Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona
Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona
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亚利桑那州 Rich Hill 浅近系和表近系过程中铅同位素对天然金形成的见解
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发表时间：
2013
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通讯作者：
E. Dewitt
中科院分区：
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作者：
G. Kamenov;E. Melchiorre;Faye Ricker;E. Dewitt
砂矿中的金块的成因是个谜，因为它们可以从原生脉存款中机械分离出来，也可以是在砂矿环境中形成的次生成因。铅同位素可以是一个有价值的示踪剂之间的静脉和砂金的同位素比值匹配将表明，砂金块只是机械分离的热液脉和未修改的风化。
勘查元素分析表明，静脉和砂金丰富的山区，亚利桑那州，含有高达数千ppm的铅，因此都适合铅同位素分析。我们测量了砂金块的铅同位素，并将结果与富山地区原生脉金和硫化物以及脉石矿物和寄主岩石进行了比较。原生脉金和方铅矿的Pb同位素组成相似，206 Pb/204 Pb比值为18.364 ~ 18.486，207 Pb/204 Pb比值为15.574 ~ 15.593，208 Pb/204 Pb比值为37.150 ~ 37.359。这表明，在地质历史的深成热液事件中，矿石金属来自类似的来源。与原生脉状矿化相比，该地区岩石的现今铅同位素组成显示出不同的值。由于可能的蚀变过程和矿化事件发生以来的长时间，当地岩浆岩的Pb同位素组成的校正和模拟没有提供一个清晰的图像，可能的热液矿石金属来源。然而，在当地的岩性单位不同的现今铅同位素，提供了一个很好的示踪剂，用于破译砂金的起源。
所有的砂金块体显示出非常不同的铅同位素组成时，其各自的热液脉源的脉金和方铅矿为代表。砂金Pb同位素变化范围很大，206 Pb/204 Pb为17.985 ~ 20.057，207 Pb/204 Pb为15.494 ~ 15.749，208 Pb/204 Pb为36.863 ~ 38.606。砂矿块金遵循现今在该地区岩石和沉积物中观察到的铅同位素趋势。此外，大多数砂金块金的铅同位素远比脉金、硫化物甚至脉石矿物的放射性成因更强。这表明，相当大的一部分，如果不是全部，铅在砂金是最近收购的砂金沉积物。原生和砂矿样品中Pb同位素组成的差异表明金在表生阶段发生了明显的化学变化。考虑到一些可能的情况下，目前所观察到的砂金和当地岩石之间的铅同位素相似性的最合理的解释是形成的一个显着的一部分，这些金块的总质量的砂金环境。
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 206Pb/204Pb ranging from 18.364 to 18.486, 207Pb/204Pb from 15.574 to 15.593, and 208Pb/204Pb 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 lithological units, however, provide an excellent tracer for deciphering the origin of the placer gold.
All the placer gold nuggets show very distinct Pb isotope compositions when compared to their respective hydrothermal vein source represented by the vein gold and galena. The placer gold Pb isotopes show very wide variations, with 206Pb/204Pb ranging from 17.985 to 20.057, 207Pb/204Pb from 15.494 to 15.749, and 208Pb/204Pb from 36.863 to 38.606. The placer nuggets follow the present-day Pb isotope trend observed in rocks and sediments in the area. In addition, the Pb isotopes in majority of the placer gold nuggets are far more radiogenic compared to the vein gold, sulfides, and even gangue minerals. This is indicating that a significant part, if not all, of the Pb in the placer gold was relatively recently acquired from the placer sediments. The observed distinct Pb isotope compositions between primary and placer samples suggest that gold undergoes significant chemical changes during the supergene stage. Considering a number of possible scenarios, the most plausible explanation for the observed present-day Pb isotope similarity between placer gold and local rocks is formation of a significant part of the total mass of these nuggets within the placer environment.