Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines
Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines
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DOI：
10.2113/econgeo.106.8.1399
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发表时间：
2011-12
期刊：
影响因子：
5.8
通讯作者：
D. Cooke;C. Deyell;P. Waters;Rene I. Gonzales;K. Zaw
中科院分区：
文献类型：
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作者：
D. Cooke;C. Deyell;P. Waters;Rene I. Gonzales;K. Zaw
碧瑶地区包含多种浅成热液矿、斑岩矿和夕卡岩矿床，以及大型、广泛地层束缚的高级泥质岩盖。岩浆作用、矿化和蚀变的发生是由于南海板块和吕宋岛北部下方斯卡伯勒海岭在过去 3 年来的俯冲作用而发生的。快速抬升和折返导致浅成热液脉覆盖了数个斑岩铜金矿床。碧瑶地区的大部分浅成热液金银矿床（包括两个最大的金矿床 Antamok 和 Acupan）均为中硫化态石英-碳酸盐-阿杜拉利亚-伊利石-贱金属硫化物脉，含有金金和少量金银碲化物。相比之下，凯利矿区的高硫化矿化包括硫铝矿、铁矾石、金金矿和贵金属碲化物，并与高级泥质蚀变有关。尽管形成斑岩矿床和浅成热液矿床的矿化流体具有不同的温度和盐度，但稳定的放射成因数据提供了岩浆对每种矿床类型的直接贡献的证据。超热液成矿流体是稀（通常为 600°C）高盐卤水（30 至 >70 wt% 氯化钠当量）和低密度蒸气。碧瑶地区南部和中部的斑岩、夕卡岩和中间硫化浅成热液矿脉中硫化物的硫同位素组成大多在+1至+6/mil之间，与矿化流体中以H2S为主的情况一致（即还原条件）。相比之下，位于碧瑶岩盖附近的高硫化矿床、斑岩矿床和中硫化矿床中的硫化物大多具有负硫同位素值（−6.9 至 +0.8‰），与氧化性（以 SO42−为主）矿化流体一致。 Acupan 的中硫化浅成热液矿脉横切了 Ampucao 矿化良好的斑岩铜金矿脉。根据辐射年龄测定（Ampucao：0.51 ± 0.26 Ma；Acupan：0.65 ± 0.07 Ma）无法区分这两个沉积物，并且被解释为同生的，由与山脊俯冲相关的快速隆起和折返引起的两种环境的伸缩。测得的 δ 34S 硫化物（+1.1 至 +6.6‰）、δ 34S 硫酸盐（+10.4 至 +31.8‰）值和硬石膏的初始锶比率（0.70378–0.70385）与 Ampucao 斑岩和 Acupan 浅热液脉中这些成分的相同且主要是岩浆来源一致。氦同位素提供了浅热液脉中地幔衍生成分的进一步证据（R/Ra 值为 6.0 和 6.7）。氧、氘和碳同位素为 Ampucao 的岩浆水为主以及 Acupan 的岩浆-大气混合水提供了证据，这些水因沸腾而沉淀出贵金属。在阿库潘浅成热液矿脉形成期间，岩浆水相对于大气水的比例和贵金属品位均随时间而降低。在整个碧瑶地区观察到的斑岩矿脉和浅成热液矿脉之间的交叉关系的普遍观察表明，斑岩型矿化向中硫化型矿化的演化是该地区的普遍现象，并对其丰富的金属禀赋做出了重大贡献。
The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration. Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, 600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (−6.9 to +0.8‰), consistent with oxidizing (SO42−-predominant) mineralizing fluids. Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured δ 34Ssulfide (+1.1 to +6.6‰), δ 34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite (0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle-derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metal endowment.