POLY-METALLIC NODULE - MANGANESE NODULE - FERROMANGANESE NODULE - MARINE NODULE

  Poly-Metallic Marine Nodules 

Marine Gold Deposits	Ferromanganese Concretions	Poly-metallic Compounds	Poly-Metallic Nodules	Poly-Metallic Sulfide deposits	Photos
	Manganese Concretions
	Iron Concretions

Gold ores have also been located along the mid-ocean ridges of the Atlantic and Pacific Oceans. One such deposit was found in association with the TAG hydrothermal field at 26o North latitude on the mid-Atlantic Ridge at the 3,670 meter water depth (Herzig 1991). The gold ores in these locations are associated with sulfide deposits formed by hydrothermal vents. These vents occur when the spreading seafloor allows water to percolate down in the crustal rocks and reach hot regions deep beneath the seafloor. The heated seawater dissolves mineral in much higher concentrations than can occur in cold water. From the spreading crustal plates the water dissolves various mineral and metals, such as sulfur, iron, copper, among others. Gold and silver are also dissolved but in very small concentrations. From samples of these hot solutions taken from the Sea Cliff hydrothermal field, on the northern Gorda Ridge, gold concentration ranges between 1 and 11ppb, and silver between 14 and 200 ppb (Zierenberg 1990). Temperatures of these solutions range from 100 to 350 deg C. Upon reaching the cold seafloor waters, much of the dissolved minerals and metals precipitate out of solution forming chimney-like vent structures. These chimneys build up and eventually fall over to form again. After enough time has passed, huge mounds of these structures form, being predominantly composed of iron and sulfide compounds. From the samples taken, gold concentrations ranged from 0.06 to 28.40 ppm. Similar, but ancient, sulfide deposits can be found in Australia, Cypress, and elsewhere. Many of these continental deposits have been commercially mined, but it was assumed that the recent oceanic deposits would not likely be of commercial grade because the evidence suggests that the continental deposits have gone through secondary concentration of the gold when ground water or surface weathering dissolves away much of the sulfide and iron matrix, leaving the deposits gold enriched (Herzig 1991). Though most of the mid-Atlantic Ridge sulfide deposits are of the lower grades that correlate to the virgin continental deposits, some, at least, appear to have undergone secondary concentration. It is theorized that this secondary concentration of gold occurs after the initial oxidation of the sulfide assemblages (0.8 to 5.5 ppm Au) and the percolation of the hydrothermal solutions redissolve and then redeposit the gold as pure native metal (at up to 23.0 ppm Au) (Herzig 1988). The discovery of this secondary concentration occurring at the bottom of the seas is important since geologists had previously assumed that sulfide gold ore deposits located on the continental plates would have had to have been exposed to weathering or ground water to be concentrated. It is now apparent that sulfide gold ore deposits may be found in previously unexplored regions. Much about the process of precipitation of gold and other metals from these solutions is unknown, however, it is believed that some sulfur-oxidizing bacteria of the genera Beggiatoa, Thiothrix or Thiovulum play an active role in this precipitation (Zierenberg 1990).

These chemosynthetic bacteria derive energy unlike their surface dwelling relatives (assuming that they are related). Instead of deriving energy from the oxidation of organic mater, or from photosyntheses, they oxidize sulfide compounds directly from the scorching hot hydrothermal liquids. How these bacteria can live and even thrive at 200o C is a matter of much discussion and investigation, but evidence suggest that these bacteria can efficiently remove gold, silver, copper, and other metals and minerals from dilute aqueous solutions. Proposed methods for this deposition vary. One such method involves the increase in pH in the micro-environment of the microbial mats that line these vent chimneys. These metals are less soluble at the higher pH's and precipitate out of solution and are then stored within the cell walls (Mullen 1989).

Other theories have been proposed regarding the role of gold precipitation from ore solutions by bacteria. Recent evidence suggests that most of the placer gold found in Alaska originated from bacterial scavenging. An analysis of the microstructure of Alaskan placer gold, and that of many of the epithermal deposits around the world, has revealed a fine structure of nearly pure gold microtubuals approximately 1 micrometer in diameter. It has been proposed that these hollow gold structures are the exact shape and size of the cellwall of bacterium genus Pedomicrobia (Watterson 1992). These bacteria are believed to derive energy from the precipitation of gold around themselves. A close examination of the microtubuals reveals branching structures of smaller diameters connected to the larger diameters. This observation is remarkable similar to the observed method of reproduction for Pedomicrobia. Instead of reproducing my fission, the splitting of the cell in two, these bacteria often reproduce by budding, a process remarkable similar in appearance to the gold microtubuals (Rennie 1992). The gold casings around the Pedomicrobia are extraordinary because of their high degree of purity, in excess of 98% gold (Pain 1988). It has been argued by these researchers that much of the Earth's placer gold deposits, have originated from similar biological processes with these or other bacteria. It is believed that the bacteria can concentrate the gold around themselves in such massive amounts because of an electrochemical reaction whereby the gold is gathered on pecifically adapted membrane receptors to which the bacteria discharges excess electrons from its biological processesthus precipitating the gold out of solution (Watterson 1992).

The possibility that certain bacteria can concentrate gold in amounts sufficient to comprise a major share of the Earth's gold ores suggest that with the right application, these or similar bacteria may be employed in the extraction of gold from low grade deposits or solutions. Already, there are commercial applications of bacteria in the mining of gold. Specifically, the bacteria Bacillus cereus is being used by the Canadian Genprobe Company to increase the yield of gold from pyrite ores (Anonymous 1989). In this case the bacteria are after the pyrite matrix that binds the gold and prevents economic recovery otherwise. Bacterial processing of these pyrite ores is relatively inexpensive and has increased yields from an average of about 65% to as much as 96% (Dworetzky 1988). Given the affinity that some bacteria have for the concentration of gold, the question arises as to whether it might be feasible to employ such a bacterium, or one specifically engineered for the task, to scavenge gold directly from the dilute concentrations present in sea water.

Conclusion:
Even at the conservative estimates of 10 ppb of gold in seawater, there is a great deal of gold in solution in the oceans. Humankind has unearthed perhaps a total of 3.3 billion ounces of gold over the course of history, an amount equivalent To a cube of gold 55 feet on a side (Dworetzky 1988), but the sea water of the Earth's oceans contain about 25 billion ounces of gold (Burk 1989). If the ability of some of these bacteria to concentrate gold around their cell membranes to the degree that they form massively dense agglomerations of hollow gold microtubuals, as the evidence suggests, then perhaps a similar bacterium may find a practical application in sea water. It is believed that these bacteria concentrated gold from solution concentrations similar to that of sea water, though perhaps not similar with regard to other constituents. If such a bacterium could be identified and grown in sufficient amounts, it might then be fixed to substrates that could then either be moved through large volumes of sea water, or placed in stationary positions in areas of relatively swift currents. Once enough time had elapsed for these bacteria to gather sufficient amounts of gold, these substrates could then be gathered and processed to recover the gold. The problems in these approaches are not trivial, and the work and research needed for an evaluation of its practicality are not simple. I believe that such research might pursue exploring the precise biochemical and bioelectrical pathways for the deposition of gold in these naturally occurring bacteria. Perhaps with a sufficient understanding of these pathways, these gold scavenging abilities might be artificially promoted or enhanced sufficiently to achieve an economic recovery of gold from sea water.

Poly Metallic Compounds

II.1.- The history of the discovery of polymetallic nodules

The first sub-marine ferromanganese concretions were discovered in 1868 on the Kara Sea (Russia). 

From 1873 to 1876, during the expedition around the world of the H.M.S. "Challenger", many small dark-brown balls, rich in manganese and iron, were collected. They were given the name of manganese nodules.

Around 1900, the scientist Agassiz found nodules in most sampling made in the Eastern Pacific ocean.

Nodules were also collected in cores and dredges by the scientific expeditions that surveyed the global ocean.

It was only after 1957 that the American John Mero succeeded in convincing some industrials of the economic interest of the nodules and led them to the exploration of the Central Pacific Ocean.

At the beginning of the sixties, manganese nodules started to be considered as potential resources for nickel, copper, cobalt and even manganese. 

The first involved companies were Kennecott and Newport Shipbuilding Company (1962) which started to carry on sampling cruises, while American scientists (Fuerstenau, Arrhenius) were studying the nodule geochemistry  and their metallurgic processing. 

From 1965, new companies undertook important researches in nodule exploration, mining and processing, and among them AFERNOD. 

In 1972, the "National Science Foundation" launched a research program that implied at least fifteen scientific laboratories in the study of nodule genesis. In the Soviet Union, Bezrukov published a voluminous book on Pacific nodules. However in this country, industrial exploration expanded only after 1982. 

II.2.- Nodule Distribution in the World Ocean

Scientific and industrial studies give a general idea on the worldwide distribution of the nodules and allow the localization of areas that contain potential ore deposits. 

Ferro-manganese concretions were found in many places of the seabed. The map, published in 1969 by Mc Kelvey, showed already the occurrence of nodules in all the oceans, some have been found also on the bottom of lakes (USA, Canada, etc...). However not all are of economic value because their low abundance and/or their small metal content (Mn, Ni, Cu, Co). 

In 1973, Horn demonstrated the predominance of the “East-West belt of the Southern part of the Northern Pacific”, between the Clarion and Clipperton fracture zones, known afterward as the "Horn zone". It was in this area that most mining companies were already working. 

II.3.- Morphology

Polymetallic nodules are small balls, dark-brown colored and lightly flattened, 5 to 10 centimeters in diameter, which lay on the seabed at 4.000 to 6.000 meters deep. Their wet density is around 2 g/cm³; their water-content is 40 % of their dry-weight and their porosity is 50 %. 

Size and shape are highly variable (roughly spherical, more or less ovoid), nodules are classified in: 

·         mononodule: simple nodule, spherical or ellipsoidal, 

·         polynodule: a nodule with several cores, 

·         composite nodule: several joined nodules. 

In section, most nodules show concentric layers called "cortex" that correspond to the successive step of growth around a "core", often microscopic. The core can be a fragment of an old nodule, a shark tooth or a rock fragment (basalt, limestone, etc...). 

The layers are formed of hydroxides of manganese and iron more or less crystallized. The more crystallized they are (todorokite, birnessite), the richer in Mn, Ni and Cu, while cryptocristallized structures (vernadite) are richer in Fe and Co. 

Several studies showed that Ni and Cu are, either adsorbed in the ferro-manganese hydroxides, or incorporated in their lattice, filling the Mn⁺⁺ sites of todorokite or birnessite.

Grades are function of the crystallization state. The more crystallized are the nodules, the highest are their metal grades. However, valuable-element recovery (Ni, Cu) seems to be easier when crystallization is incomplete. 

The growth rate of the nodules is one of the slowest phenomenon (in the order of a centimeter by several millions years). The age of Pacific Ocean nodules is 2 to 3 millions years. 

II.4.- Composition

Chemical analyses of the nodules show that they contain many components, of which the majors are: 

manganese (29.40 %); 

iron (6 %); 

nickel (1.34 %); 

copper (1.25 %); 

cobalt (0.25 %); 

titanium (0.6 %); 

aluminum (2.9 %) 

and sodium, magnesium, silicium, zinc, oxygen and hydrogen (32.16 %).

Silver

Gold

Platinum

What is the composition of nodules ?

Nodules were dried, powdered in a agate mortar, digested in a acid mixture of Hydroflouric acid, Perchloric acid and Nitric acid. These digested solutions were analyzed for a number of elements to know their concentration. The average composition of nodules from the CIOB is as follows. (source: Jauhari & Pattan 2000).

There are three types of nodules.

1. Hydrogenous type where metals are supplied from the water column. These nodules have smooth surface texture and rich in Fe, Co, Ti, P and Pb content. The Mn/Fe ratio of these nodules is ~1.

2. Diagenetic nodules where metals are supplied from the underlying sediment through the pore water by remobilization. These nodules have rough surface texture and rich in Mn, Cu, Ni and Zn content. The Mn/Fe ratio is more than 2.5.

3. Mixed type which is the combination of both hydrogenous and diagenetic type.

Some scientific results

• Nodule grade (Cu+Ni+Co %) is inversely related to abundance (kg/sq m).

• Manganese and iron show inverse relation suggesting their different source.

• Nodules with 2 to 6 cm size have high Mn, Cu , Ni and Zn concentration.

• Nodules grow with 1-2 mm per million year.

Rare earth elements are supplied to nodules in association with Fe, Ti and P from the

Poly metallic sulfide deposits

Zinc-Copper volcanogenic massive sulfide (VMS) deposits have been observed as they form at mid-ocean ridges. Chimneys formed at the ridges have as much as 29 weight % zinc and 6 weight % copper in sulfide minerals (pyrrhotite, pyrite, sphalerite, and chalcopyrite). The minerals are dissolved in fluids at temperatures as high as 380 C flowing at 1-5 m/sec.

The minerals precipitate as the hot solution comes in contact with cold sea water. Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive (Principle Investigators: D. Fornari, R. Haymon, K. Von Damm, M. Perfit, M. Lilley, and R. Lutz).

Lead, zinc, and cooper are found in VMS deposits. The deposits form in deep ocean water by the precipitation of sulfide minerals released by submarine volcanoes.

Volcanic fluids and hot seawater move through the volcanic rocks and leach metals. The deposits are associated with lava flows, breccia, water-deposited tuffs, cherts, sulfates, and limestones. VMS deposits are usually associated with quartz, anhydrite, gypsum, and barite. This photo shows pyrite in silicified tuff from a VMS deposit in eastern Java, Indonesia. The sample contains 0.55 ppm gold.
Photo by Steve Mattox.

There are three types of VMS deposits:

Kuroko-style VMS deposits are found in dacite-rhyolite domes that erupted in the deep water of back-arc (behind the main volcanic arc) basins. Kuroko deposits are zoned from copper-rich near the center, to zinc-rich, to lead-rich at the outer edges of the deposit. The fluids that form Kuroko deposits have twice the salinity of average ocean water and temperatures of 250-300 C. This cross-section of a typical Kuroko deposit is from Sato (1974) and Franklin and others (1981). Modern-day VMS deposits of the Kuroko-style have been observed as they form in the back-arc basin of the Okinawa Trough (Halbach and others, 1989).

The deposit at Bathurst in Canada is an example of lead-zinc-copper VMS. Hydrothermal solutions associated with Keweenawan flood basalts have produced copper deposits in the Lake Superior area. Between 1845 and 1968, over 13 billion pounds of copper and 16 million ounces of silver were produced from the Keweenawan district.

The porphry (not VMS) mine at El Abra in Chile will produce 500 million pounds of copper annually.

Uses for Copper, Lead, and Zinc

Copper was used as long ago as 8000 BC for tools, weapons, and ornaments. The discovery of bronze (copper and tin alloy) about 3500 BC marked the onset of the Bronze Age. Romans used lead for the plumbing and sewage systems. In Modern times, the electrical conductivity, ductility, and resistance to corrosion of copper, lead, and zinc, make them very useful in alloys. Copper is used for electrical applications (50%), general and industrial engineering applications (20%), building and construction (15%), transportation (11%) and other applications. Lead is used in storage batteries, paints, dyes, explosives, insecticides, and rubber products.

Zinc is used in galvanized steel, protective coatings for steel, and die casting. Zinc compounds are used for luminous dials, cosmetics, plastics, rubber products, soaps, and inks.

World Supply of Copper

In 1991, 9,167,000 tons of copper was produced. Chile (19.8%), USA (17.8 %), and the CIS (9.8%) are the main producers. USA, Japan, and the CIS are the main consumers.