Texture and Chemical Composition of Magnetite From Iron Skarn Deposit in the Loei and Phetchaburi Mineral Belts, Thailand: Implication for Ore Genesis and Magnetite Classification Schemes

ABSTRACT The Loei Mineral Belt hosts numerous significant iron mineral deposits in Thailand, including Khao Lek and Khao Mae Lek skarn deposits, associated with Triassic I‐type granites. In contrast, the Phetchaburi and Songkhla Mineral Belts contain iron mineralization, such as contact‐metasomatic or skarn Khao Um Khlum deposit related to Triassic S‐type granites. The mineral assemblages of magnetite veins and iron contents in the magnetite of these deposits differ and remain poorly understood. This research investigates the mineralogy, textures, and geochemistry of magnetite veins of each deposit to understand the magnetite's origin, which formed by hydrothermal fluids or magmatic‐related processes. Three distinct magnetite textures are recognized across the deposits, reflecting sequential stages of hydrothermal mineralization. Type A granular magnetite exhibits a well‐crystallized texture with growth zoning, characterized by high FeO tot content with minor impurities, and indicates formation under stable oxidizing conditions. Type B displays a massive magnetite, fine‐grained texture with lower FeO tot but elevated SiO 2 , MnO, and CaO contents, suggesting rapid crystallization under decreasing temperatures. Type C mushketovite magnetite is characterized by platy shapes with zoning and the lowest FeO tot content. The iron veins from Khao Mae Lek and Khao Lek deposits contain high FeO tot content (61.27–77.74 wt.%) resulting from a diorite intrusion into Permian limestone. Conversely, the iron veins at Khao Um Khlum deposit exhibit lower FeO tot content (55.67–70.61 wt.%) and were formed by Triassic granite intrusion into Ordovician limestone. Geochemical discrimination using Fe, Mg, Al, Mn, Ti, V, and Cr in magnetite indicates a hydrothermal origin for all deposits. The highest concentration of iron in magnetite is associated with I‐type granites, while the highest manganese with the lowest iron concentrations in magnetite is observed in the deposits linked to S‐type granite. Magnetite of the three deposits is inferred to have formed under varying temperatures, ranging from 300°C to 500°C for the granular and massive magnetite in the Khao Mae Lek and Khao Lek deposits and below 200°C–300°C for the mushketovite in the Khao Um Khlum deposit. These findings provide implications for regional exploration strategies.