FACTORS GOVERNING THE DISPOSITION OF METALLOGENIC PROVINCES
It is now well known that the earth's crust and mantle have passed through an evolutionary sequence of changes throughout geological time. These changes have influenced and are reflected in petrogenesis and the nature and extent of related mineralization. For example the association of tin mineralization with Mesozoic and late Paleozoic granites, the restriction of Banded Iron Formations to the Precambrian and the association of nickel deposits with Precambrian orthomagmatic ilmenite deposits. Consequently the paucity of certain metallogenic entities during subsequent periods may be attributed to the advanced stages of evolution of the crust and mantle, for example the lack of Phanerozoic nickel sulfide deposits may be due to the depletion of mantle sulfur during the Archaean.
The fact that the mantle and overlying crust (as indeed the mineral deposits) underwent progressive evolutionary changes with time, permits us to make certain generalizations regarding the disposition of metallogenic provinces with respect to the evolution of the mantle with the advancing age of the earth. The evolutionary changes can be conveniently discussed in terms of the Archaean, Proterozoic and Phanerozoic intervals and the environments which prevailed during these periods:
This interval is notable for the abundance of certain metals and the absence of others. Metals and metal associations developed in significant amounts include Au, Sb, Fe, Mn, Cr, Ni-Cu, and Cu-Zn-Fe. Notable absentees are Pb, U, Th, Hg, Nb, Zr, REE, and diamond. Important deposits include:
The beginning of the Proterozoic was marked by a great change in tectonic conditions. The first lithospheric plates developed whose appearence permitted the formation of sedimentary basins, leading to the deposition of platform sediments and development of continental margin geosynclines. Notable mineral deposits of the period include:
Towards the end of Proterozoic a new tectonic pattern developed which gave rise to Phanerozoic fold belts formed by continental drift. Mineralization processes during this period tended to be concentrated along such tectonic environments as rift valleys, aulacogens and associated domes, constructive and destructive plate nargins and transform faults. The disposition of deposits formed in this period are discussed under Plate Tectonics and Disposition of Metallogenic provinces below. It may therefore be stated that one of the important factors that govern the disposition of metallogenic provinces is the evolutionary stages through which the particular segment of the earth's crust has passed.
DISPOSITION OF METALLOGENIC PROVINCES VIS-A-VIS PLATE TECTONIC SETTINGS:
In addition to the above, the disposition of metallogenic provinces is governed to a great extent by global tectonics. The fact that different types of deposits form in different plate tectonic settings, relates various types of mineralization to large scale and diverse crustal structures. Thus:
Much of the material that formed at sites listed above, as indeed during the earliest stages of crustal evolution, may be conveyed by various mechanicms, such as primary plate movement, reversal of plate movement and continental collision to areas of entirely different tectonic settings from those in which the deposit formed and to which it may be genetically related. Thus mineral deposits formed along the oceanic ridges may, by various trains of circumstances, arrive at oceanic trenches at the top of a subduction zone. This material may be largely subducted and recycled or it may be thrust into melanges. Clearly any deposit formed in the new oceanic crust may eventually be mechanically incorporated into island-arcs.
Additionally, continental rifts may fragment an otherwise coherent metallogenic province and the resulting fragments may be disposed thousands of miles apart as, for instance, the tin provinces of Africa and Bolivia which constituted a single province prior to the opening of the Atlantic Ocean.
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