Nature and Origin of Mineralizing Fluids in Hyperextensional Systems: The Case of Cretaceous Mg Metasomatism in the Pyrenees

Abstract

During the Albian, the hyperextension of the Pyrenean passive margin led to a hyperthinning of the continental crust and the subsequent subcontinental mantle exhumation. The giant Trimouns talc-chlorite deposit represents the most prominent occurrence of Albian metasomatism in the Pyrenees, with the occurrence of the largest talc deposit worldwide. Consequently, this deposit, which is located on a fault zone and a lithological contact, represents one of the major drains at the scale of the Pyrenees and one of the best geological targets in order to determine the origin(s) of the fluid(s) that circulated during this period. Talc-chlorite ore is characterized by the presence of brines trapped in dolomite, quartz, and calcite fluid inclusions in the vicinity of the talc-rich zone. Considered as being responsible for the formation of talc, these fluids may be interpreted in several ways: (i) primary brines expelled from Triassic evaporites, (ii) secondary brines produced through halite leaching by diagenetic/metamorphic fluids, and (iii) brines derived from seawater serpentinization of mantle rocks. Stable isotope analyses (δ13C, δ18O, δD, and δ37Cl) and Cl/Br ratio measurements in fluid inclusions and their host minerals were carried out in order to determine the origin of the fluid(s) involved in the formation of the ore deposit. The data are consistent with a primary brine origin for the mineralizing fluid, which could have been expelled from the Triassic levels. Other hypotheses have been tested, for example, the production of brines via the seawater concentration during serpentinization. The geochemical proxies used in this study provide equivocal results. The first hypothesis is by far the most realistic one considering the numerous occurrences of Trias formations nearby, their deformation during the extension, and the drainage of the expulsed brines as evidenced by the high-salinity fluid inclusions found all around the deposit. Alternatively, the exhumation of the mantle is considered as a major source of heat and stress that favored brine migration along the major shear zones. Our results fit well with brine circulation in a hyperextensional geodynamic context, which is related to the formation of the talc-chlorite ore, the thinning of the continental crust, and the exhumation of the subcontinental mantle, in accordance with recent works.

Triassic evaporites: a vast reservoir of brines mobilized successively during rifting and thrusting in the Pyrenees

“…Traces of gas in FI are found in the different localities, mostly N 2 -CO 2 with minor CH 4 (Reynès), CH 4 and N 2 (Jau) and both N 2 , CO 2 and CH 4 are present in Trimouns FIs with various gas ratios. These new data confirm the presence of high salinity fluids in Mg-metasomatic zones such as those found at Trimouns (Parseval, 1992, Boiron et al, 2007, Quesnel et al, 2019. Such brines appear as predominant in fault zones in the eastern part of the Pyrenees.…”

“…At Trimouns, several data agree with the features of the primary evaporitic brines, such as the richness in Na-K-(Mg), Cl isotopes values compatible with evaporated seawater, and a low Cl/Br ratio indicative of evaporated seawater having passed halite saturation, at less for a part of the data (Quesnel et al, 2019). The calcium content is relatively high as in most other similar brines and is generally interpreted as Na-Ca exchange result through brine-rock interactions such as albitisation (McCaig et al, 2000, Boiron et al, 2010.…”

“…The results of this compilation indicate that Triassic evaporites constitute a significant source of chlorine that may have been moved at several periods since Triassic times: first, close to sedimentation periods are at the origin of the local formation of red beds concentrations (Subias et al, 2015); second, during Mesozoic at unknown ages, remobilisation of early metal stock as proposed by Munoz et al(2015) and Cugerone (2019); third, during the Cretaceous rifting, major convective fluid movements occurred under enhanced thermal gradients produced by the mantle exhumation (Quesnel et al, 2019); fourth, during Cenozoic, fluid displaced along with major thrusts during convergence (McCaig et al, 2000), in particular along sliding zones such as clay and evaporite formations, and their vicinity such as damaged zones. Chlorine is still present at varying concentrations in all thermal waters exploited all along the chain, in localities the most frequently very close to Triassic units pinched along major faults and thrusts.…”

“…First of all, serpentinisation is not supposed to release significant amounts of Mg as reactions may be written with conservative Mg. Primary evaporitic brines are also particularly enriched in Mg, which concentration increases continuously with the evaporation rate, the bischofite (MgCl 2 -6H 2 O) saturation being never reached (Fontes and Matray, 1993). Even in the case of the syn-rift formation of Trimouns Mg-metasomatic halos around 115-100 Ma (Schärer et al, 1999;Boutin et al, 2016), Quesnel et al (2019) precluded serpentinisation as the main source of chlorine of brines at the origin of talc formation. Thus, most geochemical parameters of brine fluid inclusions, although not incompatible with other processes such as dehydration processes, are close to Triassic evaporated seawater.…”

Micro‐scale chemical and physical patterns in an interface of hydrothermal dolomitization reveals the governing transport mechanisms in nature: Case of the Layens anticline, Pyrenees, France

“…To date, the large‐scale dolomitization process is still under debate, with remaining questions about the source of fluids and its Mg content (Whitaker et al ., 2004; Gomez‐Rivas et al ., 2014; Koeshidayatullah et al ., 2020b) on the pathways allowing for long‐term renewed fluid migrations in tectonic context (Machel & Mountjoy, 1986; Whitaker et al ., 2004; Breesch et al ., 2010; Beaudoin et al ., 2014; Gomez‐Rivas et al ., 2014; Chen et al ., 2016; Pinto et al ., 2017; Scribano et al ., 2017; Debure et al ., 2019; Quesnel et al ., 2019), and on the fluid transport limitation of the phenomenon at either outcrop‐scale (Koeshidayatullah et al ., 2020b) or crystal‐scale (Putnis & Putnis, 2007; Merino & Canals, 2011; Kondratiuk et al ., 2015). At the crystal‐scale, two conceptual models compete to account for dolomitization: the growth‐driven pressure‐solution (Merino & Canals, 2011); and the dissolution–precipitation mechanism (Putnis & Putnis, 2007).…”