Opaque and ore minerals associated with intermediate and acid igneous rocks
The primary iron-titanium oxide minerals (magnetite ulvöspinel, ilmenite-haematite and rutile), together with their secondary oxidation products (maghaemite, haematite, pseudobrookite, TiO2 minerals and sphene), are similar to those found in basic rocks. Chromite, however, is less common but other metal oxides, namely cassiterite, columbite and bixbyite, are more abundant. Early sulphides include pyrrhotite, pyrite, chalcopyrite and marcasite. (Haggerty, 1976b).
Although the mineralogy of granites is variable, they are essentially quartz and feldspar rocks that carry micas, amphiboles and pyroxenes, together with a wide variety of accessory minerals. Accurate characterization of the minor and accessory phases within granites should complement geochemical studies, especially if mass-balance calculations are to be performed. Reflected light investigation of the opaque and accessory phases has an important role in helping to discriminate fertile from infertile granites, in terms of their potential for economic metalliferous deposits or for geothermal energy (the so-called high heat producing granites). Accessory minerals are found as 10-100µm diameter grains between quartz and feldspars, or commonly within mafic and especially altered mafic minerals. Pleochroic halos in mafic minerals usually suggest the presence of one or more accessory minerals and repay careful study. Amongst the more common primary opaque and accessory minerals are zircon, allanite, sphene, tourmaline, epidote; ilmenite-haematite, magnetite-ulvöspinel, cassiterite, columbite-tantalite, uraninite, thorite, TiO2 minerals, pyrochlore group minerals, wolframite; apatite, monazite, xenotime; pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and molybdenite. In addition, numerous rare earth element-bearing carbonates and fluorocarbonates and U-Nb-Ti-bearing oxide minerals have been recorded. The accessory minerals occur as loose aggregates, often with symplectite-like intergrowths, at crystal boundaries of the silicates, or lie along the cleavage planes and grain boundaries of mafic minerals.
Few granites have escaped post-magmatic alteration by hydrothermal fluids and so show alteration and localized migration of their primary phases. Alteration includes the replacement of ilmenite by sphene and TiO2 minerals, and titanomagnetite by leucoxene. The crystallization of TiO2, minerals, ilmenite and haematite along the cleavages of altered mafics, especially biotite, suggest local redistribution of elements from silicates into oxides. Haematite and sulphides, notably pyrite, marcasite and chalcopyrite, are introduced during hydrothermal alteration. Many of the accessory minerals are translucent to semiopaque and so polished thin sections are more useful than polished blocks. Accessory minerals are often irregular in their distribution, complexly intergrown with each other and, because of their low and similar reflectances and strong internal reflections, are difficult to characterize in reflected light. Oil immersion and high magnification are required for the discrimination of fine-scale properties like zoning. Transmitted light petrography and chemical characterization by SEM or electron microprobe are also needed. Autoradiographs and Lexan overlays are used for locating the sites of uranium- and thorium-bearing phases. (Ishihara, 1977; Basham et al., 1982; Mackenzie et al., 1984; Chatterjee and Strong, 1985)
The classical sequence of late-stage magmatic to post-magmatic alteration and mineralization involves pegmatite formation followed by greisenization, albitization and, finally, hydrothermal quartz-sulphide veining. (Eugster, 1985; Plant, 1986)
Granite pegmatites are economically important sources of quartz, feldspar and micas. In addition, they are major producers of Li, Be, Cs, Pb, W, Sn, Th, Nb-Ta, rare earth element (REE) minerals and gemstones. Minerals in pegmatites are often coarse-grained. Simple pegmatites are essentially quartz, potash feldspar, sodic plagioclase and biotite, together with muscovite, tourmaline and garnet. Complex pegmatites are zoned and have the same essential mineralogy as simple pegmatites, together with more exotic silicates. Opaque and semi-opaque minerals include Th-bearing phases, thorianite and thorite; U-bearing phases including uraninite and uranothorite; Nb- and Ta-bearing phases, columbite, tantalite, pyrochlore group minerals and ilmenorutile; Sn-bearing phases, primarily cassiterite; W bearing phases, wolframite and scheelite; and a wide variety of REE-phases, including monazite, xenotime, allanite, samarskite, bästnasite and parisite group minerals. Native bismuth, bismuthinite, molybdenite, stannite group minerals and many base metal sulphides are found in minor amounts. (Cerny (ed.), 1982; Brown and Ewing (eds.), 1986)
Late stage fluids metasomatically alter acidic and alkaline plutonic rocks and the resulting metasomatites are found in apical areas of the parent pluton. Albitization, microclinization and greisenization are amongst the most important alteration processes to be associated with mineralization. The alteration sequence is not uniform; in Nigeria, albitization is followed by microclinization and then by greisenization, whereas in Comwall albitization is later than microclinization. (Jackson, 1986)
In Nigeria, sodium-rich fluids have altered a variety of granites into quartz-albite rocks containing minor amounts of Li-rich mica. Accompanying the alteration is an enrichment of Nb (Ta), Zr, Hf, Th, U, PEE, Fe, and Mn. Different compositions of the primary granites are reflected in different mineralogies in their secondary albitites; for example, niobium mineralization in peraluminous granite is present as columbite, in peralkaline granites as pyrochlore, and in metaluminous granites as fergusonite.
At Ririwai, potassic metasomatism has converted a biotite granite into a quartz, microcline and white mica rock with subordinate amounts of topaz and sericite. Minor amounts of tin and tungsten were introduced.
This is a widespread and important ore forming process produced by hot acidic fluids. Greisen is a quartz and white mica (muscovite, sericite, zinnwaldite) rock, forming irregular masses, veins or stockworks cutting apical regions in granites. Topaz, tourmaline, fluorite, biotite and albite can be present in significant amounts in addition to ore minerals. Greisens can carry an extensive opaque mineralogy, including cassiterite, wolframite, scheelite, columbite, haematite, magnetite, brannerite, molybdenite, pyrrhotite, pyrite, marcasite, arsenopyrite, chalcopyrite, sphalerite, galena, stannite, tetrahedrite group minerals, native bismuth and bismuthinite. Cassiterite, wolframite and scheelite-bearing greisens, and wolframite, molybdenite and native bismuth-bearing greisens are economically the most important. (temprok, 1985; Taylor and Strong, 1985)
Cross-cutting quartz veins in granites are common and carry metal oxides, base metal sulphides and arsenides. Associated wallrock alteration includes greisenization and tourmalinization. These are the classical tin-tungsten-molybdenum-bismuth-arsenic-bearing hydrothermal veins associated with apical areas of granites. The ore mineralogy of the veins is similar to that of greisens except that sulphides become volumetrically the more abundant and oxide minerals less so. The mineralogy of granite-associated quartz veins is extensive. Common minerals include cassiterite, wolframite, molybdenite, stannite group minerals, bismuthinite, chalcopyrite, sphalerite, galena, pyrrhotite, pyrite, marcasite, arsenopyrite, lollingite and tetrahedrite group minerals. These are accompanied by minor amounts of sulphosalts and native metals.
Much of the world's copper and molybdenum are produced from very large tonnage low grade sulphide ores associated with high level, acid to intermediate, porphyritic intrusions of Tertiary to Quaternary age; these are the porphyry deposits. Three main types are recognized: porphyry copper, porphyry molybdenum and smaller porphyry-style tin-tungsten stockwork deposits. By-product gold, silver and base metals are produced.
These are mined for copper with by-product molybdenum, gold and silver; some contain tungsten as wolframite. A central, small, porphyritic stock is surrounded by a series of genetically related alteration zones and ore shells that carry hypogene sulphide (and minor oxide) mineralization in disseminated quartz veinlets and healed microveinlets. In some deposits, larger veins carrying gold and silver mineralization are found in the outermost alteration zone.
Moving from the porphyritic stock outwards, the hydrothermal wallrock alteration zones are: potassic, phyllic, argillic and propylitic. Each carries a distinctive hypogene sulphide-oxide assemblage. The inner potassic zone, characterized by the development of secondary orthoclase and biotite together with quartz, albite, sericite,. anhydrite and apatite, is roughly coincident with an innermost low-grade chalcopyrite, pyrite, molybdenite, magnetite and bornite zone. Molybdenite, then chalcopyrite and finally pyrite increase in amounts towards the phyllic zone, but generally chalcopyrite is more abundant than pyrite.
Rocks in the phyllic zone comprise quartz, sericite and pyrite (with up to 10% by volume pyrite). The main economic ore shell occurs within this zone, often at its junction with the potassic zone. Pyrite is accompanied by chalcopyrite, molybdenite, bornite, chalcocite, magnetite, enargite and sphalerite within abundant quartz veinlets. The argillic zone comprises montmorillonite, illite, chlorite and kaolinite and carries pyrite with chalcopyrite and bornite with trace amounts of chalcocite, enargite, molybdenite, tennantite, sphalerite, galena and wolframite. The propylitic alteration zone comprises chlorite, epidote, calcite and pyrite. Pyrite is the main sulphide and trace amounts of bornite, molybdenite, sphalerite, galena, magnetite, specularite, rhodochrosite and rhodonite are also present.
The outermost part of this zone carries chalcopyrite, galena and sphalerite veins with gold and silver mineralization. Superimposed upon this primary mineralization can develop an extensive supergene 'chalcocite lanket'. This comprises chalcocite, djurleite, digenite with minor blaubleibender covelline, cuprite and native copper. Often overlying the chalcocite blanket and primary zones is a 'limonite' gossan containing jarosite, copper carbonates- copper oxides, native copper and haematite. (Titley and Hicks (eds.), 1966; Drummond and Godwin, 1976; Ney et al., 1976; Hollister, 1979; Beane and Titley, 1981; Titley (ed.), 1982)
These are rarer than porphyry copper deposits and produce byproduct tungsten. They are Mesozoic to Tertiary in age and have been divided into 'Climax-type' (stockwork type), associated with multiple porphyritic granitic intrusions with molybdenite and pyrite as major sulphides, accompanied by wolframite, cassiterite, stannite and bismuth sulphosalts; and the ,quartz-monzonite' type, associated with granites, granodiorites and quartz monzodiorites. In addition to molybdenite and pyrite, these carry minor amounts of scheelite, bismuth sulphosalts and chalcopyrite. Both types of porphyry molybdenum deposits are accompanied by potassic, phyllic, argillic and propylitic alteration but also by more minor silicification, quartz vein, greisenization and garnet-rich zones. The majority of the molybdenite occurs as stockwork quartz-molybdenite veins. The potassic zone contains K-feldspar, quartz, magnetite and biotite, and the phyllic zone abundant sericite and pyrite. Both zones carry minor molybdenite. In the argillic zone, kaolinite, montmorillonite and sericite replace feldspar, and biotite is altered to muscovite, sericite plus minor rutile, leucoxene, pyrite, carbonates and fluorite. The propylitic zone carries chlorite, epidote and pyrite accompanied by calcite, clays and sericite. Greisen veins comprise quartz, molybdenite, topaz, pyrite, magnetite and muscovite, whereas in the garnet zones spessartine is accompanied by galena, sphalerite and rhodochrosite. Supergene mineralization in porphyry molybdenum deposits is weak to absent. (Wallace et al., 1978; Sillitoe, 1980; White et al., 1981)
Other deposits, notably tin- and tungsten-bearing, show many similarities to porphyry copper and molybdenum deposits. They are associated with high level porphyritic stocks that are accompanied by broadly concentric alteration and mineralization zones. Potassic alteration is rarer, although greisenization is more common than for porphyry copper deposits. (Sillitoe et al. 1975, Grant et al. 1980; Noble et al. 1984)