The basement of the Franceville basin, located in southeastern Gabon, is mainly composed of Archaean TTG, gneiss and granitoids (Bouton et al., 2009; Caen-Vachette et al., 1988). The granitic rocks were emplaced during two main events (Bouton et al., 2009; Chevallier et al., 2002; Prian and Johan, 1989; Thiéblemont et al., 2009); the first one occurred during the Mesoarchaean (2928 ± 6 Ma to 2870 ± 5 Ma) and produced calc-alkaline granitoids, generally of grey colour with characteristics similar to those of the Archaean TTG series (Trondjemite, Tonalite, Granodiorite) and quartz-diorites with migmatic facies (Thiéblemont et al., 2009). They are locally crosscut by basic dykes (diorite). The second plutonic event occurred during the Neoarchaean (2800 to 2550 Ma) and produced alkaline to calc-alkaline pink and red magmatic rocks: syenites, syenogranites to granites, and pegmatites (Thiéblemont et al., 2009).

Overlying the Archaean basement, the 2000-m thick Franceville sedimentary basin (Ossa Ossa et al., 2013) is comprised of five unmetamorphosed sedimentary and volcano-sedimentary sequences. From bottom to top, these are (Weber, 1968; Weber and Gauthier-Lafaye, 2013):

• • the FA formation (500–1000 m thick), directly deposited on the Archaean granitoids, consists of red, green and black sandstones and conglomerates (Gauthier-Lafaye and Weber, 1989);
• • the FB formation (400 to 700 m thick), dominated by black shale with interbedded breccia and sandstone of turbiditic origin (Azzibrouck-Azzilez, 1986; Mossman et al., 2005; Parize et al., 2013; Thiéblemont et al., 2009);
• • the FC, FD and FE formations (less than 300 m), mainly composed of stromatolitic chert, black shale and sandstone, respectively (Bertrand-Sarfati and Potin, 1994; Préat et al., 2011).

The diagenetic event resulted in a silicification of the detrital quartz grains and the formation of authigenic illite and chlorite in the matrix (Gauthier-Lafaye, 1986; Gauthier-Lafaye and Weber, 1989).

The least altered granitoids are grey or pink-to-red coloured. They are tonalite, granodiorite, and granite. The primary minerals are quartz, plagioclase, biotite, alkali feldspar, and locally amphibole. Zircon, apatite, uranothorite and monazite are the common accessory mineral phases. Locally, the grey granitoids present a gneissic structure.

The granitic basement is affected by strong alteration that shows a well-developed zoning from the base of studied drill core in the granitoids (15-m depth) to the FA-granitoid unconformity. At the bottom of the drill holes, the granitoids are affected by an early propylitic alteration. The uppermost part of the granitoidic basement is largely affected by an illitic alteration. Here, the alteration imparts a green or red colour to the rock.

The propylitic alteration is characterized by pervasive crystallization of Mg-chlorite, epidote, allanite, calcite, and titanite. Chlorite and epidote have also precipitated in fissures. Locally, late calcite replaces feldspar or occurs along cracks in association with chlorite. The temperature domain of the propylitic alteration, and associated chlorite–epidote paragenesis, has been estimated using the chlorite geothermometer of Cathelineau (1988) in the 250–300 °C. The microprobe data are presented in Table 2.

The illitic alteration is mainly pervasive and characterized by an illite–phengite–haematite assemblage associated with corrosion of quartz crystals in the sandstone above the unconformity and in the plutonic rocks of the basement (Fig. 1). Illite crystallized as plagioclase replacement, whereas phengite and haematite replaced previously chloritized biotite. The illitic alteration has affected the granitic basement pervasively 4 to 5 m below the unconformity and locally extends down to a depth of 7 m along fractures. Late-stage carbonates, locally associated with anhydrite, occur in microfractures that crosscut illitized rocks.

Illitic alteration is also observed in FA sandstone above the unconformity and in FB black shale. The illitic matrix has filled all intergranular porosity and has replaced the feldspars. Illite in FA sandstones is associated with calcite, anhydrite and barite.

Fluid inclusions have been studied in primary quartz of plutonic rocks from the basement and in primary detrital quartz of one sandstone sample. All inclusions show generally two phases (an aqueous liquid and a vapour phase representing 5 to 10 vol. % of the inclusion volume). Three-phase inclusions (liquid + vapour + salt) and single-phase inclusions (liquid phase only) were rarely observed. The fluid inclusions from the propylitized samples show sizes up to 20 μm, whereas those from the illitized samples close to the unconformity and from the base of the FA sandstone have smaller sizes (5–10 μm). The two-phase fluid inclusions of propylitized samples are characterized by a wide range of ice-melting temperatures (–39 < T_{m ice} < –0.1 °C) and homogenization temperatures (95 < T_h < 206 °C; Fig. 2 and Table 1). The fluid inclusions of the illitized samples close to the unconformity and from the base of the FA sandstone are characterized by a large range of ice melting temperatures (–23 < T_{m ice} < –0.1 °C) and homogenization temperatures (132 < T_h < 220 °C; Fig. 2 and Table 1). The highest salinity fluids have not been observed in this alteration zone.

The fluid inclusions with the highest salinity (i.e., the lowest T_{m ice}) tend to show the lowest homogenization temperature (Fig. 2). The same trend was also observed for the fluids from the Franceville Basin (Openshaw et al., 1978; Mathieu, 1999; Mathieu et al., 2000), the Athabasca Basin (Canada) (Boiron et al., 2010), the Kombolgie Basin (Australia) (Boiron et al., 2010) and in the basement below an unconformity in the northwestern part of the French Massif Central (France) and at Soultz (France) (Boiron et al., 2010; Cathelineau & Boiron, 2010).

Monazite crystals were analysed from samples KA6 437.85 (0.85 m below the unconformity) strongly illitized and GR1 631 (1 m below the unconformity) less illitized. In the GRI 631 sample, some feldspars are replaced by calcite and illite, and most of microcracks, filled by calcite and locally quartz, crosscut the illitized feldspars. In sample KA6 437.85, monazite occurs as euhedral, elongated, millimeter-size crystals scattered in the illitic matrix with no evidence of alteration. In sample GR1 631, the monazite crystals are xenomorphic, often grouped in aggregates, and they are locally associated with pyrite, illite and uranothorite.

The monazites from these two altered samples are characterized by low Th contents (ThO₂ < 1.4 wt%) compared to the detrital monazites from the FA sandstones (average ThO₂ = 4 wt%) described by Mathieu et al. (2000). The euhedral monazite crystals of the illitized sample (KA6 437.85) are richer in Th (up to 1.4 wt %) and poorer in U (23 < U < 138 ppm) than those of the sample GR1 631 with the illite–carbonate alteration (25 < Th < 2263 ppm and 199 < U < 466 ppm). Plotted in the ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U diagram, the U–Pb isotopic data of euhedral monazite from sample KA6 437.85 give two groups of discordia ages (Fig. 3a): from 2998 ± 25Ma (MSWD = 0.72) to 2922 ± 24 Ma (MSWD = 1.4) and 2621 ± 30 Ma (MSWD = 1.5). The isotopic data from xenomorphic monazite from sample GR1 631 are plotted in a ²⁰⁷Pb/²⁰⁶Pb versus ²⁰⁶Pb/²³⁸U Tera–Wasserburg diagram because of their high common Pb contents. An age of 1870 ± 26Ma (MSWD = 1.04) is obtained (Fig. 3b).

From the deepest part of the bore holes to the unconformity, three successive alteration parageneses are observed, i.e.:

• (i) an early propylitic alteration characterized by Mg-chlorite–epidote ± titanite ± allanite paragenesis disseminated in the rock and by carbonate + chlorite crystallization infilling of veinlets;
• (ii) a diagenetic alteration characterized by a pervasive illitization, haematite crystallization and quartz dissolution;
• (iii) a late diagenetic alteration characterized by calcite ± anhydrite vein filling crosscutting illitized granitoids.

Petrographic observations show the superimposition of the illitic alteration on the propylitic one and the increasing intensity of the illitization toward the unconformity.

The two sets of monazite ages at 2998–2922 ± 25 Ma and 2621 ± 30 Ma in the pervasively illitized sample close to the unconformity are similar to ages of emplacement of the plutonic rocks in the basement (Bouton et al., 2009; Caen-Vachette et al., 1988). The illitic alteration has not modified the isotopic system of monazite. The monazite of the sample GR1 631 affected both illitization and carbonatation gives a younger age of 1870 ± 26 Ma. This age is similar to those reported by Bonhomme et al. (1982), who have dated the thin illitic fraction (0.2 and 0.4 μm) by the Rb–Sr method. These results suggest that the diagenetic fluids responsible for the carbonatation have percolated the basement and have favoured the precipitation of late poor Th and rich U monazite. No age between 2.5 and 2.1 Ga (age of the Franceville Basin), which could be related to a possible palaeoweathering episode, has been recorded.

The fluid inclusions measured in the different primary quartz crystals of the granitoids from the unconformity to 15 m downwards into the basement display a wide range of T_{m ice} values, from very low to high salinities. These data are similar to those previously obtained on the fluids identified in the different generations of quartz from the Franceville Basin (Gauthier-Lafaye, 1986; Gauthier-Lafaye & Weber, 1989; Mathieu, 1999; Mathieu et al., 2000; Openshaw et al., 1978). The high salinity diagenetic fluids characterized by –60 < T_{m ice} < –46 °C are Ca-dominated with a Na/Ca ratio around 0.5, and the –36 < T_{m ice} < –27 fluids are Na-dominated, with a Na/Ca ratio from 2 to 9 (Mathieu et al., 2000). The fluids with T_{m ice} < –21 °C contain divalent cations, such as Ca²⁺ and or Mg²⁺, besides Na⁺ (Yanatieva, 1946; Oakes et al., 1990) and the ones with T_{m ice} from –15 to–21 °C, referring to the Athabasca Basin, suggest a higher Na/Ca ratio (Mercadier et al., 2010).

The identification in the quartz crystals of the altered plutonic rocks below the unconformity of highly saline and moderate-temperature (130–180 °C) fluids with characteristics similar to those of the diagenetic brines from the Franceville Basin demonstrates that these brines have massively percolated into the basement at least down to –15 m below the unconformity. Similar interaction has been already observed in various localities, such as in the Rhine Graben (France) and below the Athabasca Basin (Saskatchewan, Canada), where brines penetrated the basement down to more than 5 km and 250 m below the unconformity, respectively (Cathelineau and Boiron, 2010; Mercadier et al., 2010; Pagel and Jaffrezic, 1977). The trend defined by T_{m ice}/T_h pairs (Fig. 2) could indicate a mixing between diagenetic brine and a low-saline meteoric fluid (Boiron et al., 2010). Such trend has been also described in many basins: southern Norway (Munz et al., 1995, Oliver et al., 2006), Spain (Bouch et al., 2006), France, northwestern Massif Central (Munoz et al., 1999), Athabasca (Mercadier et al., 2010; Richard et al., 2013).

Even if the high- and low-salinity fluids have been identified in the deepest samples, the low-salinity/high-temperature fluids and mixing fluids are predominantly identified in the highly illitized and dequartzified rocks located on each side of the unconformity, i.e., in the FA sandstone and shallow samples from the basement. These results suggest that the fluids with higher temperature and low salinity, of possible meteoric origin according to Mathieu et al. (2000), may be responsible for quartz dissolution and illitization. Their percolation seems to have been more intense at the unconformity and partly erased the highly saline diagenetic fluids, as the latter fluids are rare in the samples located within the few metres either side of the unconformity. On the contrary, in the deepest rocks, the brines trapped in quartz FIP have the highest salinities in samples 15 m below the unconformity, with no evidence of illitization. That argues in favour of the dominant role of the unconformity on the Na–Ca diagenetic fluid circulations between 2.1 and 1.9 Ga. In the basement rocks of the Athabasca Basin, brines having the highest salinity are only observed in the most altered samples (Mercadier et al., 2010). These authors explained the presence of these brines by:

• (i) the result of an important gain in Cl during alteration of the biotite into chlorite and illite during the brine stage;
• (ii) an important water loss during the alteration of the basement rocks, as demonstrated by the important difference between loss on ignition (LOI) for fresh and altered rocks (1% to 10%).

In the basement of the Franceville Basin, the high salinity of diagenetic fluids observed at 15 m depth may be explained by a similar fluid/rock interaction. On the contrary, the low salinity of fluids observed in the most altered rocks that are located along the unconformity is explained by the massive infiltration of meteoric fluids that mix with the diagenetic brines. The haematite precipitation coexisting with the illitic alteration invading along the unconformity testifies the oxidative conditions that may be associated with the diagenetic and/or meteoric fluids. On the other hand, the petrographic features of the altered basement below the unconformity and dating do not give any argument in favour of an Archaean weathering event between 2.5 and 2.1 Ga.