Page 431 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 413

deposits in Archean cratons is complicated by the pos- ments to the effects of rising mantle plumes (Pirajno,
sibility of unique geologic or tectonic process operating 2004).
in the early Earth (Section 11.1). These differences With the development of a narrow ocean basin
require models that incorporate the unique aspects of between the rifted continental fragments, new mineral
Archean geology and tectonics (Herrington et al., deposits are created at the mid-ocean ridge. The present
1997). day example of this environment is the Red Sea. Here
13 pools of hot brines (Fig. 13.10) have been located
along the central ridge where it is intersected by trans-
13.2.2 Autochthonous and form faults. These contain zinc-copper-lead sediments
of possible economic value, for example the Atlantis II
allochthonous mineral deposits Deep, which contains sulfide layers with zinc contents

of up to 20% which are 20 m thick and cover an area of
2
The various plate tectonic environments in which many over 50 km . It is generally agreed that the metals are of
metalliferous deposits are found are shown in Fig. 13.9. volcanic origin and have been concentrated into brines
The initial rifting of a continent includes the emplace- by the thermally induced circulation of seawater
ment of alkaline and peralkaline igneous rocks and the through the volcanic rocks and thick evaporite sequences
establishment of high geothermal gradients (Sections found in this region (Cowan & Cann, 1988). As the
7.4.2, 7.2, respectively). Ore minerals are generated ocean basin evolves, these deposits may become buried
from this magmatism and from the large-scale circula- by sediment and reappear in collisional orogens where
tion of hydrothermal fluids that are energized by it. the tectonism obscures their original setting. Also asso-

One group of igneous rocks frequently associated with ciated with this advanced phase of rifting are sediment-

extensive mineralization includes carbonatites. These are hosted massive sulfide deposits which occur in thick
unusual rocks composed of more than 50% carbonate continentally derived clastic sediments on passive con-
minerals that form ring complexes within alkaline tinental margins. They comprise single or multiple
rocks. The important elements found in this environ- lenses of pyrite, galena, and sphalerite ores with minor
ment are phosphorus (as apatite), niobium (pyrochlore), silver and copper. These are not common and probably

rare earths (monazite, bastnaesite), copper, uranium, reflect the influence of metal-rich-formation waters

thorium, and zircon. Also found are magnetite, fl uorite, powered by long-lived geothermal systems.
barite, strontianite, and vermiculite. Carbonatites may As an ocean basin continues to grow, contemporane-
also contribute to the sodium carbonate, chloride, and ous mineralization takes place at the mid-ocean ridge,

fluoride found in vast quantities in the lakes of the East and has been observed at certain locations along the
African Rift system, although it is possible that these Pacifi c (Corliss et al., 1979), Atlantic (Scott et al., 1973),
derive from weathering of the alkaline rocks. Directly and Indian Ocean ridges. The mineralization is of
related to the magmatism are porphyry and vein-type hydrothermal origin and its location depends upon the
molybdenum deposits associated with subalkaline gran- availability of oceanic crust of high permeability overly-
ites, copper-nickel deposits associated with mafi c intru- ing the magma chamber which allows fl uids to perco-
sions, and hydrothermal copper deposits. Within the late with relative ease. Hydrothermal processes of low
sediments related to rifting, stratiform copper deposits intensity lead to the formation of ferromanganese
of great volume are associated with specifi c shale nodules, and encrustations of iron and manganese on
or sandstone horizons. These disseminated ores are pillow basalts at the layer 1–layer 2 interface. Higher

believed to form during the first marine transgression intensity hydrothermal activity has been observed at
into the continental interior and overlie red-bed hori- some locations, such as on the East Pacifi c Rise where
zons, and are probably derived from the copper-rich rift discharge is of two types. Black smokers are vents where

basalts in response to the elevated heat flow of the rift. pyrrhotite particles are discharged, producing ores
Carbonate hosted lead-zinc-barite ores are also found in which may be zinc- or iron-rich and containing lesser
the intracratonic rifts and rifted continental margins amounts of cobalt, lead, silver, and cadmium. At white

(Section 7.7), typified by the deposits of the Upper Mis- smokers little sulfide material is discharged, and the main

sissippi Valley of North America. Attempts also have precipitate is barite.
been made to link a wide range of mineral deposits that In open ocean conditions ferromanganese nodules
are related to magmatism in continental rift environ- and encrustations form on top of basalt or sediments