Metallogeny is the study of how mineral deposits form and are distributed across regions and globally, focusing on their relationship to the Earth’s crust’s petrologic and tectonic features over time and space.
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The evolutionary sequence of changes in the Earth’s crust and mantle over geological time is now widely recognized. These transformations have played a significant role in petrogenesis, as well as influencing the character and scale of associated mineralization. To illustrate, the relationship between tin mineralization with Mesozoic and late Paleozoic granites, the confinement of Banded Iron Formations to the Precambrian, and the association of nickel deposits with Precambrian orthomagmatic ilmenite deposits are indicative of these influences. Consequently, the scarcity of specific metallogenic occurrences in subsequent periods can be ascribed to the advanced phases of crust and mantle evolution. For instance, the absence of Phanerozoic nickel sulfide deposits might be explained by the depletion of mantle sulfur during the Archaean.
The progressive evolutionary changes experienced by the mantle and the overlying crust, including mineral deposits, allow us to draw specific generalizations about the disposition of metallogenic provinces in relation to the progressive development of the earth’s mantle with time. These evolutionary transformations can be effectively examined in the context of the Archaean, Proterozoic, and Phanerozoic periods, along with the corresponding environments that existed during these eras.
The Archean:
During this period, there is a significant presence of specific metals and a lack of others. Notable concentrations of metals and their associations include Au, Sb, Fe, Mn, Cr, Ni-Cu, and Cu-Zn-Fe. Noteworthy omissions are Pb, U, Th, Hg, Nb, Zr, REE, and diamond. Significant deposits comprise:
- Chromium and nickel in serpentinites and chromite (Great Dyke).
- Komatiitic and tholeiitic lavas hosting Ni-Cu deposits (Kalgoorlie Belt in Australia, Southern Canada, Zimbabwe-Rhodesia, and the Baltic Shield).
- Gold deposits found in greenstone belts (Golden Mile Dolerite of Kalgoorlie).
- Volcanogenic-associated Cu-Zn deposits (Albiti Orogen in Southern Canada).
The Proterozoic:
The start of the Proterozoic era witnessed a significant shift in tectonic conditions. Initial lithospheric plate formation allowed for the creation of sedimentary basins, facilitating the deposition of platform sediments and the development of continental margin geosynclines. Remarkable mineral deposits during this period encompass:
- Gold uranium conglomerates, exemplified by the well-known Witwatersrand Basin.
- Sedimentary manganese deposits found in South Africa, Brazil, and India.
- Carbonate-hosted stratiform lead-zinc deposits, as seen in McArthur River, Mount Isa, and West Germany.
- Deposits of Cr-Ni-Pt-Cu, notably in the Great Dykes of Zimbabwe Rhodesia and the Bushveld Complex in South Africa.
- Titanium-iron deposits in Norway and Canada.
- Banded Iron Formations observed in India, Africa, and Canada.
- Sedimentary copper deposits, such as those in the Katanga System of Zambia and Shaba, and the Belt Series of Northwest USA.
- Sedimentary manganese deposits located in Central India and Namibia.
- Tin deposits associated with alkaline and peralkaline granites and pegmatites, found in West Africa and Brazil.
The Phanerozoic:
In the Proterozoic’s later stages, a distinctive tectonic pattern emerged, leading to the formation of Phanerozoic fold belts through continental drift. Throughout this era, mineralization processes tended to concentrate in specific tectonic settings, such as rift valleys, aulacogens, associated domes, constructive and destructive plate margins, and transform faults. The distribution of deposits during this period is discussed in the context of Plate Tectonics and the disposition of Metallogenic Provinces below. Thus, it can be asserted that one of the key factors influencing the disposition of metallogenic provinces is the evolutionary history of the respective segment of the Earth’s crust.
Deposition of Metallogenic Provinces in Relation to Plate Tectonic Settings:
Furthermore, the positioning of metallogenic provinces is significantly influenced by global tectonics. The diversity of deposit types forming in distinct plate tectonic settings establishes relations between various mineralization and extensive crustal structures. As a result:
- Tin, fluoride, and niobium deposits are linked to intra-continental hot spots and aulacogens.
- Carbonatite-associated deposits and Sullivan-type Pb-Zn-Ag deposits are correlated with aulacogens.
- Lead-zinc and evaporite deposits are associated with intercontinental rift zones (Red Sea Type).
- Porphyry Cu, Mo, Sn, and W deposits are found in compressional arc systems (Andean Type).
- Podiform Cr and Pt deposits, as well as Cyprus-type Cu-Fe deposits, occur in obducted ophiolites in continental collision zones.
- Lead-zinc-silver deposits and uranium deposits are situated in molasse sediments in continent-continent collision zones.
A considerable portion of the material originating from the mentioned sites, including the early stages of crustal evolution, can be transported through various mechanisms. These mechanisms encompass primary plate movement, reversal of plate movement, and continental collision, leading the material to areas with entirely different tectonic settings compared to the sites of its initial formation and genetic association. For instance, mineral deposits formed along oceanic ridges may, through a series of circumstances, end up at the top of oceanic trenches within a subduction zone. This material may undergo subduction and recycling, or it might be thrust into melanges. Notably, any deposit initially formed in the emerging oceanic crust could eventually become mechanically incorporated into island arcs.
Moreover, continental rifts have the potential to fragment an otherwise unified metallogenic province, leading to the dispersal of resulting fragments over significant distances. An illustrative example is the tin provinces of Africa and Bolivia, which once formed a singular province before the opening of the Atlantic Ocean, but is now situated thousands of miles apart.