Petrographic and magnetic fabric evidence for magmatic emplacement and hydrothermal alteration in the Shadan Au porphyry deposit, 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 Au-rich deposit, located in northeastern Iran. Fresh quartz monzodioritic intrusions exhibit primary magmatic fabrics characterized by oblate AMS ellipsoids, predominantly aligned with magnetite. Progressive hydrothermal alteration—including potassic, phyllic, propylitic, and argillic zones—systematically modifies these fabrics. These alterations are associated with the destruction of magmatic 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. AMS fabrics are controlled by both ferromagnetic (magnetite) and paramagnetic silicate minerals and indicate a predominantly compressional brittle–ductile deformation regime, locally overprinted by late-stage extensional structures. The spatial distribution of magnetic fabrics further highlights the influence of regional and local structural controls on fluid pathways and alteration development within the deposit. 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.