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首页> 外文期刊>Economic geology and the bulletin of the Society of Economic Geologists >Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona
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Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona

机译：铅同位素在亚利桑那州里奇希尔（Rich Hill）的低基因和超基因过程中对天然金形成的见解

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The origin of gold nuggets in placers is enigmatic as they can be mechanically separated from the primary vein deposit or can be of secondary origin, formed in the placer environment. Pb isotopes can be a valuable tracer as a match in the isotopic ratios between vein and placer gold will indicate that the placer nuggets are simply mechanically separated from the hydrothermal vein and unmodified by weathering. Reconnaissance elemental analyses revealed that vein and placer gold from the Rich Hill district, Arizona, contain up to thousands of ppm Pb and therefore both are suitable for Pb isotope analyses. We measured Pb isotopes in placer gold nuggets and compare the results to primary vein gold and sulfides and also gangue minerals and host rocks in the Rich Hill area. Primary vein gold and galena show similar Pb isotope ratios with ~(206)Pb/~(204)Pb ranging from 18.364 to 18.486, ~(207)Pb/~(204)Pb from 15.574 to 15.593, and ~(208)Pb/~(204)Pb from 37.150 to 37.359. This indicates that the ore metals were derived from a similar source during a hypogene hydrothermal event in the geologic past. Present-day Pb isotope compositions of local rocks in the area show distinct values compared to the primary vein mineralization. Age-corrected and modeled Pb isotope compositions for local magmatic rocks do not provide a clear picture for a possible hydrothermal ore metal source due to possible alteration processes and the long time elapsed since the mineralization event. The distinct present-day Pb isotopes in local lithologi-cal units, however, provide an excellent tracer for deciphering the origin of the placer gold.
机译：砂金在砂矿中的来源是神秘的，因为它们可以与原矿沉积物机械分离，也可以是在砂矿环境中形成的次生金矿。铅同位素可能是有价值的示踪剂，因为静脉和砂金的同位素比值的匹配将表明砂矿块只是从热液脉中机械分离而不受风化作用的改变。侦查元素分析显示，亚利桑那州里奇希尔地区的矿脉和砂金含有高达数千ppm的Pb，因此两者均适用于Pb同位素分析。我们测量了砂金矿块中的Pb同位素，并将结果与​​Rich Hill地区的原生脉金和硫化物以及脉石矿物和宿主岩进行了比较。主脉金和方铅矿显示相似的Pb同位素比，〜（206）Pb /〜（204）Pb的范围为18.364至18.486，〜（207）Pb /〜（204）Pb的范围为15.574至15.593，〜（208）Pb /〜（204）Pb从37.150至37.359。这表明在地质过去的一次次生热液事件中，矿石金属是从类似的来源中提取的。与初级矿脉成矿相比，该地区当地岩石的当今铅同位素组成显示出不同的价值。由于可能的蚀变过程和自矿化事件以来经过的时间长，因此对年龄局部校正的岩浆岩石建模的铅同位素模型并不能为可能的热液矿石金属来源提供清晰的图像。然而，当地石化单位中独特的当今Pb同位素为解密砂金的来源提供了极好的示踪剂。

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