M.C. Boiron1, M. Cathelineau1, F. Noronha2, D.A. Banks3, E. Vindel4, and J.A. Lopez4
Thus, a selection of representative test sites already studied for metallogenic purposes have been selected and revisited to provide a better reconstruction of the P-T-X-time conditions prevailing from the peak of metamorphism to the long term activity of the paleogeothermal system of the Hercynian belt. Main objectives were the identification of the processes at the origin of the chemical features of the fluids and the identification of fluid sources using the data from fluid inclusions and mineral assemblages. Around 8 test sites have been considered, for a detailed examination of the fluid chemistry, combining individual inclusion characterization (microthermometry, Raman spectroscopy) together with the analysis of bulk leachates on the same samples.
Sampling and choice of representative examples
Numerous occurrences of late Hercynian quartz vein sets, often with associated wall rock alteration, attest to significant fluid movement during retrograde metamorphism and late geothermal activity within the Hercynian basement in western Europe. Two main segments of the belt have been chosen : i) the French Massif Central (Limousin area) with the Marche and St Sylvestre zone known for their U deposits, the Bourneix and Villeranges-Chatelet district (Au deposits), ii) the Iberian Peninsula including the Spanish Central System and the northwestern part of the Iberian massif known for several major size W deposits (Panasqueira, Borralha, Mirandela) or districts (Spanish Central System) and Au prospects (Corcoesto, Tomino).
These two areas have been extensively studied and prospected in the last 15 years mostly for Au ores. It is noteworthy that the same succession of events have been recognized in the different occurences attesting an extended and similar fluid migration at the scale of the different zones of the Hercynian belt. Successive changes in the fluid chemistry and the P-T conditions, correlated to the evolution of the hercynian orogen have been documented.
Production of CHON fluids
Fluids synchronous or slighltly predating the syntectonic granite intrusions were found in microfissured metamorphic rocks. These belong to the C-H-O-N system and have high densities which indicate they were trapped at relatively deep structural levels. Fluids are mostly dominated by CO2-H2O and display a high density of these volatile phase (Fig.1). It is thought that CHON fluids result from the circulation of waters through metamorphic rocks and acquire a part of their chemical features (CO2/ CH4 ratios for instance) during water-rock interaction. It is highly probable that these waters are not true metamorphic fluids resulting from the dehydration processes. P-T conditions derived from FI are compatible with a regional metamorphism of intermediate temperature and low pressure (400 < T< 550oC and P~300-400 MPa). A late ductile deformation stage resulting in the drainage of a part of the regional fluids in relation with the activity of major discontinuities, frequently under higher thermal gradients linked to the syntectonic granite intrusions. Locally, regional fluids mix with a CH4-N2 which is typically produced by devolatilization processes and specific oxydidation-reduction reactions within the graphite rich units (Boiron et al., 1996) of thermometamorphic aureoles around granites. During this stage, the main deposition of quartz (+/-arsenopyrite and pyrite) occured in the main faults hosting gold deposit. Dense C-H-O-(N) fluids were trapped at pressures above 200 MPa and temperatures of 350oC to 450oC. Typical fluid inclusions contain H2O-NaCl-CO2-(CH4) liquids (Lc-w or Vc-w) showing two or three fluid phases at room temperature. CO2 homogeneizes to the vapour phase in the range -4 to 30oC, TmCO2 in the range -61.5 to -56.8oC, TmCl in the range 4 to 12oC and Th in the range 250 and 380
Evolution of CHON fluids during uplift and the late granite intrusions
Post-tectonic granites generally intrude the metamorphic series at a slighly higher structural level than those above described. The first stage of W deposition is always characterized by the presence of aquo-carbonic fluids. The transport of W is probably related to aquo-carbonic fluids at much lower pressures than during the earlier stages previously reported and ranging from 100 to 50 MPa (higher structural levels). Dilution and nearly isobaric temperature decrease are probably at the origin of the deposition of tungsten, according to the model proposed by Dubessy el al. (1987).
A progressive enrichment in water and CH4 content and decrease in the density of the volatile phase is demonstrated by a series of fluid compositions recorded in Au deposits as well as in W ones. H2O-NaCl-CH4-(CO2) liquids (Lw-(c-m)) occur as two phase inclusions, with a low density volatile phase displaying strong fluctuations of the CH4/CO2 ratio, which is rather high in inclusions displaying high Tmcl (7-16oC) values. Th ranges between 200 and 360oC and the P-T conditions estimated around 300-380oC, 70 to 150 MPa. Thus, CH4 rich liquids are interpreted as the result of progressive dilution of early fluids yield to fluid compositions characterized by rather low volatile contents, the volatile phase being dominated by methane due to the sluggish kinetics of fluid-graphite reactions below 400oC.
Fig.1 : Summary of the evolution of the fluid composition (density of the volatile phase versus composition)
In W deposits, the spatial relationships between CH4 bearing inclusions and the main sulphides, indicate these sulphides crystallized from CH4 bearing fluids. This indicates that the sulphide stage is not disconnected from the first hydrothermal cycle.
Uplift and extended microfissural fluid circulation
A brittle microfracturing stage is then well developed allowing aqueous fluids to circulate in relatively shallow crustal levels (less than 5 km). They are associated with the main stage of gold deposition, and Pb-Ag sulphosalts. These late fluid stages are ubiquitous, and attest of a significant microfissural fluid migration. Lw1 fluids are observed as two-phase aqueous inclusions, with Tmice between -5.7 and -1.5oC corresponding to a salinity of 2.6 to 8.8 wt % eq.NaCl and moderate Th, ranging from 100-280oC. These low salinity aqueous fluids were trapped at low temperatures (180 to 250oC) and pressures (hydrostatic pressures). Similar mixing trends between two end members have been recorded in all the studied areas and provide evidence of the general cooling and dilution processes.
It must also be emphasized that in most ores and rocks, later fluids, unconnected with the main Hercynian cycle (Mesozoic hydrothermal systems) have been found as scarse FIP. Lw2, small (<5-10 micron), biphase inclusions have lower Tm ice than Lw1, with a range -22 to -12oC. Th is in the range 90-150oC.
General fluid chemistry evolution
Results show that the Hercynian granites may be regarded as playing a different role in ore forming processes than previously thought. The main role of the granites was to supply a heat source rather than metal or fluids, as no typical magmatic signature have been found except the earliest stages associated with the post-tectonic granites (brines associated with pegmatites and greisens).
Consideration of halogen chemistry shows that most fluids found in quartz veins (containing Au, or W) display features similar to fluids equilibrated with upper crustal rocks, such as those found in the Canadian or Baltic shields. Thus, most fluids migrating through rocks at the end of the Hercynian orogeny are waters equilibrated at decreasing P and T within the shallow crustal series. The main driving force behind their migration are major discontinuities acting as drainage zones, the emplacement of the syntectonic and then post tectonic plutons, the uplift and general decompression of the Hercynian units. The last stage is characterized by a progressive dilution of the crustal fluids by oxidizing solutions penetrating the basement from the surface.
Acknowledgements : This work has been supported by the EC program (Joule programme, n°ree; JOU-CT93-0318) "Fluid behaviour in the upper crystalline crust : A multidisciplinary approach".