Episodic fluid charging and mixing triggers calcite-bearing agates in the Permian Emeishan Basalt, SW China

Agate, a distinctive form of banded chalcedony, derives its characteristic appearance from the deposition of silica and the presence of impurities. A significant variety of agate-bearing calcite was identified in the Permian Emeishan Basalt located in the southwestern Sichuan Basin. This calcite-bearing agate provides valuable insights into the complex interplay and evolution of fluids and minerals. This study aims to provide a detailed description and analysis of the microscopic textural and geochemical characteristics of the agate, while elucidating the sequence of mineral deposition, fluid properties, and the genetic mechanism underlying its formation. The findings reveal that the agate primarily consists of lamellar calcite and banded chalcedony, exhibiting a variety of textures, thicknesses, and colours. Through the analysis of the transformation and precipitation of epidote and allanite, and the identification of four types of calcites and five micro-silica textures, it has been confirmed that the early stage of fluid in geodes involved the mixing of meteoric water with post-magmatic hydrothermal fluid. The middle and late stages involve the mixing of magmatic-hydrothermal fluid with organic-acid fluid. The charging of silica-rich hydrothermal fluids and the generation of hydrocarbons from organic matter cause fluctuations in SiO₂ concentration, pH, and crystallization rates, contributing to the diversity of silica minerals, such as chalcedony and quartz. The interlayered growth observed between chalcedony and calcite is attributed to secondary metasomatism, which occurs after the dehydration and transformation of chalcedony due to increasing temperatures. Notably, the internal texture of the agate is somewhat influenced by pressure variations caused by the episodic charging of the fluid.

玛瑙（Agate）是一类特殊的条带状玉髓（banded chalcedony），其标志性外观源于二氧化硅沉积与杂质的赋存。在四川盆地西南部的二叠纪峨眉山玄武岩（Permian Emeishan Basalt）中，已识别出一类重要的含玛瑙方解石变种。这类含方解石的玛瑙为解析流体与矿物间的复杂相互作用及演化过程提供了宝贵视角。本研究旨在对该玛瑙的微观结构（microscopic textural）与地球化学特征（geochemical characteristics）开展详细描述与分析，同时阐明矿物沉积序列、流体性质及其形成的成因机制（genetic mechanism）。研究结果表明，该玛瑙主要由层状方解石（lamellar calcite）与条带状玉髓组成，展现出多样的结构形态、厚度与色彩。通过对绿帘石（epidote）与褐帘石（allanite）的转化及沉淀过程进行分析，并识别出四类方解石与五种微二氧化硅结构，研究证实晶洞流体的早期阶段涉及大气降水（meteoric water）与岩浆期后热液流体（post-magmatic hydrothermal fluid）的混合。中晚期阶段则涉及岩浆热液流体（magmatic-hydrothermal fluid）与有机酸流体（organic-acid fluid）的混合。富二氧化硅热液流体的充注以及有机质生成烃类（hydrocarbons）的过程，会导致二氧化硅浓度、pH值与结晶速率发生波动，进而促成玉髓、石英等二氧化硅矿物的多样性。玉髓与方解石间的互层生长现象，可归因于次生交代作用（secondary metasomatism），该作用发生于温度升高引发玉髓脱水转变之后。值得注意的是，玛瑙的内部结构在一定程度上受到流体周期性充注所导致的压力变化的影响。