Mining in Manitoba
There are about 140 sites of presently or recently active hydrothermal venting on the sea floor and more are being found every year by marine geologists, geophysicists and biologists conducting basic research on a fundamental Earth process. This is one important way that heat is ventilated from the interior. The vented hot water, at temperatures as high as 420°C but more commonly 350°C, carries dissolved in it high concentrations of metals, reduced sulfur and other elements that are precipitated when it encounters the 2°C bottom water. The mineralogy and fundamental geology of the larger deposits would be familiar to anyone who works with volcanogenic massive sulfides (vms) on land.
|Most of the 140 or so known sites are just
geochemical anomalies -- metalliferous sediments, vents with little or no mineralization,
thin crusts, small chimneys and mounds that would fit comfortably inside a small house.
Some, however, are large, such as the main active mound at the Trans-Atlantic Geotraverse
(TAG) site on the mid-Atlantic Ridge east of Miami or the Magic Mountain site on Explorer
Ridge about 200 km west of Vancouver Island. These are comparable in size and shape to
Toronto's Skydome, a covered baseball and football stadium, which makes them to be in the
3 - 5 million metric ton (mmt) range and there are several more large mounds at both
sites. This size is comparable to the typical 1 - 3 million metric ton vms found today on
land in Canada but, of course, nothing at all like the giant Brunswick #12 deposit (134.1
mmt) in New Brunswick or Kidd Creek deposit (117.5 mmt) in Ontario.
The largest of all the ocean vms deposits, the Atlantis II Deep in the Red Sea, is 94 million metric tons. These metal-rich muds were carefully assessed by the German company Preussag on behalf of the Sudanese and Saudi Arabian governments in the 1970s. At 0.45% copper, 2.07% zinc, 39 grams/ton silver and 0.5 grams/ton gold, but with higher grade sections, the deposit was considered to be uneconomic. Regardless, that is a similar grade to the average for the 85 deposits totaling 1765 million metric tons comprising the Iberian Pyrite Belt in Spain and Portugal (0.8% Cu, 2.0% Zn, 0.7% Pb, 26 g/t Ag, 0.5 g/t Au).
The true grade, in a mining sense, for any of deposits, except for the Atlantis II Deep, is not known. Most of the sampling has been by submersible and dredge which typically scrapes the exterior of a mound or takes part of a chimney. These tend to be quite spectacular. Fifty-nine analyses from TAG, for example, averaged 6.3% Cu, 25.2% Zn, 0.04% Pb, 378 Parts per million (ppm) Ag, 5.7 ppm Au. However, ODP drilling into TAG found that the interior was mostly relatively barren pyrite and anhydrite. Analyses of the ODP core average 2.2% Cu, 0.6% Zn, 0.0% Pb, 9 ppm Ag, 0.5 ppm Au. These discouraging results, from a potential economic point of view, do not seem to have been repeated at Middle Valley, though. It is probable that, just as is the case for land deposits, some seafloor vms, like TAG, will be teasers with some high grade parts but mostly low grade while others will be spectacular.
The eastern Manus Basin (EMB) is an approximately 4000 km2 pull-apart basin between two transform faults in arc crust of New Britain and New Ireland. Felsic volcanic ridges, mostly dacite and rhyodacite, are surrounded by andesite and basalt, very reminiscent of the famous Noranda vms district of Archean age.
Basic field work at EMB has consisted of detecting hydrothermal particulate
plumes using an instrument package that was "tow-yoed" between 20 and 300 metres
off bottom behind a ship, deep-tow camera-video traverses and sampling by dredge and
sediment corer. A joint Japanese-French submersible expedition in 1995 on which Roger Moss
and Ray Binns participated conducted detailed mapping and sampling. Extensive zones of
hydrothermally active and dead chimneys, massive sulfides and altered hyaloclastite were
discovered on two large ridges (PACMANUS and SuSu Knolls). The massive sulfides at
PACMANUS, predominantly chimneys without underlying mounds, lie on the flanks of effusive
felsic volcanic domes at a water depth near 1700 metres. Initial analyses of 26 random
samples averaged 10.9% Cu, 26.9% Zn, 1.7% Pb, 230 ppm Ag and 15 ppm Au.
The technology for extracting bulk samples from the sea floor does not yet exist in total but some engineering can be borrowed from the ill fated manganese nodule programs of the 1970s. The Japanese are developing an ocean miner which is said could be used both for manganese nodules and polymetallic sulfides. Actually, polymetallic sulfide recovery from the rather shallow seabed at EMB should be easier than for manganese nodules where depths are on the order of 5500 metres. The Japanese may be in a position to try out their new miner on a vms deposit of their own. Their Metal Mining Agency has recently announced that they have begun a 5 year feasibility study of a large deposit they have found in the Okinawa Trough at about 1600 metres water depth between Taiwan and Kyushu.