Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 85

Box 4.2 VMS Deposits of the Urals
The Urals in Russia host six enormous VMS deposits, each containing more
than 100 mt of ore, and many smaller deposits. These deposits formed in
volcanic-dominated sequences that evolved during the Silurian and Devonian
as oceanic island arcs collided with the Precambrian continent of central
Russia. The deposits contain different metals that can be correlated with both
the nature of the associated volcanic rocks and their tectonic setting. In
certain deposits the dominant ore metals are Cu and Zn, and these are
associated with tholeiitic mafic volcanics that erupted in an early arc setting.
Other deposits are polymetallic and contain significant concentrations of Pb,
Ag, and Au in addition to Cu and Zn. These are hosted by bimodal mafic-
felsic calc-alkaline volcanics and sediments in forearcs or rifted arcs that
developed during arc-continental collision.
A remarkable feature of many deposits is their excellent preservation due
to an absence of metamorphism and deformation following their deposition.
This has meant that their textures, structures and compositions show minimal
disturbance, which has provided a window to the ore-forming processes.
Herrington et al. ( 2005 ) have described how clastic and hydrolytic processes
that preceded diagenesis on the ancient sea floor modified the morphology
and mineralogy of the deposits. The excellent preservation also allowed the
preservation of fossilized tube worm stuctures and other examples of the
fauna that constitute part of the unique ecological systems surrounding
hydrothermal vents. The similarity between the Silurian fossils and modern
vent fauna attests to the slow evolution of this part of the biosphere.
between ascending hydrothermal fluid and seawater; (5) the massive sulfide deposit
itself, formed at or near the sea floor; and (6) bedded sediments formed by precipita-
tion of sulfides and other components from the hydrothermal plume.
A VMS deposit forms in the following way. Magma intrudes at a shallow level
in the oceanic crust. It heats seawater that is present in pores and fractures in the
volcanic and sedimentary rocks and causes the water to circulate through the
volcanic pile (Fig. 4.6 ). As it does so it draws down seawater into rocks flanking
the intrusion, thus setting up a convective system. The cold seawater percolates
down through the oceanic crust through open fissures and the slightly alkaline water
precipitates its sulfates and carbonates as it descends. Its temperature progressively
increases and as the fluid approaches the magma chamber at 2-3 km depth, it has
been transformed to hot hydrothermal fluid whose temperature is 350-400 C and
whose pH has decreased to 4-6. As it approaches the critical point its volume
increases drastically, driving it back up towards the surface. The hot, acid, corrosive
liquid leaches metals from the volcanic or sedimentary rocks and these metals are
transported upwards, probably as metal halide complexes. The fluids ascend along
fractures until they reach the seafloor. On expulsion they cool rapidly and mix with