Origin of Fluorine Contained in Rocks within the Eulwangsan, Yongyudo

용유도 을왕산에 분포하는 암석 내 불소 기원

Abstract

This study was conducted to investigate the natural origin of fluorine contained in the rocks within the Eulwangsan area via petrological and mineralogical analyses. The main geology of the Yongyudo Eulwangsan area is Triassic biotite granite. Biotite granite and mylonite are the major rock types containing fluorine at high levels (up to 1,700 and 2,400 mg/kg for biotite granite and mylonite, respectively). In the case of the biotite granite, a high concentration of fluorine can be contributed to fluorite, and the results of microscopic analyses show that the fluorite was observed as small veinlets filling cleavages and micro-fractures within alkali-feldspars and plagioclases, or observed together with quartz in ore veins, indicating the secondary formation of fluorite by hydrothermal fluids. In mylonite, on the other hand, a high fluorine concentration is attributable to sericite. Microscopic analyses revealed that the boundary between sericite and surrounding quartz was not clear, the sericite occurred filling the micro-fractures of quartz and encapsulating small quartz cataclasts. These results indicate that the sericite was also formed as a result of hydrothermal alteration. Consequently, it is speculated that the high fluorine level in the rocks of the Eulwangsan area of Yongyudo is of natural origin due to hydrothermal processes.

본 연구는 인천국제공항 3, 4단계 건설 사업부지 토취원인 을왕산 지역의 암석 내에 존재하는 불소의 자연적 기원을 규명하고자 암석학적, 광물학적 분석을 실시하였다. 용유도 을왕산은 트라이아스기 흑운모화강암이 분포하는 지역으로, 흑운모화강암과 압쇄암의 불소의 농도는 각각 1,700과 2,400 mg/kg 정도로 매우 높게 나타난다. 흑운모화강암에는 형석으로 인해 불소함량이 높게 나타나는데, 현미경 관찰 결과, 형석은 알칼리장석이나 사정석의 벽개와 균열을 따라 채워진 세맥 형태로 나타나거나, 석영과 함께 들어온 광맥 형태로 존재하는 것으로 보아 열수에 의하여 이차적으로 형성된 것으로 해석된다. 압쇄암은 견운모에 의해 불소함량이 높게 나타난다. 현미경하에서 관찰 결과 견운모는 주변에 있는 석영과의 경계가 뚜렷하지 않고 견운모가 석영의 균열에 채워져 있으며, 압쇄된 작은 결정의 석영을 포획하고 있는 특징을 보이며, 이는 견운모 또한 열수에 의한 이차적인 변질작용으로 형성된 것임을 지시한다. 따라서 용유도 을왕산 화성암류 내 존재하는 불소는 열수에 의해 이차적으로 생성된 자연발생적 기원인 것으로 판단된다.

Fig. 2. The photographs showing the outcrops of representative rocks observed in the study area. (a) Ash gray (EW-1) and (b) Rose pink fresh biotite granite (EW-2); (c) Brown weathered biotite granite (EW-3); (d) Pegmatite (EW-5); (e) Granite porphyry (EW-6); (f) Andesite (EW-8); (g) Biotite schist (EW-9); (h) Mylonite (EW-10).

Fig. 3. Microscopic photographs of biotite granites and mylonite. (a and e) crossed nicols; (b, c, d, and f) open nicols. (a and b) Mineralogy of biotite granite (EW-1); (c) Small veinlet-type fluorite in biotite granites (EW-1); (d) Fluorite within ore veins with quartz in biotite granites (EW-2); (e and f) Sericite including cataclastic quartz grains or sericite filled in the micro-fractures of quartz (EW-10). Qz=quartz; Pl=plagioclase; K-f=K-feldspar; Ser=sericite; Bt=biotite; Chl=chlorite; Fl=fluorite.

Fig. 4. The BSE images on the rocks containing fluorine. (a~c) Small veinlet type of fluorite in biotite granites; (d) Sericite, quartz, and iron oxides in mylonite.

Fig. 5. The x-ray diffractograms of biotite granites (EW-1, 2, 3) (a), mylonite (EW-10) (b), and separation of peaks between fluorite and K-feldspar (c). The peaks of sericite and muscovite are identical, and only that of muscovite is presented. (a) The peaks of quartz, plagioclase, alkali-feldspar, sericite, biotite, and chlorite are shown; (b) Only the peaks of quartz and sericite appear; (c) The peak of fluorite is overlapped with that of K-feldspar. The shoulder of fluorite can be observed in the biotite granites containing fluorite, but it cannot be seen in the mylonite without fluorite.

Fig. 1. (a) Location of the study area (Incheon Yongyudo); (b) The geological map of the study area (modified after Kim and Choi, 2014; Cho and Lee, 2016).

Table 1. The results of XRF analyses (wt%)

Table 2. The results of XRD quantitative analyses on biotite granites (EW-1, 2, and 3) and mylonitic rock (EW-10) (wt%)

Table 3. The EPMA analytical results on the average fluorine concentrations(n: 5~10) of minerals in biotite granites (EW-1, 2, and 3) and mylonitic rock (EW-10) (wt%)