Pseudo-cubic trigonal pyrite from the Madan Pb–Zn ore field (Rhodope Massif, Bulgaria): morphology and twinning

A new occurrence of pyrite crystals with rhombohedral habit, up to several centimeters in length, is described from the Madan Pb–Zn ore field (Rhodope Massif, south Bulgaria), where it constitutes a late pyrite generation. As observed in the past in other deposits, the ideal rhombohedron is derived from the pyritohedron by suppression of half of its faces (six “polar faces”) around a ternary axis. In studied crystals, together with six main “equatorial faces”, additional minor faces correspond to cube faces as well as polar faces. Such a dissymmetry indicates that the crystallographic point group of these crystals is 3‾, a subgroup of the eigensymmetry 3‾2/m of a rhombohedron taken as geometric face form. Twinning by metric merohedry confirms such a symmetry decrease and permits the definition of this type of pyrite as a dimorph of cubic pyrite, i.e., pseudo-cubic trigonal pyrite (pyrite-R). Twin operations belong to the set of symmetry operations absent in point group 3‾ relative to pyrite symmetry m3‾: reflection about the {100} plane or two-fold rotation about the &lt;100&gt; direction. Four twin types have been distinguished (name, chromatic point group): three contact twins (reflection, m′; rotation, 2′; trapezoidal, (m(2)m(2)2(2))(4)), as well as one penetration twin (crossed, 2′/m′). Composition planes always correspond to {100}, but there are two types of twin interfaces. More complex twinned samples may develop erratically during crystal growth. Other twin variations as well as genetic aspects of such a type of pyrite are discussed.</p

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal-As-Sb-Au mineralization during Hercynian strike-slip deformation.

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritized lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. Following the massive sulphidation stage (Fe-As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb-Sb-Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390-350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300-275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key-control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralization of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in BIF-hosted gold deposits

Original mineralogical features of a hydrothermalised oolitic ironstone : The deposit of Saint-Aubin-des-Châteaux (Armorican Massif, France).

Despite its large area, the Armorican Massif provided only very few new mineral species: plombogummite, laumontite, bertrandite, natrodufrenite and lulzacite. This last species was recently discovered by one of us (Y. L.) in a sandstone quarry at Saint-Aubin-des-Châteaux (near Châteaubriant, Loire-Atlantique department) (Moëlo et al., 2000), and various studies permitted to reveal original mineralogical features, related to the superimposition of hydrothermal processes on an Ordovician oolitic ironstone interstratified in the sandstone sequence. This ironstone belongs to a very large sedimentary Fe deposit lying at the East margin of the Armorican Massif. At Saint-Aubin, ooliths are constituted essentially by siderite and chlorite, with abundant Srrich fluorapatite and organic matter. During Hercynian orogenesis, faulting controlled hydrothermal processes, which induced pronounced mineralogical changes, especially massive sulphidation of the ironstone (Gloaguen, 2002). Due to the abundance of primitive apatite and other peculiar geochemical features, it permitted the crystallisation of various Sr or lanthanoide phosphates. Lulzacite, Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10, is very well crystallised in small quartz veins crosscutting the ironstone, together with siderite and pyrite. It is isostructural with jamesite (Léone et al., 2000). It occurs as massive aggregates (up to some cm3), as well as euhedral crystals up to 1 cm in size. It was formed by short-range remobilisation of synsedimentary apatite (process of lateral secretion). Later, its decomposition lead to the formation of goyazite (with amethyst colour), together with late fluorapatite, and some berthierine. Lanthanoide phosphates were formed directly within the ironstone. The very rare scandium phosphate pretulite, ScPO4, has grown together with xenotime-(Y) in epitaxy on detrital zircon crystals (Moëlo et al., 2002). SEM as well EPMA revealed concentric zonation of pretulite and xenotime crystals, indicative of a multi-stage process. Monazite-(Ce) is present as minute anhedral neoformed aggregates, while detrital crystals are rare. Detrital zircon shows growth zones enriched with Sc-, Y- and HREE- phosphate components. EPMA of zircon and pretulite suggested a complete solid solution according to the heterovalent substitution rule: Zr + Si -> Sc + P. This solid solution has been confirmed experimentally in high temperature conditions (Dubost, 2003). A general metallogenic study is in progress (Gloaguen, 2002; Gloaguen et al., in prep.). After the main Fe-S-(As) stage, a Zn-Pb-(Cu) stage presents some Sb sulfosalts (boulangerite, bournonite and tetrahedrite), as well as traces of electrum. While the formation of lulzacite and subordinated goyazite is directly related to the hydrothermal process acting in the deposit of Saint-Aubin, that of xenotime and monazite begins very early within the ironstone formation, as observed in the neighbouring Fe deposit of Rougé, what may also permit the discovery of new pretulite occurrences. Thus, it seems that hydrothermalism of such ironstones is a major key and control when studying rare minerals associated with ironstones. Moreover, others phosphates have been previously described within ironstones: wolfeite (Fe2+,Mn2+)2(PO4)(OH) (Brousse and Chauvel, 1969), lazulite (Chauvel, 1968) and are not yet recognized in the Saint-Aubin-des-Châteaux quarry. We propose that differences between these phosphates minerals parageneses are controlled by ironstone initial chemistry and hydrothermalism. Such hydrothermalism is favoured and associated with late orogenic context. In this way, the large European palaeozoic ironstone belt is probably an important target for potential new and rare minerals species. Actually, all armorican iron mines are closed and a re-examination of ironstones samples preserved in museums could probably allow the discover of new species. Brousse, R. and Chauvel, J.-J. Bull. Soc. fr. Minéral. Cristallogr. 92, 1969, pp 93-94. Chauvel, J.-J. Mém. Soc. géol. Minéral. Bretagne, 16, 1968, 243 p. Dubost, V. Unpublished Research report, Magistère de Physico-Chimie Moléculaire, Paris XI-ENS Cachan, Institut des Matériaux de Nantes, 2003, 32 p. Gloaguen, E. Unpublished Research report, DEA Géosystèmes, Université d'Orléans, 2002, 41 p. Léone, P, Palvadeau, P. and Moëlo, Y. C. R. Acad. Sci. Paris, IIc, 2000, 301-308. Moëlo, Y., Lasnier, B., Palvadeau, P., Léone, P. and Fontan, F. C. R. Acad. Sci. Paris, 330, 2000, 317-324. Moëlo, Y., Lulzac, Y., Rouer, O., Palvadeau, P., Gloaguen, E. and Léone, P. Can. Mineral., 40, 2002, 1657-1673

APPORTS DE LA MICROSONDE ÉLECTRONIQUE À L'ÉTUDE CRISTALLOCHIMIQUE DES SULFURES COMPLEXES NATURELS D'ANTIMOINE ET PLOMB

L'analyse ponctuelle à la microsonde électronique en mode séquentiel automatisé a permis l'étude chimique approfondie des sulfures complexes naturels d'antimoine et plomb : 16 éléments détectés, 11 composés nouveaux identifiés. Le rôle des éléments mineurs, comme constituants spécifiques ou comme éléments de substitution, a été précisé, en particulier pour Cu, Ag, Mn, Tl, Cl et Cd. Les variations de composition chimique agissent de manière déterminée sur l'évolution du type structural par rapport au type idéal (NaCl). La température joue un rôle analogue : son effet sur l'évolution structural peut être compensé par certaines variations du chimisme et notamment par adjonction d'éléments supplémentaires en faible teneur.Quantitative point X-ray microanalysis following an automated sequential approach was used for detailed chemical analyses of complex Pb-Sb sulfides : 16 elements were detected, and 11 new mineral compounds identified. Elements present in minor concentrations, such as Cu, Ag, Mn, Tl, Cl and Cd, act either as spectific constituents of a mineral or in solid solution. Chemical composition changes selectively modify the structural transformations as compared to the structure type of NaCl. A similar dependence with temperature has also been shown. The structural changes vs. temperature may be cancelled by adding small quantities of specific additional elements

Original mineralogical features of a hydrothermalised oolitic ironstone : The deposit of Saint-Aubin-des-Châteaux (Armorican Massif, France).

Despite its large area, the Armorican Massif provided only very few new mineral species: plombogummite, laumontite, bertrandite, natrodufrenite and lulzacite. This last species was recently discovered by one of us (Y. L.) in a sandstone quarry at Saint-Aubin-des-Châteaux (near Châteaubriant, Loire-Atlantique department) (Moëlo et al., 2000), and various studies permitted to reveal original mineralogical features, related to the superimposition of hydrothermal processes on an Ordovician oolitic ironstone interstratified in the sandstone sequence. This ironstone belongs to a very large sedimentary Fe deposit lying at the East margin of the Armorican Massif. At Saint-Aubin, ooliths are constituted essentially by siderite and chlorite, with abundant Srrich fluorapatite and organic matter. During Hercynian orogenesis, faulting controlled hydrothermal processes, which induced pronounced mineralogical changes, especially massive sulphidation of the ironstone (Gloaguen, 2002). Due to the abundance of primitive apatite and other peculiar geochemical features, it permitted the crystallisation of various Sr or lanthanoide phosphates. Lulzacite, Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10, is very well crystallised in small quartz veins crosscutting the ironstone, together with siderite and pyrite. It is isostructural with jamesite (Léone et al., 2000). It occurs as massive aggregates (up to some cm3), as well as euhedral crystals up to 1 cm in size. It was formed by short-range remobilisation of synsedimentary apatite (process of lateral secretion). Later, its decomposition lead to the formation of goyazite (with amethyst colour), together with late fluorapatite, and some berthierine. Lanthanoide phosphates were formed directly within the ironstone. The very rare scandium phosphate pretulite, ScPO4, has grown together with xenotime-(Y) in epitaxy on detrital zircon crystals (Moëlo et al., 2002). SEM as well EPMA revealed concentric zonation of pretulite and xenotime crystals, indicative of a multi-stage process. Monazite-(Ce) is present as minute anhedral neoformed aggregates, while detrital crystals are rare. Detrital zircon shows growth zones enriched with Sc-, Y- and HREE- phosphate components. EPMA of zircon and pretulite suggested a complete solid solution according to the heterovalent substitution rule: Zr + Si -> Sc + P. This solid solution has been confirmed experimentally in high temperature conditions (Dubost, 2003). A general metallogenic study is in progress (Gloaguen, 2002; Gloaguen et al., in prep.). After the main Fe-S-(As) stage, a Zn-Pb-(Cu) stage presents some Sb sulfosalts (boulangerite, bournonite and tetrahedrite), as well as traces of electrum. While the formation of lulzacite and subordinated goyazite is directly related to the hydrothermal process acting in the deposit of Saint-Aubin, that of xenotime and monazite begins very early within the ironstone formation, as observed in the neighbouring Fe deposit of Rougé, what may also permit the discovery of new pretulite occurrences. Thus, it seems that hydrothermalism of such ironstones is a major key and control when studying rare minerals associated with ironstones. Moreover, others phosphates have been previously described within ironstones: wolfeite (Fe2+,Mn2+)2(PO4)(OH) (Brousse and Chauvel, 1969), lazulite (Chauvel, 1968) and are not yet recognized in the Saint-Aubin-des-Châteaux quarry. We propose that differences between these phosphates minerals parageneses are controlled by ironstone initial chemistry and hydrothermalism. Such hydrothermalism is favoured and associated with late orogenic context. In this way, the large European palaeozoic ironstone belt is probably an important target for potential new and rare minerals species. Actually, all armorican iron mines are closed and a re-examination of ironstones samples preserved in museums could probably allow the discover of new species. Brousse, R. and Chauvel, J.-J. Bull. Soc. fr. Minéral. Cristallogr. 92, 1969, pp 93-94. Chauvel, J.-J. Mém. Soc. géol. Minéral. Bretagne, 16, 1968, 243 p. Dubost, V. Unpublished Research report, Magistère de Physico-Chimie Moléculaire, Paris XI-ENS Cachan, Institut des Matériaux de Nantes, 2003, 32 p. Gloaguen, E. Unpublished Research report, DEA Géosystèmes, Université d'Orléans, 2002, 41 p. Léone, P, Palvadeau, P. and Moëlo, Y. C. R. Acad. Sci. Paris, IIc, 2000, 301-308. Moëlo, Y., Lasnier, B., Palvadeau, P., Léone, P. and Fontan, F. C. R. Acad. Sci. Paris, 330, 2000, 317-324. Moëlo, Y., Lulzac, Y., Rouer, O., Palvadeau, P., Gloaguen, E. and Léone, P. Can. Mineral., 40, 2002, 1657-1673

Lead-antimony sulfosalts from Tuscany (Italy). XIV. Disulfodadsonite, Pb11Sb13S30(S2)0.5, a new mineral from the Ceragiola marble quarry, Apuan Alps: occurrence and crystal structure.

The new mineral species disulfodadsonite, Pb11Sb13S30(S-2)(0.5), has been discovered in a cavity of the Liassic marbles quarried in the Ceragiola area, near the town of Seravezza, Apuan Alps, Tuscany, Italy. It occurs as acicular crystals, up to 3-4 mm in length and a few micrometers in width, associated with boulangerite, calcite, and sphalerite. Disulfodadsonite is metallic black. Under the ore microscope, it is white with red internal reflections visible on the grain edges; no pleochroism could be distinguished. Anisotropy is weak but distinct, with rotation tints from brown to dark blue. Electron microprobe analyses collected on two different grains give (wt %): Pb 46.42(20), Sb 32.29(32), As 0.41(2), S 20.19(13), Cl 0.03(2), total 99.34(58), and Pb 46.76(55), Sb 32.30(32), As 0.40(4), S 20.64(9), Cl 0.02(1), total 100.12(35). On the basis of Sigma Me = 48 apfu, the structural formulae are respectively Pb-21.74(8)(Sb25.73(8)As0.53(2))(Sigma=26.26)S61.11(24)Cl0.08(4) and Pb-21.83(25)(Sb25.66(25)As0.51(4))(Sigma=26.17)S62.27(41)Cl0.06(2), close to Pb22Sb26S62 (against Pb23Sb25S60Cl for dadsonite). The crystal structure study gives a triclinic unit cell, space group P-1, with a 4.1192(3), b 17.4167(14), c 19.1664(16) angstrom, alpha 96.127(6), beta 90.015(7), gamma 91.229(7)degrees, V 1366.9(2) angstrom(3). Main diffraction lines of the powder diagram, corresponding to multiple hkl indices, are (relative visual intensity): 3.820 (ms), 3.649 (s), 3.416 (s), 3.381 (vs), 2.857 (ms), 1.897 (ms). The mean crystal structure of disulfodadsonite has been solved by X-ray single-crystal study on the basis of 3389 reflections with a final R-1 = 0.102. It agrees with the general features of dadsonite but without any visible superstructure. The unit-cell content is Pb11Sb13S30(S-2)(0.5) (Z = 1). There are five pure Pb and five pure Sb sites, two mixed (Sb/Pb) sites, and sixteen S positions. The structure can be described as formed by rod-layers, with two types of rod-layers alternating along the b stacking direction. Disulfodadsonite is the Cl-free homeotype of dadsonite, Pb23Sb25S60Cl, stabilized by the disulfide ion (S-2)(2-), and related by the coupled substitution Pb2+ + Cl -> Sb3+ + (S-2)(2-). Its formation is dependent upon a high value of f(S-2), like other associated minerals at Seravezza (native sulfur, enargite, moeloite, etc)