경남 밀양지역 납석광상과 안산암질 모암의 지구화학적 연구

A geochemical study on pyrophyllite deposits and andesitic wall-rocks in the Milyang area, Kyeongnam province

Several pyrophyllite deposits occur around the Milyang area where Cretaceous andesitic rocks and spatially related granitic rocks are widely distributed. Pyrophyllite ores consist mainly of pyrophyllite, and quartz with small amount of sericite, pyrite, dumortierite, and diaspore. The andesitic rocks and spatially related granitic rocks in this area suggest that they could be formed from the same series of a calc-alkaline magma series. The contents of SiO₂, Al₂O₃, LOI(loss on ignition) are enriched, and K₂O, Na₂O, CaO, MgO, Fe₂O₃ are depleted in altered andesitic rocks and ores. Enrichment of As, Cr, Sr, V, Sb and depletion of Ba, Cs, Ni, Rb, U, Y, Co, Sc, Zn are characteristic during mineralization. The pyrophyllite ores can be discriminated from the altered-and unaltered wall-rocks by an increasing of (La/Lu)cn from 4.18~22.13 to 8.98~55.05. In R-mode cluster analysis, Yb-Lu-Y, La-Ce-Hf-Th-U-Zr, TiO₂-V-Al₂O₃, Sm-Eu, CaO-Na₂O-MnO, Cu-Zn-Ag, K₂O-Rb are closely correlated. In the discriminant analysis of multi-element data, P₂O₅, As, Cr and Fe₂O₃, Sr are helpful to identify the ores from the unaltered-and altered wall-rocks. In the factor analysis, the factors of alteration of andesitic rocks and ore mineralization were extracted. In the change of ions per unit volume, SiO₂, Al³⁺ and LOI are enriched and Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺ and Fe³⁺ are depleted during the alteration processes. The Milyang and the Sungjin pyrophyllite deposits could be mineralized by hydrothermal alteration in a geochemical condition of low activity ratio of alkaline ions to hydrogen ion with reference to spatially related granitic rocks.