Abstract

Agate mineralization in Central Karelia, Northwest Russia is related to the Paleoproterozoic volcanic rocks of the Ludicovian Superhorizon (2.05–1.95 Ga) within the Onega structure. Agates and parental volcanic rocks were studied with optical and electron microscopy, electron microprobe, X-ray diffraction, Raman spectroscopy, and ICP-MS. It is shown that fine-grained quartz, and fibrous and fine-flake chalcedony are the major minerals in the agate structure. Inclusions of coarse-crystalline calcite, and microinclusions of chlorite, iron oxides and hydroxides, hydroxylapatite, epidote, mica (phengite), apatite, pyrite, chalcopyrite, titanite, and leucoxene are present in agates. The presence of contrast rhythms within agate amygdules is marked by compositional variation in impurity mineral phases and different microtextures of silica layers represented by different-grained aggregates of quartz, fine-flake and fibrous chalcedony, and quartzine. This indicates stage-by-stage mineral crystallization at variable temperatures and pressures during agate formation, which may also reflect the heterogeneity of the initial hydrothermal fluid. High concentrations of Ti, Cr, Mn, Ni, and Cu (10–120 ppm) and the low concentrations of Li, Co, Ga, Zn, Sr, Zr, Mo, and Sn (0.5–10 ppm) are characteristic of quartz–chalcedony agates. Calcite in agates is characterized by high concentrations of Mn (1253–6675 ppm), Sс, Ti, Ni, Sr, Y, La, Ce, and Nd (5–56 ppm). The chondrite-normalized REE distribution pattern in agates shows a decay profile from La to Lu and a negative Eu anomaly in some samples. Low contents of rare metals and REE in agates compared to parental volcanic rocks indicate a gradual chemical depletion of circulating fluids during agate formation.

中文翻译：

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来自卡累利阿中部Onega结构的古元古代火山岩

摘要

俄罗斯西北卡累利阿中部的玛瑙矿化与Onega结构内Ludicovian Superhorizo​​n（2.05-1.95 Ga）的古元古代火山岩有关。用光学和电子显微镜，电子显微探针，X射线衍射，拉曼光谱和ICP-MS研究了火山灰和母体火山岩。结果表明，细粒石英，纤维状和片状玉髓是玛瑙结构中的主要矿物。玛瑙中存在粗晶方解石和亚氯酸盐，氧化铁和氢氧化物，羟基磷灰石，山竹，云母（斑云母），磷灰石，黄铁矿，黄铜矿，钛铁矿和白二烯的微量夹杂物。玛瑙杏仁内存在对比节律，其特征是杂质矿物相的组成变化以及以不同粒度的石英，细鳞片和纤维状玉髓，石英质的聚集体为代表的二氧化硅层的不同微观结构。这表明在玛瑙形成过程中，在可变的温度和压力下，逐步进行矿物结晶，这也可能反映出初始热液流体的非均质性。高浓度的Ti，Cr，Mn，Ni和Cu（10–120 ppm）和低浓度的Li，Co，Ga，Zn，Sr，Zr，Mo和Sn（0.5–10 ppm）是石英的特征ch玛瑙。玛瑙中的方解石的特征是高浓度的Mn（1253-6675 ppm），Sс，Ti，Ni，Sr，Y，La，Ce和Nd（5-56 ppm）。玛瑙中的球粒陨石归一化REE分布图显示了从La到Lu的衰减曲线，并且在一些样品中具有负Eu异常。与亲本火山岩相比，玛瑙中的稀有金属和稀土元素含量低，表明玛瑙形成过程中循环流体的化学消耗逐渐减小。