Uranium-Pb dating of unusual coarse-grained apatite crystals from hydrothermal dolostone breccia in the barite-rich Magcobar zone at the Silvermines deposit, Ireland, indicates an age of 331 ± 5.6 Ma for hydrothermal alteration. This age is in agreement with an Re-Os age on pyrite-sphalerite but differs from previous estimates that were based on palemomagnetism and sphalerite Rb-Sr geochronology at Silvermines. The new U-Pb age indicates the deposit largely formed epigenetically rather than as a synsedimentary deposit. The trace element composition of the apatite provides additional constraints on the geochemical evolution of the Silvermines hydrothermal system indicating that it was initiated with the expulsion of oxidizing fluids responsible for early hematite-silica precipitation. This was followed by the formation of hydrothermal dolostone breccias containing barite and the apatite dated here. The infiltration of metal-bearing hydrothermal fluids, likely derived from the basement, overlapped and followed the development of hydrothermal dolostone breccias, interacting with Carboniferous seawater during dolomitization and barite precipitation. Near-sea-floor biogenic activity, possibly enhanced due to escape of hydrothermal fluids, resulted in the reduction of seawater sulfate and formation of the sulfide-bearing fluids that facilitated the development of the Silvermines Zn-Pb deposit.

中文翻译：

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爱尔兰银矿矿床磷灰石 U-Pb 定年：热液成因模型

对爱尔兰银矿矿床富含重晶石的马格科巴带热液白云岩角砾岩中异常粗粒磷灰石晶体进行的铀-铅定年表明，热液蚀变的年龄为 331 ± 5.6 Ma。该年龄与黄铁矿-闪锌矿的铼-锇年龄一致，但与之前基于银矿古地磁学和闪锌矿铷-锶地质年代学的估计不同。新的 U-Pb 年龄表明该矿床主要是后生形成的，而不是同沉积矿床。磷灰石的微量元素组成对银矿热液系统的地球化学演化提供了额外的限制，表明它是随着氧化流体的排出而开始的，而氧化流体负责早期的赤铁矿-二氧化硅沉淀。随后形成了含有重晶石和磷灰石的热液白云岩角砾岩。可能来自基底的含金属热液的渗透，重叠并跟随热液白云岩角砾岩的发育，在白云石化和重晶石沉淀过程中与石炭纪海水相互作用。近海底的生物活动可能由于热液的逃逸而增强，导致海水硫酸盐减少并形成含硫化物流体，从而促进了银矿锌铅矿床的开发。在白云石化和重晶石沉淀过程中与石炭系海水相互作用。近海底的生物活动可能由于热液的逃逸而增强，导致海水硫酸盐减少并形成含硫化物流体，从而促进了银矿锌铅矿床的开发。在白云石化和重晶石沉淀过程中与石炭系海水相互作用。近海底的生物活动可能由于热液的逃逸而增强，导致海水硫酸盐减少并形成含硫化物流体，从而促进了银矿锌铅矿床的开发。