Application of geological and shortwave infrared (SWIR) spectroscopy mapping in the Yechangping giant porphyry-skarn Mo-W deposit, East Qinling, China

Abstract

The Yechangping deposit, located in the East Qinling Orogen belt, is a giant porphyry-skarn Mo-W deposit in China. This deposit features two spatially zoned alteration-mineralization systems: (1) porphyry-type assemblages and (2) magnesium (Mg)-skarn-type associations, with high-grade ore bodies predominantly hosted within Mg-skarn. Poor understanding of alteration-mineralization relationships has hindered exploration progress. This study employs shortwave infrared (SWIR) spectroscopy to map alteration minerals in Yechangping and evaluate its exploration potential. SWIR spectroscopic analysis identified multiple alteration mineral groups, including white mica (illite, muscovite and phengite), smectite (montmorillonite and saponite), kaolin (kaolinite and dickite), amphibole (hornblende, tremolite and actinolite), phlogopite, serpentine, talc, chlorite, carbonate minerals (dolomite, calcite and siderite) and sulfates (gypsum). White mica shows a strong spatial association with porphyry-related alteration and mineralization, whereas chlorite, talc, and serpentine are extensively developed within skarn zones and are closely linked to the formation of skarn-related ore. The Illite Spectral Maturity (ISM) values (>0.8) of white mica effectively map porphyry-related potassic and phyllic alteration zones, revealing the distribution of granitic porphyry and its adjacent magnesium skarn. High-grade Mo-W ore coincides spatially with retrograde magnesium skarn can be indicated by chlorite Depth2330/Depth2250 ratios (>2.0), talc Fe-OH absorption wavelengths (>2294.5 nm), and serpentine Mg-OH band ranges (2315–2321 nm). These mineralogical indices define vectors to mineralization centers. This study provides the first documentation of using SWIR spectral mapping in the exploration of porphyry-magnesium skarn Mo-W deposits.