Córindon em metassedimentos

Córindon em metassedimentos

CORUNDUM IN ALUMINA-RICH METASEDIMENTS by G.J. Simandl1 and S. Paradis2 1 British Columbia Geological Survey, Victoria, B.C., Canada 2 Geological Survey of Canada, Sydney, B.C., Canada

Ref: córindon, sedimentos, metabauxitas, gneisses

Simandl, G.J. and Paradis, S. (1999): Corundum in Alumina-rich Metasediments; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10. IDENTIFICATION SYNONYMS: Corundum-bearing schists and paragneisses; corundum in metapelites and metabauxites are covered by this model. COMMODITIES (BYPRODUCTS): Industrial-grade corundum (gem corundum) and emery. EXAMPLES: (British Columbia - Canada/International): Blu Star (082FNW259); Elk Creek, Bozeman and Bear Trap deposits (Montana, USA), Gangoda and Tannahena occurrences (Sri Lanka). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Corundum occurs as porphyroblasts or idiomorphic, xenomorphic or skeletal crystals within high grade, regionally metamorphosed belts. It is confined to specific metamorphic layers and concordant lenses of alumina-rich gneisses and schists. It is rarely of gem quality. Emery is a fine-grained, black, granular rock composed of intergrowths of corundum, magnetite, hercinite or hematite that commonly forms in medium to high grade metamorphic environments. TECTONIC SETTINGS: Corundum in gneisses occurs mostly in fold belts or deep cratonic (catazonal) environments exhumed in thrust belts or by erosion. Emery and related meta-bauxites may be found in wide variety of tectonic environments. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Most of these deposits form in high-grade, mainly granulite facies, dynamothermal metamorphic (catazonal) environments. Metasedimentary belts containing aluminous strata or lenses, in some cases intruded by igneous rocks, are particularly favourable. Emery deposits are also known to occur in similar and lower grade metamorphic environments. AGE OF MINERALIZATION: Corundum is considered syn-metamorphic. The protolith may be Precambrian or younger. Rocks that were exposed at the surface during periods of extreme chemical weathering are particularly favourable. HOST/ASSOCIATED ROCK TYPES: Corundum-bearing gneisses and schists are associated with sillimanite-garnet-biotite gneisses, kyanite-mica schists, quartzites, clinopyroxenites, pegmatites, syenites or alkaline intrusions, anorthosites, charnockites, migmatites, granitic and intermediate intrusive rocks, quartz-mica schists, granulites, aplites, marbles, cordierite-bearing gneisses, amphibolites and wollastonite-scapolite rocks. The lithologies hosting metasedimentary emery lenses are commonly lower metamorphic grade equivalents of above listed rocks. DEPOSIT FORM: Corundum-bearing, stratabound and discontinuous layers and lenses in gneisses are from 20 centimetres to a few metres in thickness and may be traced for tens to hundreds of metres along strike. These layers are commonly strongly deformed, with coarse-grained "sweat outs" which may cut across the gneissic texture. Emery may form lenses from 5 to more than 50 metres thick and more than 100 metres in length.   TEXTURE/STRUCTURE: Gneissosity and schistosity is generally parallel to the compositional layering and corundum mineralization; however, if migmatization or granitization was involved, corundum zones may be irregular or vein-like. The texture of corundum-bearing rocks varies from fine-grained, equigranular to coarse-grained (approaching pegmatitic), locally displaying pseudo-orbicular texture. Corundum crystals may be idiomorphic, xenomorphic or skeletal and may vary from near gem quality to those with abundant solid inclusions. Emery deposits in the low metamorphic grade areas, may contain corundum pseudomorphs after diaspore. ORE MINERALOGY (Principal and subordinate): Industrial grade corundum is a dominant constituent of corundum-bearing gneiss. Same gneiss may also contain specimen quality materials and exceptionally near gem quality stones. Corundum is also the essential constituent of emery ores. GANGUE MINERALOGY (Principal and subordinate): In corundum-bearing schists and gneisses: feldspar, quartz, ± sillimanite, ± muscovite, ± biotite, ± rutile, ± titanite, ± zircon, ± apatite, ± tourmaline, ± magnetite, ± kyanite, ± calcite, ± dolomite, ± chlorite, ± prehnite, ± amphibole, ± pleonaste, ± cordierite, ± sapphirine, ± chloritoid. In emery-type deposits: magnetite, spinel (typically hercinite), ± hematite are the most common impurities. Diaspore, staurolite, kyanite ± hydrargite, ± garnet, ± mica, ± chloritoid, ± chlorite, ± calcite, ± epidote may be also present. ALTERATION MINERALOGY: Corundum crystals commonly alter to muscovite along fractures and twinning planes. Retrograde corundum alteration to diaspore and margarite is also known. Vermiculite-rich layers may form at the contact between corundum-bearing and mafic rocks or marbles. WEATHERING: Post-depositional exposure of rocks to intense weathering produces high-alumina protolith required to form isochemical metamorphic emery and corundum deposits. Corundum is resistant to chemical and mechanical weathering. Weathering facilitates crystal recovery from the hardrock deposits. Corundum may be enriched in residual soils or eroded and deposited as placer-type deposits. A large proportion of alluvial gem corundum is sometimes interpreted to be derived from corundum layers within garnet-sillimanite-biotite gneisses (Dahanayake and Ranasinghe, 1981). ORE CONTROLS: The principal controls are the chemical composition (high alumina and low silica content) of the protolith and a high regional metamorphic grade, typically granulite facies. On the other hand, emery deposits may form at temperatures as low as 420°C. ASSOCIATED DEPOSIT TYPES: Sillimanite deposits (P02). Corundum and garnet placer deposits (C01 and C02) are sometimes derived from these corundum deposits. Crystalline flake graphite (P04), vein graphite deposits (P05), and muscovite (Q03) and quartz feldspar pegmatites (Q04) may occur in the same geological settings. GENETIC MODELS: In most cases, coarse corundum-bearing metasediments are believed to form by the isochemical metamorphism of alumina-rich regoliths, including bauxite protoliths formed under conditions of tropical weathering. Hydrothermal alteration zones containing clays, alunite and diaspore and igneous rocks, such as nepheline syenites and anorthosites, are also considered as favourable, premetamorphic protoliths. Alternatively, some of the deposits are interpreted to have formed by preferential concentration of alumina in restites associated with extreme metamorphism, migmatization and granitization. COMMENTS: Corundum occurrences formed in shallow, low pressure hydrothermal environments, such as the Empress porphyry deposit, British Columbia and the Semiz-Buru deposit in Kazakhstan, are not covered by this model. In most of geological environments, corundum occurs in silica-undersaturated rocks. Corundum may coexist with quartz at unusually high pressures (Shaw and Arima, 1988). EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Corundum and associated minerals, such as sillimanite, ± garnet, ± sapphirine and ± pyroxene in heavy mineral concentrates from stream, lake, till and residual soils. Emery may be also detected in heavy mineral concentrates. GEOPHYSICAL SIGNATURE: Magnetite-bearing emery deposits may be detected by magnetometer surveys. OTHER EXPLORATION GUIDES: Aluminous lithologies within metasedimentary sequences in high grade metamorphic belts. These aluminous lithologies commonly contain high alumina silicate assemblages. Contacts between silica-deficient intrusions and alumina-rich metasediments. ECONOMIC FACTORS TYPICAL GRADE AND TONNAGE: Typical, individual corundum-bearing lenses and layers contain 5 to 28% corundum and most contain less than 7 500 tonnes of ore. In emery rock, the corundum content may reach 70%. The emery is crushed, but in most cases the corundum is not separated from gangue minerals. ECONOMIC LIMITATIONS: Most of the gneiss-hosted corundum deposits contain industrial grade corundum with little or no high quality gem-quality stones. Mining is typically by open cast, because of relatively low prices of industrial grade corundum (U.S.$ 150.00 to 275.00 per tonne). Residual and placer deposits are not only less expensive to exploit, but typically contain a higher proportion of gem-quality material due to the break-up of micro-fractured stones during stream transport. Synthetic corundum competes with natural corundum in gem applications and has replaced it in most high technology industrial applications. END USES: Corundum materials are used as abrasives, in refractory applications, as hardener for heavy-duty concrete floors and as anti-skid material on bridges and entrances to toll booths. IMPORTANCE: In industrial applications both emery and corundum have to compete with a number of higher-performance synthetics, such as silicon carbide and fused alumina, or lower priced natural materials, such as garnet. Natural and synthetic diamond are also competing for the same market. As a result, combined consumption of corundum and emery in the USA is estimated in order of 10 000 tonnes per year. The U.S. Bureau of Mines considers corundum deposits as a possible substitute for bauxite in high-alumina refractories. The United States imports over 90% of refractory grade bauxite. Unlike bauxite, corundum does not require thermal processing. Preliminary flotation studies of corundum-bearing gneisses were successful in producing refractory grade materials (Smith and Liewellyn, 1987).