Critical role of caldera collapse in the formation of seafloor mineralization: The case of Brothers volcano
Critical role of caldera collapse in the formation of seafloor mineralization: The case of Brothers volcano
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DOI：
10.1130/g46047.1
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发表时间：
2019-08-01
期刊：
影响因子：
5.8
通讯作者：
Reyes, Agnes G.
中科院分区：
文献类型：
--
作者：
de Ronde, Cornel E. J.;Humphris, Susan E.;Reyes, Agnes G.
海底弧火山所承载的热液系统通常包含大量的岩浆流体。这些系统中的高Cu-Au含量和强酸性流体与现今陆地上开采的某些斑岩型铜和浅成低温热液型金矿床的浅部流体相似。沿着Kermadec弧的海底部分（新西兰近海）有两种主要类型的热液系统：岩浆影响系统和海水主导系统。兄弟火山拥有这两种类型。在这里，我们报告的结果，从一系列钻孔的核心国际海洋发现计划到这两种类型的热液系统。我们发现，热液蚀变的主机英安质火山碎屑岩和熔岩的程度反映了原生岩性孔隙度和对比的空间和时间的贡献，岩浆流体，热液流体和海水。我们提出了一个两步模型，链接到火山破火山口的演化热液流体制度的变化。在破火山口形成之前，最初的热液活动主要由岩浆气体和高盐卤水组成。前者在向海底上升时与海水混合，后者则被隔离在地下。破火山口塌陷后，海水通过断层控制的渗透性渗入火山，与围岩和分离的盐水相互作用，将伴生金属输送到海底，并在破火山口壁和边缘形成富含铜、锌、金的烟囱，这一过程一直持续到今天。这两个步骤的过程可能是常见的海底弧火山口火山的火山成因块状硫化物矿床，它是特别有效的集中矿化在或附近的海底。
Hydrothermal systems hosted by submarine arc volcanoes commonly include a large component of magmatic fluid. The high Cu-Au contents and strongly acidic fluids in these systems are similar to those that formed in the shallow parts of some porphyry copper and epithermal gold deposits mined today on land. Two main types of hydrothermal systems occur along the submarine portion of the Kermadec arc (offshore New Zealand): magmatically influenced and seawater-dominated systems. Brothers volcano hosts both types. Here, we report results from a series of drill holes cored by the International Ocean Discovery Program into these two types of hydrothermal systems. We show that the extent of hydrothermal alteration of the host dacitic volcaniclastics and lavas reflects primary lithological porosity and contrasting spatial and temporal contributions of magmatic fluid, hydrothermal fluid, and seawater. We present a two-step model that links the changes in hydrothermal fluid regime to the evolution of the volcano caldera. Initial hydrothermal activity, prior to caldera formation, was dominated by magmatic gases and hypersaline brines. The former mixed with seawater as they ascended toward the seafloor, and the latter remained sequestered in the subsurface. Following caldera collapse, seawater infiltrated the volcano through fault-controlled permeability, interacted with wall rock and the segregated brines, and transported associated metals toward the seafloor and formed Cu-Zn-Au-rich chimneys on the caldera walls and rim, a process continuing to the present day. This two-step process may be common in submarine arc caldera volcanoes that host volcanogenic massive sulfide deposits, and it is particularly efficient at focusing mineralization at, or near, the seafloor.