Relations between Au / Sn-W mineralizations and late hercynian granite: Preliminary results from the Schistose Domain of Galicia-Trás-os-Montes Zone, Spain

International audienceAu and W-Sn mineralization of the Schistose Domain of Galicia-Trás-os-Montes are spatially related to late hercynian granites. The Bruès (Au) and the Mina Soriana W-(Sn) deposits are studied. Both show some similarities and are assumed to form in the same tectonic and metamorphic context, on top of the granites. The role of the granite as a source for mineralizing fluids and rheological control for vein emplacement is re-adressed and discussed

Late-Hercynian gold mineralisation (Brués, Galicia, NW Spain): P-T-X conditions from fluid inclusion studies

The Brués Au-As deposit is located in the Boborás granite cupola, intrusive in the Schistose Domain of Galicia Trás-os-Montes (NW Spain). Gold is hosted in quartz veins at the granite exocontact. A sequence of quartz generation and mineral deposition was established: i) early Q1 (tension gashes and shear veins); ii) Q2a quartz accompanying arsenopyrite deposition in tension joints crosscut Q1; iii) Q2b in arsenopyrite fracture filling; iv) Q3 as clear quartz filling cavity (with phengite and bismuthinite) in arsenopyrite; v) native gold deposition, together with native Bi and tetradymite, a late Q4 quartz is likely coeval. Fluid inclusion trapped within the four quartz generation yield to the reconstruction of the P-T-X-t fluid evolution at Brués. Three main stages of fluid circulations are recognised. Two stages of aquo-carbonic fluid circulation: i) CO2-H2O-(CH4-N2) (c-w inclusions) are observed in early quartz generations (Q1 and Q2a quartz), trapped under lithostatic pressure (100-250 MPa) and in temperature range 400-450C; ii) CO2-CH4-H2O-(N2) are mainly observed in Q2a and Q3 quartz, trapped under hydrostatic conditions (50 to 100 MPa, 400-380C). Late stage of aqueous fluid circulation (2 to 12 wt% eq. NaCl, and Th 150 to 310C) is recorded in Q1 to Q3 as secondary inclusion planes and as primary in Q4. The P-T-t path appears as the succession of: (i) a near isothermal (ca. 400C) drastic pressure decrease (250 to 50 MPa), followed by (ii) an isobaric temperature decrease down to ca. 150C. These P-T-X evolution and correlated mineral deposition are broadly similar to other West European Variscan gold deposit (e.g. Boiron et al., 2003). In particular it is likely that at Brués as in other deposits, gold was deposited at the end of the evolution, at low P and T, and from aqueous fluids. On the other hand, no clear evidence of a locally derived magmatic brine was found until now in the Brués hydrothermal system, although the origin of chlorine in mildly saline aqueous fluids need to be further investigated. Boiron, M.-C., Cathelineau, M., Banks, D. A., Fourcade, S. and Vallance, J. (2003). Chemical Geology, 194, 119-141

Compared model of formation of AU, SN-W, and TA-NB-Li-SN ore deposits within the tras-os-montes domain of the hercynian orogen (NW Spain). The role of intrusion on the mineralogical and fluid inclusion characteristics.

International audienceThe study area is located in the Galicia-Trás-os-Montes Zone (GTMZ zone, Arenas et al. 1986; Farias et al. 1987 Fig. 1) a part of the Iberian hercynian massif. The GTMZ belongs to the internal zone of the Hercynian belt and is composed of a relative autochthonous and parautochthonous units overthrusted by allochthonous complexes. Studied area is located in the Schistose Domain (parautochthonous Marquínez García 1984) which is composed by a monotonous sequence of schists. Rocks of this domain exhibit a well-developed regional schistosity related to nappes emplacement (D1 and D2 events) and are affected by NS-trending crenulation and folds (D3 event) characterized by a high-temperature metamorphism leading to local development of migmatites. Four generations of granites (G1 to G4), well identified in NW Spain by their textural, geochemical characteristics and crosscutting relationships are present in the studied area. G1 to G3 granites are coeval with late with D3 event. G1 granites are syn-kinematic porphyric biotite granites. G2 granites are syn-D3 two micas granites and leucogranites (Capdevila and Floor 1970; Barrera Morate et al. 1989). G3 granites are biotite-dominant two mica granites (Barrera Morate et al. 1989). G4 granites are post D3 (Capdevila and Floor 1970; Bellido Mulas et al. 1987; Barrera Morate et al. 1989). Gold mineralizations are spatially associated with G3 granites, and Bruès, the main deposit, is located on the North-western edge of the Boborás granite roof (fig. 1). Sn-W deposits are represented, by disseminated and vein-type mineralizations. Sn,Ta,Li,Nb±W disseminated mineralizations are hosted by REE-pegmatites-aplites (Fuertes-Fuente and Martín-Izard 1998), crosscut by Sn-bearing quartz veins, spatially associated with G2 granites. The main pegmatite field is the Couso district, located on the Eastern edge of the La Estrada-Cerdedo G2 granite (fig 1). Sn-W±Ta±Nb vein-type deposits are also spatially associated with G2 granites. The most important deposits are located on the Eastern edge of the composite G1-G2 Beariz granite (fig 1)

Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the Castromil gold deposit, northern Portugal

Castromil (northern Portugal) is one of several important orogenic gold deposits located within the ‘‘Central Iberian'' geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways, and the reconstruction of the P–T–X conditions during mineralisation were obtained by combining the geometric characteristics of veins and microstructures together with a detailed study of the inclusion fluids. Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone, characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic fluids dominated by CO2 and CH4 that were probably generated by high-temperature fluid–rock interaction (400–500 jC) with graphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the deposit. At temperatures between 400 and 500 jC, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of 10F1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and represents the main phase of quartz–arsenopyrite–pyrite deposition. The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275 jC). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system.The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments. To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of economic grade (10–15 g/t) gold ore