從礦脈中液包體重建臺灣廬山地熱區之熱液歷史

Abstract

本研究以廬山地熱區為研究區域，整合礦脈產狀與特徵，以及液包體分析，探討熱液流體的活動模式，並重建地熱區內流體的演化歷程。首先依照採集之礦脈產狀是否受到塑性變形，以及晶體型態是否有方向性排列，將礦脈分為三類，並根據其與區域構造的演化關係，反映出礦脈在造山帶中由深至淺的抬升歷程與岩體環境的轉變，因此將礦脈分為三個形成時期，並透過液包體顯微測溫實驗觀察構造演變下流體性質的改變。 液包體顯微測溫結果顯示，三期礦脈之原生液包體最低均化溫度呈現下降趨勢，第一期約210°C、第二期約190°C至第三期約140-150°C，第三期礦脈次生液包體約130°C，代表流體溫度隨著地層抬升隨之降低。由冰融溫度檢測之流體鹽度顯示，各時期礦脈皆有可達10.0wt.% NaCl eq.左右的較高鹽度流體，可能指示不同時期的流體皆由同一深源供給，且推測因向斜區域構造的限制，使流體來自同一區域的向斜軸內；第三期出現<4.5wt.%的低鹽度流體，反映淺部天水的混合。各區域第三期礦脈液包體的溫度顯示馬海濮溪具有較高之溫度，可能與經過的斷層有關。 綜合礦脈時序、液包體鹽度與流體溫壓條件，本研究提出廬山地熱區的熱液演化模式：早期深部由變質流體主導，鹽度可達10.0 wt.%，經液包體均化溫度壓力校正的形成溫度可達285°C，隨後岩體抬升與節理開展，熱液沿裂隙上升，後期因岩層接近地表，天水滲入並混合，形成低溫、低鹽的流體特徵，即鹽度<4.5 wt.%，流體溫度為150°C，甚至接近現今儲集層的溫度170°C。

This study focuses on the Lushan geothermal area and integrates vein structures, field occurrence, and fluid inclusion analysis to investigate hydrothermal fluid behavior and reconstruct its evolutionary history. The collected veins are first classified into three types based on whether they experienced ductile deformation and whether their crystals display preferred orientations. Their geometric relationships with regional structures are then used to infer progressive exhumation from deep to shallow levels and changes in host-rock conditions, allowing definition of three vein-forming stages that frame the subsequent fluid inclusion microthermometry. Microthermometric data show a decreasing trend in minimum homogenization temperatures of primary inclusions from about 210 °C in Stage I, to 190 °C in Stage II, and 140–150 °C in Stage III, while secondary inclusions in Stage III yield around 130 °C, indicating cooling associated with uplift. Melting temperatures reveal consistently high values (5.0–10.0 wt.% NaCl eq.) across all stages, suggesting a deep, saline fluid source likely constrained within the synclinal axial zone. In contrast, low-salinity inclusions (<4.5 wt.% NaCl eq.) in Stage III reflect mixing with meteoric water. The Stage III vein fluid inclusion temperatures in different areas indicate that the Mahebu River site records higher temperatures, which are likely related to the faults passing through this area. Combining the timing of vein formation with fluid inclusion salinity and P–T conditions, this study proposes a hydrothermal evolution model for the Lushan geothermal system: early fluids were dominated by deep metamorphic fluids with salinities up to 10.0 wt.% and pressure-corrected trapping temperatures up to 285 °C, followed by upward migration along fractures during exhumation. In the late stage, near-surface conditions allowed infiltration and mixing of meteoric water, generating lower-temperature, lower-salinity fluids (salinity <4.5 wt.% NaCl eq., temperatures around 150 °C) that may comparable to present reservoir temperatures of approximately 170 °C.