SCREED: Supergene enrichment of carbonatite REE deposits

稀土元素(REE)，特别是钕和镝，对于风力涡轮机等可再生能源设备和运输用电机的开发至关重要。目前，稀土元素既来自低浓度风化花岗岩(离子吸附粘土)矿床，也来自高浓度碳酸盐岩矿床，特别是中国白云鄂博硬岩蚀变碳酸盐岩中世界上最主要的稀土矿床。中国以外的唯一一家主要矿商是澳大利亚的韦尔德山风化碳酸盐岩。像韦尔德山这样的风化碳酸盐岩是世界上最丰富的稀土矿床之一，有几个正在进行积极的勘探。作为NERC全球伙伴关系种子基金项目WREED的一部分，我们对碳酸盐岩赋存的稀土矿床的风化产物进行了初步调查。已鉴定出三种端元风化产物(1)碳酸盐矿物溶解留下原生稀土矿物，形成残留矿床；(2)稀土磷酸盐和氟碳酸盐矿物的溶解和再沉淀导致新的水合稀土磷酸盐或碳酸盐矿物产生表生富集；以及(3)粘土和铁锰氧化物盖层的形成(无论是矿床本身的风化，还是来自围岩的土壤迁移)，它们可能容纳吸附在矿物表面的稀土元素(c.f。离子吸附沉积)。高品位的风化碳酸盐岩矿床通常由一系列土壤和风化层组成，由于风化过程中磷灰石和独居石的溶解和重新沉淀，可能是富磷的(澳大利亚韦尔德山)，被后来的沉积物覆盖，后者可能因湖泊沉积物中残留矿物的积累而富含稀土元素(托托尔，俄罗斯)。矿带的矿物学与未风化的碳酸盐岩有关，但不同于未风化的碳酸盐岩，包括磷酸盐、刚玉岩族矿物、碳酸盐、氟碳酸盐和氧化物。在本研究中，我们将利用块状岩石地球化学、顺序浸出技术、矿物化学和微生物学技术，研究碳酸盐岩中不同风化稀土矿床类型的形成过程及其对稀土经济品位和环境影响的影响。主体岩石地球化学将被用来量化元素相对于基岩的富集度和贫化率，并研究风化碳酸盐岩中离子吸附方式矿化的可能性。矿物化学技术将被用来研究风化的时间，稀土元素的宿主矿物，相对于原生矿物的潜在的有益或有害的化学变化，以及环境控制风化方式的指标。这些数据将与现有的地表形态和风化深度记录相结合，产生将气候、地貌和地球化学联系起来的总体成因模型，从而能够预测碳酸盐风化带的资源潜力。结果将与行业和公众沟通，以提高人们对脱碳所需资源的认识，以及提高开采效率和减少影响的潜在途径。

The rare earth elements (REE), and in particular neodymium and dysprosium, are essential for renewable energy devices such as wind turbines and the development of electric motors for transport. At present the REE are sourced from either low concentration weathered granitoid (ion adsorption clay) deposits, or from high concentration carbonatite-related deposits, especially the World's dominant REE mine in hard-rock, altered carbonatite at Bayan Obo, China. The one major mine operator outside of China is the Mount Weld weathered carbonatite, Australia. Weathered carbonatites such as Mount Weld are some of the world's richest REE deposits and several are subject to active exploration. As part of the NERC Global Partnerships Seedcorn fund project WREED, we have carried out preliminary investigations of weathering products on carbonatite hosted REE deposits. Three end member weathering products have been identified (1) carbonate mineral dissolution leaves behind primary REE minerals, forming residual deposits; (2) dissolution and reprecipitation of REE phosphates and fluorcarbonate minerals results in the formation of new hydrated REE-phosphate or -carbonate minerals producing supergene enrichment; and (3) the formation of clay and iron-manganese oxide caps (either from weathering of the deposit itself, or from soil transport from surrounding rocks) that may hold the REE adsorbed to mineral surfaces (c.f. the ion adsorption deposits). High grade, weathered carbonatite deposits typically consist of a range of soil and weathered horizons, that may be phosphate-rich due to dissolution and re-precipitation of apatite and monazite during the weathering process (Mount Weld, Australia), overlain by later sediments that may be REE enriched either by accumulation of residual minerals in lake sediments (Tomtor, Russia). The mineralogy of the ore zone is linked to, but distinct from, the unweathered carbonatite rock, and includes phosphates, crandallite-group minerals, carbonates and fluorcarbonates and oxides. In this study we will utilise bulk rock geochemistry, sequential leaching techniques, mineral chemistry and microbiology to investigate the processes producing different weathered REE deposit styles in carbonatites and their influence on the economic REE grade and environmental impact of deposits. Bulk rock geochemistry will be used to quantify element enrichments and depletions relative to bedrock, and to investigate the potential for ion adsorption style mineralisation in weathered carbonatites. Mineral chemical techniques will be used to investigate the timing of weathering, host minerals of the REE, potential beneficial or harmful changes in chemistry relative to primary minerals, and proxies for the environmental controls on weathering style. These data will be combined with existing records of surface morphology and weathering depth to produce overall genetic models linking climate, geomorphology and geochemistry that will allow prediction of the resource potential of the carbonatite weathered zone. The results will be communicated with industry and the public to raise awareness of the resource requirements of decarbonisation, and potential routes to increased extraction efficiency and reduced impact.