Page 436 - Global Tectonics

418 CHAPTER 13

Outer arc Arc Basin

Podiform chromite Position of possible
spreading axis with
Mélange with Cyprus-type deposits
oceanic sulfide Besshi-type
deposit massive sulfides
Oceanic sulfide Porphyry copper ore
deposits
[Cyprus-type] Kuroko-type Plutons with
being transported massive Sn, W, Bi veins
in oceanic crust sulfides
Continental
Oceanic crust Batholiths Oceanic crust crust
Descending lithospheric
plate
Ascending
calc-alkaline
magma

Asthenosphere
Partial melting

Fig. 13.13 Development and emplacement of mineral deposits in a subduction-related setting (redrawn from Evans,
1987, using data from Sillitoe, 1972a, 1972b, with permission from the Economic Geology Publishing Co. and the
Institute of Mining and Metallurgy).

quantities in the oceanic crust, and is similarly absent in episode of faulting. They probably originate in mag-
island arcs. An oceanic origin of the tin appears unlikely. matically associated brines. In backarc basins that form
One hypothesis is that tin is derived from deep in a above oceanic subduction zones the crust is similar to
Benioff zone which is migrating away from a continent oceanic, although generated in a different fashion, and

during backarc spreading. The fluorine originating at so mineral deposits would not be expected to differ
these levels would extract tin from deep levels of still greatly from those in oceanic crust. The mineralization
hot granite plutons and deposit it at the surface in their in these settings may be similar to that formed during
vicinity. Another hypothesis is that the generation of tin the early development of a spreading ridge, and thus
requires the presence of thick continental crust, such as may be related to magmatic and exhalative volcanic
is present in the tin belts of the Andes, Alaska and upper processes.
Myanmar, and a shallow dipping Benioff zone, which Zones of continental collision and terrane accretion
acts as a source of heat and volatiles. In this case the tin (Sections 10.4, 10.6) also host a wide range of metallifer-
may originate from pre-existing concentrations in the ous deposits. These belts may display allochthonous
lower continental crust. terranes containing mineral associations that formed
Within ensialic backarc basins vein-type gold and during the early stages of crustal accretion, such as
silver deposits are common, such as are found in the ophiolites, ferromanganese nodules, subduction-related
Great Basin of Nevada. These are associated with deposits, and mineralization related to the early stages
andesites, dacites, and rhyolites, and pre-date the major of rifting. Deposits that originate during the continental