Autochthonous and allochthonous mineral deposits

The various plate tectonic environments in which many metalliferous deposits are found are shown in Fig. 13.9. The initial rifting of a continent includes the emplacement of alkaline and peralkaline igneous rocks and the establishment of high geothermal gradients (Sections 7.4.2, 7.2, respectively). Ore minerals are generated from this magmatism and from the large-scale circulation of hydrothermal fluids that are energized by it. One group of igneous rocks frequently associated with extensive mineralization includes carbonatites. These are unusual rocks composed of more than 50% carbonate minerals that form ring complexes within alkaline rocks. The important elements found in this environment are phosphorus (as apatite), niobium (pyrochlore), rare earths (monazite, bastnaesite), copper, uranium, thorium, and zircon. Also found are magnetite, fluorite, barite, strontianite, and vermiculite. Carbonatites may also contribute to the sodium carbonate, chloride, and fluoride found in vast quantities in the lakes of the East African Rift system, although it is possible that these derive from weathering of the alkaline rocks. Directly related to the magmatism are porphyry and vein-type molybdenum deposits associated with subalkaline granites, copper-nickel deposits associated with mafic intrusions, and hydrothermal copper deposits. Within the sediments related to rifting, stratiform copper deposits of great volume are associated with specific shale or sandstone horizons. These disseminated ores are believed to form during the first marine transgression into the continental interior and overlie red-bed horizons, and are probably derived from the copper-rich rift basalts in response to the elevated heat flow of the rift. Carbonate hosted lead-zinc-barite ores are also found in the intracratonic rifts and rifted continental margins (Section 7.7), typified by the deposits of the Upper Mississippi Valley of North America. Attempts also have been made to link a wide range of mineral deposits that are related to magmatism in continental rift environ ments to the effects of rising mantle plumes (Pirajno, 2004).

With the development of a narrow ocean basin between the rifted continental fragments, new mineral deposits are created at the mid-ocean ridge. The present 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 transform faults. These contain zinc-copper-lead sediments of possible economic value, for example the Atlantis II Deep, which contains sulfide layers with zinc contents of up to 20% which are 20 m thick and cover an area of over 50 km2. It is generally agreed that the metals are of volcanic origin and have been concentrated into brines by the thermally induced circulation of seawater through the volcanic rocks and thick evaporite sequences found in this region (Cowan & Cann, 1988). As the ocean basin evolves, these deposits may become buried by sediment and reappear in collisional orogens where the tectonism obscures their original setting. Also associated with this advanced phase of rifting are sedimenthosted massive sulfide deposits which occur in thick continentally derived clastic sediments on passive continental margins. They comprise single or multiple lenses of pyrite, galena, and sphalerite ores with minor silver and copper. These are not common and probably reflect the influence of metal-rich-formation waters powered by long-lived geothermal systems.

As an ocean basin continues to grow, contemporaneous mineralization takes place at the mid-ocean ridge, and has been observed at certain locations along the Pacific (Corliss et al., 1979), Atlantic (Scott et al., 1973), and Indian Ocean ridges. The mineralization is of hydrothermal origin and its location depends upon the availability of oceanic crust of high permeability overlying the magma chamber which allows fluids to percolate with relative ease. Hydrothermal processes of low intensity lead to the formation of ferromanganese nodules, and encrustations of iron and manganese on pillow basalts at the layer 1-layer 2 interface. Higher intensity hydrothermal activity has been observed at some locations, such as on the East Pacific Rise where discharge is of two types. Black smokers are vents where pyrrhotite particles are discharged, producing ores which may be zinc- or iron-rich and containing lesser amounts of cobalt, lead, silver, and cadmium. At white smokers little sulfide material is discharged, and the main precipitate is barite.

In open ocean conditions ferromanganese nodules and encrustations form on top of basalt or sediments

Mineral Deposits Tectonics
Fig. 13.9 (a-j) Schematic cross-sections through plate boundary-related tectonic settings of mineralization (redrawn from Mitchell & Garson, 1976).

Fig. 13.10 Locations of hot brine pools and metalliferous sediments in the Red Sea (redrawn from Bignell et al., 1976, with permission from the Geological Association of Canada).

Fig. 13.10 Locations of hot brine pools and metalliferous sediments in the Red Sea (redrawn from Bignell et al., 1976, with permission from the Geological Association of Canada).

where strong ocean currents prevent the accumulation of clastic sediments. These deposits are of hydrous origin and accumulate slowly, sometimes forming extensive pavements. As well as iron and manganese, they also contain smaller amounts of copper, nickel, and cobalt.

In addition to the exhalative processes of mineralization described above, ore bodies may form within the oceanic lithosphere as it is created. Much of our knowledge of these deposits is derived from studies of ophio-lites (Section 2.5), which are interpreted as allochthonous slices of oceanic or backarc basin lithosphere tectoni-cally emplaced within continental crust during colli-sional orogenesis. One of the most intensively studied bodies of this type is the Troodos complex of Cyprus (Fig. 13.11), and other examples include ophiolites of northwestern Newfoundland, the Semail Ophiolite of Oman, and Ergani Maden in Turkey. At high levels in the lithosphere, massive sulfide deposits (marcasite, chalcopyrite and sphalerite) occur on top of or within the pillow lavas of layer 2. It has been suggested that these sulfides formed either in a manner similar to the brines of the Red Sea or by precipitation from hydrothermal solutions which became enriched in metals by circulating within the volcanic rocks. The deeper, plutonic portions of ophiolites contain economic deposits of chromite, which occur as podiform bodies and tabular masses within the harzburgites and dunites of the upper lithospheric mantle. These deposits may have formed by partial melting of primitive mantle material or by crystal fractionation within the magma chamber underlying ocean ridges (Section 6.10). Similarly associated with the magma chamber are nickel and platinum sulfides. The model for mineralization of the oceanic lithosphere derived from ophiolite studies is shown in Fig. 13.12 (Rona, 1984).

The formation of ore deposits composed of nickel, copper and platinum group elements also can be linked to the mafic and ultramafic magmatism that results in the formation of LIPs (Section 7.4.1). Examples of these types of deposit include the 250 Ma Noril'sk deposit in Siberia, which currently produces 70% of the world's palladium, and the Proterozoic Bushveld intrusion (Section 11.4.1) of South Africa, which produces large quantities of platinum and chrome (Naldrett, 1999).

Exploration models that integrate the characteristics of these deposits with the formation of LIPs rely on detailed information about the sources of the magma and the deep plumbing systems that transport it through the crust (Pirajno, 2004).

Several forms of mineralization are present in subduction zone environments, their types depending upon whether the overriding lithosphere is continental or oceanic. Hedenquist & Lowenstern (1994) have reviewed the role of magmas in the formation of hydrothermal ores in such environments. The most important mineralizations are the porphyry coppers. These are relatively rare, low grade deposits that are gold-rich and molybdenum-poor when associated with island arcs and gold-poor and molybdenum-rich in Andean-type mountain ranges. They are found, for example, in the Andes themselves, as well as in the Philippines, Taiwan, Puerto Rico, and the Ryuku and Burman arcs. The broad uniformity of these deposits worldwide suggests that the controls on their location are related to the primary subduction-related magmatism and do not require the presence of any specific crustal or magma type. Magma may be emplaced and porphyry coppers

Chromite Mineralisation Ophiolites
Fig. 13.11 Mineralization in the Troodos Ophiolite (redrawn from Searle & Panayiotou, 1980, with permission from the Ministry of Agriculture and Natural Resources, Cyprus).

Sediment Metalliferous sediment (Cu, Fe, Mn, Pb, Pb, Zn, Ba, Co, Ag, Au) Stratiform oxides, hydroxides, silicates (Fe, Mn) Massive, disseminated, and stockwork sulfides (Cu, Fe, Zn, Ag, Au)

Dikes

Gabbro and serpentinite

Cumulates Chromite

Lherzolite and harzburgite-

Nickel and platinum sulfides

Minerals Found Lithosphere

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).

Layer 1: sediment Layer 2: mafic rocks (basalt) Layer 3: mafic and ultramafic rocks

Upper mantle: ultramafic rocks

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 deposits will most likely be formed where magma ascent is concentrated over a prolonged period of time. Richards (2003) reviews many of the large-scale mag-matic and tectonic processes leading to the formation of porphyry deposits at convergent margins.

Another important class of deposits found associated with oceanic subduction zones (Fig. 13.13) is stratiform massive sulfides of zinc, lead and copper known, after their type area of occurrence in Japan, as Kuroko-type ores. These ores also are known as volcanic-hosted or volcanogenic massive sulfide (VMS) deposits. They reflect deposition in a shallow marine environment and occur interbedded with pyroclastics and silicic calc-alka-line lavas. Many appear to occur during a late stage of volcanic arc evolution. Halbach et al. (1989) suggest that they formed in a backarc basin (Section 9.10), and cite the Okinawa Trough as a modern analogue. They may have been deposited by saline submarine hot springs arising from the separation of aqueous ore fluids during the final stages of magmatic fractionation or from the leaching of older volcanic rocks. Kuroko-type ores may be incorporated into continents during continent-island arc collisions, such as at Río Tinto in Spain, Umm Samiuki in Egypt, and the Buchans mine in Newfoundland.

There also exist other forms of stratiform massive sulfides that differ in their depositional environment from Cyprus- or Kuroko-type. They are associated with intermediate to basic volcanic rocks with carbonaceous mudstones, clastic limestones, or quartzites, all of which suggest a deep water environment unlike ocean ridges, ocean basins, or island arcs. They have been termed Besshi-type deposits. They may have formed in a trench or a tensional environment, but their origin remains, as yet, obscure.

There are several types of deposit that are specific to Andean-type subduction. These include stratabound copper sulfide deposits, such as are found in Chile, which are closely related to episodic calcalkaline volcanism and occur within porphyritic andesite lavas. The principal minerals are chalcosite, bornite, and chal-copyrite, and they contain significant amounts of silver. The intercalation of these deposits with shallow marine and terrestrial deposits suggests their formation in small lagoons. Tin and tungsten mineralization occurs in the eastern Andes of Peru and Bolivia on the landward side of the porphyry copper belt. It appears to be derived from the same Benioff zone region as the magmas, and may owe its existence to the anomalously shallow dip of the subduction zone in this region (Section 10.2.2).

In the backarc environment of Andean-type subduction zones in the Pacific there are granite belts that contain deposits of tin and tungsten with lesser molybdenum, bismuth, and fluorite. The origin of the tin, in particular, is controversial. Tin is present in only minute

Outer arc Arc Basin

Autochthonous Mineral Deposit Diagram
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 island arcs. An oceanic origin of the tin appears unlikely. One hypothesis is that tin is derived from deep in a Benioff zone which is migrating away from a continent during backarc spreading. The fluorine originating at these levels would extract tin from deep levels of still hot granite plutons and deposit it at the surface in their vicinity. Another hypothesis is that the generation of tin requires the presence of thick continental crust, such as is present in the tin belts of the Andes, Alaska and upper Myanmar, and a shallow dipping Benioff zone, which acts as a source of heat and volatiles. In this case the tin may originate from pre-existing concentrations in the lower continental crust.

Within ensialic backarc basins vein-type gold and silver deposits are common, such as are found in the Great Basin of Nevada. These are associated with andesites, dacites, and rhyolites, and pre-date the major episode of faulting. They probably originate in mag-matically associated brines. In backarc basins that form above oceanic subduction zones the crust is similar to oceanic, although generated in a different fashion, and so mineral deposits would not be expected to differ greatly from those in oceanic crust. The mineralization in these settings may be similar to that formed during the early development of a spreading ridge, and thus may be related to magmatic and exhalative volcanic processes.

Zones of continental collision and terrane accretion (Sections 10.4, 10.6) also host a wide range of metalliferous deposits. These belts may display allochthonous terranes containing mineral associations that formed during the early stages of crustal accretion, such as ophiolites, ferromanganese nodules, subduction-related deposits, and mineralization related to the early stages of rifting. Deposits that originate during the continental collision also are present. Solomon (1990) noted that, in the southwestern Pacific rim, porphyry copper-gold deposits mostly form after a reversal of arc polarity following a collisional event. Sometimes associated with porphyry coppers are mercury deposits (as cinnabar or quicksilver), which may have originated in a similar manner. Granite bodies commonly are emplaced during and after a collisional event. Associated with these granites are tin-tungsten deposits of cassiterite and wolframite and, in some cases, vein-type deposits of uranium. This mineralization, like the granites, may be derived from the partial melting of the lower continental crust.

The Paleozoic Lachlan Orogen of southeast Australia illustrates the types of base and precious metals that form and are preserved in long-lived accretionary orogens (Section 10.6.3). Bierlein et al. (2002) describe orogenic gold deposits that evolved within developing accretionary wedges while major porphyry copper-gold deposits formed in an oceanic island arc located offshore of the Pacific margin of Gondwana. As deformed oceanic sequences, volcanic arcs, and microcontinents accreted onto the Australian margin, sediment-hosted copper-gold and lead-zinc deposits formed in short-lived intra-arc basins, while volcanogenic massive sulfide deposits were produced in forearc regions. Compression leading to the inversion (Section 10.3.3) of these basins also triggered pulses of orogenic gold mineralization. This and other studies (Groves et al., 2003) illustrate that gold-rich deposits can form during any stage of orogenic evolution.

Oceanic transform faults are favorable environments for mineralization because they may be associated with high heat flow and provide highly fractured and permeable conduits for both the downward percolation of seawater and the upward migration of mineralizing fluids. Iron sulfide concretions have been reported from the Romanche Fracture Zone of the equatorial Atlantic which may have originated by this mechanism. The brine pools of the Red Sea appear to be located where transform faults intersect the central ridge, and it is possible that the metals ascend along these faults. Indeed, base metal deposits are found along the continental continuation of the faults. A similar mechanism has been postulated for the brines of the Salton Sea, California. It is probable that the ultramafic intrusions occurring in fracture zones (Section 6.12) contain high proportions of nickel, cobalt, and copper.

For mineralization in the Archean cratons, analogies with the plate tectonic settings of some Phanerozoic deposits are possible. For example, many Archean greenstone belts host volcanogenic massive sulfides (Kuroko-type), copper-zinc-lead sulfides, and gold deposits that also occur throughout the Phanerozoic record. However, many aspects of Archean metallogenesis require further investigation. Porphyry coppers, which typically have a clear association with subduction zone environments, are extremely rare in the Archean, except for a few controversial examples (Herrington et al., 1997). In addition, nickel-sulfide deposits hosted by komatiites in Archean greenstone belts (Section 11.3.2) have no modern analogues. Some studies (de Ronde et al., 1997) have suggested that fluid circulation in the Archean occurred at a larger scale than during other times in Earth's history, which would have influenced the formation of hydrothermal ore deposits. These features may reflect fundamentally different tectonic and/or crustal processes operating during the Archean compared to Phanerozoic times (Section 11.3).

Banded iron formations (BIFs) are common in Archean cratons (Section 11.3.2), although they also occur in rocks as young as Devonian. These rocks contain magnetite, hematite, pyrite, siderite, and other iron-rich silicates. Two main types have been identified (Pirajno, 2004). An Algoma type is associated with volcanic sequences in backarc environments. A Superior type is associated with sedimentary sequences deposited on the continental shelves of rifted continental margins. The development of BIFs on a global scale during Late Archean and Early Proterozoic times also may reflect a period of enhanced mantle plume activity.

Proterozoic mineral deposits are widely interpreted as forming in plate tectonic environments, particularly those related to divergent plate margins and subduction zones (Gaal & Schulz, 1992). Possible exceptions to this approach may include massif-type anorthosite complexes, which are associated with iron-titanium deposits of magnetite and ilmenite. These magma-hosted ore deposits may have originated during episodes of lower crustal melting (Section 11.4.1). Some studies have related such magmatism to the break-up of supercontinents, to zones of continental rifting, and to mantle plumes (Pirajno, 2004).

Another type of magma-hosted deposit includes diamonds that occur in kimberlite pipes. Kimberlites consist of small potassic, ultramafic intrusions that originate from the mantle. These intrusions occur in virtually every Archean craton as well as throughout the Phanerozoic. In some areas, such as in parts of North and South America, there is good evidence that the majority of kimberlites were generated during times of enhanced hot spot or mantle plume activity (Sections 5.5, 5.7), possibly associated with the break-up of the supercontinent Gondwana. The relationships among kimberlite magmatism, supercontinent assembly and dispersal, the relative stability of the crust and mantle, and diamond productivity are discussed further by Heaman et al. (2003).

Was this article helpful?

0 0
Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook


Responses

  • sophie himmel
    What is allochthonous deposit?
    2 years ago
  • gruffo
    Which of the minerals found in sandstones is autochthonous?
    2 years ago
  • petra
    What is atochathonous mineral?
    2 years ago

Post a comment