ULTRA - Ultramafic-hosted mineral Resource Assessment

ULTRA - 超镁铁质托管的矿产资源评估

• 批准号: NE/S004998/1
• 负责人: Robert Knight
• 金额: $ 3.39万
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FS004998%2F1
• 关键词: ULTRA; Ultramafic; hosted; mineral; Resource

Summary: Hydrothermal seafloor massive sulphide (SMS) deposits on mid-ocean ridges (MOR) are paradoxical; their size seems to be inverse to the amount of volcanic activity. While hydrothermal SMS are more frequent at fast-spreading MOR, the largest deposits occur where volcanism appears to be a minimum. Here, at so-called amagmatic segments on slow- and ultra-slow spreading ridges, ultramafic rocks from the lower-crust and upper-mantle are exhumed by long-lived faulting; a process that is thought to affect 50% of the length of slow-spreading ridges. Ultramafic-hosted seafloor massive sulphides (muSMS) in these settings form some of the largest deposits known, hosting high metal concentrations of Au, Cu, Ni, E-tech elements (Co, Pt). Whereas the magmatic driving force for volcanic-hosted SMS deposits is well established, it remains contentious for the muSMS. Similarly, while there are models for the sub-surface structure and extent of volcanic-hosted SMS, little is known about muSMS. For some muSMS, vent fluid chemistry indicates the potential for extensive sub-seafloor metal precipitation, possibly by interaction with pH barriers due to serpentinisation of the host rock. Furthermore, the physical, chemical and microbial mechanisms affecting muSMS after their formation are poorly constrained. Our study aims to test the hypothesis that muSMS deposits form extensive sub-surface mineralisation and undergo significant post-formational modification at and beneath the seafloor under the influence of highly variable pH conditions as a result of interaction with ultramafic rocks and during serpentinisation. Our plan is to combine novel geophysical techniques (electromagnetic induction and inverted down-hole seismic tomography) with surface mapping and sub-seafloor drilling (recovering host rocks, sulphides, sediment and fluids) to image the 3D structure and composition of the deposit and its surroundings. The mineralogy, geochemistry and isotope signatures of the samples will reveal the paragenetic history of the deposits including formation, recrystallisation, metal mobilisation, alteration and penetration by seawater. Hydrothermal fluid samples will reveal the nature of the heat source driving deposit formation and host-rock interactions and, combined with studies of metalliferous sediment, constrain metal mobility during later alteration. Ages of these processes will be constrained by radiometric dating. Rates of processes will be constrained by in situ and lab-based, abiotic oxidation and microbial alteration experiments. We will draw these observations together using thermo-physio-chemical numerical modelling to construct a coherent understanding of the formation and preservation of these large polymetallic muSMS deposits in todays-oceans. Our approach requires two cruises to the largest known and best characterised muSMS field at 13degrees30minutesN, Mid-Atlantic Ridge (MAR). Despite being technically ambitious, our experience from the EU-funded Blue Mining project and the involvement of both academic and industrial partners, contributing in-kind data and costs, significantly de-risks the research.

摘要：大洋中脊 (MOR) 上的热液海底块状硫化物 (SMS) 沉积物是矛盾的；它们的大小似乎与火山活动的强度成反比。虽然热液 SMS 在快速扩散的 MOR 中更为频繁，但最大的沉积物出现在火山活动似乎最小的地方。在这里，在慢速和超慢速扩张脊上的所谓非岩浆段，来自下地壳和上地幔的超镁铁质岩石通过长期断层作用被挖掘出来。这一过程被认为会影响缓慢扩张的山脊 50% 的长度。这些环境中的超镁铁质海底块状硫化物 (muSMS) 形成了一些已知最大的矿床，含有高浓度的金、铜、镍、电子技术元素（Co、Pt）。尽管火山SMS矿床的岩浆驱动力已得到充分证实，但对于muSMS矿床仍然存在争议。同样，虽然存在火山托管 SMS 的地下结构和范围的模型，但人们对 muSMS 知之甚少。对于一些 muSMS，喷口流体化学表明存在大量海底金属沉淀的可能性，这可能是由于主岩蛇纹石化与 pH 屏障相互作用所致。此外，影响 muSMS 形成后的物理、化学和微生物机制很少受到限制。我们的研究旨在检验这样的假设：muSMS 沉积物形成广泛的地下矿化，并在由于与超镁铁岩相互作用和蛇纹石化过程中 pH 条件高度变化的影响下，在海底及其下方经历了显着的形成后改变。我们的计划是将新型地球物理技术（电磁感应和倒置井下地震层析成像）与地表测绘和海底钻探（回收主岩、硫化物、沉积物和流体）相结合，对矿床及其周围环境的 3D 结构和成分进行成像。样品的矿物学、地球化学和同位素特征将揭示矿床的共生历史，包括形成、重结晶、金属移动、蚀变和海水渗透。热液样本将揭示驱动沉积物形成的热源性质以及主岩相互作用，并结合含金属沉积物的研究，限制后期蚀变期间的金属流动性。这些过程的年龄将受到放射性测年法的限制。过程速率将受到原位和实验室、非生物氧化和微生物改变实验的限制。我们将使用热物理化学数值模型将这些观察结果结合在一起，以对当今海洋中这些大型多金属 muSMS 矿床的形成和保存建立一个连贯的理解。我们的方法需要两次巡航，前往位于北纬 13 度 30 分钟、大西洋中脊 (MAR) 的已知最大、特征最鲜明的 muSMS 场。尽管在技术上雄心勃勃，但我们从欧盟资助的蓝色矿业项目中获得的经验以及学术和工业合作伙伴的参与，贡献了实物数据和成本，显着降低了研究风险。