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As in the case of emeralds, most mineral deposits (Ni, Cr, Co, V, Mn, Au, Ag, Sn, Cu, Zn, Sn, Ta, Nb, Pb, Th, U, Mo, Hg, B, Be) can be considered as exploitable concentrations of elements that otherwise are found only at minor or trace concentration levels in rocks of the mantle and crust. The formation of a mineral deposit requires therefore the coincidence of four principle factors: 1) the existence of a rock source containing the relevant element, 2) a liquid medium capable of dissolving and fractionating the element, 3) a structure to focus the passage of the fluid, and 4) a rock filter to extract and concentrate the element. (The formation of an exploitable deposit may involve several such cycles, plus a final stage of concentration involving the removal of host rock material by a shallow circulating fluid system.) In the case of gold deposits associated with ultramafic rocks occurring along the Larder Lake Break (fault zone) of the Kirkland Lake region of Ontario, magnetite produced by the serpentinization of olivine traps sulphur to form pyrite, which in turn traps arsenic to form arsenopyrite, which in turn filters the gold. Magnetite represents the primary filter material in many other gold deposits. (It should be noted as a matter of environmental concern that this process is merely a delaying action or buffer, notwithstanding a complex one, on the access of sulphur from the mantle to the atmosphere, since the sulphur in many mineral deposits as well as the background sulphur in continental crust is ultimately freed to the atmosphere by the natural process of erosion. The action of man as miner simply speeds this process. For example, while the coal mines of South Wales in Britain have all been closed, the mines have to be continually pumped to prevent flooding of the galleries and the consequent rapid removal of sulphur as sulphuric acid to the surface river sytems. A simple question of surface area and access to oxygen.)
Since all crustal materials other than meteorite debris must inevitably be derived from the mantle, many of the elements of economic interest pass through a first-order concentration cycle related to plate tectonic processes, where the source is the mantle, the liquid medium is basaltic melt rock, the focus is the spreading ridge feeder zone, and the filter is the oceanic crust formed above the feeder zone. Primary deposits formed during such 1st-order cycles include chromite, filtered and trapped in the feeder zone and periodically fed into the overlying migrating oceanic crust, and, should the basaltic melt contain sufficient sulphur, perhaps a nickel and PGE-bearing intercumulus sulphide melt fraction.
Further concentration may take place during three independent 2nd-order cycles. Firstly, the development of sea-water convection cells will cause hydrothermal mobilization of elements such as Au, Cu and Zn and re-precipitation of these elements higher in the oceanic crust, or at the surface of the oceanic crust (e.g. black smokers; Red Sea metalliferous brines; Jerome, Arizona). Released into sea water some elements such as Mn and Cu may also be precipitated in continental margin sediments. Secondly, the dehydration of hydrated oceanic crust during its subduction will cause the transfer of chloride and sulphate bearing fluids and their dissolved metal element load into the overlying mantle. Thirdly, should oceanic crust be obducted onto a continental margin (e.g. New Caledonia), intense weathering under tropical conditions may cause hydration and leaching of olivine and the concentration of Ni as a residual component in garnierite serpentine (.3% Ni in olivine converted to 6% Ni in garnierite). In other cases of obducted oceanic crust (e.g. Bou Azzer, Maroc) hydrothermal processes may lead to the formation of important nickel and cobalt sulphide deposits.
The formation of supra-subduction zone melts in the contaminated mantle initiates a 3rd cycle. The melts are transferred to crustal magma chambers and to the surface as volcanic rocks, where once again elements are subject to solution by hydrothermal systems and re-deposition in favourable structural sites. In this case metal zonation tends to mimic depth of subduction and arc maturity. Copper is concentrated in immature arcs (low K) and those parts of arcs nearest to the arc trench, whereas lead and zinc are concentrated in mature arcs (high K) and at sites furthest from the arc trench. In the case of arcs built on continental margins, the remobilization of material in the underlying continental crust may allow further element fractionation leading to the formation of tin deposits.
Subduction related hydrothermal ore deposits are classified as:
Porphyry - adjacent to or hosted by the intrusion, 2-5 km depth
Skarn - adjacent to intrusion in carbonate rock, 1-5 km depth, Fe, Cu, Sn, W, Mo, Au, Ag, Pb-Zn
Pluton-related veins - fractures in and near intrusion, variable depth, Sn, W, Mo, Pb-Zn, Cu, Au
Epithermal, high sulphidization - above parent intrusion, < 1.5 km depth, Au-Cu, Ag-Pb, Hg
Epithermal, low suphidization
- Distant from magmatic heat source, < 2 km depth;
low salinity -Au (Ag, Pb-Zn);
moderate salinity - Ag-Pb-Zn (Au)
Massive sulphide - near extrusive domes, near sea floor, Zn-Pb-Ag (Cu, Au)
The hydrothermal fluids emanating from arc magmas may consist of a low-density vapour and a dense hypersaline liquid. Both immiscible phases are capable of mixing with meteoric water, particularly the vapor phase, which will tend to discharge to the surface as volcanic fumaroles, or form acidic water capable of leaching the host rock. Magmatic fluids dominate the early history of hydrothermal systems, whereas meteoric water becomes more important with time and distance from the magma (epithermal). Only a small proportion of hydrothermal systems actually form ore, and the fluids responsible for mineralization may be present only for short periods during the lifetime of the hydrothermal system, possibly at times of individual tectonic or hydraulic fracturing events.
Finally, a fourth cycle may be engendered by the elevation of mountain systems as a result of the collision of arcs and continental margins. The collision may lead to the superimposition of a meteoric water hydrologic system that flushes metal laden brines into carbonate rocks of the bordering continental cratons, leading to the formation of mineral deposits rich in zinc and lead (Mississippi Valley type ores). The collisional stages of plate systems may also lead to the formation of hydrothermal mineral deposits related to a phase of crustal melting of extensively underplated, hydrated oceanic crust. This is commonly referred to as post-orogenic magmatism.
Special conditions of high heat flow during the Archean, and related intense hydrothermal activity, led to the extraordinary abundance of komatiite Ni, PGE (Platinum Group elements), felsic volcanogenic-Cu and Zn, sedimentary iron ores and fracture related Au deposits. This is a temporal feature as distinct from a process related feature. It is also thought that some deposits (sediment hosted Cu [Keweenawan; Zambia; Morocco] and Pb-Zn deposits; anorthosites; uranium in weathered profiles; U-Cu-Au Olympic Dam deposits) are related to the break up and dispersal of periodically assembled supercontinents.
Structural Provinces of North America.
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