From magmatic flow to hydrothermal collapse: AMS constraints on structural and alteration dynamics in the Shadan Au-rich porphyry deposit, South Khorasan, Eastern Iran

Magmatic processes and subsequent hydrothermal alterations within porphyry deposits are well-documented through the analysis of mineral chemistry and the magnetic fabrics of their host intrusions. This study integrates structurs, petrography, mineral chemistry, and anisotropy of magnetic susceptibility (AMS) data to investigate the evolution of magmatic and hydrothermal fabrics in the Shadan porphyry gold deposit, located in northeastern Iran. Fresh monzonitic to monzodioritic intrusions exhibit primary magmatic fabrics characterized by oblate AMS ellipsoids, predominantly aligned with magnetite. Progressive hydrothermal alteration—including sodic–calcic, potassic, phyllic, propylitic, and argillic zones—systematically modifies these fabrics, resulting in prolate ellipsoids and a reduction in bulk magnetic susceptibility. These alterations are associated with the destruction of magnetite, hematitization, the replacement of biotite by sericite, and Fe–Mg re-equilibration in amphibole, biotite, and chlorite. Quartz–carbonate and late argillic zones exhibit very weak magnetic susceptibility due to the oxidation and obliteration of primary magnetic minerals. The findings indicate that AMS effectively records the superimposed magmatic and hydrothermal processes, providing quantitative insights into the physicochemical evolution, alteration zoning, and mineralogical transformations in porphyry gold systems. This methodological approach underscores the significance of magnetic fabrics in guiding predictive exploration