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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The Lepanto Cu-Au deposit, Philippines: A fossil hyperacidic volcanic lake complex
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The Lepanto Cu-Au deposit, Philippines: A fossil hyperacidic volcanic lake complex

机译：菲律宾Lepanto Cu-Au矿床：化石高酸性火山湖综合体

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摘要

Hyperacidic lakes and associated solfatara in active volcanoes are the expression of magmatic gas expansion from source to surface. Here we show for the first time, that the vein system that comprises the ～ 2 Ma high-sulfidation, Lepanto copper-gold deposit in the Mankayan district (Philippines) was associated with a contemporary hyperacidic volcanic lake complex-possibly the first such lake recognized in the geological record. A 15-20‰ difference in sulfur isotopic composition between barite and sulfides and sulfosalts in the vent fumarole encrustations supports the interpretation that SO_2-rich volcanic gas vented into the base of the lake and marginal to it and ties the mineralization directly to magmatic gas expansion, fracture propagation, and mineralization that occurred through a series of decompression steps within the feeder fracture network. These data confirm that crater lake environments such as Kawah Ijen (Java, Indonesia) provide modern day analogs of the Lepanto and other high sulfidation Cu-Au depositing environments. We also provide extensive analysis of sulfosalt-sulfide reactions during vein formation within the hyperacidic lake complex. Pyrite ± silica deposited first at high temperature followed by enargite that preserves the vapor-solid diffusion of, for example, antimony, tin, and tellurium into the vapor from the crystallizing solid. Subsolidus, intra-crystalline diffusion continued as temperature declined. Pyrite and enargite are replaced by Fe-tennantite in the lodes which initially has low Sb/(Sb + As) atomic ratios around 13.5% close to the ideal tennantite formula, but evolves to higher ratios as crystallization proceeds. Fumarole encrustation clasts and sulfosalts in the lake sediment are more highly evolved with a larger range of trace element substitutions, including antimony. Substitution of especially Zn, Te, Ag, and Sn into tennantite records metal and semi-metal fraction-ation between the expanding magmatic gas and deposited sulfide sublimates provides a rare insight into the fate of metals and semi-metals in the shallower parts of fracture arrays that feed modern hyperacidic lakes. These data support a growing understanding of the formation of high-sulfidation gold deposits as the consequence of single-phase expansion of gas from magmatic-gas reservoirs beneath the surface of active volcanoes without the intervention of a later aqueous fluid including groundwater. Aggressive sulfide-sulfosalt reactions, including pitting and the almost complete dissolution of earlier minerals, are persistent characteristics of the vein assemblages and precious metals typically occur late in pits or along brittle fractures. These characteristics support a hypothesis of mineral deposition at temperatures of the order of 600 ℃ in contrast to available fluid inclusion data from enargite that record temperatures following phase transitions in the sulfosalt during the retrograde devolution of the deposit in the presence of groundwater.
机译：活火山中的高酸性湖泊和伴生的solfatara是岩浆气从源头到地表膨胀的表现。在这里，我们首次显示，由Mankayan区（菲律宾）〜2 Ma的高硫化，Lepanto铜金矿床组成的静脉系统与当代的高酸性火山湖群有关，可能是第一个被认可的湖泊在地质记录中。火山喷口地壳中重晶石与硫化物和亚硫酸盐之间的硫同位素组成差异为15-20‰，这支持以下解释：富SO_2的火山岩气体排入湖泊底部及其边缘，并将矿化与岩浆气膨胀直接相关，裂缝的扩展和矿化是通过给料机裂缝网络内的一系列减压步骤发生的。这些数据证实，火山口湖环境，例如Kawah Ijen（印度尼西亚爪哇省）提供了Lepanto和其他高硫化铜金沉积环境的现代类似物。我们还提供了对高酸性湖泊中静脉形成过程中亚硫酸盐-硫化物反应的广泛分析。首先在高温下沉积的黄铁矿±二氧化硅，然后凝结为硬石膏，以保持例如锑，锡和碲的气固扩散，使之从结晶固体进入蒸汽。随着温度下降，亚固相线的晶体内扩散继续进行。黄铁矿和堇青石在铁矿中被铁钙锰矿替代，铁矿最初具有低的Sb /（Sb + As）原子比，接近理想的钙锰矿分子式，约为13.5％，但随着结晶的进行逐渐演变为更高的比率。湖底沉积物中的富马壳结壳碎屑和亚硫酸盐随着包括锑在内的大量痕量元素替代而高度演化。特别是将Zn，Te，Ag和Sn替换为球铁矿记录了膨胀的岩浆气体和沉积的硫化物升华物之间的金属和半金属分馏，从而提供了对裂缝较浅部分中金属和半金属的命运的罕见见解供给现代高酸性湖泊的阵列。这些数据支持人们对高硫化金矿床的形成有越来越多的了解，这是由于活火山表面下岩浆气藏中气体的单相膨胀而没有后来的含水流体（包括地下水）的介入的结果。剧烈的硫化物-硫酸盐反应（包括点蚀和早期矿物的几乎完全溶解）是静脉聚集体的持久特征，而贵金属通常发生在点蚀后期或脆性断裂处。这些特征支持了一个假设，即600℃左右的温度下的矿物沉积，这与可从辉石中获得的流体包裹体数据相反，该数据记录了在存在地下水的情况下，矿床逆行转移过程中，硫磺盐发生相变之后的温度。

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