Eric Gloaguen, Yannick Branquet, P. Boulvais, Y. Moëlo, J.J. Chauvel, P.J. Chiappero, Eric Marcoux - PDF

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. Eric Gloaguen, Yannick

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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. Eric Gloaguen, Yannick Branquet, P. Boulvais, Y. Moëlo, J.J. Chauvel, P.J. Chiappero, Eric Marcoux To cite this version: Eric Gloaguen, Yannick Branquet, P. Boulvais, Y. Moëlo, J.J. Chauvel, et al.. 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.. Mineralium Deposita, 2007, 42, pp.4, /s . hal HAL Id: hal Submitted on 19 Mar 2007 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal As Sb Au mineralisation during Hercynian strike-slip deformation Eric Gloaguen 1, 5, Yannick Branquet 1, Philippe Boulvais 2, Yves Moëlo 3, Jean- Jacques Chauvel 2, Pierre-Jacques Chiappero 4 and Eric Marcoux 1 (1) Institut des Sciences de la Terre d Orléans (ISTO), UMR 6113 CNRS/Université d Orléans, Bâtiment Géosciences, rue de Saint Amand, BP 6759, Orleans Cedex 2, France (2) Géosciences Rennes, UMR 6118, CNRS/Université de Rennes 1, Rennes Cedex, France (3) Laboratoire de Chimie du Solide, Institut des Matériaux Jean Rouxel, CNRS UMR 6502, 2, rue de la Houssinière, BP 32229, Nantes Cedex 3, France (4) Département Histoire de la Terre, USM 201-UMR 7160, Muséum National d Histoire Naturelle, 61 rue Buffon, Paris, France (5) Present address: BRGM, 3, avenue Claude-Guillemin, BP6009, Orleans Cedex 2, France Abstract 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 pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After 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 C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop ( 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 that 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 mineralisation of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits. Keywords : Oolitic ironstone - Orogenic gold - Lead isotopes - France Introduction Iron-rich sedimentary rocks ( 15% Fe) have two main types: (1) banded iron formations (BIFs) of Precambrian age and (2) ironstones, which are mostly oolitic and with an age ranging from Late Precambrian to Tertiary (see reviews by Gross 1996a; Christie and Brathwaite 1997 for details and sub-types). BIFs host important Au deposits, commonly associated with strong sulphidation (massive replacement of oxides and/or in veins) and hydrothermal alteration (e.g. Foster and Gilligan 1987; Thompson et al. 1990; Ledru and Bouchot 1993; Newton et al. 1997; McCuaig and Kerrich 1998). The syngenetic, with remobilisation or not, vs epigenetic origin of BIF-hosted gold deposits is still debated, but the geochemical and rheological controls of Au mineralisation in BIFs are now well established (Fripp 1976; Kerrich and Fryer 1979; Phillips et al. 1984; Groves and Vearcombes 1990; Macdonald 1990; Ledru and Bouchot 1993). In contrast, syngenetic and epigenetic sulphidation of oolitic ironstones is not well documented. In oolitic ironstone formations (e.g. Palaeozoic ironstones like Clinton USA, Wabana Canada, Coal Measures UK), only depositional and diagenetic sulphides, particularly framboidal pyrite, are reported, reflecting sulphate availability and organic carbon content in the environment (Lyons 1957; Spears and Caswell 1986; Spears 1989; Cotter and Link 1993; Gross 1996b). One may thus question why oolitic ironstones, in contrast to BIFs, are apparently less affected by sulphidation or polymetallic mineralisation processes. A key observation is that, on the contrary to Aubearing Precambrian BIFs, the oolitic ironstones (e.g. the Lorraine Luxembourg so-called Minette in the Paris basin and Clinton ironstones in the Appalachian foreland basin) are located far from orogenic internal zones where deformation, magmatism and metamorphism enhance fluid flow and metal transport. In this study, we present structural, petrological and geochemical data on the hydrothermal alteration and the polymetallic epigenetic mineralisation that developed in the Lower Ordovician oolitic ironstone of the Saint-Aubindes-Châteaux area, Armorican Massif, Hercynian belt of western France. Geological setting The Saint-Aubin-des-Châteaux aggregate quarry is located in the Central Armorican Domain (CAD), about 6 km north from the Northern Branch of the South Armorican Shear Zone (NBSASZ, Figs. 1a,b and 2a). The NBSASZ and the North Armorican Shear Zone, which bound the CAD, are the main structures of the Hercynian Armorican Massif and correspond to Middle to Late Carboniferous right-lateral wrenching (Jégouzo 1980; Cartier 2002; Gumiaux 2003). The CAD is mostly composed of a Neo-Proterozoic pelitic basement covered by Palaeozoic detrital sediments that underwent deformation and anchizonal greenschist metamorphism during the Carboniferous (Le Corre et al. 1991). Fig. 1 a Sketch map of the main structural domains of the Armorican Massif. b Geological map of the Eastern central part of the Armorican Massif, modified from Chantraine et al. (1996). Mineral deposits from Méloux et al. (1979). Note the strong spatial relation between Hercynian granites and tin deposits (i.e. La Villeder) and between Late Hercynian shear zones and some gold deposits Palaeozoic sedimentation Lower Ordovician sandstones overlie unconformably the Neo-Proterozoic basement (Fig. 2b). The so-called Grès armoricain Formation, of Arenig age, records an important marine transgression (Guillocheau and Rolet 1982; Durand and Noblet 1986). The formation includes three members (Kerforne 1915; Chauvel 1971), which are from bottom to top (Fig. 2b): (1) the lower member made of thick sandstone layers with fine mudstone intercalations; (2) the intermediate member made of sandstone, siltstone and mudstone alternations and (3) the upper member made of thinner micaceous sandstone layers with mudstone intercalations. The Ordovician conditions of sedimentation are characterised by a nearshore depositional environment with storm effects in a domain of internal platform protected by barriers (Chauvel and Le Corre 1971; Guillocheau and Rolet 1982; Joseph 1982). Oolitic ironstones form sand infratidal media, on the top of transgressive sequences during a period of sedimentary stabilisation (Joseph 1982). The marine conditions of sedimentation prevailed up to the Middle Devonian. Wrenching, along master shear zones bordering the CAD, induced the location of Upper Devonian to Stephanian marine depocentres within individualised longitudinal basins (e.g. Laval basin, Fig. 1b). Fig. 2 a Geological map of Châteaubriant area synthesised and simplified from Dadet et al. (1987), Dubreuil et al. (1988), Herrouin et al. (1988) and Trautmann et al. (1987). b Stratigraphic log of the Palaeozoic sequence of the Central Armorican Domain, modified from Dadet et al. (1987). The detailed sequence of ironstones layers (A to D) is modified from Chauvel (1971, 1974) Structures and deformation Palaeozoic sedimentary rocks are well preserved and exposed within N110 E-trending upright folds, south of Rennes (Fig. 1b). The large buckling wavelength is controlled by the thick and competent Grès armoricain Formation (up to 600 m, Noblet 1983). These folds and the associated N110 E-trending regional cleavage, which bears a sub-horizontal stretching lineation, are linked to ductile shearing along the NBSASZ. This deformation corresponds to regional-scale dextral simple shear that affected the whole CAD during the Carboniferous (Le Corre 1978; Gapais and Le Corre 1980; Choukroune et al. 1983; Gumiaux 2003). In the CAD, strain intensity increases from north to south and reaches a maximum near the NBSASZ (Gapais and Le Corre 1980, Fig. 1b). From macro- to microscopic scale, the CAD is affected by important vertical fracturing (strike-slip faults and extensional fractures, see Figs. 1b and 2a). The major regional fractures have a dominant N160 E orientation and are oblique to fold axes. Choukroune et al. (1983) demonstrated that this fracturing was compatible and coeval with the CAD-scale Carboniferous dextral simple shear that occurred via successive incremental deformation during clockwise rotation of principal strain axes. Synkinematic granite intrusions (about Ma, Carron et al. 1994) were emplaced along the NBSASZ during Carboniferous dextral wrenching (e.g. Lizio granite, Fig. 1b). These supracrustal intrusions are of laccolith shape and are partially coeval with regional cleavage development within the CAD (Le Corre 1978; Berthé et al. 1979; Vigneresse and Brun 1983; Roman- Berdiel et al. 1997). Ore deposits of the area Four types of metallic mineralisation, mostly small scale, have been recognised along the NBSASZ (Chauris and Marcoux 1994, Fig. 1b): (1) Ordovician sedimentary deposits (Zr- and Ti-rich sandstones and oolitic ironstones); (2) Lower Silurian volcanosedimentary Zn Pb Cu (Ag Au) deposits, localised within the Saint-Georges-sur-Loire unit; (3) Quartzcassiterite veins associated with leucogranites (e.g. La Villeder, 200 t Sn produced during the nineteenth century) and (4) Late Hercynian polymetallic (Pb, Sb, Cu, Zn, Au) veins and stockworks (e.g. La Lucette, Pontpéan, Fig. 1b) linked to regional or local shear zones and/or emplacement of the late Hercynian granites. Local setting of the oolitic ironstone The quarry, locally named the Bois-de-la-Roche quarry, is located in the southern limb of the Châteaubriant anticline (Fig. 2a). Iron ore mining occurred within the Lower Member of the Grès armoricain Formation in two main pits split by the La Chère river (Figs. 2b and 3a). The intermediate member lies conformably on the lower member. Fig. 3 a Geological map of the Saint-Aubin-des-Châteaux quarry modified from Gloaguen (2002). The quarry is hosted within the Lower and Intermediate Members of the Grès armoricain Formation. Oolitic ironstone A-layer is symbolised by a black dashed line. Note the important density of conjugate dextral and sinistral strike-slip faults. Bedding of sandstones from both members is plotted in stereogram projection (equal area, lower hemisphere). b Cross-section of the north-western part of pit 1 (location on Fig. 3a), showing oolitic ironstone A-layer near the top of the Lower Member (thick grey line). Massive sulphide lenses within ironstone A-layer are in black. Note the strong spatial relationship between massive sulphide lenses and strike-slip faults. c Crosssection of the western part of the pit 2 showing flower structure related to strike-slip faults. A- and B-layer represented by thick grey lines Bedding is generally WNW trending with a broad low SSW dip, from horizontal to 30 (Fig. 3a). Locally, the dip direction is disturbed by tilting along a N155 E-trending major vertical fault (Fig. 4a) and collapse and folding induced by secondary faults (Figs. 3 and 4b). Most faults are dextral strike-slip faults with a small normal component that produced negative flower structures (Fig. 3c). These faults belong to the regional fracture system previously discussed. Extensional fracture orientation and fault-slip data analysis are presented below. Fig. 4 a Major fault of the quarry (dextral strike-slip fault with a low normal component). The large amount of sulphides (mainly pyrite) within the fault zone is responsible for its dark colour. b View of the northern front of the pit 2, showing folds induced by strike-slip faults. Ironstone A-layer is present on the upper part of the photograph, arrows indicate main massive sulphide lenses. c View of the southern front of the quarry showing the spatial relation between massive sulphide lenses (contours underlined in white) and the sulphide-bearing strike-slip faults crosscutting the ironstone A-layer. d En echelon quartz-chlorite tension gashes crosscut and deformed by a sinistral quartz shear vein (dotted line) containing traces of pyrite and galena (stage 2) Four main horizons of oolitic ironstones, hereafter named A to D from top to bottom (Fig. 2b), have been recognised and mined within the Lower Member (Davy 1880; Bellanger 1911; Kerforne 1914; Puzenat 1939; Chauvel 1971, 1974). Only the A and B horizons are exposed at the Saint-Aubin-des-Châteaux quarry (A in pit no. 1, A and B in pit no. 2, Fig. 3b and c). Because of the low quality of B-horizon exposures, we have focused on the A-oolitic ironstone horizon (OIH). OIH varies from dark to black colour and is around 2 m thick with a maximum of 2.55 m (Chauvel 1971, 1974). At outcrop scale, OIH is heterogeneous because of the occurrence of lenticular stratoid massive sulphide bodies, mainly composed of pyrite (Figs. 3b,c and 4c). These sulphide lenses within the OIH have been formerly interpreted as syngenetic massive sulphide deposits (Herrouin et al. 1989). However, Moëlo et al. (2000) and Gloaguen (2002) argued that hydrothermal alteration produced epigenetic sulphidation of the OIH. Consistently, a strong spatial correlation is observed between the development of the sulphide lenticular bodies within OIH and vertical fault traces across OIH (Figs. 3b and 4c). In both pits, the whole sedimentary sequence, including OIH, is affected by important quartz veining expressed by numerous extensional veins and shear veins. As faults, these veins are filled with hydrothermal parageneses including base-metal sulphides (Fig. 4d). In the quarry, sandstones and OIH underwent diagenesis and very low-grade metamorphism (Le Corre 1978) with no cleavage development. The oolitic ironstone Petrography In the quarry, the lower member of the Grès armoricain Formation is homogeneous and composed of dark grey massive sandstone beds (maximum 1.5 m thick) intercalated with thin dark pelite beds. The sandstone beds are mainly composed of weakly recrystallised quartz grains ( μm), the remaining (10%) corresponding to chlorite, detrital muscovite, tourmaline, zircon, rutile and organic matter responsible for the dark colour of the unit. Sedimentary structures like ripple marks, mud cracks and load casts are common on bedding surfaces. Where not affected by hydrothermal alteration, the primary texture of OIH corresponds to oolites with a nucleus of variable nature (detrital quartz, zircon, monazite and diagenetic apatite, siderite or chamosite) and a poorly laminated chamosite cortex (Fig. 5a). The matrix is made of siderite cement in which organic matter particles are widespread. Oolitic texture distribution is not homogeneous, the matrix may largely dominate locally with few sparse oolites only. Fig. 5 a Photographs showing the progressive sulphidation of the oolitic ironstone, with, from left to right, (1) non-altered chlorite siderite oolitic facies (plane polarised transmitted light [PPTL]); (2) oolitic facies with pyrite (Py) surrounding oolites (plane polarised reflected light [PPRL]) and (3) massive sulphide (mainly pyrite) with oolite ghosts underlined by graphite, graphitoids and residual chlorite [PPRL]. b Handle sample showing both oolitic ironstone (left), massive sulphide (right) and the sulphidation reaction rim. This reaction rim contains macroscopic apatite (Ap), red chlorite (Chl) and disseminated arsenopyrite (Apy) and pyrite. Dissolution cavities correspond to siderite (ironstone matrix) dissolved by hydrothermal fluids Diagenesis is responsible for the growth of Sr-bearing fluorapatite (Chauvel and Phan 1965). This is consistent with geochemical anomalies observed in bulk-rock analyses (see below). Geochemistry of OIH Analyses of major, trace and rare earth elements of nine samples from the oolitic ironstone A- layer from the CAD are reported in Table 1. Seven of them have been taken outside of Saint- Aubin-des-Châteaux to obtain a regional geochemical average. Table 1 Major, minor and trace elements of the Ordovician A-layer oolitic ironstone of the Central Armorican Domain Sample Pfe 01 Pfe 02 Pfe 03 Pfe 04 Pfe 07 Pfe 09 Pfe 11 Pfe 22 Pfe 24 Location St-Aubin- des- Châteaux pit 1 St-Aubin- des- Châteaux pit 2 Angrie La Boserie Angrie La Boserie Chazé- Henry Chazé Sion-les- Mines Claray Teillay La Brutz Saint-Malode-Phily Montserrat Angrie La Gare d Angrie Av. n = 7 localities SE SiO 2 (wt%) Al 2 O Fe 2 O 3 a MnO MgO CaO Na 2 O nd nd nd nd 0.05 nd nd nd nd K 2 O nd nd nd nd nd nd TiO P 2 O LOI Total As (ppm) Ba Be 3.35 nd Bi nd Co Cr Cs nd nd Cu Ga Ge Hf Mo Nb Ni Pb nd nd nd nd Sb Sn Sr Ta Th U V W Y Zn Zr La Ce Pr Nd Sample Pfe 01 Pfe 02 Pfe 03 Pfe 04 Pfe 07 Pfe 09 Pfe 11 Pfe 22 Pfe 24 Location St-Aubin- des- Châteaux pit 1 St-Aubin- des- Châteaux pit 2 Angrie La Boserie Angrie La Boserie Chazé- Henry Chazé Sion-les- Mines Claray Teillay La Brutz Saint-Malode-Phily Montserrat Angrie La Gare d Angrie Av. n = 7 localities Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Samples Pfe 01 and Pfe 02 are non-altered and hydrothermally altered ironstone, respectively, from the Saint- Aubin-des-Châteaux quarry. The regional average composition and standard error of the A-Layer ironstone are calculated on seven samples (Pfe 03 to Pfe 24). Whole-rock compositions were determined by ICP-AES for major elements and by ICP-MS for trace and rare elements at CRPG Nancy (France). nd Below detection limit; Av. average; SE standard error a Fe 2 O 3 = total Fe expressed as Fe 2 O 3 SE Two samples are from the Saint-Aubin-des-Châteaux quarry. The first one, Pfe 01, comes from the upper part of the Pit no. 1 (Fig. 3). This sample is a fine-grained oolitic chamosite siderite ironstone, without hydrothermal alteration. The second sample, Pfe 02, comes from a stron
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