大屯火山群黃鐵礦地球化學特徵對金礦床的成因隱示

Abstract

黃鐵礦廣泛存在於各種熱液礦床中，其礦物學及地球化學特性對於探討金礦形成具有重要指標意義。本研究選取位於臺灣北部大屯火山區之硫磺子坪地區為研究區域，該地區緊鄰過去曾被西班牙人開採的三重橋（八煙）金礦區，且與金瓜石高硫化礦床具有相似的地質與換質特徵，因此推測兩者可能具有共同的熱液成礦背景。然而，該區域黃鐵礦形成機制及其與金礦化之間的關係尚未明確。

本研究針對硫磺子坪地區之黃鐵礦進行系統性的微量元素分析，藉以釐清區域成礦條件並評估金礦之潛力。研究透過掃描式電子顯微鏡（SEM）、能量頻譜分析儀（EDS）、電子微探儀（EPMA）及雷射剝蝕電感耦合電漿質譜儀（LA-ICP-MS）等技術，分析深775公尺岩芯樣品內的黃鐵礦。分析結果顯示，黃鐵礦形貌隨深度呈系統性變化，由淺層稀少的脈狀與斑狀晶體，逐漸轉變為中深層密集的自形立方、八面體與五角十二面體晶體，深部則展現穩定結晶環境下的大量脈狀與斑狀自形晶體，晶粒大小介於100至1000微米。此外，部分樣本出現明顯的砷振盪環帶，最高砷含量達2.97 wt%，伴隨顯著的鎳（最高2,601 ppm）、鈷（最高815 ppm）、銅（最高766 ppm）及金（最高172 ppm）等微量元素之富集。

元素比值分析顯示，本研究區域黃鐵礦之S/Fe原子比平均為1.999（範圍1.936–2.108），反映以熱液成因之特性。Co/Ni比值平均為4.226（範圍0.004–69.851），且大部分樣品比值大於1，符合典型的熱液黃鐵礦特徵；然而，也有部分樣品呈現較低比值，顯示本地區曾經歷複雜且多階段的熱液演化。儘管砷含量極高，但金含量始終偏低且位於Au-As溶解度門檻之下，意味著金多以固溶態形式存在於黃鐵礦晶格內，可能與區域高溫條件或熱液中背景金濃度較低有關。

整體而言，硫磺子坪地區的黃鐵礦特性揭示此地曾經歷中至高溫、多階段的熱液環境。此外，晚期熱液降溫導致鐵元素耗減並伴隨鈷、鎳、銅等元素的明顯富集與砷分帶現象。綜合上述分析，本研究推論該地區具有高硫淺溫礦床的礦化潛力，但尚未有足夠證據支持具經濟價值之金礦床。未來研究將進一步結合此區域之分析資料，並與金瓜石礦區礦化帶樣本進行比較分析，更深入探討大屯火山區之金礦成礦潛力與區域地質意義。

Pyrite is ubiquitous in hydrothermal deposits, and its mineralogical and geochemical traits provide key clues to gold mineralization. This study focuses on pyrite from the Liuhuangziping hydrothermal area of Taiwan’s Tatun Volcanic Area (TVA), adjacent to the historic Sanchungchiao (Ba-yan) gold mine (formerly mined by the Spanish). Because the district shares geological and alteration features with the high-sulfidation Chinkuashih deposit, a common hydrothermal origin is suspected, yet pyrite-forming processes and their link to gold remain unresolved.

This study presents a systematic depth-resolved mineralogical and trace-element dataset for Liuhuangziping pyrite, obtained from drill-core samples down to 775 m. Analytical techniques included scanning-electron microscopy (SEM), energy-dispersive spectroscopy (EDS), electron-probe micro-analysis (EPMA), and laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS).

Pyrite morphology changes systematically with depth. Shallow intervals (≤ 150 m) contain sparse vein-hosted and disseminatedgrains. Depths between 200 m and 600 m host abundant euhedral cubic, octahedral, and pentagonal-dodecahedral crystals, whereas deeper intervals (> 600 m) exhibit abundant vein-hosted and disseminatedeuhedral crystals formed under more stable conditions; crystal sizes range from 100 µm to 1 mm. Pronounced oscillatory zoning is observed, with As concentrations reaching 2.97 wt.%, accompanied by enrichments of nickel (up to 2,601 ppm), cobalt (up to 815 ppm), copper (up to 766 ppm), and gold (up to 172 ppm).

Geochemically, the atomic S/Fe ratio averages 1.999 (range 1.936–2.108), confirming a hydrothermal origin. Co/Ni ratios average 4.226 (0.004–69.851); most values > 1 match typical hydrothermal pyrite, whereas low ratios point to multi-stage fluid evolution. Despite exceptionally high arsenic levels, gold consistently remains below the Au–As solubility threshold, implying that Au is mainly incorporated in solid solution—likely a consequence of high temperatures or limited Au in the fluids.

These results show that Liuhuangziping pyrite crystallized in a multistage, moderate- to high-temperature hydrothermal system. Late-stage cooling caused iron depletion and concomitant enrichment of Co, Ni, Cu, and oscillatory As zoning. The mineralogical and chemical signatures are consistent with a high-sulfidation epithermal environment, yet evidence for economic gold mineralization is currently lacking. Future comparative work with Chinkuashih pyrite is required to clarify the TVA’s ore potential and regional metallogenic significance.