Mineralogy, geochemistry, and evolution of the Mn-Fe phosphate minerals within the pegmatite in Cheolwon, Gyeonggi Massif

Mn-Fe phosphate mineral complexes included within the pegmatite are observed at Jurassic Cheolwon two-mica granite in Gyeonggi Massif, South Korea. The genetic evolution between the Cheolwon two-mica granite and pegmatite, and various trend of Mn-Fe phosphate minerals is made by later magmatic, hydrothermal, and weathering process based on mineralogical, geochemical analysis. The Cheolwon two-mica granite is identified as S-type granite, considering its chemical composition (metaluminous ∼ peraluminous), post-collisional environment, low magnetic susceptibility, and existence of biotite and muscovite. The K-Ar age (ca. 153 Ma) of pegmatite is well coincident with age of the Cheolwon two-mica granite (151±4 Ma). It indicates that these two rocks are originated from the same magma. Pegmatite indicates the LCT geochemical signature, and was classified as muscovite-rare element class / Li subclass / beryl type / beryl-columbite-phosphate subtype pegmatite. The triplite {(Fe_0.4 ²⁺,Mn_1.6)(PO₄)(F_0.9)} is dominant phosphates in later magmatic stage which partly altered to leucophosphite {KFe³⁺ ₂(PO₄)²OH·2H₂O} and jahnsite {(Fe³⁺ _0.7,Mn_2.3)(PO₄)₂OH·4H₂O} by hydrothermal alteration. In particular, near fractures, the triplite has been separatelty replaced by the phosphosiderite (Fe³⁺PO₄·2H₂O) and Mn-oxide minerals during weathering stage.