Opaque and ore minerals associated with basic and ultrabasic igneous rocks




Opaque associations in extraterrestrial rocks

Click hereOxides and sulphides and other opaque minerals are found in all basic and ultrabasic rocks. They vary in amount from minor accessory phases to massive ores, The identification of opaque phases within these rocks is important, for potentially they can provide information regarding pressure, temperature and fo2 and fs2, conditions during their formation. The opaque minerals also control the geomagnetic properties of these rocks.


Opaque associations in extraterrestrial rocks


Meteorites

Click hereThese extraterrestrial rocks comprise silicates (olivine, pyroxenes, plagioclase) with iron-nickel alloys, metal oxides and sulphides. There are four classes of meteorite which, in decreasing abundance of opaque minerals, are irons, stony irons, chondrites and achondrites. The opaque mineralogy varies between the classes, but all the assemblages are distinctly different from terrestrial rocks. The main minerals are kamacite (alpha-Fe), taenite (gamma-Fe), troilite, chromite and spinel. Associated minor minerals include schreibersice, magnetite, ilmenite, daubreelite, graphite, oldhamite, cohenite, niningerite, alabandite, pentlandite, mackinawite, sphalerite, chalcopyrite and native copper. Many of these primary minerals rapidly oxidize to limonite under terrestrial conditions.

Click hereThe well known Widmanstätten pattern comprises broad bands of kamacite bounded by lamellar taenite oriented along octahedral planes. Plessite is a fine intergrowth between kamacite and taenite. (McCall, 1973; Ramdohr, 1973; El Goresy, 1976b)


Lunar rocks

Click hereMuch of the investigated lunar material has a mineralogy similar to that of meteorites and may be meteoric in origin; this is especially true of the lunar regolith samples. However, indigenous lunar rocks (mainly basalts) contain ilmenite-geikielite, native iron (kamacite, taenite) and troilite, together with chromite-ulvöspinel, armalcolite-anosovite and rutile. (Frondel, 1975)


Opaque associations in unmineralized terrestrial rocks

Kimberlites

Click hereKimberlites are ultrabasic potassic rocks comprising megacrysts of olivine, chrome diopside, phlogopite, garnet and enstatite in a fine-grained matrix of the same silicates plus carbonates and serpentine. The main opaque phases are ilmenite-geikielite, chromian spinels and chromite; they are found both within the groundmass and as megacrysts. Ilmenite is characteristically magnesium-rich, with a high geikielite component, and occurs as xenocrysts or is intergrown with magnetite or rutile ± armalcolite. Minor phases include rutile, magnetite, perovskite, armalcolite, pyrite, chalcopyrite, heazlewoodite and native metals. Magnesium-rich ilmenites have lower reflectances than iron-rich ones. (Haggerty, 1976b; Dawson, 1980)


Mafic and ultramafic xenoliths

Click hereMafic and ultramafic gabbro, pyroxenite, peridotite and Iherzolite inclusions are found in basalts, especially alkaline basalts, and are believed to be deep-seated or mantle in origin. Their mineralogy is dominated by spinel together with chromite, magnetite, ilmenite, haematite and pyrrhotite. (Haggerty, 1976b)


Basalts and dolerites

Click hereModal analyses of fine-grained basalts or highly altered basalts can be difficult to obtain using normal thin sections and transmitted light petrography. Accurate modal analyses are easier to achieve using a well polished thin section.


Primary assemblages

Click hereThe main primary opaques of basic rocks (gabbro, dolerite and basalt) are chrome spinels, magnetite-ulvöspinel (titanomagnetite) and ilmenite-haematite solid solution minerals, rutile and the sulphide breakdown products of the monosulphide solid solution in the Fe-Ni-S system (mss), or intermediate solid solution in the Cu-Fe-S system (iss). These sulphides include pentlandite, pyrrhotite, pyrite, chalcopyrite, cubanite and mackinawite.

Click hereTexturally, early spinels form small euhedral crystals within olivine and pyroxene and sulphides occur as small droplets within, or about, silicates. Magnetite-ulvöspinel forms euhedral equant crystals with curved fractures; they are often zoned and in fine-grained lavas characteristically are skeletal and cruciform in shape. Ilmenite forms laths or lobate lath-shaped crystals, or symplectite-like intergrowths with silicates.

Click hereIlmenite-haematite solid solution produces characteristic exsolution textures of rods or lens-shaped bodies oriented along the (0001) plane of the host mineral. Coarse-grained exsolved lenses often have further fine-grained exsolution bodies within them. Grains containing ilmenite exsolution discs in a haematite host are known as ilmenohaematite and those with haematite discs in host ilmenite are called haemoilmenite. (Skinner and Peck, 1969; Irving et al., 1970; Elsdon, 1975; Haggerty, 1976b; Czamanske and Moore, 1977; Morrison and Thompson, 1986)


Alteration and oxidation of basalt

Click hereIn altered basic rocks, oxidation, oxidation-exsolution and autometasomatism (including the introduction of secondary sulphides) are the processes that often dominate the opaque mineralogy. Primary oxides undergo complex mineralogical changes on oxidation which partially depend on pressure and temperature. The alteration is initiated along grain boundaries and fractures and results in a number of different secondary minerals which have similar optical properties and are often intergrown on a fine scale. These minerals are most accurately identified by microanalysis.

Click hereThe initial oxidation products of magnetite-ulvöspinel solid solution (titanomagnetite) include titanomaghaemite, ilmenite and haematite. Characteristically, ilmenite forms fine trellis-like 'exsolution' lamellae oriented along (111) of the host magnetite and these coarsen with increasing oxidation. On yet further oxidation, the lamellae are pseudomorphed by fine-grained mixtures of rutile, titanohaematite and pseudobrookite. With extreme oxidation, the end product is a mixture of haematite with minor pleonaste after magnetite, and rutile after ilmenite.

Click hereAlteration of ilmenite produces complex intergrowths of ferrian ilmenite, ferrian rutile, rutile, titanohaematite and pseudobrookite. Extreme alteration results in mixtures of haematite and TiO2, minerals that pseudomorph the original grain. Chromium spinels oxidize to lighter coloured 'ferritchromits'.

Click hereAlteration of the rocks forms secondary magnetite and haematite along fractures in olivine (often the only indication of olivine in very altered basic rocks), neomorphic fine-grained TiO2, (mainly anatase), thin haematite rims about silicates and sphene mantling ilmenite. Secondary pyrite, marcasite and chalcopyrite are introduced. (Ade-Hall et al., 1971; Haggerty, 1976a)


Weathering of basalts

Click hereWeathering alters iron oxides to haematite, and both oxides and sulphides to limonite (a mixture of goethite and lepidocrocite). Secondary copper sulphides replace earlier copper minerals.


Magmatic deposits associated with basic and ultrabasic rocks

Click hereA number of mineral deposits have formed by magmatic, segregation processes; they are often called orthomagmatic deposits. These deposits are important sources of chromium, titanium, vanadium, the platinum group metals, copper, nickel (and diamonds). Much titanium, diamond and significant amounts of the platinum group metals are mined from sedimentary placer deposits, formed by the weathering of these primary deposits. (Wilson (ed.), 1969)


Iron-titanium-chromium oxides in ultramafic rocks

Magmatic oxide ores fall into a number of classes and associations.

(a) Magmatic segregations of chromite, magnetite, and ilmenite associated with layered basic intrusions. The oxides display little chemical variation.
(b) Chromite with magnetite and very minor ilmenite within podiform deposits. The spinels show a wide variation in their Al2O3:Cr2O3 ratios.
(c) Magmatic segregations of ilmenite-haematite and ulvöspinel-magnetite minerals associated with anorthosites.
(d) Magnetite and haematite ores associated with apatite.


Chromite deposits

Click hereChromite is a classical orthomagmatic mineral and chromite ores are divided into two main classes:
(a) Stratiform deposits, often called Bushveld-type
(b) Podiform deposits, often called Alpine-type.


Chromite deposits: Stratiform deposits

Click hereStratiform deposits These deposits are few in number but large in size and are associated with Archaean to Proterozoic in age layered basic intrusions within Precambrian cratonic areas. Within the intrusions, an upper mafic zone overlies a lower ultramafic zone of dunites, pyroxenites and peridotites that carry chromitite layers. These layers are up to 1m thick, laterally extensive, and comprise euhedral cumulate chromite with interstitial olivine, ortho- and clinopyroxenes and plagioclase.

Click hereChromite, as euhedral grains within the silicate matrix, is the main oxide, accompanied by magnetite, ilmenite, pyrrhotite, pentlandite and chalcopyrite. Platinum group minerals occur in trace amounts often associated with iron, iron-nickel, copper and copper-iron sulphides. Platinum and palladium are the most abundant platinum group metals in these ores.


Chromite deposits: Bushveld, Republic of South Africa

Click hereThe Bushveld Complex (extending for 180 000 km2) is a series of overlapping funnel shaped intrusions oriented along ENE and WNW structural trends and is the result of long-lived magmatism involving a number of different magmas. It consists of the Laminated Marginal Sequence, the main Layered Series and the overlying Bushveld Granite. The Layered Series has five zones, including the economically important Critical Zone which is divided into an upper anorthosite subzone and lower pyroxenitic one. Chromitite layers occur in both subzones but are mined mainly from the lower pyroxenite subzone. The layers have a simple mineralogy, essentially chromite with interstitial bronzite and plagioclase, and very minor amounts of sulphides.


Chromite deposits: Zimbabwe

Click hereThe Great Dyke is over 530 km in length and up to 9 km wide. It comprises four layered ultramafic and mafic (gabbro) sequences. Stratiform chromite layers occur within the lower dunite, harzburgite, pyroxenite and serpentinite units. A nickel, copper and platinum group metal-bearing horizon occurs within ultramafic rocks just below their contact with the gabbros.


Podiform chromite deposits

Click hereThese deposits are smaller than stratiform deposits, in the size range of 1O2 to 1O6 tonnes, and are found in ophiolite sequences of Palaeozoic to Tertiary age. The chromite ores are often associated with dunite bodies within peridotite and both rocks are extensively serpentinized. Chromite aggregates are often rounded and give a spotty appearance to the rocks; deformation of these aggregates is common. In most podiform deposits (that have been described), ruthenium, osmium and iridium minerals are more abundant than those of platinum, palladium and rhodium. Chromite that shows alteration to ferritchromit rims, is accompanied by lesser amounts of pentlandite, heazlewoodite and chalcopyrite and trace amounts of laurite, irarsite-hollingworthite, sperrylite, iron-platinum alloys and Ru-Ir-Os alloys. (Coleman, 1977)

Click hereShetland Islands: A dismembered Caledonian ophiolitic sequence is emplaced in Moinian/Dalradian rocks. The ophiolite comprises serpentinized harzburgite overlain by a layered ultramafic sequence and gabbros. Harzburgite has thin dunite lenses; these lenses carry discontinuous chromite layers and pods.


Titanium deposits

Click herePrimary ilmenite deposits are uncommon. The orebodies occur as magmatic segregations in the form of lenses, dykes or sills within gabbro-norite-anorthosite sequences or within anorthosites. Magnetite and iron, nickel and copper sulphides occur in minor amounts in the ores. (Lister, 1966; Philpotts, 1967).


Magnetite deposits

Small magnetite deposits are common within gabbroic bodies. Magnetite in the large magnetite layers within the Upper Zone of the Bushveld is vanadium-rich and is exploited for vanadium in addition to iron.


Sulphide assemblages in unmineralized and weakly mineralized ultramafic rocks

Click hereThe sulphide assemblage in weakly mineralized ultramafics differs from that found in mineralized ultramafics where pyrrhotite, pentlandite, pyrite and magnetite-dominated nickel sulphide deposits are found. In barren or weakly mineralized rocks, the main opaque phases are magnetite, pentlandite, heazlewoodite with minor awaruite, millerite, valleriite and cobaltian pentlandite. This sulphide assemblage is associated with post-magmatic processes, notably serpentinization and talc-carbonate alteration of ultramafic rocks, rather than magmatic segregation processes. (Groves et al., 1974; Eckstrand, 1975; Groves and Keays, 1979)


Sulphides associated with serpentinized dunite. Unst, Shetland Islands, Britain

Click hereDunites have suffered extensive hydrothermal alteration with the formation of serpentine minerals, chlorites, talc and magnesium-rich carbonates. The alteration processes have altered the chromite to ferritchromit and have precipitated magnetite and sulphides, arsenides and antimonides of nickel, iron and copper. They also remobilized the platinum group metals.


Nickel-copper sulphide deposits

Click hereThese deposits are important sources of nickel and copper, together with by-product platinum group metals, gold and iron (from pyrrhotite). Nickel sulphide deposits fall into two main associations, both of which have a number of subclasses. These are: (a) a dunite-peridotite association with two subclasses, the volcanic peridotite subclass and intrusive dunite subclass; (b) a gabbroic association including mafic and ultramafic complexes, layered basic intrusions and the unique Sudbury Complex (Naldrett and Cabri, 1976; Naldrett, 1981; Arndt and Nisbet (eds.), 1982).


Dunite-peridotite association

Deposits of this association are found within Archaean greenstone belts.


Volcanic peridotite

This class is associated with komatiite flows, where the nickel deposits are most commonly found at the base of the first flow. Deposits are small, high grade and contain gold and platinum group metals. (Groves and Hudson, 1981; Arndt and Nisbet (eds.), 1982; Groves et al., 1984)


Intrusive dunite

These are lens-shaped intrusions of dunite into felsic volcanics or metasediments. The primary mineralogy is similar to the volcanic peridotite class: namely, monoclinic and hexagonal pyrrhotite, pentlandite, chalcopyrite and magnetite. These are accompanied by nickel-cobalt sulphides and trace amounts of platinum group minerals. (Billington, 1984)


Gabbro association

Bushveld, Republic of South Africa

Click hereWithin the Bushveld Complex, platinum group minerals are concentrated in the Merensky Reef, the UG2 Chromitite Layer, the Platreef and in discordant dunite pipes. By-product copper and nickel are produced from the Merensky Reef, and chromite from the UG2 horizon. The platinum group minerals vary throughout the Bushveld but include sperrylite, braggite, stibiopalladinite, laurite, cooperate and iron-platinum alloys and are associated with chromite, pyrrhotite, chalcopyrite, pentlandite and minor gold plus copper-iron, nickel and iron sulphides. (Cabri, 1972; von Gruenewaldt, 1977, 1979; Kinloch, 1982; Stumpfl and Rucklidge, 1982)


Stillwater Complex, Montana, USA

Click hereThe complex consists of a sequence of northwards-dipping differentiated layered mafic (gabbro and norite) and ultramafic (peridotite, bronzitite and anorthosite) rocks which have a strike length of 48 km. The sequence has been divided into three main units: a basal unit containing large sulphide concentrations, an ultramafic unit with chromitite layers and an upper banded unit with disseminated sulphides and economic platinum group mineralization. (Hess, 1960; Jackson, 1961; Page, 1971; Page et al., 1976; Todd et al., 1982; Talkington and Lipin, 1986)


Sudbury Complex, Ontario, Canada

Click hereThe Sudbury Igneous Complex contains sulphide deposits which are mined for nickel, copper, cobalt, iron and platinum group metals. The complex is believed to be unique, as it is interpreted as being the result of a meteorite impact. The Complex is an irregular-shaped lopolith 60 x 30 km in size and 4 km thick and consists of a lower (outer) norite overlain by gabbro and with an upper (inner) granophyre capping. The complex is overlain by the Onaping Formation of volcaniclastic rocks and sediments. Within the mafic and ultramafic complex, sulphide mineralization is found close to the lower contact, within a suite of sulphide-rich norites and gabbros, known as the Sublayer and also in a series of steeply inclined dyke-like quartz diorite ,offsets'. Sulphides have segregated out from the Sublayer onto the undulating footwall of the complex to give massive sulphide ores, above which are disseminated sulphides. Disseminated sulphides also occur in the hangingwall quartz norite. The quartz diorite offsets have massive sulphides ores and disseminated sulphide-rich rocks.

Click hereThe ores comprise hexagonal and monoclinic pyrrhotite, pentlandite, chalcopyrite, pyrite and magnetite together with ilmenite, cubanite, cobaltite, niccolite and gersdorffite. Platinum groups minerals occur in trace amounts and include sperrylite, michenerite, moncheite, merenskyite, mertieite II and froodite. (Hawley, 1962; Hawley and Stanton, 1962; Souch et al., 1969; Gasparrini and Naldrett 1972; Cabri and Laflamme, 1976).


Listwaenites

These are carbonatized ultramafic rocks found as lenses along major tectonic contacts in ophiolite complexes. They comprise magnesium-iron-calcium carbonates and quartz with accessory serpentine, talc, magnesium chlorite, fuchsite, potassium feldspar and epidote. Gold occurs within pyrite- or arsenopyrite-rich listwaenites, where it is associated with nickel, cobalt and iron arsenides. Minor amounts of molybdenite, galena and sphalerite are present. (Buisson and Leblanc, 1985, 1986)


Carbonatites

Click hereCarbonatites are carbonate-rich magmatic rocks associated with alkalic magmatism. They form complex multiple plutonic intrusions in areas of major rifting and are Proterozoic to Present in age. Their extreme differentiation is reflected in their exotic mineral assemblages. Sövites, comprising calcite, apatite, biotite, aegirine-augite, magnetite and pyrochlore, form an early intrusive phase, later intrusives carry iron-bearing carbonates, rare earth minerals and uranium- and thorium-bearing phases. Fenitization (a potassic wallrock alteration) of the country rocks, commonly accompanies emplacement of carbonatites.They are mined principally for their magnetite, apatite, niobium (pyrochlore), rare earth metals (bästnasite, monazite and parisite group), thorium and zirconium minerals. The economic importance of these minerals varies between different carbonatites. (Tuttle and Gittins (eds), 1966)