This study unravels the spectral footprint of the Paraná deposit, northeastern Brazil, and provides strategies for emerald exploration in mafic- and/or ultramafic-hosted deposits using point spectral data and hyperspectral imaging. Potential host rocks (phlogopite and actinolite-phlogopite schists) were discriminated from other associated lithotypes after petrography and whole-rock geochemistry, using binary MgO versus Al₂O₃ and PC1 versus PC3 diagrams. Spectrally, phlogopite schist is marked by Fe-OH absorption at ~2,250 nm and Mg-OH absorptions at ~2,330 and ~2,388 nm. When actinolite is present, a shoulder at ~2,296 nm is recorded, and the first Mg-OH absorption feature is shifted to ~2,315 nm. Emerald crystals have their spectral signature attenuated in the presence of phlogopite. In a 5% emerald and 95% schist mixture simulation, two emerald absorption features are recognizable at ~1,150 (H₂O absorption) and ~2,155 nm. The emerald indices EI1, EI2, and EI3 were chosen for the automated identification of emeralds in the Paraná samples based on the features at ~1,150, ~1,460, and ~2,155 nm. For the host schists, three indices were provided (MI_depth, MI_ratio, and ACI) considering the Mg-OH features of phlogopite and actinolite, allowing automated distinction between potential mineralized and other associated rocks of the Paraná deposit. Our results show that, operating in high-resolution mode (pixels of ~1 mm), imaging spectroscopy can detect the presence and characterize emerald in handpicked samples and drill cores based on chromium- and iron-derived absorption features in the visible to near infrared range. It thus represents a swift, reliable, nondestructive, and relatively cost-effective method for exploring for emerald and, potentially, other gems.

中文翻译：

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使用高分辨率光谱方法勘探构造岩浆相关祖母绿矿床的进展：揭开巴拉那矿床（巴西东北部）的光谱足迹

这项研究揭示了巴西东北部巴拉那矿床的光谱足迹，并为使用点光谱数据和高光谱成像在镁铁质和/或超镁铁质矿床中勘探祖母绿提供了策略。_{在岩相学和全岩地球化学之后，使用二元 MgO 与 Al 2} O ₃以及 PC1 与 PC3 图将潜在的主岩（金云母和阳起石-金云母片岩）与其他相关岩型区分开来。从光谱上看，金云母片岩的特征是 Fe-OH 吸收在 ~2,250 nm 处，Mg-OH 吸收在 ~2,330 和 ~2,388 nm 处。当阳起石存在时，记录到 ~2,296 nm 处的肩部，并且第一个 Mg-OH 吸收特征移动到 ~2,315 nm。当金云母存在时，祖母绿晶体的光谱特征会减弱。在 5% 祖母绿和 95% 片岩混合物模拟中，可以在 ~1,150（H ₂ O 吸收）和 ~2,155 nm 处识别出两个祖母绿吸收特征。根据 ~1,150、~1,460 和 ~2,155 nm 的特征，选择祖母绿指数 EI1、EI2 和 EI3 用于自动识别巴拉那样品中的祖母绿。对于宿主片岩，考虑到金云母和阳起石的 Mg-OH 特征，提供了三个指数（MI_深度、MI_比率和 ACI），从而可以自动区分巴拉那矿床的潜在矿化岩和其他伴生岩。我们的结果表明，在高分辨率模式（约 1 毫米的像素）下运行，成像光谱仪可以根据铬和铁在可见光到近红外范围内的吸收特征来检测精心挑选的样品和钻芯中祖母绿的存在并对其进行表征范围。因此，它代表了一种快速、可靠、无损且相对经济高效的祖母绿和其他宝石勘探方法。