Magmatic evolution and metal systematics of back-arc volcanic rocks of north-east Japan and implications for deposition of massive sulphide Kuroko ores
Magmatic evolution and metal systematics of back-arc volcanic rocks of north-east Japan and implications for deposition of massive sulphide Kuroko ores
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日本东北部弧后火山岩的岩浆演化、金属系统学及其对块状硫化物黑子矿沉积的意义
DOI：
10.1007/s00410-022-01980-z
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发表时间：
2022
影响因子：
3.5
通讯作者：
M.A.
中科院分区：
文献类型：
--
作者：
Agangi;A.;Manalo;P.C.;Takahashi;R.;Veeravinantanakul;A.;Elburg;M.A.
日本东北部的Hokuroku地区拥有重要的火山岩型块状硫化物Zn-Pb-Cu矿床，被认为是世界上Kuroko（黑色矿石）矿床的原型。双峰，长英质为主的火山序列，主机的矿石沉积在大陆裂谷形成大陆伸展在中新世弧后开口的最后阶段，导致日本海的形成。在这项研究中，我们定义了一些基本的火山作用的强度参数（温度，压力的结晶，流体饱和度，fO 2）的岩石质地的基础上，全岩样品，矿物和熔融包裹体的分析。基于熔融包裹体分析，我们评估了岩浆演化和脱气过程中的金属行为，并评估了可能的矿床意义。从黑矿下伏和上覆两个单元采集的长英质凝灰岩和熔岩样品中的斜长石熔体地质温度测量表明温度为880-940 °C，Fe-Ti氧化物平衡表明氧逸度约为100 ° C。FMQ + 1.5。熔体包裹体具有高SiO2流纹岩成分（> 75重量%，在无水的基础上），和规范的矿物成分在花岗岩三图的曲线图表明低压力的岩浆失速和结晶（< 1千巴）在CO2组成。熔体夹杂物金属含量绘制与不相容元素Y表明不同的金属在分馏和脱气过程中的对比行为。锌主要保留在熔体结晶过程中，而其他金属，如铅，铜，锡和钼，被释放到一个exsolving流体相。因此，后者可能是从脱气岩浆转移到热液系统。相对较热的岩浆储存在较浅的地方，会在海底引起强烈的热液循环，这是矿床沉积的先决条件。
The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880–940 °C, and Fe–Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO2rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition.