DataSheet3_Sphalerite as a non-traditional critical metal source: Correlative microscopy (EPMA, EBSD, and APT) of cobalt-enriched sulfides from the sediment-hosted copper-cobalt Dolostone Ore Formation deposit, Namibia.docx

Sphalerite from the sediment-hosted Dolostone Ore Formation (DOF) Cu-Co-Zn deposit, in northwestern Namibia, has previously been shown to contain extremely high concentrations of the critical metal Co (up to 1 wt%). These concentrations are the highest reported in sphalerite to date, and the how and why of sphalerite being able to incorporate such high concentrations of Co are poorly understood. We use correlative electron probe microanalysis, electron backscattered diffraction, and atom probe to reconstruct the likely incorporation mechanisms and modes of occurrence of such high Co concentrations in natural sphalerite. While over twenty samples were studied, the comprehensive analytical workflow was executed on one representative sample to gain a detailed understanding of Co enrichment. The sulfides of the studied sample are Co-rich pyrite, chalcopyrite, Co-rich sphalerite, linnaeite, cobaltpentlandite, and cobaltite, mentioned in order of abundance. Detailed petrography of these sulfides indicates that they formed through three stages during the main Cu-Co-Zn ore stage of the DOF. Cobalt was initially contained in pyrite that grew during Ore Stage 1 and was later affected by oxidizing fluids (Ore Stage two). This led to remobilization and growth of linnaeite (Co2+Co3+2S4). A later change in fO2 (Ore Stage three) led to the breakdown of linnaeite and the further growth of accessory cobaltite along with the Co-rich sphalerite and chalcopyrite. The hyper-enriched Co-sphalerite then is the last major sink for Co in the DOF deposit. A low Fe and Co and high Zn sub-grain boundary network within the Co-rich sphalerite was identified by EPMA and EBSD. This sub-grain network is believed to have formed during a later, secondary metamorphic stage (Cu-Zn (-Pb) Ore Stage 4), which developed during ductile deformational mineralization styles such as pressure shadows and veins. Our APT data reconstructions show no evidence for Co-inclusions within the Co-sphalerite, and spatial ion correlation analyses of the data suggest that Co occurs in the sphalerite through simple substitution of Zn. This study demonstrates that sphalerite may contain significant concentrations of the Co through simple substitution, potentially representing an important non-traditional Co source in future critical metal exploration.

纳米比亚西北部沉积容矿白云岩地层组（Dolostone Ore Formation, DOF）铜-钴-锌矿床中的闪锌矿（sphalerite），此前已被证实含有极高浓度的关键金属钴（Co），最高可达1重量百分比。该浓度为目前已报道的闪锌矿中钴含量的最高值，而闪锌矿能够赋存如此高浓度钴的机制与原因仍不甚明确。
本研究采用联用的电子探针显微分析（Electron Probe Microanalysis, EPMA）、电子背散射衍射（Electron Backscattered Diffraction, EBSD）与原子探针（Atom Probe, APT）技术，重建了天然闪锌矿中这类高浓度钴的可能赋存机制与产出方式。本次研究共分析二十余件样品，并针对一件典型样品开展了完整的分析流程，以深入解析钴的富集特征。
研究样品中的硫化物按丰度排序依次为富钴黄铁矿、黄铜矿、富钴闪锌矿、钴硫尖晶石（linnaeite）、钴镍黄铁矿（cobaltpentlandite）与辉钴矿（cobaltite）。对这些硫化物的详细岩相学研究表明，它们形成于DOF矿床主铜-钴-锌矿化阶段的三个矿化期次。
钴最初赋存于矿化期1形成的黄铁矿中，后续被氧化流体改造（矿化期2），这一过程导致钴被活化迁移，并促使钴硫尖晶石（Co₂Co₂S₄）形成。随后氧逸度发生变化（矿化期3），使得钴硫尖晶石发生分解，伴生的辉钴矿与富钴闪锌矿、黄铜矿进一步生长。最终，超富钴闪锌矿成为DOF矿床中钴的主要最终储库。
通过EPMA与EBSD分析，研究人员在富钴闪锌矿内部识别出了低铁钴、高锌的亚晶界网络。该亚晶界网络被认为形成于后期的次生变质阶段（铜-锌（-铅）矿化期4），该阶段发育于韧性变形矿化样式（如压力影与脉体）的形成过程中。
本研究的原子探针数据重建结果显示，富钴闪锌矿内部不存在钴的包裹体；对数据的空间离子相关性分析表明，钴通过简单的类质同象置换锌的方式赋存于闪锌矿晶格中。
本研究证实，闪锌矿可通过简单类质同象置换赋存高浓度钴，这一发现可能为未来关键金属勘查提供一类重要的非传统钴资源。