| The Western deposit in the Songshunangou district represents a significant gold resource within the North Qilian metallogenic belt. Mineralization occurs within Late Ordovician quartz diorite porphyry and adjacent basaltic andesite near the contact zone. Documented gold resources exceed 20 tonnes, defining a large-scale deposit. However, the genetic classification lacks systematic mineralogical support, and ore-forming fluid evolution remains poorly constrained, impeding deep and peripheral exploration. This study focuses on magnetite veins within the potassic alteration (Mt1) and the potassic alteration overprinted by propylitic alteration (Mt2) in the Western deposit. Mineralogical studies, combined with electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses, reveal distinct compositional differences. Mt1 exhibits higher concentrations of Ti (2830×10-6–18797×10-6), Al (1309×10-6–18797×10-6), Si (1851×10-6–24993×10-6), and Mn (130×10-6–2982×10-6), with lower levels of Co (0.53×10-6–280×10-6), Ni (37.08×10-6–135.71×10-6), and Sn (3.52×10-6–39.90×10-6). In contrast, Mt2 shows significantly lower Ti (614×10-6–4464×10-6), Al (539×10-6–7268×10-6), Si (2155×10-6–10544×10-6), and Mn (137×10-6–467×10-6) content compared to Mt1, while containing slightly higher Co (90.97×10-6–196.58×10-6), Ni (20.95×10-6–163.04×10-6), and Sn (2.70×10-6–77.02×10-6). Formation temperatures indicate that Mt1 crystallized under relatively high-temperature conditions (average 395 °C). In contrast, the lower temperature range of Mt2 (average 337 °C) records overprinting by later moderate to low-temperature hydrothermal fluids. This shift manifests as decreasing temperature and increasing oxygen fugacity during the fluid evolution. Elemental geochemical diagrams exhibit characteristics diagnostic of a porphyry gold deposit system. Both magnetite types (Mt1 and Mt2) are of hydrothermal origin. Furthermore, magnetite Mt1, defined by its high Ti, Al, and Mn content, serves as a diagnostic indicator for identifying altered and mineralized porphyry systems in exploration targeting. |