Abstract This study examines the whole-rock geochemistry, Fe-oxide texture and crystal morphology, and Fe-oxide (U-Th)/He date variation of jasperoids, as well as the relationships between these characteristics, in order to evaluate jasperoids as an indicator for gold mineralization. Jasperoid samples with a range of textures, appearances, and expected gold concentrations were collected from two gold districts in Nevada: the northern Carlin trend (Gold Quarry Mine), and the Battle Mountain district (Marigold Mine; Battle Mountain prospect). In our jasperoid samples, whole-rock concentrations of Au are positively correlated with As, Ag, Pb, Sb, U, W, and Tl. The following Fe-oxide textures are associated with anomalous gold concentrations: disseminated, submicron crystals; feathery and acicular crystal morphologies; concentric zonation from hematite to goethite; and botryoidal textures. Calculated closure temperatures for the (U-Th)/He system in observed jasperoid Fe-oxide morphologies within our dataset range from 26 to 136 °C, indicating variable susceptibility to He diffusion at near-surface conditions. Fe-oxide (U-Th)/He dates range from 29.4 ± 0.5 Ma to 0.11 ± 0.01 Ma, displaying intra- and intersample variation. We attribute the date variation to multiple factors including He implantation, parent isotope exchange, and He loss through closure temperature sensitivity and alpha-ejection. The intra- and intersample variation within the Fe-oxide (U-Th)/He dates limits our ability to make geologic interpretations. Our case study identifies important limitations of the (U-Th)/He method in jasperoids and provides insight on how He date variation might be avoided in future studies.

中文翻译：

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内华达州两个金矿区碧玉中的铁氧化物：表征、地球化学和 (U-Th)/He 定年

摘要 本研究考察了碧玉的全岩地球化学、氧化铁结构和晶体形态、氧化铁 (U-Th)/He 日期变化以及这些特征之间的关系，以评估碧玉作为金矿化指标。从内华达州的两个金矿区收集了具有一系列质地、外观和预期金浓度的 Jasperoid 样品：北卡林矿区（金采石场）和巴特尔山区（万寿菊矿；巴特尔山前景）。在我们的碧玉样品中，Au 的全岩浓度与 As、Ag、Pb、Sb、U、W 和 Tl 呈正相关。以下氧化铁质地与异常的金浓度有关：弥散的亚微米晶体；羽毛状和针状晶体形态；从赤铁矿到针铁矿的同心环带；和葡萄状纹理。在我们的数据集中观察到的碧玉铁氧化物形态中计算的 (U-Th)/He 系统的闭合温度范围为 26 至 136 °C，表明在近地表条件下对 He 扩散的敏感性不同。氧化铁 (U-Th)/He 日期范围从 29.4 ± 0.5 Ma 到 0.11 ± 0.01 Ma，显示样本内和样本间的变化。我们将日期变化归因于多种因素，包括 He 植入、母体同位素交换以及 He 因闭合温度敏感性和 α 喷射而损失。铁氧化物 (U-Th)/He 日期内的样品内和样品间变化限制了我们进行地质解释的能力。