mineralizing stages and guide exploration when interpreted alongside structural, textural, and geochemical evidence.