Modern seafloor hydrothermal systems: new perspectives on ancient ore-forming processes

Seafloor massive sulfides are deposits of metal-bearing minerals that form on and below the seabed as a result of heated seawater interacting with oceanic crust. These occurrences are more variable than previously thought, and this variability is not necessarily reflected in the analogous volcanogenic massive sulfide deposits that are preserved in the ancient rock record. The geological differences affect both the geochemistry and the size of seafloor massive sulfide deposits. Current knowledge of the distribution, tonnage, and grade of seafloor massive sulfides is inadequate to rigorously assess their global resource potential due to the limitations in exploration and assessment technologies and to our current understanding of their 3-D characteristics

Insights into extinct seafloor massive sulfide mounds at the TAG, Mid-Atlantic Ridge

Over the last decade there has been an increasing interest in deep-sea mineral resources that may contribute to future raw metal supply. However, before seafloor massive sulfides (SMS) can be considered as a resource, alteration and weathering processes that may affect their metal tenor have to be fully understood. This knowledge cannot be obtained by assessing the surface exposures alone. Seafloor drilling is required to gain information about the third dimension. In 2016, three extinct seafloor massive sulfide mounds, located in the Trans-Atlantic Geotraverse (TAG) hydrothermal area of the Mid-Atlantic Ridge were drilled. A mineralogical and textural comparison of drill core and surface-grab samples revealed that in recent ceased mounds high-temperature copper assemblages typical for black smoker chimneys are still present whereas in longer extinct mounds the mineralogy is pre-dominated by an iron mineral assemblage. Zinc becomes remobilized early in the mound evolution and forms either a layer in the upper part of the mound or has been totally leached from its interior. Precipitation temperatures of sphalerite calculated using the Fe/Zn ratio can help to identify these remobilization processes. While the Fe/Zn ratios of primary sphalerites yield temperatures that are in very good agreement with fluid temperatures measured in white smokers, calculated temperatures for sphalerites affected by remobilization are too high for SMS. Overall drilling of SMS provides valuable information on the internal structure and mineralogy of the shallow sub-surface, however, additional drilling of SMS, at a greater depth, is required to fully understand the processes affecting SMS and their economic potential

Dispersion and intersection of hydrothermal plumes in the Manus Back-Arc Basin, Western Pacific

The composition of hydrothermal plumes reflects the physical and chemical characteristics of seafloor hydrothermal fluids, which in turn reflects the host rock and subseafloor reaction conditions as well as the water column processes that act to alter the plumes as they disperse and age. Here, we show that the turbidity, current, pH value, dissolved Fe (dFe), and dissolved Mn (dMn) compositions of hydrothermal plumes can be used to understand the spatial distribution and source of hydrothermal systems in the submarine geological environment. Data were obtained from 18 hydrocast stations, among which the water column samples were collected at 8 stations during the MANUS cruise of R/V KEXUE in 2015. The results showed that the Satanic Mills plume and Fenway plume rose approximately 140 m and 220 m above the seafloor, respectively. In the Satanic Mills plume, dFe remained longer than dMn during lateral plume dispersal. There was a clear intersection of the Satanic Mills plume and Fenway plume between 1625 m and 1550 m in the PACMANUS hydrothermal field, and the varied dispersion trends of the mixed plumes were affected by current velocities at different depths. The physical and chemical properties of the seawater columns in the Manus Basin were affected by the input of high-Mn, high-Fe, and low-Mg vent fluids. The turbidity and dFe, dMn, and dissolved Mg concentrations in the sections of the plumes proximal to the Satanic Mills, Fenway, and Desmos vent sites were generally higher (turbidity, Mn, and Fe) and lower (Mg) than those in the sections of the plumes that were more distal from the vent sites. This implied that the plumes proximal to their vent fluid sources, which were interpreted to have relatively young ages, dispersed chemically over time, and their concentrations became more similar to those of the plumes that were more distal from their vent fluid sources

Geological fate of seafloor massive sulphides at the TAG hydrothermal field (Mid-Atlantic Ridge)

Highlights • Generic geological model of hydrothermally extinct seafloor massive sulphide. • Sub-surface characterisation by combined drilling and geophysics. • New resource estimate for slow-spreading mid-ocean ridges. • Holistic approach to seafloor mineral deposits assessment. Abstract Deep-sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply. Increasing demand for these metals, many of which are required to enable a low-carbon and high-technology society and to relieve pressure on land-based resources, may result in deep sea mining within the next decade. Seafloor massive sulphide (SMS) deposits, containing abundant copper, zinc, gold and silver, have been the subject of recent and ongoing commercial interest. Although many seafloor hydrothermally systems have been studied, inactive SMS deposits are likely to more accessible to future mining and far more abundant, but are often obscured by pelagic sediment and hence difficult to locate. Furthermore, SMS deposits are three dimensional. Yet, to date, very few have been explored or sampled below the seafloor. Here, we describe the most comprehensive study to date of hydrothermally extinct seafloor massive sulphide deposits formed at a slow spreading ridge. Our approach involved two research cruises in the summer of 2016 to the TAG hydrothermal field at 26°N on the Mid-Atlantic Ridge. These expeditions mapped a number of hydrothermally extinct SMS deposits using an autonomous underwater vehicle and remotely operated vehicle, acquired a combination of geophysical data including sub-seafloor seismic reflection and refraction data from 25 ocean bottom instruments, and recovered core using a sub-seafloor drilling rig. Together, these results that have allowed us to construct a new generic model for extinct seafloor massive sulphide deposits that indicate the presence of up to five times more massive sulphide at and below the seafloor than was previously thought

Polymetallic mineralisation in the Chillagoe district of north-east Queensland: insights into base metal rich intrusion-related gold systems

The Chillagoe district, located in northeast Queensland, is host to several polymetallic mineral deposits, which comprise three major metallogenic commodities: 1. Cu-Au, 2. Zn-Pb-Cu and 3. Sn-W-Mo. Metals such as W, Mo, Bi, Te, (± As, Sb) vary in concentrations within and between deposits. The deposits occur in a wide range of styles such as porphyries, breccias, skarns, and veins. Previous workers suggested that granites related to four igneous supersuites of the Permo-Carboniferous Kennedy Igneous Province were the source of the diverse metal commodities, whereas the calc-alkaline, metaluminous to weakly peraluminous, I-type, weakly oxidised to reduced, compositionally evolved nature of the oldest supersuite suggests that it alone could be the source for the polymetallic deposits. The mineralisation could be recognised as having affinities to intrusion-related gold deposits (IRG), adding to the increasing number of such systems recognised within the Tasmanides of eastern Australia. Although an IRG system could explain the polymetallic nature of the deposits in general, the economic grade of base metals hosted by deposits of the Chillagoe district is quite unusual for an IRG system. This thesis investigated three polymetallic deposits (Recap, Mungana and Red Dome) of the Chillagoe district by combining petrographical, electron-microprobe (EMP), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and radiogenic isotope studies (Re-Os, U-Pb and Lu-Hf) in order to improve the understanding of the metal diversity of IRGs, and provide insights into the processes of magma formation and evolution related to such systems. Petrographical studies showed that skarn formation at all three deposits is very similar and show no differences to the processes in other intrusion-related skarn systems. Mineralisation on the other hand differs regarding their major commodities, with Redcap and Mungana being base metal dominant, but also hosting minor gold mineralisation, whereas Red Dome is dominated by copper mineralisation and also comprises gold mineralisation and a very minor base metal commodity. The difference in the metal commodities and economic grade was caused by the emplacement of two compositionally different magmatic phases. The primary magmatic event (~ 326-320 Ma), characterised by weakly reduced and strongly fractionated rocks, provided the metallogenic diversity typical for an IRG system with some of the metals being incorporated into silicate phases (Sn, Zn) and As-Cu-Sb sulphide minerals (Au). The second magmatic event (more oxidised and less fractionated, ~312-305 Ma) caused remobilisation of metals related to the primary event, as well as adding S and possibly Zn, Cu, Pb to the system at Redcap and Mungana, whereas at Red Dome in addition to the sulphur, Cu and maybe Au was introduced. Studies of the mineral chemistry showed that composition of garnets and sphalerite have the potential to be used as exploration tool to locate causative intrusions. Due to the complexity of the systems investigated here, the concept should be tested again to establish its use in other multiphase IRG systems. Trace element concentrations and radiogenic isotope data (Lu-Hf) of zircons on the other hand showed that the diverse metallogeny of the IRG systems can be linked to the most fractionated phase in the magmatic system, with the magma originally being derived from a crustal metasedimentary source (in Chillagoe: ~2.0-2.5 Ga). The great advantage of determining such information from zircons rather than from whole rock data is that the latter can become easily affected by metamorphism, (hydrothermal) alteration and/or weathering, whereas physicochemically robust zircons preserve their primary information

Polymetallic Cu-Zn-Ag-Pb, Sn-W-Mo and Cu-Au-Bi mineralisation associated with I- and A-type granites in the Chillagoe region, north Queensland, Australia

Polymetallic mineralisation of at least three major styles (Cu-Zn-Ag-Pb, Sn-W-Mo, and Cu-Au-Bi) occurs in different types of host rocks (skarns, breccias, porphyries) in the Chillagoe region of North Queensland, Australia. Relating mineralisation to the nearest plutonic rocks can give incorrect results, as a chain of overprinting events is responsible for the polymetallic mineralisation in the Chillagoe district. Of particular interest is that the spatially related granites of the Tate Batholith are mostly I-types (one Atype), but the metallogenic association includes elements normally associated with S-type granites (especially type II). This research is testing the potential genetic links between intrusions and mineralisation. It is possible that the Sn-W-Mo associations (and perhaps others) are related to metal contributions from the country rocks, rather than fluids released from intrusions. Alternately, if all these metals are derived from the granitoids, then a revision of the metallogenic connections between magma type and mineralisation is required

Formation, remobilisation and alteration processes at inactive hydrothermal vents: insights from elemental analysis of Cu-(Fe-)S sulfides from TAG, Mid-Atlantic Ridge

Chalcopyrite is the main Cu mineral in mafic-hosted marine hydrothermal systems. Its trace element budget and that of its alteration products may hold valuable information on formation, remobilisation and alteration processes of the hydrothermal system. In this study, we analysed chalcopyrite from five inactive seafloor massive sulfide (SMS) sites from the TAG hydrothermal field on the Mid-Atlantic Ridge by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 24 elements. Twelve of them are discussed in detail. In general, trace element concentrations range between sub-parts per million (ppm) to several hundreds of ppm. The elements Se and Co are incorporated into the lattice at high temperatures of > 300 °C, whereas As, Ge, Ga substitute into the structure at intermediate to low temperatures. Other elements, e.g. Zn, are either accommodated into the mineral lattice or form inclusions, whereas V and Mn, which originate from seawater, get adsorbed onto the mineral surface. Idaite, chalcocite, and covellite exhibit similar trace element patterns to those of the precursor chalcopyrite. However, the secondary copper minerals show enrichment of Ag and Mo. Factors controlling the incorporation are predominantly related to changes in physicochemical conditions with the host rock composition playing only a minor role

Formation, remobilisation and alteration processes at inactive hydrothermal vents: insights from elemental analysis of Cu-(Fe-)S sulfides from TAG, Mid-Atlantic Ridge

Chalcopyrite is the main Cu mineral in mafic-hosted marine hydrothermal systems. Its trace element budget and that of its alteration products may hold valuable information on formation, remobilisation and alteration processes of the hydrothermal system. In this study, we analysed chalcopyrite from five inactive seafloor massive sulfide (SMS) sites from the TAG hydrothermal field on the Mid-Atlantic Ridge by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 24 elements. Twelve of them are discussed in detail. In general, trace element concentrations range between sub-parts per million (ppm) to several hundreds of ppm. The elements Se and Co are incorporated into the lattice at high temperatures of > 300 °C, whereas As, Ge, Ga substitute into the structure at intermediate to low temperatures. Other elements, e.g. Zn, are either accommodated into the mineral lattice or form inclusions, whereas V and Mn, which originate from seawater, get adsorbed onto the mineral surface. Idaite, chalcocite, and covellite exhibit similar trace element patterns to those of the precursor chalcopyrite. However, the secondary copper minerals show enrichment of Ag and Mo. Factors controlling the incorporation are predominantly related to changes in physicochemical conditions with the host rock composition playing only a minor role

Rock physic samples from TAG, Mid-Atlantic Ridge, and various onshore samples

The list attached contains physical properties (density, porosity, magnetic susceptibility, p-wave velocity and electrical resistivity) of seafloor massive sulphide, jasper and basalt samples. The seafloor samples have been obtained during cruise JC138, Mid-Atlantic Ridge, 26N (cruise report https://www.bodc.ac.uk/resources/inventories/cruise_inventory/report/16052/ ) in July 2016 and from onshore analogues. The data has been measured at the National Oceanography Centre under the lead of Dr Laurence North