Spatial and Temporal Distribution of Ore Deposits - a Global Perspective.

 

It has been recognized since a long time that there have been certain periods of time in the geological past during which the deposition of a metal or group of metals was most pronounced; and also that specific regions of the world possess a notable concentration of deposits of one or more metals.

Mineral deposits are not distributed uniformly through the Earth's crust. Rather, specific classes of deposit tend to be concentrated in particular areas or regions called metallogenic provinces. These groupings of deposits occur because deposit-forming processes, such as the emplacement of magma bodies and the formation of sedimentary basins, are themselves controlled by larger processes that shape the face of the Earth. The shape and location of such features as continents and oceans, volcanoes, sedimentary basins, and mountain ranges are controlled, either directly or indirectly, through the process of plate tectonics. For example, the distribution of hydrothermal mineral deposits, which form as a result of volcanism, is controlled by plate tectonics because most of the Earth's volcanism occurs along plate margins. In addition, porphyry copper deposits are formed as a result of volcanism along a subduction zone (i.e., the zone where one plate descends beneath another); this gives rise to metallogenic provinces parallel to subduction plate edges. Evidence indicates that plate tectonics has operated for at least two billion years, so that the locations and features of most metallogenic provinces formed over this period can be explained, at least in part, by this geologic process. Factors controlling the distribution of deposits formed more than two billion years ago are still a matter for research, but they too may have been linked to plate tectonics.

Metallogenic epochs are units of geologic time during which conditions were particularly favourable for the formation of specific classes of mineral deposit. One conspicuous example of a metallogenic epoch is the previously mentioned 700-million-year period, from 2.5 to 1.8 billion years ago, when all of the great Lake Superior-type BIFs were formed. Because the iron in these deposits was deposited from seawater (an impossibility today, since the atmosphere is too oxidizing to allow seawater to transport iron), it is probable that a specific composition of the atmosphere and ocean peculiar to that period defined the BIF metallogenic epoch. Another great deposit-forming period occurred between about 2.8 and 2.65 billion years ago, when a large number of volcanogenic massive sulfide deposits formed; the probable cause of this metallogenic epoch was a period of extremely active submarine volcanism.

A METALLOGENIC EPOCH is therefore a period of time in the geological

  past during which notable ore formation took place; and

 

A METALLOGENIC PROVINCE is a region in which notable ore formation

  took place during one or more metallogenic epochs.

 

The recognition and delineation of metallogenic epochs and provinces is of fundamental importance to economic geologists searching for metals.  It is probable that any further discoveries of important metallic deposits will be made within these regions (PROVINCES) and stratigraphic horizons (EPOCHS).  It is necessary to make continental drift reconstructions in order to delineate pre-drift provinces.

 

EXAMPLES OF METALLOGENIC EPOCHS AND PROVINCES

 Banded Iron Formations: These consist of quartz and hematite rich layers and are found on all the continents of the world (particularly restricted to Archaean Greenstone Belts).  There is therefore, no distinct iron province recognizable as such.  The best development of these, however took place in the period 2600-1800 m.y. ago (most forming  2200 m.y. ago) in early Proterozoic basins.  There is therefore, a distinct iron epoch ( 2200 m.y.).  Some minor ferruginous sediments of the hematite-chamosite-siderite type are restricted to the Jurassic horizons of Europe.

 Sulfide Nickel Ores of Mafic-Ultramafic Association: These are confined to a few Precambrian provinces, all restricted to Archaean Greenstone Belts:

     a) Superior-Ungava area of Canada,

     b) Western Australian Shield,

     c) Kola Peninsula-Siberia region of Scandinavia-Russia, and

     d) Zimbabwe-Rhodesia region of Southern Africa.

Most of these deposits are Precambrian (oldest 2500 m.y.), few are Paleozoic to Mesozoic.  However, those with mantle derived sulfur are restricted to Archaean and Proterozoic.  Hence they show a good development of metallogenic provinces but are not characterized by any metallogenic epoch.

 Stratiform Sulfide Deposits of Volcanic Affinity: These are present on all continents, around old volcanic nuclei, volcanic arcs and eugeosynclines.  They are also well spread through time (2700 m.y. to present).  However two `bursts' of mineralization are recognizable:

     a) 1700 - 1500 m.y. Eg. Sullivan, Broken Hill & Mount Isa, and

     b) 500 - 300 m.y. Eg. N. Appalachians, Caledonides & E. Australia

 Precious Metal Telluride Deposits of Volcanic Affiliation: These are characterized by three provinces and two epochs:

     a) Ontario-Quebec region

     b) Western Australian region  - 2500 m.y.

     c) Circum-Pacific Belt - Tertiary

 The "Porphyry Coppers": These are characterized by two regions and two provinces:

  1. Circum-Pacific region (particularly in Western Americas, including Chile-Central America, Mexico, Arizona, Nevada, N. British Columbia) - Mesozoic, and

  2. Southwest Pacific Province (Philippines, New Guinea and Solomon Islands) - Tertiary.

 Plutonic Vein Deposits: These contain a multiplicity of metals and are associated with the folded mountain belts of all ages.  Hence no epochs or provinces are recognizable.

 Tin Deposits of Granitic Affiliation: Commercial tin deposits have a strong relationship with post-tectonic granites.  The following associations are conspicuous and constitute tin epochs:

     6.6% with mid Paleozoic (Caledonian) granites,

     18.1% with late Paleozoic (Hercynian) granites, and

     63.1% with Mesozoic granites.

  Tin deposits are confined to at least three distinct provinces:

     a) Tin province of Eastern Australia (Paleozoic),

     b) Tin province of Central South Africa,

     c) Tin province along the Western coast of South Africa, and

     d) Tin province along the Eastern coast of South America.

Note: c & d constitute one province that broke up during the westward drift of South America during early Cretaceous (130-120 m.y.)

 Titanium Deposits of Anorthositic Association: These are characterized by a strong relationship with post-tectonic andesite-labradorite anorthosites emplaced around 1600 + 200 m.y. ago, being characterized by a distinct metallogenic epoch.  They are confined to mid Proterozoic mobile belts of Laurasia and Gondwanaland.  Eg:

     a) Bergen, Egersund and Lofoten (Norway),

     b) St. Urbain and Allard Lake (Quebec), and

     c) Iron Mountain, Wyoming and Sanford Lake (New York).

When plotted on a pre-Permian continental drift reconstruction, these anorthosites appear to be confined to two linear belts in the northern and southern hemispheres, thus outlining two metallogenic provinces for titanium.

 Manganese Deposits of Sedimentary Affiliation: Two types of commercial manganese deposits are known - one of volcanic affiliation while the other of shelf association.   Manganese deposits of volcanic affiliation are found on all continents, volcanic islands and in all geological periods.  But they are most conspicuous in the Precambrian (no metallogenic epoch or province).

 Manganese deposits of shelf association are more restricted in space and time, most of them occurring along the northern hinterland of the Black Sea having formed in the lower to mid Oligocene.  They are therefore characterized by one metallogenic epoch and one

metallogenic province.

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