Opaque and ore minerals associated with metamorphic rocks

The opaque assemblages are often simple and reflect the mineralogy of the unmetamorphosed parent rock, especially if the metamorphism is low grade. However, both the prograde and the retrograde metamorphic history of the rocks will be reflected by the final mineral assemblage. Sediments and basic igneous rocks which have suffered low grade metamorphism characteristically carry magnetite-ulvöspinel, ilmenite-haematite, TiO₂ polymorphs, sphene, carbonaceous material, pyrite, marcasite, pyrrhotite and base metal sulphides. Metamorphism alters anatase to rutile and pyrite to pyrrhotite, and increases the reflectance of carbonaceous matter. In calcium-rich igneous rocks, ilmenite and TiO₂ minerals form sphene. With increasing metamorphic grade, pseudobrookite solid-solution minerals, magnetite, pleonaste, haematite-ilmenite, graphite and corundum are formed. Fine-grained oxides, notably ilmenite and TiO₂ minerals, are exsolved from phyllosilicates and other mafic minerals on retrograde metamorphism. Hence, polished thin sections are better than polished blocks for the investigation of unmineralized metamorphic rocks. (Rumble, 1976; Fleet et al., 1980; Mongkoltip and Ashworth, 1983)

Many syngenetic mineral deposits, especially older ones, have suffered regional metamorphism. For example, the Witwatersrand gold-uranium ores and the Zambian Copper Belt ores have suffered greenschist facies metamorphism, and the Lake Superior iron formations amphibolite facies metamorphism. The degree of metamorphic alteration depends not only on the metamorphic grade but also on the original composition of the ore. Susceptible deposits can show the effects of both prograde and later retrograde metamorphism and hence their original paragenesis is extensively modified, if not destroyed. Therefore, great care should be taken in interpreting the results from mineralogical geothermometers or geobarometers obtained from metamorphosed deposits.

Metamorphism (and associated tectonism) alter the mineralogy of the primary ore by the formation of new minerals or by homogenizing the composition of existing phases. Neomorphic assemblages are a common result of the metamorphism of iron and manganese deposits; here, silicate minerals are formed at the expense of the primary oxide, hydroxide or carbonate minerals. In sulphide assemblages, neomorphic changes include pyrite altering to pyrrhotite, tetrahedrite-tennantite altering to arsenopyrite, chalcopyrite, sphalerite and berthierite, and the formation of spinel, magnetite and gahnite. However, the most striking features of metamorphosed sulphide ores are their changes in texture and fabric. (Gole, 1981)

Sulphide deposits respond to metamorphism by recrystallization and to tectonism by deformation. Refractory minerals, like pyrite, arsenopyrite, sphalerite and magnetite, show little change with low grade metamorphism and retain their primary compositions. With greater metamorphism they lose their zoning, and compositionally re-equilibrate (and retain this composition through retrograde metamorphism); they coarsen in grain size and anneal to give porphyroblasts or mosaic-like textures with characteristic 120' triple junctions. These minerals tend to fracture rather than flow in response to stress. Softer sulphides, like pyrrhotite, chalcopyrite and galena, compositionally re-equilibrate under lesser degrees of metamorphism and hence show the effects of both prograde and retrograde metamorphism. Therefore, they have limited value in determining initial formation conditions. They deform by twinning and kink-banding, but primarily they flow, which leads to the mechanical break up of the ore and country rock. This produces the very diagnostic durchbewegung and ball textures and the presence of injected sulphides along cleavages and fractures within harder ore and gangue phases. Chemical etching of sulphide ores is helpful in determining the change in their fabrics with metamorphism. This is especially true for massive ores containing the isotropic minerals, pyrite, galena and sphalerite. (Vokes, 1968, 1969, 1973; Mookherjee, 1976)

The central Appalachians contain massive stratiform and stratabound sulphides which have undergone regional greenschist to amphibolite grade metamorphism. The metamorphosed ores show thermal and recrystallization effects, and deformational or stress features, but little change in mineralogy. Individual deposits are pyrite-rich to pyrrhotite-rich with variable amounts of chalcopyrite, sphalerite, galena and magnetite.

Broken Hill has produced major amounts of lead, zinc and silver and by-product gold, cobalt, cadmium, copper and antimony. Two main ore types are found: the Thackaringa quartz-siderite-galena veins and the Broken Hill stratiform massive sulphide orebodies. The Broken Hill orebodies occur within highly deformed, granulite facies Precambrian rocks of the Willyama Complex. Within this complex, the Broken Hill Mine Sequence comprises six stratiform massive sulphide orebodies, eight banded iron formations and a quartz-gahnite horizon, together with abundant pelites, psammites, felsic gneisses and amphibolites. A garnet-plagioclase gneiss, the Potosi Gneiss, lies close to the orebodies. Later pervasive supergene alteration, down to more than 100m has produced a complex association of secondary minerals, including cerussite, coronadite and native silver. The sequence is interpreted as a metamorphosed exhalative chemical sediment associated with acid volcanics and mafic tuffs and tuffites. Over fifty opaque and ore minerals have been recorded. (Johnson and Klingner, 1975)

The thermal metamorphism of pure limestones and sandstones results in very simple metamorphic assemblages with few or no opaque phases. Contact metamorphism of argillaceous or dolomitic limestones yields more complex silicate assemblages associated with a fairly simple opaque mineralogy. This comprises magnetite, TiO₂ minerals, pyrite, pyrrhotite, marcasite, chalcopyrite, bornite, galena, sphalerite associated with calcite, dolomite, quartz, silica, feldspar, garnet, epidote, pyroxene and other calc-silicates. Mineralized skarns, however, show a wide range of mineral assemblages which are controlled both by the nature of the intrusion and the composition of the country rock (although the latter is usually a carbonate). They are the result of thermal metamorphism accompanied by metasomatism. Skarns are divided into exoskarns, found within the country rock, and endoskarns, found within the intrusion. Most skarns are the end result of multiple events, with oxide and sulphide mineralization superimposed on earlier silicate skarnification. Skarns are important ores for tungsten (scheelite-powellite), although they have been major producers of iron, copper, tin, molybdenum, lead, zinc and uranium. Cobalt, gold and platinum-bearing skarns are known, but are rare. (Einaudi et al., 1981; Einaudi and Burt, 1982)