Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits, southwestern Japan
Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits, southwestern Japan
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DOI：
10.1016/j.oregeorev.2008.09.009
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发表时间：
2008-12
影响因子：
3.3
通讯作者：
Y. Morishita;T. Nakano
中科院分区：
文献类型：
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作者：
Y. Morishita;T. Nakano
大多数低硫化浅成热液矿床的岩浆-矿床关系仍不清楚，部分原因是对此类矿床的许多稳定同位素研究表明热液中大气水占主导地位。然而，热液系统最终是由岩浆侵入驱动的，因此，即使在与岩浆没有明显联系的矿床中，浅成热液金矿床也可能是由岩浆活动产生的。我们利用结构模拟和同位素数据重新研究了两种典型的低硫化浅成热液金矿床（串木野矿床和菱狩矿床）的成因。日本西南部的九州发现了许多浅成热液金矿床，包括串木野矿床和菱狩矿床。串木野矿床由新近纪安山岩火山岩内的裂隙充填矿脉组成，这些火山岩覆盖在不整合的白垩纪沉积基底之上。矿脉由含金、银的石英和方解石组成，并含有少量的阿杜拉石、绢云母和硫化物。尽管矿脉的碳和氧同位素数据表明矿液的起源是陨石，但有限元模拟表明矿脉系统可能是对岩浆侵入的直接反应而形成的。特别是，地球物理数据表明，侵入的岩浆抬升了基底岩石，从而产生裂缝和矿脉以及布格正异常，并提供驱动成矿热液系统所需的热量。这项研究的第二个组成部分是调查串木野和菱狩浅成热液系统的性质和演化。同位素数据记录了热液的地球化学演化。我们得出的结论是，深层沉积基岩的存在可能影响了 Kushikino 和 Hishikari 矿石流体的锶和碳同位素比率。 87Sr/86Sr比值和δ13C-δ18O趋势表明，Hishikari矿床中的主要矿脉可以与浅层贫矿脉区分开来。同位素表明，造成附近浅层和贫瘠环流系统中贫瘠矿脉的流体仅受浅层主岩控制。这种多同位素系统学提供了一个强大的工具，可以用来确定热液活动中心，从而记录热液流体的演化。
The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data. Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system. The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The87Sr/86Sr ratios and δ13C–δ18O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.