Armanda Dória

Characterization and timing of the different types of fluids present in the barren and ore-veins of the W-Sn deposit of Panasqueira, Central Portugal

The Panasqueira W-Sn deposit is the largest quartz-vein type deposit of the Iberian Peninsula and the most important wolframite deposit in Western Europe. The ore-veins are almost exclusively sub-horizontal. Besides ore-bearing sub-horizontal veins, the Panasqueira mine also contains barren quartz veins. There are essentially two generations of barren quartz: quartz, contemporaneous with the earliest regional metamorphism (QI), and recrystallized quartz, contemporaneous with the thermal metamorphism related to the granite intrusion (QII). Fluid inclusion studies (microthermometry and Raman) were undertaken in order to distinguish fluids contemporaneous with the barren quartz from those contemporaneous with the ore-bearing quartz (QIII). Fluid inclusion data indicate that the barren and ore-bearing quartz fluids are dominantly aqueous (93 to 98 mol% H2O), with a nearly constant bulk salinity (8 to 12 wt% eq. NaCl), with the quantity of volatile component (determined by Raman spectrometry) higher in QIII, but never greater than 5 mol%. However, the CO2/CH4 + N2 ratio is different for each type of quartz. Volatiles are dominated by CH4 (10 to 96 mol% ZCH4 and/or N2 (3 to 87 mol% ZN2) in the barren quartz and by CO2 (60 to 73 mol% ZCO2) in ore-bearing quartz. The bulk chemical composition of the fluids in QIII is comparable to that found commonly in hydrothermal fluids associated with wolframite mineralization, where Na>K>Ca and HCO3>Cl>SO4. A dispersion in TH (226 to 350 °C) found in QIII, together with a variation in the degree of filling (0.5 to 0.7) and with the consequent variation of fluid densities (0.70 to 0.79), may result from changes in the fluid pressure regime below lithostatic pressure, suggesting vein filling related to tectonic events.

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

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 Tres-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 metamorphismOPa 300-350 MPa and Ta 500-5508CU: 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 4508C. 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 ,3008C 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. q 2000 Elsevier Science B.V. All rights reserved.

Quantitative Determination of Gaseous Phase Compositions in Fluid Inclusions by Raman Microspectrometry

ABSTRACT Raman microspectrometry has been revealed as a powerful technique for performing qualitative analysis and estimation of relative molar fractions of the gaseous species present in minerals fluid inclusions. In this work, the methodology and calibration procedures used for the quantification of the different species in fluid inclusions are described, paying special attention to the estimation of the CO2 molar fraction. A discussion about the Fermi resonance of CO2 vibrations (ν1-2ν2; 1285–1388 cm−1) is also included.

Building up of a nested granite intrusion: magnetic fabric, gravity modelling and fluid inclusion planes studies in Santa Eulália Plutonic Complex (Ossa Morena Zone, Portugal)

The Santa Eulalia Plutonic Complex (SEPC), located in the Ossa Morena Zone (south Portugal), is composed of a medium- to coarse-grained pink granite (G0-type) and a central grey medium-grained biotite granite (G1-type). Available Rb–Sr data indicates an age of 290 Ma. An emplacement model for the SEPC is proposed, taking into account magnetic fabric, 2D gravity modelling and fluid inclusion planes studies. The G0 and G1 types demonstrate different magnetic behaviour: G0 is considered a magnetite-type granite and G1 is an ilmenite-type granite. The formation of G0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma. The 2D gravity modelling and subvertical magnetic lineations show that the feeder zone of the SEPC is located in the eastern part of the pluton, confirming the role of the Assumar and Messejana Variscan faults in the process of ascent and emplacement. The magma emplacement was controlled by ENE–WSW planar anisotropies related to the final brittle stages of the Variscan Orogeny. The emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field. The magnetic fabric however suggests emplacement of the G0-type first, closely followed by emplacement of the G1-type, pushing the G0 laterally which becomes more anisotropic towards the margin. The G1-type became flattened, acquiring a dome-like structure. The SEPC is a nested pluton with G0-type granite assuming a tabular flat shape and G1-type forming a rooted dome-like structure. After emplacement, SEPC recorded increments of the late Variscan stress field documented by fluid inclusion planes in quartz.