Page 435 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 417

Sediment Seafloor spreading 0 Layer 1:
sediment
Metalliferous sediment Axis
(Cu, Fe, Mn, Pb, Pb, Zn, Ba, Co, Ag, Au) Layer 2:
Stratiform oxides, hydroxides, mafic rocks
silicates (Fe, Mn) (basalt)
Massive, disseminated, and Thickness (km) Layer 3:
stockwork sulfides (Cu, Fe, Zn, Ag, Au) mafic and
Dikes 5 ultramafic rocks
Gabbro and serpentinite Upper mantle:
ultramafic rocks
Cumulates
Chromite
Lherzolite and harzburgite
Nickel and platinum sulfides
0
5
10
Fig. 13.12 Schematic block diagram showing the potential distribution of mineral deposits in the oceanic lithosphere
(redrawn from Rona, 1984, with permission from Elsevier).

may form anywhere along the volcanic arc, but large from Cyprus- or Kuroko-type. They are associated with
deposits will most likely be formed where magma intermediate to basic volcanic rocks with carbonaceous
ascent is concentrated over a prolonged period of time. mudstones, clastic limestones, or quartzites, all of
Richards (2003) reviews many of the large-scale mag- which suggest a deep water environment unlike ocean
matic and tectonic processes leading to the formation ridges, ocean basins, or island arcs. They have been
of porphyry deposits at convergent margins. termed Besshi-type deposits. They may have formed in
Another important class of deposits found associ- a trench or a tensional environment, but their origin
ated with oceanic subduction zones (Fig. 13.13) is strat- remains, as yet, obscure.
iform massive sulfi des of zinc, lead and copper known, There are several types of deposit that are specifi c to
after their type area of occurrence in Japan, as Kuroko- Andean-type subduction. These include stratabound
type ores. These ores also are known as volcanic-hosted copper sulfide deposits, such as are found in Chile,

or volcanogenic massive sulfide (VMS) deposits. They which are closely related to episodic calcalkaline

reflect deposition in a shallow marine environment and volcanism and occur within porphyritic andesite lavas.
occur interbedded with pyroclastics and silicic calc-alka- The principal minerals are chalcosite, bornite, and chal-
line lavas. Many appear to occur during a late stage of copyrite, and they contain significant amounts of silver.

volcanic arc evolution. Halbach et al. (1989) suggest that The intercalation of these deposits with shallow marine
they formed in a backarc basin (Section 9.10), and cite and terrestrial deposits suggests their formation in small
the Okinawa Trough as a modern analogue. They may lagoons. Tin and tungsten mineralization occurs in the
have been deposited by saline submarine hot springs eastern Andes of Peru and Bolivia on the landward side
arising from the separation of aqueous ore fl uids during of the porphyry copper belt. It appears to be derived
the final stages of magmatic fractionation or from the from the same Benioff zone region as the magmas, and

leaching of older volcanic rocks. Kuroko-type ores may owe its existence to the anomalously shallow dip
may be incorporated into continents during continent– of the subduction zone in this region (Section 10.2.2).
island arc collisions, such as at Río Tinto in Spain, In the backarc environment of Andean-type subduc-

Umm Samiuki in Egypt, and the Buchans mine in tion zones in the Pacific there are granite belts that
Newfoundland. contain deposits of tin and tungsten with lesser molyb-
There also exist other forms of stratiform massive denum, bismuth, and fluorite. The origin of the tin, in

sulfides that differ in their depositional environment particular, is controversial. Tin is present in only minute