A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal F. Noronha a, * , M. Cathelineau b , M.-C. Boiron b , D.A. Banks c , A. Do ´ria a , M.A. Ribeiro a , P. Nogueira a , A. Guedes a a Faculdade de Cie ˆncias, Centro de Geologia da Universidade do Porto, 4099-002 Porto, Portugal b UMR 7566 and CREGU, BP 23, 54501 Vandoeuvre-les-Nancy Cedex, France c School of Earth Sciences, Leeds University, Leeds LS2 9JT, UK Abstract Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Pene- dono), together with a deposit linked to sub-vertical structures in silicified metasediments at Tre ˆs-Minas, and several intra- metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P– T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasedi- ments). They are: Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315–310 Ma) at P– T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphism P  300–350 MPa and T  500–550°C: No clear evidence was found for gold deposition during this stage. Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P– T conditions between 100 and 300 MPa and 300 and 450°C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous- carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found. Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P– T conditions were 100 MPa and 300°C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb–Bi–Ag dominated). The fractures frequently contain chlorite (^ sericite) especially where they crosscut earlier sulphides (arsenopyrite). These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation.  2000 Elsevier Science B.V. All rights reserved. Keywords: gold metallogenesis; northern Portugal; Variscan terranes; fluid inclusions; halogen geochemistry; retrograde metamorphism Journal of Geochemical Exploration 71 (2000) 209–224 0375-6742/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0375-6742(00)00153-9 www.elsevier.nl/locate/jgeoexp * Corresponding author. E-mail address: fmnoronh@fc.up.pt (F. Noronha).