Massive sulfide deposits were discovered from the diving saucer Cyana on the accreting plate boundary region of the East Pacific Rise near 21 degrees N. The deposits form conical and tubular structures lying on a basaltic basement. Mineralogical and geochemical analyses showed two main types of intimately associated products: a polymetallic sulfide-rich material composed of pyrite and marcasite in association, zinc-rich phases, and copper-rich compounds, and an iron-rich oxide and hydroxide material (also called gossan) composed largely of goethite and limonite. Silicate phases such as opaline, silica, iron-silicon clay, and trace amounts of mica and zeolite are encountered in both types of material. Possible mechanisms for the formation of the sulfide deposits on the East Pacific Rise are discussed.
The grey monazite of Britanny, determined in 1963, makes up small millimetric nodules in ordovician, dinantian and perhaps also precambrian shales, located in the central armorican trough. In the Ordovician, the mineralized layers are mainly localized in the Llanvirnian. These grey-coloured nodules, with a 4.65 specific weight, are in the shape of flat ellipsoids. Their structure is most often either grained polycrystalline, or as radiated mosaics, or as a crown (central monocrystal surrounded by small crystals). Inclusions involve phyllites, quartz, rutile, graphite, iron oxides, averaging to about 10°/0 of the nodule. The chemical composition is characterized by its high grade in Eu 20 3 (0.5%) and in thorium. The monazite shales, poorly metamorphic, include some chloritoid and a little graphite. The monazite nodules are scattered (content ranging from 50 to 200 g/t), arranged across the schistosity and surrounded by pressure shadows with phyllitous infilling. Within the ordovician stratigraphy, the llanvirnian monazite shales overlie iron ore and sandstons rich in detrital rutile-zircon-monazite, deposits in which diagenesis phenomena occur. A noteworthy sedimentary differentiation characterizes the epicontinental deposits of the ordovician sea invading the weathered and flattened brioverian relieves. The early diagenesis of the deposits is of prime importance in the genetic pattern proposed for monazite: 1) gel of rare earth phosphate in marine silts; 2) transformation of this gel into fibro-radiated globules of rhabdophanite; 3) evolution of the rhabdophanite into monazite; 4) occurrence of the syntectonic metamorphism, partial rotation of the nodules. Further occurrences of grey monazite are known from Siberia, Africa (Morocco, Gaboon, Congo) and Madagascar. At present, the placers only display some economical interest, as it is the case in Brittany.La monazite grise de Bretagne, d&erminfie en 1963, forme des petits nodules millim&riques dans des schistes ordoviciens, dinantiens et peut-&re aussi pr6cambriens, situ~s dans la fosse centrale armoricaine. Dans l'Ordovicien, les niveaux mindralis6es se localisent surtout dans le Llanvirnien. Les nodules, de couleur grise et de densitfi 4,65, ont la forme d'ellipsoides aplatis. La structure est le plus souvent soit polycristalline engren6e, soit en mosaique radifie, soit encore en couronne (monocristal central entour6 de petits cristaux). Les inclusions comprennent phyllites, quartz, rutile, graphite, oxydes de fer, reprfisentant en moyenne environ 10% du nodule. La composition chimique se caract~rise par la richesse en EuzO a (0,5O/o) et la pauvretfi en thorium. Les schistes ~i monazite, faiblement m6tamorphiques, renferment du chloritoide et un peu de graphite. Les nodules de monazite s'y trouvent dispers6s (teneurs de 50 ~ 200 g/t), dispos6s en travers de la schistosit6, et <
Chabourneite, Tl₂₁(Sb, As)₉₁S₁₄₇ is triclinic, P1, with a = 16.346, b = 42.602, c = 8.534 Å, α = 95.86°, β = 86.91°, γ = 96.88°, Z = 1, Dx = 5.121 g.cm⁻³. The strongest lines in the powder pattern are : 4.196(5) (0.10.0 ; 2.8.0), 4.067(6) (0.[-4].2 ; [-4].2.0), 3.928(7) ([-4].4.0), 3.573(10) (2.[-6].2 ; 2.10.0), 3.485(5) (0.6.2 ; 0.[-8].2), 3.358(7) ([-2].12.0), 2.853(7) (0.10.2 ; [-4].2.2), 2.808(8) ([-4].12.0), 2.754(5) (4.10.0 ; [-4].[-4].2), 2.709(7) (6.0.0), 2.345(7) (6.[-2].2), 2.135(9) (0.[-18].2) Å. Microprobe analyses show the substitution Tl⁺ + (Sb, As)³⁺ ⇆ 2 Pb²⁺. Chabourneite is a thallium-rich member of the solid solution series with the general formula Tl[21-x] Pb[2x] Sb[49-x+y] As[42-y] S₁₄₇. Microprobe analysis of the thallium end-member from the Jas Roux deposit gave : T1 23.87 ; Pb 0.00 ; Sb 32.92 ; As 17.63 ; S 26.05 ; total 100.47% corresponding to the ideal formula Tl₂₁Sb₄₉As₄₂S₁₄₇. In reflected light, chabourneite is white with weak reflection pleochroism and reflectances between Rmax and Rmin of stibnite : 31.3-41.6 (420) ; 30.7-40.2 (460) ; 30.3-39.3 (500) ; 29.8-38.0 (540) ; 29.2-36.7 (580) ; 27.8-35.3 (520) ; 26.4-33.6 (660) ; 25.3-32.4 (700 nm). Strongly anisotropic with bluish and greenish polarization colours ; internal reflections red. Microhardness (Vickers) 95 kg.mm⁻². Measured specific gravity 5.104 g.cm⁻³. Macroscopically black, conchoidal fracture, luster submetallic to greasy. Chabourneite occurs in dolomitic limestones at Jas Roux (Hautes-Alpes, France), associated with pierrotite, parapierrotite, stibnite, pyrite, sphalerite, twinnite, zinckenite, madocite, andorite, smithite, laflittite, routhierite, aktashite, wakabayashilite, realgar, orpiment and at Abuta, Hokkaido island (Japan), associated with getchellite, sphalerite and barite. The name is for Chabournéou glacier near the Jas Roux deposit (type deposit for chabourneite). Type material is preserved at École Nationale Supérieure des Mines in Paris.
Des travaux miniers r4cents darts les gisements du Laurium (Attique-Grhce) ont mis en 6vidence une min4ralisation compos4e essentiellement de magndtite et localis6e dans un ensemble volcano s4dimentaire 5 proximit4 d'un massif de granodiorite. L'4tude min&alogique et p&rographique d&aillde des min6rali-sations et de leur enveloppe a permis de retracer l'4volution g4ndtique de ces mindralisations et de distinguer Ies r61es m4tallog4niques respectifs du volcanisme et du m6tamorphisme de contact.Recent mining work in the ore deposits of Laurium (Attic-Greece) reveal a mineralisation mainly composed of magnetite, located in a volcano-sedimentary formation, close to a stock of granodiorite. The detailed mineralogical and petrographical studies of the mineralisation and its environment permit to relate the genetic evolution of the ore, and to distinguish the different metallogenic acts played by the volcanism and the contact metamorphism.
La pirquitasite, Ag₂ZnSnS₄, quadratique, I[-4]2m ou I[-4], a = 5,786, c = 10,829 Å, à structure type stannite, a été découverte dans le gisement de Pirquitas (Argentine), en association avec hocartite, pyrite, marcasite, wurtzite, franckéite, miargyrite, aramayoïte, chalcostibite, stannite, kësterite, rhodostannite et cassitérite. En lumière réfléchie, elle est gris brunâtre ; macles polysynthétiques, anisotropie forte du rouge brique au vert clair. Réflexions internes rouges. Microdureté Vickers : 218 kg/mm² (P = 25 g). Raies les plus intenses du diagramme de poudre : 3,267 (10) (112), 2,901 (4) (200), 2,049 (6) (220), 1,976 (8) (204), 1,735(8) (132), 1,289 (4) (240), 1,165 (4) (244). Pouvoir réflecteur (Rmax et Rmin) : 24,6 — 21,7(420), 23,8 — 21,8(500), 24,1 — 22,2 (540), 24,1 — 22,9 (600), 21,6 — 20,1 (700 nm). Elle forme une solution solide très étendue (12 à 67 % mol.) avec la hocartite. On observe une immiscibilité de la hocartite avec la stannite. La rhodostannite de Pirquitas forme une solution solide avec le terme extrême Ag₂FeSn₃S₈ jusqu'à la composition (Ag₁,₁₄₃ Cu₀,₈₉₉)₂,₀₄₂ Fe₀,₉₉₆ S₃,₀₁₄ S₇,₉₄₆. Les propriétés optiques de cette phase sont données.
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