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By Gerald R. Dickens

Massive sulfide ores and Fe-Mn oxyhydroxide precipitates represent two extremes of a spatial continuum of hydrothermal deposition along oceanic ridges. Elemental variations across this continuum are of interest to economic geologists as a prospective geochemical exploration tool. Theoretical considerations suggest that certain physical and chemical parameters can be used to model compositional variations within the far-field (i.e., Fe-Mn oxyhydroxide) component as a function of distance from ridge axis; however, testing and verification of such models generally has been precluded because the few available distal hydrothermal sediment sequences have been sampled at low resolution and/or are highly diluted with biogenic components (e.g., siliceous and calcareous tests). Recent ocean drilling (ODP Leg 145; Sites 885/886) in the North Pacific successfully recovered an extensive record of "biogenic-free" distal hydrothermal precipitates from an area approximately 60 km south of the Chinook Trough at longitude 168.1 °W. Geophysical and geomorphological evidence suggests the Chinook Trough is the southern lithospheric scar marking initiation of late Cretaceous north-south rifting, implying that recovered hydrothermal materials record distal hydrothermal precipitation from the now-subducted Kula-Pacific spreading ridge. The Chinook Trough hydrothermal sequence is a continuous 15 my record of "biogenic-free" hydrothermal Fe-Mn oxyhydroxide precipitation mixed with eolian clays. It was deposited during the late Cretaceous, early Paleocene (80-65 Ma) as the perpendicular distance from ridge axis increased from 75 to 600 km. A high-resolution geochemical analysis of this hydrothermal sequence reveals distinct trends for the up-core (equivalent to increasing distance from ridge) distribution patterns of Fe, Sc, As, and rare earth elements (REE). Iron concentrations exhibit an S-shape distribution pattern with respect to increasing

Jan 1, 1994

By B. J. Bluck

Diamond mega-placers, in order to rank amongst the primary diamond deposits, are defined as => 70 m carats at =>95% gem quality. There is only one mega-placer known and that is found along the coast of southwestern Africa fringing the Kaapvaal craton. Placers are classified as residual when they are left on the craton, transient when being eroded into the exit drainage and terminal. Terminal placers, in being the final depositories of diamonds off the craton, have the greatest probability of being a mega-placer and such is the case in the placer deposits near the Orange River mouth, southwestern Africa. There are four main groups of conditions leading to the development of a mega-placer: the craton, the drainage, the nature of the environment at the terminus and the timing. The craton, in its role as the producer of diamonds is significant in respect to its size, diamond-fertility, the degree to which diamonds from successive kimberlite intrusions have been retained there and their availability to the final drainage yielding the mega-placer. Cratons in being buoyant have a tendency to leak diamonds into surrounding basins but in being incompressible often have orogens converge onto them resulting in lost sediment being returned as foreland basin fills. In order to concentrate a bulk of diamonds the drainage basin has to be large with a high proportion of it on the craton. The drainage yielding the mega-placer should ideally not have been preceded by older drainage and other systems that may have robbed the craton of earlier diamonds. The exits of diamonds from the craton need to be few in order to focus on their supply and the drainage network needs to be efficient in order to remove the maximum volume of diamonds off the craton. This efficiency and the focused termination is best achieved by the rejuvenation of a pre-existing drainage network. The drainage should be young enough to yield host sediments friable enough to allow brittle diamonds to be recovered from them.

Jan 1, 2004

By Curtis Clement

Williamson & Associates, Inc. has developed a marine mineral exploration tool using an induced polarization technology (IP) under license from the US Geological Survey. Developed by Jeff Wynn at the USGS, this method has gone through several prototyping stages and is now in commercial use to map offshore ilmenite deposits. Preliminary results of a recent resource mapping program are discussed as well as potential applications in the detection and quantification of other minerals in marine sediments.

By Tetsuo Yamazaki

Deep-sea rare-earth element-rich mud (REE Mud) distributes in pelagic clayey sediment column on ocean seafloor at 4,000-6,000 m deep. The thickness ranges 5-80 m and the burial depth 0-100 m. The REE content ranges 600-2,250 ppm in Pacific and one of the richest, maximum 6,500ppm, has been found in Marcus Is. exclusive economic zone. By applying a conventional hydraulic excavation and lifting methods, the economy of REE Mud mining around Marcus Is. is preliminary examined. Because the waste content in REE Mud is high and the disposal cost in Japan is very expensive, the mining is recognized far away from economy.

Sep 14, 2011

By Robert M. Owen

The reconstruction of ocean history has been a major focus of marine research over the past decade. A significant outcome of these investigations is the recognition that certain types of marine mineral deposits are linked to regional to global scale changes in ocean composition and climate conditions. Manganese-bearing deposits are of particular interest in this regard because changes in their occurrence, distribution and/or composition are responsive to both biochemical and geological factors. Analyses of such deposits whose formation spans critical age/epoch boundaries thus provide clues concerning the interplay of different process during anomalous periods in Earth history. Recent studies demonstrating this deposit - event - process relationship include: (1) Changes in the bulk chemical composition of ferromanganese crusts during the Late Miocene-Early-Pliocene (-7-3 Ma) that coincide with a period of enhanced biological productivity in the eastern equatorial Pacific upwelling zone; (2) The formation of large statiform manganese deposits at the Cenornanian- Turonian Boundary (-92 Ma) in association with widespread oceanic anoxia; and (3) Increased deposition of hydrothermal precipitates caused by tectonic reorganizations at the Paleocene - Eocene boundary (-55 Ma) that resulted in global-scale changes in atmospheric and oceanic circulation and climate.

Jan 1, 1995

By James R. Hein

Low- to intermediate-temperature, diffuse-flow hydrothermal fields and associated mineral precipitates have been found at many locations along oceanic spreading centers, back-arc basin spreading axes, and midplate hot-spots. However, this type of hydrothermal field has not been found in oceanic fracture zones until recently. We mapped and collected samples from an active hydrothermal field in the Blanco Fracture Zone (BFZ) located in the northeast Pacific. The BFZ connects the Gorda and Juan de Fuca spreading centers and is divided into east and west segments by Cascadia Depression, a small spreading center. Small pull-apart basins occur along both the east and west segments. A complex set of basins near the western end of the BFZ includes from east to west, Surveyor Depression, East Blanco Depression (EBD), and West Blanco Depression. The hydrothermal field described here is located in the EBD.

Jan 1, 2001

By Marco Figoni

The increased interest on deep water minerals resources is a challenge for both mining and oil industry. The exploration and production of oceans minerals require a synergy between of different technologies. While mining industry is leading the dredging and processing part of the system, oil industry is contributing through development of ore lifting technology based on existing subsea knowledge. The paper presents an overview, based on published projects around the world, of different approaches to marine mining. Several exploration techniques and offshore production systems were already designed and assessed, all of them with apparently positive results. Exploration phase is well advanced and in continuous expansion allowing the discovery of increasing resources around the world of different nature but production can be considered to be in an early stage. Different from onshore mining, subsea operations require a closer cooperation with international and local authorities to preserve as much as possible the environment. There are still many uncertainties at respect and all the exploration process must be carefully evaluated to allow subsea mining to develop in a sustainable manner. Technically, mining at seabed was already demonstrated and the process is based on three main components, namely seafloor mining tools, riser and lifting system and production vessel support. This layout is based on the concept that ore must be disaggregated at seafloor first, transported to surface in form of slurry that must be dewatered before loading to be transported to processing plants onshore. The challenges are rising because the environment and safety must be taken into consideration as a priority when evaluating the efficiency of the system. The most critical challenges regarding the production operations, especially in deep water are the deep-sea excavation process, the transport of large slurry volumes, the slurry abrasivity, the power supply management and the subsea

Sep 14, 2011

By Kokichi Iizasa

Kuroko-type deposits exist in Japanese EEZ. Some of them are present in the Izu-Ogasawara oceanic island arc south of Tokyo. Major two Kuroko-type deposits occur in the area. One is the Sunrise hydrothermal field occurring on the caldera floor of Myojin knoll, which is likely to be the developed stage in sulfide mineralization. Another is present in the crater of Suiyo seamount that is the early stage of sulfide mineralization. The two deposits are here presented on the growth history based on samples recovered by Benthic Multicoring System (BMS).

Jan 1, 2002

By Fred C. Dobbs

To effectively manage the potential environmental impacts of deep-sea mining of manganese nodules, we need to know substantially more about the effects of sediment redeposition on abyssal communities and the time scales of recovery from such effects. As an initial step towards a predictive understanding of mining impacts on abyssal benthos, we are conducting a series of biological studies within the framework of the Benthic Impact Experiment (BIE). Mining operations will cause resuspension of sediments across extensive areas of the abyssal Pacific Ocean. The resultant plumes likely will produce redeposition layers, ranging from a few sediment grains in thickness to > 1 cm, over thousands of square kilometers of the seafloor. The impact of such redeposition on abyssal benthic communities is exceedingly difficult to predict. While there has been speculation that redeposition of 1 mm or less will exert deleterious effects, there has been very little testing, and the BIE will provide the first in situ test of redeposition impacts. In a series of cruises before and after controlled resedimentation, we are collecting biological samples to determine the undisturbed state of the ecosystem, the immediate effects of disturbance, and the time-course of recovery resulting from quantified levels of resedimentation.

Jan 1, 1992

By Akira Usui

Iron and manganese are very mobile metallic elements in the surface aqueous environment, particularly in the oceans. The elements are easily precipitated as oxides or dissolved back into water, controlled mainly by redox conditions, and the oxide precipitates often selectively accumulate other metallic elements. We have found clear evidence of fine-scale variations in chemistry, mineralogy, isotope composition and microstructures within various iron-manganese nodules and crusts, such as in the deep-sea basins and seamounts of the Pacific Ocean, and in shallow waters in the Baltic Sea. The patterns of variation with depth (that is time-lapse profiles of the deposits) are often correlated among samples and sometimes between remote localities. These fine-scale variations most probably reflect past depositional conditions and possibly image the environmental change, although they remain to be correlated to any physicochemical, geological, or oceanographic parameters. The hydrogenetic ferromanganese crusts have grown widely in the northwestern Pacific, accelerated by intensive circulation of oxygenated bottom waters during the last several millions of years, or earlier. The fine-scale (in millimeter) in-depth compositional and textural patterns with Be-10 growth-rate curves suggest significant changes in growth rate and also occasionally show a common microstratigraphic pattern among samples. Some of the samples indicate a correlation in growth pattern between remote locations, while others suggest short-range difference of oceanographic conditions with water depths.

Jan 1, 2004

By Sophia Katsouri

Hydrothermal circulation that produces sulfide deposits in the marine environment can also produce sulfide deposits in lakes. The major differences in terms of process are (1) in lakes the hydrothermal fluid is meteoric water of low salinity whereas in the marine setting it is saline and (2) the rocks that are leached of metals to produce the deposits are continental crust in one case and oceanic crust in the other. We have examined sulfides that are presently forming in two lakes ? Lake Tanganyika in east Africa and Lake Oyunuma in Hokkaido, Japan. Tanganyika is the second largest lake in the world and the second deepest rift lake, after Lake Baikal in southeastern Siberia. Two hydrothermal fields are known in its northern basin at Pemba and Cape Banza (Tiercelin et al., 1993). At Pemba, CO2 and CH4-rich, low temperature (53-88°C) hydrothermal fluids are forming decimeter-tall chimneys at 2-46 m water depth. Our microscopic and x-ray diffraction study of samples from the Pemba site reveal marcasite and pyrite to be the most common sulfides. Minor minerals include barite, hematite, bornite and chalcopyrite. Quartz and feldspar are also identified and were probably incorporated from the surrounding lake sediments. At Cape Banza, aragonite chimneys, up to 70 cm high, are venting fluids at 66-103°C.

Jan 1, 2001