ULTRA - Ultramafic-hosted mineral Resource Assessment

摘要：大洋中脊海底热液块状硫化物（SMS）矿床是自相矛盾的；它们的大小似乎与火山活动量成反比。虽然热液SMS在快速扩张的MOR更频繁，但最大的矿床发生在火山作用最小的地方。在这里，在缓慢和超缓慢扩张脊上所谓的岩浆段，下地壳和上地幔的超铁质岩石被长期断裂作用掘出；这一过程被认为影响了50%的缓慢扩散脊的长度。在这些环境中，超镁质海底块状硫化物（muSMS）形成了一些已知的最大矿床，含有高浓度的Au， Cu, Ni， E-tech元素（Co, Pt）。虽然火山寄存的SMS矿床的岩浆驱动力已经确立，但对于muSMS来说仍然存在争议。类似地，虽然有火山主导的短地震的地下结构和范围的模型，但对短地震知之甚少。对于一些muSMS来说，喷口流体化学表明海底下可能存在广泛的金属沉淀，可能是由于寄主岩石的蛇纹石化而与pH屏障相互作用造成的。此外，对形成后影响muSMS的物理、化学和微生物机制的研究也很少。我们的研究旨在验证这样的假设，即muSMS沉积物形成广泛的地下矿化，并在与超基性岩石相互作用和蛇纹石化过程中，在高度变化的pH条件的影响下，在海底和海底下经历了显著的形成后改造。我们的计划是将新的地球物理技术（电磁感应和倒置的井下地震层析成像）与地面测绘和海底钻探（回收寄主岩石、硫化物、沉积物和流体）结合起来，对矿床及其周围环境的三维结构和组成进行成像。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、再结晶、金属动员、蚀变和海水渗透。热液样品将揭示驱动矿床形成和矿岩相互作用的热源的性质，并结合含金属沉积物的研究，约束后期蚀变过程中的金属流动性。这些过程的年龄将受到辐射测年法的限制。过程的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。我们将利用热物理化学数值模拟将这些观察结果汇总在一起，以构建对这些大型多金属muSMS沉积物在当今海洋中的形成和保存的连贯理解。我们的方法需要两次巡航到已知的最大和最具特征的muSMS油田，在13度30分，大西洋中脊（MAR）。尽管在技术上雄心勃勃，但我们从欧盟资助的蓝色采矿项目中获得的经验以及学术界和工业界合作伙伴的参与，提供了实物数据和成本，大大降低了研究的风险。

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.