Mineralogy, Geochemistry, and Evolution of the Mn-Fe Phosphate Minerals within the Pegmatite in Cheolwon, Gyeonggi Massif

Abstract

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 similar to 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 {KFe23+(PO4)(2)OH center dot 2H(2)O} and jahnsite {(Fe-0.7(3+), Mn-2.3)(PO4)(2)OH center dot 4H(2)O} by hydrothermal alteration. In particular, near fractures, the triplite has been separatelty replaced by the phosphosiderite (Fe3+PO4 center dot 2H(2)O) and Mn-oxide minerals during weathering stage.