A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two int ra-granitic gold deposits (Grovelas and Penedono), together with a deposit Linked to sub-vertical structures in silicified metasediments at Tres-Minas , and several intrametamorphic occurrences at Vila Pouca de Aguiar. They al l possess similar mineral assemblages, deformational state, fluid flow char acteristics, ore fluid composition and have comparable P-T conditions. Thre e successive crystallisation stages are recorded during the formation of go ld-bearing structures independent of their location or host rocks (granites or metasediments). 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 cond itions reflecting high temperature and low pressure. They are similar to th ose from pluton induced metamorphism (P = 300-350 MPa and T = 500-550 degre esC). 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 I) at P-T cond itions between 100 and 300 MPa and 300 and 450 degreesC. Local sulphide dep osition (arsenopyrite II and pyrite II) occurred in clear quartz, but was n ever 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 t he involvement of magmatic fluids was found. Stage 3 - intense microfissuring of the earlier vein infillings occurred, a ssociated with the main episode of gold deposition. The P-T conditions were <100 MPa and <300 degreesC 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 fractur es frequently contain chlorite (+/- sericite) especially where they crosscu t earlier sulphides (arsenopyrite). These processes and fluid types are similar in both the granites and metamo rphic host rocks. Therefore, the gold ores appear to be the result of succe ssive periods of fluid circulation, in this case related to the uplift of t he Variscan basement in response to high heat flow and the intrusion of gra nites. Without exception, these fluids have been re-equilibrated with the m etamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation. (C) 2000 Elsevier Science B.V. All rights reserved.