Abstract Carbonation of ultramafic mine tailings has the potential to offset greenhouse gas emissions from mining by trapping CO2 within the crystal structures of Mg-carbonate minerals and hydrotalcite supergroup minerals, which form as weathering products in tailings storage facilities. Here, we present a detailed geochemical and mineralogical assessment of tailings from the Woodsreef Chrysotile Mine, New South Wales, Australia, demonstrating that coupling mineralogical and elemental datasets improves the accuracy of carbon accounting in mined landscapes. Detailed analysis of tailings mineralogy using quantitative X-ray diffraction (XRD) and total carbon analyses reveals that previous assessments of passive mineral carbonation at Woodsreef have been underestimated. Maximum values for the abundance of total carbon (up to 0.4 wt. %), as well as the abundances of secondary carbonate minerals (i.e., up to 1.9 wt. % hydromagnesite and up to 2.6 wt. % pyroaurite, measured with XRD) are observed between approximately 2 cm and 30 cm depth in profiles collected within experimental plots. However, an amorphous Mg-carbonate phase, that cannot be detected using XRD, is also present at comparably high abundances to depths of at least 1 m. This phase is readily observed using scanning electron microscopy, it contributes a measured carbon content of approximately 0.2 wt. % at up to 1 m depth, and it has a predominantly atmospheric carbon isotopic signature (F14C >0.80). We find that using only XRD data results in the sequestered CO2 being underestimated by nearly four times compared to estimates incorporating total carbon measurements, highlighting the important role of amorphous Mg-carbonates in the carbon cycles of mines. Combining XRD and total carbon data, we provide an estimate for passive carbon sequestration by both crystalline and amorphous carbonates in the Woodsreef tailings (11.7 kg CO2/m2, considering the upper 1 m3) and suggest that future studies should employ both XRD and total carbon measurements for carbon accounting. The Woodsreef Chrysotile Mine was also the test site for a field-based geochemical treatment system designed to promote mineral carbonation. A solar powered, independently operating geochemical treatment system is designed and deployed to deliver controlled acid (0.08 M H2SO4) or water leaching treatments, and maintain soil pore saturation within optimal levels (approximately 18 – 36%) to enhance the weathering rate of mine tailings. While the applied treatment did not accelerate capture and mineralization of CO2 from air, it could be coupled to technologies that enhance the supply of CO2 for mineral carbonation. We apply our new strategy for carbon accounting to this experimental site in order to assess changes in mineralogy and the spatial scale on which carbon accounting must be done to accurately measure carbon sequestered during weathering of ultramafic rock. Our work provides important lessons and context for future trials of accelerated tailings dissolution and mineral carbonation, which will benefit the next stage of development in the scale-up of this technology.

中文翻译：

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澳大利亚新南威尔士州 Woodsreef 温石棉矿开采景观的碳核算以及地球化学处理系统的部署以增强风化

摘要 超镁铁矿尾矿的碳化有可能通过将二氧化碳捕获在碳酸镁矿物和水滑石超群矿物的晶体结构中来抵消采矿产生的温室气体排放，这些矿物在尾矿储存设施中作为风化产物形成。在这里，我们展示了澳大利亚新南威尔士州 Woodsreef 温石棉矿尾矿的详细地球化学和矿物学评估，证明结合矿物学和元素数据集提高了开采景观中碳核算的准确性。使用定量 X 射线衍射 (XRD) 和总碳分析对尾矿矿物学进行详细分析表明，先前对 Woodsreef 被动矿物碳化的评估被低估了。总碳丰度的最大值（高达 0.4 wt. %），在实验地块内收集的剖面中，在大约 2 厘米和 30 厘米深度之间观察到次生碳酸盐矿物的丰度（即，高达 1.9 重量％的水菱镁矿和高达 2.6 重量％的焦金铁矿，用 XRD 测量）。然而，使用 XRD 无法检测到的无定形碳酸镁相也以相对较高的丰度存在于至少 1 m 的深度。使用扫描电子显微镜很容易观察到这一相，它贡献了大约 0.2 重量％的碳含量。% 在最深 1 m 处，并且它具有主要的大气碳同位素特征（F14C > 0.80）。我们发现，与结合总碳测量的估计值相比，仅使用 XRD 数据导致封存的 CO2 被低估了近四倍，强调无定形碳酸镁在矿山碳循环中的重要作用。结合 XRD 和总碳数据，我们提供了 Woodsreef 尾矿中结晶和无定形碳酸盐被动碳封存的估计值（11.7 kg CO2/m2，考虑到上部 1 m3），并建议未来的研究应同时使用 XRD 和总碳碳核算的测量。Woodsreef 温石棉矿也是旨在促进矿物碳化的现场地球化学处理系统的试验场。太阳能供电、独立运行的地球化学处理系统被设计和部署用于提供受控酸 (0.08 M H2SO4) 或水浸出处理，并将土壤孔隙饱和度保持在最佳水平（约 18 – 36%），以提高尾矿的风化率. 虽然所应用的处理并没有加速从空气中捕获和矿化二氧化碳，但它可以与提高矿物碳酸化二氧化碳供应的技术相结合。我们将我们的碳核算新策略应用于该实验场地，以评估矿物学的变化以及必须进行碳核算以准确测量超基性岩风化过程中封存的碳的空间尺度。我们的工作为未来加速尾矿溶解和矿物碳化的试验提供了重要的经验教训和背景，这将有利于该技术扩大规模的下一阶段的发展。我们将我们的新碳核算策略应用于该实验场地，以评估矿物学的变化以及必须进行碳核算以准确测量超基性岩风化过程中封存的碳的空间尺度。我们的工作为未来加速尾矿溶解和矿物碳化的试验提供了重要的经验教训和背景，这将有利于该技术扩大规模的下一阶段的发展。我们将我们的新碳核算策略应用于该实验场地，以评估矿物学的变化以及必须进行碳核算以准确测量超基性岩风化过程中封存的碳的空间尺度。我们的工作为未来加速尾矿溶解和矿物碳化的试验提供了重要的经验教训和背景，这将有利于该技术扩大规模的下一阶段的发展。