Recherche infos sur les plumasites et buschites

[alchimiste]

bonjour, je recherche toute info, photo sur les plumasites et buschites , particulierement francaises.

evidement, vous devinez pourquoi

merci d'avance aux pointures de ce forum

Victor

[ANDRE HOLBECQ]

plumosite moi connaître mais pas plumasite. Ce terme n'est pas accepté : dans les plumosites il y a soit la jamesonite soit la boulangerite.

[esor6]

salut Alchimiste et André,

sujet très intéressant pour qui aime les cœurs bleus foncé!

Il y a, semble-t-il, plusieurs appellations pour désigner ce genre de formation.

Un copié/collé (un peu raide!) sur les plumasites (j'ai de la doc en pdf sur le sujet pour ceux que çà intéresse):

 

ULTRAMAFIC-RELATED CORUNDUM

Q09

by G.J. Simandl1 and S. Paradis2

1 British Columbia Geological Survey, Victoria, B.C., Canada

2 Geological Survey of Canada, Mineral Resources Division, Sidney, B.C., Canada

 

Simandl, G.J. and Paradis, S. (1999): Ultramafic-related Corundum (Contact Metamorphic/Metasomatic); in Selected British Columbia Mineral Deposit Profiles, Volume 3, G.J. Simandl, Z.D. Hora and D.V. Lefebure and T. Höy, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.

IDENTIFICATION

SYNONYMS: Plumasite and marundite deposits, contact-metamorphic corundum and emery, "desilication" or metasomatic sapphire.

COMMODITIES (BYPRODUCTS): Rubies, sapphires, industrial grade corundum and emery.

EXAMPLES (British Columbia - Canadian/International): Corundum Hill (North Carolina, USA), Emery Hill (New York, USA), Natal and Birdcage camp (South Africa), Umba (Tanzania), Kinyiki Hill and Penny Lane ruby mine (Kenya).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Sapphire, ruby and industrial grade corundum occur within, or adjacent to, aplite, pegmatite, albitite, plumasite or marundite dykes, sills and rarely plugs cutting mafic and ultramafic rocks and their metamorphosed equivalents. Industrial grade corundum is also found commonly along contacts of mafic/ultramafic intrusions with metapelites or other felsic country rocks. It may occur both within country rock and the intrusion.

TECTONIC SETTINGS: These deposits occur in orogenic belts where felsic rocks are thrust against silica-undersaturated rocks and within the stable cratons.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Corundum is commonly found in quartz-free reaction zones located along contacts of silica-deficient rocks, such as ultramafic and mafic rocks, with pegmatite, paragneiss, syenite gneiss or other felsic rocks. Country rocks are typically affected by medium to high grade regional metamorphism.

AGE OF MINERALIZATION: Archean or younger. Abrasive-grade corundum deposits are commonly contemporaneous with contact metamorphism, while gem-quality corundum may post-date metamorphism and the peak of the tectonic activity.

HOST/ASSOCIATED ROCKS: Common host rocks are vermiculite ± chlorite ± asbestos-bearing rocks, plumasite (coarse grained rock consisting of anhedral corundum crystals in an oligoclase matrix), marundite (corundum in margarite matrix), syenite, pegmatite, aplite or hornfels. Associated rocks are ultramafics, a variety of mafic lithologies including gabbro, amphibolite, anorthosite, serpentinite, anthophyllite-chlorite-talc schist, peridotite and dunite and peraluminous orthogneisses or paragneisses.

DEPOSIT FORM: Most of the dyke-associated or fracture-controlled deposits that crosscut ultramafic and mafic rocks are planar or lens-shaped; rarely forming vertical plugs. They are less than a metre to 10 metres in thickness and may extend from few metres to several tens of metres along strike. These deposits exhibit several types of mineralogical zoning from the center of the deposit outwards:

a. Corundum-chlorite > spinel - chlorite > enstatite > talcose rock > friable dunite > dunite;

b. plumasite > biotitite > pegmatite > serpentinite;

c. aplite> plumasite>spinel-magnetite rock > vermiculite and/or chlorite > actinolite >

talc>serpentinite;

d. barren pegmatite> marundite > talc-chlorite zone>amphibolite (pegmatite may not be present).

Lenticular or irregularly shaped, corundum-bearing pockets may be also present along the tectonic contacts between gneiss and serpentinite. Some of the gem-quality and most of the industrial grade corundum and emery deposits occur near the contacts of mafic and ultramafic intrusions with country rocks. Emery may form veins, layers and irregular or lens-shaped masses within both endo- and exometamorphic reaction zones. Most of the corundum is typically found in metapelites adjacent to such intrusions.

TEXTURE/STRUCTURE:  Sapphire and ruby may form rhombohedral or hexagonal prisms or they may occur as clear portions of large, poikilitic corundum crystals that may exceptionally reach over a metre in length. In South African plumasites the corundum crystals commonly vary from 3 millimetres to 10 centimetres. In marundite, corundum occurs as coarse hexagonal crystals embedded in scaly or rosette-shaped aggregates of margarite. Emery rock is typically equigranular, fine-grained (<1mm). It may form layers, veinlets or lenses and irregular zones of massive ore in intrusive and country rock.

ORE MINERALOGY:

In plumasite and marundite: sapphires, rubies, specimen- quality or industrial grade corundum.

Within contact metamorphic zones of mafic and ultramafic intrusions: mostly emery or silimanite-corundum rock or coarse industrial-grade corundum.

Along tectonic contacts: rubies, sapphires, specimen and industrial grade corundum.

GANGUE MINERALOGY [Principal and subordinate]:

In plumasites: mainly plagioclase, ± biotite, ± amphibole, ± fuchsite, ± tourmaline. Some of the solid inclusions identified within sapphires and rubies are zircon, rutile, apatite, boehemite, monazite, hematite, mica, calcite, pyrrhotite and graphite.

In marundites: margarite, ± feldspar, ± biotite, ± apatite, ± garnet, ± tourmaline, ± fuchsite, ± kyenite (?), ± talcose material and possibly anthophyllite.

In metasomatic zones cross-cutting ultramafic rocks without plumasite core: vermiculite, ± chlorite. The main solid inclusion in gem corundum is vermiculite.

In contact metamorphic deposits: a) In emery ores: Hercinite, pleonaste, magnetite, hematite/ ilmenite, ilmenohematite. hypersthene, sapphirine, sillimanite, cordierite, garnet, biotite, feldspar, staurolite, gahnite. Some of the minor constituents in emery ore may be due to hostrock inclusions. In sillimanite-corundum rock: rutile and ilmenite are trace constituents

ALTERATION MINERALOGY: Corundum may retrograde into diaspore or mica. In marundites it is commonly partially replaced by gibbsite and margarite.

WEATHERING: Some uneconomical primary gemstone and industrial grade deposits may form viable residual or placer deposits.

ORE CONTROLS: There are three major spatial controls: 1) fracture zones control metasomatic and plumasite mineralization within the mafic/ultramafic rocks; 2) tectonic contacts control mineralization pockets located between gneisses and serpentinites; and 3) contact metamorphic zones around mafic intrusions are also favourable.

GENETIC MODELS: A number of theories explaining the origin of these deposits have been proposed over the years. The three models that appear the most likely are:

a) Desilication of granitic pegmatites or pegmatitic fluids by interaction with silica-undersaturated country rocks. This is particularly popular theory to explain the origin of fracture-controlled mineralization associated with marundite, plumasite, vermiculite rock, pegmatite or aplite crosscutting ultramafic country rocks.

In the contact metamorphic/metasomatic settings, the high ratio of Al2O3/(CaO+Na2O+K2O) and low silica content that favor corundum formation may be achieved by magmatic interaction of mafic or ultramafic rocks with metapelites or by partial melting of the pelitic country rocks.

c) Where the felsic rocks were thrust against ultramafic rocks, reaction zones may have formed under open system conditions during regional metamorphism.

ASSOCIATED DEPOSIT TYPES: Placer-type corundum deposits (C01 and C02) and corundum-bearing residual soils. Vermiculite (M08), nepheline syenite (R16) and pegmatites (O01, O02, O03 and O04) may be genetically related to some of the corundum deposits covered by this profile. Corundum-bearing metapelites (P06) may also be present in the same geological setting.

COMMENTS: Emery is a black granular rock formed by intergrowths of corundum with magnetite, hercinite or hematite. Emery deposits may also form during regional metamorphism of aluminous sediments, such occurrences are described in the profile P06 (this volume). It is used mainly as an abrasive or for anti-skid surfaces. "Anolite", a highly-priced ornamental stone formed from a famous ruby-bearing zoisite amphibolite from Longido (Tanzania), is closely associated with serpentinites (Keller, 1992). Due to the lack of outcrops, it is not clear if this deposit belongs to the metasomatic type of mineralization described in this profile. Marble and skarn-hosted ruby/sapphire deposits, such as those described by Okrush et al. (1976), also may be similar in origin. Some of these deposits may have formed by essentially isochemical regional metamorphism, while others may be pegmatite and aplite-related metasomatic zones. Marble hosted deposits should be considered as a distinct deposit type.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Corundum-bearing lithologies are silica-undersaturated and characterized by their high Al2O3/(CaO+Na2O+K2O) ratio. Saphire, ruby, corundum or emery may be found in heavy mineral concentrates from stream sediments or tills. As well, the solid inclusions within corundum crystals, corundum texture, and associated minerals in the concentrates may be indicative of the type of primary source, such as gem corundum hosted by alkalic rocks (Q10), corundum in aluminous metasediments (P16) and gem corundum in marbles.

GEOPHYSICAL SIGNATURE: Ultramafic rocks associated with this deposit type may be detected and possibly delimited by magnetic or electromagnetic surveys. Magnetite-bearing emery deposits may be detected using a magnetometer.

OTHER EXPLORATION GUIDES: Some vermiculite occurrences may be worth examining for gem corundum.

ECONOMIC FACTORS

TYPICAL GRADE AND TONNAGE: Grades are rarely reported for hard rock-hosted sapphire and ruby deposits. They are difficult to determine as these deposits are often high-graded and mined sporadically. A substantial proportion of the production is sold on the black market. Grades of up to 2000 carats of rough gems per ton are reported from the weathered extension of sapphire and ruby rock occurrences at Umba (Tanzania). In another portion of the same property 100 000 carats were recovered from soil above apparently barren veins, but the grade is not reported. In South Africa, plumasites contain 5 to 80% corundum with typical grades around 30 to 40%. Larger deposits may contain 5 to 10 thousand tonnes, but average tonnage is more likely less than 2 thousand tonnes. These deposits were mined in the first half of the 19th century to about 40 metres. Typical content of eluvial deposits associated with plumasites varies from 10 to 20% by volume. The emery deposits of Emery Hill (Peekskill area) consisted of veins (some less than 2cm thick), pods and thin layers parallel to the schistosity. The emery consisted of varying proportions of spinel (0 to 65%), magnetite (20-30%) and corundum (15 to 65%).

ECONOMIC LIMITATIONS: Together with emerald, red beryl and diamond, ruby and sapphire are the most valuable gemstones. The most valuable rubies are dark purplish red ("pigeon's blood red"). The most desirable color for sapphire is "Kashmir blue". Star rubies and sapphires exhibit asterism better than any other gems. The color of many natural corundum gems is artificially enhanced by heat treatment. . Due to the highly variable grades and relatively small deposit size, these hard rock deposits are commonly mined by open-cast methods and in some cases by primitive underground methods.

END USES: Depending on quality, corundum may be used as a gemstone, abrasive or friction material on non-slip surfaces. Sillimanite-corundum rock is a relatively highly priced material for refractory applications. Some corundum-bearing rocks are used as ornamental stones.

IMPORTANCE: Most corundum gems are recovered from regoliths, residual soils or gravels, and as byproducts of placer mining (C01, C02). They may be also found in alkali basalts, lamprophyres (Q10) and rarely in aluminous metamorphic rocks (P06) and marbles. However, deposits of this type, remain worthwhile targets for prospectors and small exploration companies. Clear, nearly inclusion-free corundum crystals are produced synthetically, and compete with natural gems. Silicon carbide and artificial corundum manufactured from bauxite has largely replaced corundum and emery in most industrial abrasive applications. Today, the combined consumption of industrial grade corundum and emery in the USA is estimated to be less than 10,000 tonnes/year.

REFERENCES

Andrews, P.R.A. (1991): Summary Report No. 15: Minor Abrasives-Corundum, Emery, Diatomite, Pumice, Volcanic Ash and Staurolite; Canada Centre for Mineral and Energy Technology, Division Report MSL 91-110 ®, 91 pages.

Barker, F. (1964): Reaction Between Mafic Magmas and Pelitic Schist, Cortland, New York; American Journal of Science, Volume 262, pages 614-634.

De Villiers, S.B. (1976): Corundum; in Mineral Resources of the Republic of South Africa, Coetze, C.B., Editor, Geological Survey of South Africa, Volume 7, pages 341-345.

DuToit, A.L. (1918): Plumasite (Corundum-aplite) and Titaniferous Magnetite Rocks from Natal; Transactions of the Geological Society of South Africa, Volume 21, pages 53-73.

Grant, J.A. and Frost, B.R. (1990): Contact Metamorphism and Partial Melting of Pelitic Rocks in the Aureole of Laramie Anothosite Complex, Morton Pass, Wyoming; American Journal of Science, Volume 290, pages 425-427.

Hall, A.L (1920): Corundum in Northern and Eastern Transwal; Union of South Africa Geological Survey, Memoir 15.

Keller, P.C. (1992): Gemstones of East Africa; Geosciece Press Inc., 144 pages.

French, A.E. (1968): Abrasives; in Mineral Resources of the Appalachian Region; United States Geological Survey, Professional Paper 580, pages 261-268.

Game, P.M. (1954): Zoisite-amphibolite with Corundum from Tanganyika; Mineralogical Magazine, London, Volume 30, pages 458-466.

Gillson, J.L. and Kania, J.E.A. (1930): Genesis of the Emery Deposits near Peekskill, New York; Economic Geology, Vol. 25, pages 506-527.

Hughes, R.W (1990): Corundum. Butterworth-Heinmann, London, 314 pages.

Keller, P.C. (1990): Gemstones and Their Origin; Van Nostrand Reinhold, New York, 144 pages.

Larsen, E.S. (1928): A Hydrothermal Origin of Corundum and Albitite Bodies; Economic Geology, Volume 23, pages 398-443.

Okrusch, M., Bunch, T.E. and Bank, N. (1976): Paragenesis and Petrogenesis of a Corundum-bearing Marble at Hunza (Kashmir); Mineralium Deposita, Volume 11, pages 278-297.

Pattison D.R.M. and Harte, B. (1985): A Petrogenic Grid for Pelites in the Ballachulish Aureole and other Scottish Thermal Aureoles; Journal of Geological Society of London, Volume 142, pages 7-28.

Pattison D.R.M. and Tracy, J. (1991): Phase Equilibria and Thermobarometry of Metapelites; in Contact Metamorphism, D.M. Kerrick, Editor, Reviews in Mineralogy and Petrology, Volume 26, Mineralogical Society of America, pages 105-204.

Robb, L.J. and Robb, V.M. (1986): Archean Pegmatite Deposits in the North-Eastern Transwaal; in Anhausser, C.R. and Maske, S. Editors, Mineral Deposits of Southern Africa Volume I, Geological Society of South Africa, Johannesburg, pages 437-449.

Rossovskiy, L.N. and Konovalenko, S.I. (1977): Corundum Plagioclasite of the Southwestern Pamirs; Doklady Academie Science. U.S.S.R., Earth Science Section, Volume 235, pages 145-147

Sinkankas, J. 1959: Gemstones of North America; D.Van Nostrand Company Inc, New York, 675 pages.

Solesbury, F.W. (1967): Gem Corundum Pegmatites in NE Tanganyika; Economic Geology, Volume 62, pages 983-991.

October 10, 1999

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[Deposit Profiles]

Last Updated June 13, 2003

 

[esor6]

Simonet en parle aussi (en français!) dans sa thèse sur les corindons.

au fait, je suis aussi preneur d'infos!

bien à vous,

[alchimiste]

salut esor6

c'est bien de ca dont il s'agit en effet, le coeur bleu.

je voulais juste avoir des photos de ces deur roches histoire de voir de visu a quoi ca ressemble...

merci pour tes infos

[esor6]

je n'en ai pas mais tu devrais trouver çà dans la thèse de Simonet et dans le RM spécial corindon et spinelle.

Ce sont des roches claires, feldpathiques.

Au plaisir,