花蓮天星礦場黑雲母礦脈成因探討

Abstract

本研究在台灣玉里帶中發現了變質熱液成因的黑雲母礦脈，其為脈狀產狀且組成為90% 金雲母、6% 鉀長石與3% 方解石，是前人研究中尚未報導過的新發現。 根據岩象與岩石學分析，藉由綠簾石-斜黝簾石岩內殘餘的鈉長石、鉀長石與碳物質證實其原岩為變質泥質岩，再受到富含鈣、矽的熱液換質後形成綠簾石-斜黝簾石岩。且同一期熱液換質事件形成了緊鄰的軟玉礦床。樣本中脈狀金雲母以裂隙填充的形式沉澱於受剪切之斜黝簾石岩中，寬度介於1mm-1cm，並緊鄰閃玉與蛇紋岩。於偏光顯微鏡下觀察，黑雲母粒徑介於30-80um，彼此緊密鑲嵌且呈無序排列，在平行偏光下呈現淡綠色與弱多色性。 根據電子微探儀礦物化學資料計算，此黑雲母 (XMg= 0.69)組成介於Mg-biotite與Phlogopite之間，且根據Nachit (2005)提出的TiO2-FeO-MgO分類法屬於Neoform Biotite，此外根據與之共生的綠泥石地質溫度計計算，其形成溫度約為289±10℃，證實其係由低溫熱液所形成之熱液黑雲母。而在較遠離超基性岩的變質泥質岩中同樣也觀察到了熱液黑雲母存在，然而其XMg=0.49，鎂元素比例明顯較鄰近超基性岩中的黑雲母礦脈低，指示鎂元素應是自超基性岩擴散。 綜合對此樣本的各項觀察，並綜合前人研究線索，在此臺灣閃玉熱液換質事件的後期，富含鈣、矽的變質熱液以變質泥質岩和蛇紋岩之接觸邊界為通道，伴隨由超基性岩提供的鎂、鐵元素，變質泥質岩所提供的鉀、鋁、矽元素在與變質高孔隙水壓的情形沈澱了此脈狀黑雲母於裂隙之中。根據前人研究，此事件可能發生於上新世晚期(3.3Ma)，為臺灣閃玉成礦熱液作用晚期事件。

The Yuli Belt, located at the southeastern Taiwan, is regarded as metamorphosed subduction complex, and mainly composed of metapelites and quartz-mica schists with few metamorphic mafic and ultramafic rocks and high-pressure blocks, which were formed during the late-Miocene subduction event. We firstly discovered biotite veins of metamorphic hydrothermal origins in the Yuli Belt. These biotite veins are predominantly composed of biotite (90%) with minor K-feldspar (6%) and calcite (4%). According to the petrological analyses, the host rock of vein, epidote-clinozoisite rock was recognized as meta-pelite, which contains relict albite and carbonaceous materials, then was metasomatized by hydrothermal fluid rich in Ca and Si. Petrographically, the biotite grains are 30-80 µm in diameter with interlocking texture and random orientation, and show pale-green color and weak pleochroism under PPL. Based on the analysis of electron microprobe analyzer, the compositions of these biotites (XMg= 0.69) fall between Mg-biotite and phlogopite, and belong to neo-form biotite, in terms of classification, the TiO2-FeO-MgO ternary diagram, proposed by Nachit (2005). In addition, the formation temperature is estimated as 289±10℃, coexisting with chlorite, which suggests that these biotite were formed in low-temperature hydrothermal condition. The host rocks, sheared clinozoisitic rock, of biotite veins occurred with nephrite deposits, which was formed in late Pliocene (3.3 Ma). This biotite-forming metasomatic event, therefore, could be occurred as the same stage of the hydrothermal mineralization of nephrite. The metasomatism occurred between the ultramafic rocks (Fe, Mg provider) and meta-pelites (K, Al and Si provider) interacted with the Ca-, Si- and CO2-rich metamorphic hydrothermal fluids, to precipitate biotite in the fissures under the condition of high pore water pressure.