ULTRA - Ultramafic-hosted mineral Resource Assessment

摘要：大洋中脊(MOR)上的热液海底块状硫化物(SMS)矿床是矛盾的；它们的大小似乎与火山活动量成反比。虽然热液短消息在快速蔓延的莫尔地区更为频繁，但最大的矿床发生在火山活动似乎最小的地方。在这里，在缓慢和超慢扩张山脊上的所谓岩浆段，来自下地壳和上地幔的超镁铁质岩石被长期的断裂作用挖掘出来；这一过程被认为影响了缓慢扩张山脊长度的50%。这些环境中赋存于超镁铁质海底块状硫化物(MuSm)形成了一些已知的最大矿床，蕴藏着高度集中的Au、Cu、Ni、E-tech元素(Co、Pt)。尽管火山-Sms矿床的岩浆驱动力已经确立，但对于MUSMS来说，这一点仍然存在争议。类似地，虽然已经有关于火山寄生短信的地下结构和范围的模型，但对mu短信知之甚少。对于一些MUSM来说，喷口流体化学表明可能存在广泛的海底金属沉淀，可能是由于主岩的蛇纹岩作用而与pH屏障相互作用。此外，在MuSMS形成后影响它们的物理、化学和微生物机制很少受到限制。我们的研究旨在验证这样一种假设，即由于与超镁铁质岩石的相互作用，以及在蛇纹石化过程中，在高度可变的pH条件的影响下，musms矿床形成了广泛的地下矿化，并在海底和海底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(电磁感应和井下地震层析成像)与地面测绘和海底钻探(回收主岩、硫化物、沉积物和流体)相结合，以成像矿床及其周围的三维结构和组成。样品的矿物学、地球化学和同位素特征将揭示矿床的共生史，包括形成、重结晶、金属活化、蚀变和海水渗透。热液流体样品将揭示驱动矿床形成和容矿-岩石相互作用的热源的性质，并结合变质沉积物的研究，限制后期蚀变过程中的金属活动性。这些过程的年龄将受到辐射测年的限制。过程的速度将受到现场和实验室、非生物氧化和微生物改造实验的限制。我们将使用物理化学数值模拟将这些观测结果结合在一起，以构建对这些大型多金属MUSMS矿床在当今海洋中的形成和保存的一致理解。我们的方法需要两次巡航，最大的已知和最具特色的MuSMS油田，位于北纬13度30分钟，大西洋中脊(MAR)。尽管我们在技术上雄心勃勃，但我们从欧盟资助的Blue Mining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，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 mineralization 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, recrystallization, 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 metaliferous 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 physiochemical 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 the largest known and best characterised muSMS field at 13degree30minuteN, 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.