Min�ralisations Pb-Zn ?Sous Inconformit�? des Series de Plates-Formes Carbonat�es. Exemple du Gisement de Tr�ves (Gard, France). Relations entre Dolomitisations, Dissolutions et Min�ralisations

Macquar, Jean-Claude; Lagny, Ph.

doi:10.1007/bf00202741

Cited by 9 publications

(4 citation statements)

References 7 publications

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“…(2005), no definitive argument for a direct genetic link with the bulk of the Pb ores is given. The best relative chronological indicators are based on geology (cross‐cutting relationships between ores and sedimentary formation) and tectonics, and indicate a post‐Triassic age at a minimum, most probably early Jurassic (Macquar & Lagny 1981; Macquar et al. 1990; probably post‐Hettangian (Lotharingian to Bathonian) following Charef & Sheppard 1988) for the main ore stage and probably Kimmeridgian as sulphide rich layers (sedex) are observed locally in Kimmeridgian formations (Ming‐An et al.…”

Section: Brine Migration Related To Oceanic Riftingmentioning

confidence: 99%

Fluid flows and metal deposition near basement /cover unconformity: lessons and analogies from Pb–Zn–F–Ba systems for the understanding of Proterozoic U deposits

Boiron¹,

Cathelineau²,

Richard³

2010

Geofluids

105

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Fluid circulation at basement ⁄ cover unconformities is of first importance for metal transfer and especially the formation of Pb-Zn, F, Ba and U-deposits. This is typically the case for world-class Proterozoic U deposits (Canada, Australia, Gabon) in basins, which show many similarities with younger Pb-Zn-F-Ba systems (Irish Paleozoic Pb-Zn deposits, F-Pb-Zn-Ba deposits related to extensional tectonics from Spain, western France and Silesia and fluid movements related to continental rifting in the Rhine graben). As fluid mixing near the basement ⁄ cover unconformity is one of the key factors for ore formation, a series of parameters have been considered for both systems: the time gap between basin formation and metal deposit, the origin and nature of the ore fluids, the temperature of fluid end members and the style of migration. Results show great similarities in all fluid systems: (i) a wide range of fluid salinity indicating the lack of homogeneity of fluid chemistry at the scale of the reservoirs, (ii) the deep penetration of brines through faults and dense networks of microfractures within the basement below the unconformity, (iii) local fluid-rock interaction leading to porosity increase and significant fluid changes in fluid chemistry, (iv) a pulsatory fluid regime during fluid trapping, (v) anisothermal fluid mixing revealed by a systematic temperature gap between brines and recharge fluids, (vi) stages of fluid movements facilitated by discontinuous opening related to later tectonic ⁄ telogenetic stages linked to major geodynamic events, typically without related sedimentation and burial (exception in a few cases characterized by the synchronous production and penetration of surface brines and ore genesis). By analogy with younger systems, the conditions of burial and penetration of brines in the Archean basement suggest that thermal convection drove the brine movements, and was possibly linked to extensional tectonics in a part of the giant mid-Proterozoic U-deposits.

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Section: Brine Migration Related To Oceanic Riftingmentioning

confidence: 99%

Fluid flows and metal deposition near basement /cover unconformity: lessons and analogies from Pb–Zn–F–Ba systems for the understanding of Proterozoic U deposits

Boiron¹,

Cathelineau²,

Richard³

2010

Geofluids

105

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“…Two principal stages of mineralization are recognized along with several sub‐stages (Rouvier 1980; Macquar & Lagny 1981). Stage 1 (Fig.…”

Section: Geochemistry and Mineralogymentioning

confidence: 99%

“… Geological setting of the Trèves and related zinc–lead deposits in the Cévennes region (modified from Macquar & Lagny 1981). …”

Section: Introductionmentioning

confidence: 99%

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Precipitation of lead–zinc ores in the Mississippi Valley‐type deposit at Trèves, Cévennes region of southern France

Leach¹,

Macquar²,

Lagneau³

et al. 2006

Geofluids

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The Trè ves zinc-lead deposit is one of several Mississippi Valley-type (MVT) deposits in the Cé vennes region of southern France. Fluid inclusion studies show that the ore was deposited at temperatures between approximately 80 and 150°C from a brine that derived its salinity mainly from the evaporation of seawater past halite saturation. Lead isotope studies suggest that the metals were extracted from local basement rocks. Sulfur isotope data and studies of organic matter indicate that the reduced sulfur in the ores was derived from the reduction of Mesozoic marine sulfate by thermochemical sulfate reduction or bacterially mediated processes at a different time or place from ore deposition. The large range of d 34 S values determined for the minerals in the deposit (12.2-19.2& for barite, 3.8-13.8& for sphalerite and galena, and 8.7 to )21.2& for pyrite), are best explained by the mixing of fluids containing different sources of sulfur. Geochemical reaction path calculations, based on quantitative fluid inclusion data and constrained by field observations, were used to evaluate possible precipitation mechanisms. The most important precipitation mechanism was probably the mixing of fluids containing different metal and reduced sulfur contents. Cooling, dilution, and changes in pH of the ore fluid probably played a minor role in the precipitation of ores. The optimum results that produced the most metal sulfide deposition with the least amount of fluid was the mixing of a fluid containing low amounts of reduced sulfur with a sulfur-rich, metal poor fluid. In this scenario, large amounts of sphalerite and galena are precipitated, together with smaller quantities of pyrite precipitated and dolomite dissolved. The relative amounts of metal precipitated and dolomite dissolved in this scenario agree with field observations that show only minor dolomite dissolution during ore deposition. The modeling results demonstrate the important control of the reduced sulfur concentration on the Zn and Pb transport capacity of the ore fluid and the volumes of fluid required to form the deposit. The studies of the Trè ves ores provide insights into the ore-forming processes of a typical MVT deposit in the Cé vennes region. However, the extent to which these processes can be extrapolated to other MVT deposits in the Cé vennes region is problematic. Nevertheless, the evidence for the extensive migration of fluids in the basement and sedimentary cover rocks in the Cé vennes region suggests that the ore forming processes for the Trè ves deposit must be considered equally viable possibilities for the numerous fault-controlled and mineralogically similar MVT deposits in the Cé vennes region.

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Fluid Flows and Metal Deposition near Basement/Cover Unconformity: Lessons and Analogies from Pb–Zn–F–Ba Systems for the Understanding of Proterozoic U Deposits

Boiron

Cathelineau

Richard

2010

Frontiers in Geofluids

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Min�ralisations Pb-Zn ?Sous Inconformit�? des Series de Plates-Formes Carbonat�es. Exemple du Gisement de Tr�ves (Gard, France). Relations entre Dolomitisations, Dissolutions et Min�ralisations

Cited by 9 publications

References 7 publications

Fluid flows and metal deposition near basement /cover unconformity: lessons and analogies from Pb–Zn–F–Ba systems for the understanding of Proterozoic U deposits

Fluid flows and metal deposition near basement /cover unconformity: lessons and analogies from Pb–Zn–F–Ba systems for the understanding of Proterozoic U deposits

Precipitation of lead–zinc ores in the Mississippi Valley‐type deposit at Trèves, Cévennes region of southern France

Fluid Flows and Metal Deposition near Basement/Cover Unconformity: Lessons and Analogies from Pb–Zn–F–Ba Systems for the Understanding of Proterozoic U Deposits

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