As principal editor, Franco Pirajno wrote in Chapters 2, 3 and 4 of his book Ore Deposits and Mantle Plumes published 2000 that his group of authors have examined how doming of the crust, its rupturing and formation of a rift basin are processes that can be linked to mantle plumes. The East African Rift System, and its Red Sea and Gulf of Aden extensions are a modern example of this phenomenon; the Mid Continent Rift System in North America is an ancient example. Another ancient example is the Damara-Ribeira rift systems, which subsequently evolved to ocean floor spreading, resulting in the separation of South America from southwestern Africa (Damara hotspot junction, discussed in Chapter 4). "Deposits formed in continental hot spots, rifts and aulacogens" are discussed by Mitchell and Garsan (1981), who included mineral deposits associated with intracontinental hotspots, such as Sn, Nb and U in peraluminous and peralkaline granites,
REE in carbonatite; deposits associated with intracontinental rifts and aulacogens, such as REE, U and P in carbonatite and alkaline complexes, diamonds in kimberlite and porphyry Mo in biotite granite; stratabound Cu (e.g. Kupferschiefer in Europe, Copperbelt in central Africa), stratabound Pb-Zn-Ag (e.g. Sullivan in Canada, Mt. Isa and McArthur River in Australia), lacustrine brines and evaporites (East African lakes), polymetallic hydrothermal veins in granite (e.g. Midcontinent Rift System in North America). The Great Dyke and Bushveld lgneous Complex, discussed in Chapter 8 of this book, are also included in the rifts and aulacogen settings of Mitchell and Garsan (1981).
Sawkins (1990) devoted a chapter to "Intracontinental hotspots, anorogenic magmatism and associated metal deposits", in which he included Sn in anorogenic granites, Fe-Ti in anorthosite, Ni-Cu-PGE in layered intrusions (e.g. Bushveld, Stillwater), Cu-Au-U-REE of the Olympic Dam type and deposits of carbonatite complexes. Sawkins (1990) considered rift-related magmatic and hydrothermal mineralisation in terms of early and advanced stages of rifting. The early rifting stages of Sawkins (1990) encompass hydrothermal Cu (e.g. Messina in South Africa), porphyry Mo (e.g. Oslo rift in Norway), magmatic Cu-Ni-PGE (e.g. Noril'sk in Russia), and the Archaean Witwatersrand Au-U conglomerates in South Africa. In the advanced rifting stages, he included the metalliferous deposits of the Red Sea, sediment-hosted massive sulphides (SEDEX), stratabound PbZn- Ag of Mt. Isa, McArthur River, Hilton in Australia and Sullivan in Canada, Mississippi Valley-type (MVT) Pb-Zn, and the major Fe ores of Superior-type banded iron-formations. Pirajno (1992) discussed the major stratiform and stratabound base metal deposits and banded iron formation
(including Mn deposits) under the heading of "Continental rift environments".
Sawkins' division of early and advanced stages of rifting is a neat and convenient way of classifying rift-related ore deposits, although it is more realistic to see these ore systems as a time-continuum of closely-related phenomena, rather than separate events. What the authors observed and studied are snapshots. Commonly, and again for the sake of convenience, they tended to focus on end-members of this continuum.
In this book, the authors considered two groups of end-members: one group (1 and 2 below) includes ores that are formed from anorogenic magmas in intra-continental rift settings, the other group of end-members (3, 4 and 5 below) are, perhaps the better known sedimentary-hosted sulphide deposits of rift basins. Thus, the end-member ore systems considered are:
1. those that are formed from high-temperature fluids that emanate from anorogenic igneous complexes in intra-continental settings;
2. mesothermal and epithermal systems linked to hydrothermal convective cells activated by high T/P metamorphism induced by deep-seated heat sources;
3. surface deposits formed as precipitates from the discharge of thermal springs in lacustrine environments, during incipient stages of rifting;
4. high-temperature fluids that ascend along growth faults, to exhale at higher levels in a rift-related sedimentary pile (e.g. SEDEX deposits).
5. low-temperature metalliferous brines that move laterally across basins during compaction and lateral tectonic push (e.g. Mississippi Valley-type and stratabound Cu-Co-Ag deposits).
Mixing of two or more of the various types of fluids (magmatic, metamorphic and/or meteoric) is common and result in highly complex ore systems. Ultimately, the thermal energy is provided directly by igneous intrusions or indirectly by mantle plumes in a crust-attenuated environment. These categories of hydrothermal mineralization can be considered singly or as part of regional-scale metallogenic processes that are associated with rifting.
As an example of the coverage, in this chapter (Chapter 9) linked above, the authors examined some of the tectonic environments and associated ore deposits that are included in the first two categories listed above, whereas some aspects of the last three categories are examined in Chapter 10. Thus, first , the authors considered ore systems associated with the modern East African Rift System. Then, they discussed mesothermal and epithermal mineralization associated with intraplate anorogenic volcano-plutonic complexes. The authors also examined the regional metallogeny of rift systems for which there is good evidence of mantle plume involvement, the Meso-Neoproterozoic Damara and Irumide orogens in southwestern Africa and the Mid-continent Rift System in North America. They concluded Chapter 9 with a brief review of the controversial Archaean orogenic Au-hearing Iode systems and their possible relationship to mantle plumes.
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