Abstract
Geological setting. The base-metal, fluorite and barite deposits of the SW Massif Central are hosted by EW-trending vertical fractures that transect Lower Cambrian to Lower Carboniferous nappe sequences in the Rouergue, Albigeois, northwestern Montagne Noire and Monts de Lacaune regions of the Hercynian Massif. To the north, base-metal and fluorite veins outcrop in a Cambro-Ordovician series of pelites, quartzites and metabasites. South of the Albigeois, base-metal veins outcrop in a weakly metamorphosed sequence of Cambrian black shales, sandstones ans volcanoclastics. In the eastern Monts de Lacaune, karst-vein-hosted barite deposits occur in dolomites and limestone of Devonian age and in the adjacent Triassic clastics of the Causses Basin. Mineral paragenesis and microthermometric data. Base-metal fluorite mineralisation in the SW Massif Central is associated with a number of generations of gangue quartz that are distinct in terms of their texture and minerals associations. They may be regional in extent and occur in a variety of different lithologies. Four major stages of quartz deposition have been identified. 1. Quartz I typically takes the form of a silicified tectonic breccia and is known regionally as 'B.T.H.' (br~che tectonique hypersilicifi6e). Sheared fragments of local wall-rock are cemented first by chalcedony and then by successive generations of zoned prismatic quartz giving rise to charasteristic cockscomb textures. B.T.H. may occur independently and is found as prominent EW-trending 'dykes' throughout the region. Where B.T.H. occurs in association with Pb-Zn-F-Ba mineralisation it typically forms the wall-rock to the deposit. Primary fluid inclusions have aqueous two-phase fills with salinities between 20 and 24 wt.% NaC1 equiv, and homogenisation temperatures of 85 to 130~ First melting temperatures indicate that CaC12 is also present (-52~ 2. Quartz la is referred to as lamellar quartz and consists of interlocking, randomly orientated lamellae of quartz. Lamellar quartz may occur in association with silicified breccias though relationship between the two are ambiguous. Lead-Zinc sulphides may fill angular vugs between lamellae. Quartz l a occurs in the Rouergue and Monts de Lacaune regions of the Massif Central but is not present in the Albigeois and NW Montagne Noire. Fluid inclusions are similarly CaCl2-bearing liquidvapour fills where temperature of final ice melting indicate salinities of approximately 26 wt.% NaC1 equiv. Homogenisation temperatures range from 185 to 213~ 3. Quartz 2 occurs in association with sulphide cockade ores in the NW Montagne Noire and Rouergue regions. Local host-rock, silicified breccia and lamellar quartz are brecciated and cemented by siderite and sphalerite. Coarse prismatic quartz fills terminal vugs within the sphalerite matrix. Primary inclusions show aqueous two-phase fills with salinities that vary between 20 and 22 wt.% NaC1 equiv. (including CaCI2) and homogenisation temperatures range of 117 to 166~ 4. Quartz 3 occurs in association with massive blue fluorite veins in the Albigeois, as drusy quartz fills of terminal rugs. Fluorite stage mineralisation, is seen to cross-cut sulphide ores in the deposit of Peyrebrune in the NW Montagne Noire. Fluid inclusions in quartz associated with the fluorite are CaCl2-bearing liquid-vapour inclusions with salinities of 20-26 wt.% NaC1 equiv, and homogenisation temperatures range of 125-170~ Although pressures prevailing during mineralisation are presently unknown, Munoz et al., (1994) find good correlation between homogenisation tempera-
Published Version
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