The Obiro deposit is located in the Tagawa Acidic Rocks (AR), Uetsu region, NE Japan. The Tagawa AR is composed of a volcanic phase of dacitic welded tuff and a plutonic phase of porphyritic granodiorite. Drill core and ore samples were collected from the deposit and examined by XRD, EPMA, and microthermometry. The drill core samples have suffered pervasively from sericite (illite) alteration, whereas pinkish K‐feldspar alteration halo occur close to veins. The results of EPMA and microthermometry is interpreted as that the magnatic‐hydrothermal fluids has changed as follows; the granodioritic magma intruded at about 1.0 kb and 700°C near the water‐saturated granite solidus; after cooling to about 500°C the fluids boiled according to a change in the pressure regime from lithostatic to hydrostatic; mixing with meteoric water led to sulfide mineralization at around 400°C or less. The main reasons for the mineralization in the Obiro deposit are as follows; the oxidized magma intruded at a shallower level, and thereafter hydrothermal fluids were boiled, resulting in a saline fluid. The saline fluid then dissolved metals such as Pb, Zn, Cu, and Bi, and these metals precipitated during cooling accompanied by dilution of the meteoric water and increasing pH, resulting in decreasing solubility.

中文翻译：

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日本东北部上越地区田川酸性岩石中的小圆矿床的热液活动

Obiro矿床位于日本东北部上越地区的Tagawa酸性岩石（AR）。Tagawa AR由火山岩凝灰岩的火山相和斑状花岗闪长岩的古生相组成。从矿床中收集钻芯和矿石样品，并通过XRD，EPMA和微量热分析法进行检查。钻芯样品普遍遭受绢云母（伊利石）蚀变的影响，而粉红色的钾长石蚀变晕出现在矿脉附近。EPMA和微量热法的结果被解释为：镁热流体发生了如下变化。花岗二岩浆岩在水饱和花岗岩固相线附近侵入约1.0 kb和700°C; 冷却至约500°C后，流体根据从静压到静压的压力变化而沸腾；与大气水混合会导致在约400°C或更低的温度下发生硫化物矿化。Obiro矿床成矿的主要原因如下：氧化岩浆进入较浅的水平面，然后使热液沸腾，形成盐水。盐水然后溶解了金属，例如Pb，Zn，Cu和Bi，并且这些金属在冷却过程中沉淀，伴随着流水的稀释和pH值的增加，导致溶解度降低。