HYDROTHERMAL FLUIDS and PLUMES

An important unresolved question regarding the genesis of massive sulphide ores, and particularly the rare huge deposits such as Kidd Creek in Canada (117 million metric tons), is whether a metal-rich fluid emanating directly from a magma chamber mixes with the normal hotspring water. The latter is essentially heated sea water with concentrations of metals in it that are seemingly too low to produce "giant" ore bodies.
 


Top: An opened bubble in a melt inclusion in pyroxene.
Bottom: Magnified portion of the bubble containing precipitates of dominant Cu, Ni, Zn, Fe, Na chlorides and sulphides.

Research Associate Kahui Yang is studying minute inclusions of magmatic gases trapped in crystals that were growing prior to their eruption in lavas unto the sea floor in areas where there are polymetallic sulphide deposits.
The study areas are the eastern Manus Basin offshore eastern Papua New Guinea (where there is active hydrothermal venting producing sulphide mounds and chimneys) and Bathurst, New Brunswick where the giant Brunswick #12 ore body (134 million metric tons) is being mined. Surprisingly, in both cases, the trapped magmatic fluid is predominantly carbon dioxide with lesser amounts of water, methane, and nitrogen. Lining the inclusion cavities are thin films of the very metals (except for lead) that are in the adjacent ore deposits together with sulfur and some other elements. Mixing just one percent of this metal-rich magmatic fluid with normal hydrothermal fluid would raise the total metals vented onto the sea floor by a factor of 400. Could such mixed fluids be the main source of metals in giant ore deposits? 
High temperature hot springs in the deep ocean, also known as "black smokers" because they look like a dirty industrial process, produce a cloud of fine particulate matter near the ocean floor that can be traced for kilometres down current from the source. The Scotiabank Marine Geology Research Laboratory has been among those in the forefront of developing techniques for locating such plumes as a means of finding hydrothermal vents. Fallout of particles from the plume raises the trace metal content of the underlying seafloor sediment and, if the primary dispersion patterns of these metals can be fully understood, may provide a means of locating ancient ores on land by analysing their contiguous sediments. Differences are anticipated according to the regional setting of the venting so plumes are being studied from three different tectonic environments: a sediment-starved mid-ocean ridge (Explorer Ridge offshore western Canada); a sedimented ridge (Guaymas Basin in the Gulf of California, Mexico); and a back-arc (eastern Manus Basin, Papua New Guinea).

The project demands a large number of different types of samples to be collected by surface ships and submersibles. These include water samples from within and outside the plumes for the determination of the concentration, size, distribution and composition of particulates and of surficial sediments for trace their element abundances. Special attention is being paid to anomalous concentrations of manganese, methane and rare earth elements in the sulphide particles that can help to identify their formation paths and the hydrothermal influence. The samples are being analyzed using a variety of techniques available in the Department of Geology and elsewhere at the University of Toronto.

Ancient base and precious metal massive sulfide ores being mined on land commonly have an associated distinctive, metal-rich sediment, called "exhalite", associated with them that can be traced for hundreds of metres. Because this sediment covers a much larger area than the ore deposit itself, it is easier to find so is a good guide in the exploration geologist's search for ore. Research by the Scotiabank Marine Geology Research Laboratory has shown that exhalite is formed by the fallout of particles from a hydrothermal plume, a process that can be studied on the present-day ocean floor. Mapping the distribution of certain trace elements in the exhalite may provide vectors towards ores.

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