Ore Deposits Related to Magmatic Activity

Certain accessory or uncommon constituents of magmas become enriched into bodies of sufficient size and richness to constitute valuable mineral deposits eg. Chromite and platinum.  Magmatic ore deposits are characterized by their close relationship with intermediate or deep seated intrusive igneous rocks.  They themselves are igneous rocks whose composition happens to be of particular value to man.  They constitute either the whole igneous mass or a part of it, or may form offset bodies.  They are magmatic products that crystallize from magmas. They are also called Magmatic Segregations, Magmatic Injections, or Igneous Syngenetic Deposits. Mode of Formation: Magmatic Deposits result from: 1.      Simple crystallization 2.      Concentration by differentiation of intrusive igneous masses. There are several modes of formation of magmatic deposits.  They originate during different periods of magma crystallization – in some the ore minerals crystallize early, in others late, and in still others they remained as immiscible liquids until after crystallization of the host rock. Classification of Magmatic Deposits: I.  Early Magmatic Deposits: Those which resulted from straight magmatic processes (orthotectic and orthomagmatic). These deposits have formed by: a.       Simple crystallization without concentration b.      Segregation of early formed crystals c.       Injection of material concentrated elsewhere by differentiation. A. Dissemination Deep seated crystallization will yield a granular rock in which the early formed crystals are disseminated.  If such crystals are valuable and abundant, the whole rock or a part thereof becomes the orebody.  The individual crystals may be phenocrysts eg. B. Segregation Concentration of early formed crystals in-situ.  These are early concentrates of valuable constituents of the magma that have taken place as a result of gravitative crystallization differentiation, eg. Chromite.  These orebodies are generally lenticular and small in size, commonly disconnected pod shaped lenses, stringers or    eg. II. Late Magmatic Deposits:  Those which consist of minerals crystallizing from a magma towards the close of magmatic period.  The ore minerals are later than the rock silicates and cut across them, embay them, and yield reaction rims around earlier minerals.  They are always associated with mafic igneous rocks. The late magmatic deposits have resulted from: a.       Variations of crystallization differentiation. b.      Gravitative accumulation of heavy residual liquids. c.       Liquid separation of sulfide droplets. A. Residual Liquid Segregation In certain mafic magmas, the residual liquid becomes enriched in iron, titanium and volatiles.  This liquid settles to the bottom of the magma chamber, or crystallizes in the interstices of early formed crystals.  Examples: Titaniferous magnetite layers of the Bushveld Igneous Complex, S. Africa. B. Residual Liquid Injection The iron-rich residual liquid accumulated in the above manner may be subjected to movement because of: a.       Gentle tilting (causing lateral movement). b.      Pressure and be squirted out to places of lesser pressure. In both cases it may be injected into adjacent rocks and even in the earlier consolidated parent silicate mass.  Examples: Titanomagnetite Deposits, Adirondack Region, New York; Allard Lake Deposits; Magnetite Deposits of Kiruna, Sweden. C. Immiscible Liquid Segregation Sulfide-rich magmas are immiscible in silicate rich magmas.  This gives rise to separation even before crystallization.  The accumulated sulfide may not necessarily be pure – in fact it quite often is an enrichment of sulfides in the lower parts of the magma.  Deposits formed in this manner are pyrrhotite-chalcopyrite-pentlandite nickel-copper ores confined to rocks of the gabbro family. Examples: Ni-Cu Deposits of Insizwa, S. Africa; Nickeliferous Sulfide Deposits of Bushveld, S. Africa & Norway; Nickel Sulfide Deposits of  Sudbury, Ontario. D. Immiscible Liquid Injection Examples: Vlackfontein Mine of S. Africa; Nickel Deposits of Norway.  Association of Rocks and Mineral Products: Definite associations exist between specific magmatic ores and certain kinds of rocks: 1.      Platinum occurs only with mafic to ultramafic rocks such as varieties of norite, peridotite or their alteration products. 2.      Chromite (with rare exceptions) is formed only in peridotites, anorthosites and similar mafic rocks. 3.      Titaniferous magnetite and ilmenite are found with gabbros and anorthosites. 4.      Magnetite deposits occur with syenites. 5.      Ni-Cu deposits are associated with norite. 6.      Corundum occurs with nepheline syenite. 7.      Diamond occurs only in kimberlite, a variety of peridotite. 8.      Pegmatite minerals, such as beryl, cassiterite, lepidolite, scheelite, and niobium-bearing minerals occur chiefly with granitic rocks. It is thus seen that deep-seated mafic rocks are the associates of most of the magmatic mineral deposits.  This indicates a genetic relationship with the early magmatic history of associated rocks. Characteristics of different rock types: Peridotite: A coarse grained mafic igneous rock composed of olivine with small amounts of pyroxene and amphibole. Anorthosite: A plutonic rock composed mainly of Ca-rich plagioclase feldspars. Gabbro: A black, coarse grained intrusive igneous rock, composed of calcic plagioclases and pyroxenes.  The intrusive equivalent of basalt. Syenite: A group of plutonic rocks containing alkali feldspars, a small amount of plagioclase, one or more mafic minerals, and quartz only as an accessory, if at all.  The intrusive equivalent of trachyte. Kimberlite: A peridotite that contains garnet and olivine and is found in volcanic pipes. Pegmatite: An igneous rock with extremely large grains (> 1 cm in dia).  It may be of any composition, but is most frequently granitic.