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

Abstract

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.

Rôle des facteurs hydrodynamiques dans le contrôle de la formation et de l’emplacement des dépôts d’uranium associés à des discordances: aperçus issus de la modélisation de transport réactif

“…The downwelling parts of the convection cells allow the basinal brines to penetrate into the basement; while the upwelling parts of the convection cells enable the basal fluids to be brought into the sandstone. This confirms the previous findings that basinal brines can penetrate into the underlying basement to some depth (a couple of hundred meters below the unconformity; Boiron et al 2010;Richard et al 2010;Cui et al 2012b), and also highlights the importance of the convection cells in transferring the mass between the sandstone and underlying metamorphic basement. The flow rate in the basinal sandstone is moderate (about 1.6 m/year), while in the basement it is much less (about 4.7 × 10 −4 m/year) due to its very low permeability compared with that of the other units (refer to Table 1).…”

“…Unconformity-related uranium (URU) deposits, hosted by Paleoproterozoic sedimentary basins in Canada and Australia, have been formed by large-scale circulation of diagenetic brines that percolated between basinal sandstone and underlying basement rocks (Kyser and Cuney 2009;Boiron et al 2010;Morichon et al 2010;Richard et al 2012). In addition to interaction of oxidizing uranium-bearing brines with basement-derived reducing fluids or reducing minerals in the basement, favorable physicochemical conditions (e.g.…”

Fluids in the Continental Crust

“…Sedimentary formation waters are of significance for much more than just the development of sedimentary basins and their diagenesis because they are often overpressured at depth, and lie above crystalline basement rocks which, we have already noted, are fluid sinks. The importance of formation water interactions with underlying basement rocks for the formation of ore deposits has now been widely recognised (Boiron et al, 2010). It appears likely that they also play a wider role, facilitating deformation and penetrating basement along fractures during extension.…”

Fluids and Ore Formation in the Earth's Crust

“…However, transient periods following active fracturing (e.g., in the region of aftershocks following major earthquakes, or artificially induced by hydraulic fracturing of geothermal reservoirs) raise this baseline permeability of the continental crust by 2-3 orders of magnitude (Townend and Zoback, 2000;Evans et al, 2005;Rojstaczer et al, 2008) over periods of ~1-10 3 y (Ingebritsen and Manning, 2010). This is likely to allow episodes of significant thermal convection throughout the seismogenic part of the continental crust and may be a key to basement fluid injection into basin-hosted ore systems (Deming, 1992;Koziy et al, 2009;Boiron et al, 2010).…”

“…Typical temperatures of ore deposition are between 50 and 120°C for sedimenthosted Cu deposits and replacive (MVT) Pb-Zn-deposits, and may reach 200°C in stratiform Pb-Zn deposits.. Potential metal-source volumes include immature clastic sediments, basin-filling volcanics as well as the fractured continental basement underlying the ore-bearing sedimentary basin (Boiron et al, 2010). Although continental redbeds and volcanic rocks are a typical component of the sediment package underlying Cu deposits, their hematitic mineralogy is a product of ore-related fluid flow and not necessarily a source-rock characteristic that differentiates them from the Pb-Zn deposits.…”