Page 437 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 419

collision also are present. Solomon (1990) noted that, in deposits are possible. For example, many Archean

the southwestern Pacific rim, porphyry copper-gold greenstone belts host volcanogenic massive sulfi des

deposits mostly form after a reversal of arc polarity fol- (Kuroko-type), copper-zinc-lead sulfides, and gold
lowing a collisional event. Sometimes associated with deposits that also occur throughout the Phanerozoic
porphyry coppers are mercury deposits (as cinnabar or record. However, many aspects of Archean metallo-
quicksilver), which may have originated in a similar genesis require further investigation. Porphyry coppers,
manner. Granite bodies commonly are emplaced during which typically have a clear association with subduc-
and after a collisional event. Associated with these gran- tion zone environments, are extremely rare in the
ites are tin-tungsten deposits of cassiterite and wolfram- Archean, except for a few controversial examples (Her-
ite and, in some cases, vein-type deposits of uranium. rington et al., 1997). In addition, nickel-sulfi de depos-
This mineralization, like the granites, may be derived its hosted by komatiites in Archean greenstone belts
from the partial melting of the lower continental (Section 11.3.2) have no modern analogues. Some
crust. studies (de Ronde et al., 1997) have suggested that

The Paleozoic Lachlan Orogen of southeast Austra- fluid circulation in the Archean occurred at a larger
lia illustrates the types of base and precious metals that scale than during other times in Earth’s history, which

form and are preserved in long-lived accretionary would have influenced the formation of hydrothermal
orogens (Section 10.6.3). Bierlein et al. (2002) describe ore deposits. These features may refl ect fundamentally
orogenic gold deposits that evolved within developing different tectonic and/or crustal processes operating
accretionary wedges while major porphyry copper-gold during the Archean compared to Phanerozoic times
deposits formed in an oceanic island arc located off- (Section 11.3).
shore of the Pacific margin of Gondwana. As deformed Banded iron formations (BIFs) are common in

oceanic sequences, volcanic arcs, and microcontinents Archean cratons (Section 11.3.2), although they also
accreted onto the Australian margin, sediment-hosted occur in rocks as young as Devonian. These rocks
copper-gold and lead-zinc deposits formed in short-lived contain magnetite, hematite, pyrite, siderite, and other
intra-arc basins, while volcanogenic massive sulfi de iron-rich silicates. Two main types have been identifi ed
deposits were produced in forearc regions. Compres- (Pirajno, 2004). An Algoma type is associated with vol-
sion leading to the inversion (Section 10.3.3) of these canic sequences in backarc environments. A Superior
basins also triggered pulses of orogenic gold mineraliza- type is associated with sedimentary sequences depos-
tion. This and other studies (Groves et al., 2003) illus- ited on the continental shelves of rifted continental
trate that gold-rich deposits can form during any stage margins. The development of BIFs on a global scale
of orogenic evolution. during Late Archean and Early Proterozoic times

Oceanic transform faults are favorable environments also may reflect a period of enhanced mantle plume
for mineralization because they may be associated with activity.
high heat flow and provide highly fractured and perme- Proterozoic mineral deposits are widely interpreted

able conduits for both the downward percolation of as forming in plate tectonic environments, particularly
seawater and the upward migration of mineralizing those related to divergent plate margins and subduction

fluids. Iron sulfide concretions have been reported from zones (Gaál & Schulz, 1992). Possible exceptions to this
the Romanche Fracture Zone of the equatorial Atlantic approach may include massif-type anorthosite com-
which may have originated by this mechanism. The plexes, which are associated with iron-titanium deposits
brine pools of the Red Sea appear to be located where of magnetite and ilmenite. These magma-hosted ore
transform faults intersect the central ridge, and it is deposits may have originated during episodes of lower
possible that the metals ascend along these faults. crustal melting (Section 11.4.1). Some studies have
Indeed, base metal deposits are found along the conti- related such magmatism to the break-up of superconti-
nental continuation of the faults. A similar mechanism nents, to zones of continental rifting, and to mantle
has been postulated for the brines of the Salton Sea, plumes (Pirajno, 2004).
California. It is probable that the ultramafi c intrusions Another type of magma-hosted deposit includes dia-
occurring in fracture zones (Section 6.12) contain high monds that occur in kimberlite pipes. Kimberlites
proportions of nickel, cobalt, and copper. consist of small potassic, ultramafi c intrusions that
For mineralization in the Archean cratons, analogies originate from the mantle. These intrusions occur in
with the plate tectonic settings of some Phanerozoic virtually every Archean craton as well as throughout