A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal
Introduction
A large number of gold deposits and occurrences are present in the northwest of the Iberian Peninsula. They often occur in granites (Penedono (P), Grovelas (G) — Portugal; Sousa and Ramos, 1991, Nogueira, 1997), (Corcoesto, Tomiño and Piño — Spain; Gouanvic et al., 1981, Castroviejo, 1990, Cathelineau et al., 1993, Gonzalez Clavijo et al., 1994) and in Paleozoic metasedimentary sequences (Valongo area — Portugal; Couto et al., 1990) (Vila Pouca de Aguiar area — Portugal; Dória et al., 1993, Noronha and Ramos, 1993). Some were known (and mined) in the Roman and pre-Roman times, others have been found more recently, mainly as a result of the efforts of national mining bureaus, geological surveys and exploration companies, sometimes with the involvement of universities. The two main gold deposits exploited in Portugal were Três-Minas (TM) and Jales (J) (Fig. 1c). At Três-Minas, the two large roman open-pits (Covas and Ribeirinha) represent extensive mining operations. The largest, Ribeirinha, has dimensions of corresponding to some 9.45 Mt of rock removed. The other, Covas, has dimensions of corresponding to 6.20 Mt of rock removed. The Jales mine exploited hydrothermal sub-vertical quartz veins (mainly in granites) and from 1933 to 1989 produced about 25 tons of gold and 100 tons of silver (Neiva and Neiva, 1990). Gralheira, an ancient roman gold mine located north of Jales, recently prospected by BP-BRGM and Cogema/EDM, also exploited sub-vertical quartz veins hosted by metasediments (Neiva and Neiva, 1990).
Gold mineralisation was regarded generally as Hercynian, but the origin and age is still unclear. The first studies on gold metallogeny were devoted to occurrences from Minho (Vila Verde), Beira (Escádia Grande) and Trás-os-Montes (Freixeda) (Neiva, 1945, Cerveira, 1947, Cerveira, 1952). In their studies, both authors relate the mineralisation to magmatic differentiation of a calc-alkaline granitic magma, during the Permian. For the Jales deposit, Brink (1960) proposed an early gold–silver period associated with emplacment of two-mica peraluminous granites (“older granites” 308 Ma), however, Schermerhorn (1981) suggests the gold mineralisations to be of Autunian age considering Jales to be linked genetically to the “younger granites” (290 Ma). More recently, Neiva and Neiva, 1990, Neiva, 1992 also suggested a genetic relationship between the Jales gold quartz veins and hydrothermal fluids related to the late stages of fractional crystallisation of the two-mica Jales granite.
The understanding of ore genesis requires constraints on the physical–chemical parameters controlling the transport and deposition of metals. As the information from mineral assemblages alone is in most cases insufficient, quantitative chemical analysis of fluid inclusions (FI) is the main approach for understanding and modelling fluid–rock interactions. The latter contain direct evidence of the composition and density of the fluids that are related to mineralisation.
In this study, the genesis of gold-bearing structures in northern Portugal has been re-evaluated from a comparative study of the fluid flow evolution in the different deposits. Reconstruction of the P–T–X conditions was obtained from microthermometric and Raman studies of FI combined with ion analyses on solutions obtained by the crush leach method.
In order to establish the P–T–X conditions and fluid flow evolution of the gold mineralisation, deposits at Três-Minas , Grovelas (G) and Penedono (P) and several occurrences at Vila Pouca de Aguiar (VPA) were studied. Três-Minas and Vila Pouca de Aguiar are hosted by metasediments; Grovelas and Penedono gold deposits are hosted by late-tectonic Hercynian biotite granites and two-mica syntectonic granites, respectively (Fig. 1b). The deposits at Jales (J) and Gralheira (GR) (Fig. 1c) are not considered in this study.
Section snippets
Regional geology
The Variscan orogeny was a major event in the tectonic evolution of Western Europe. The characteristics of the European Variscides are those of a classical subduction–obduction–collision belt with a stacking of large-scale thrust nappes, between 390 and 320 Ma (Dias and Ribeiro, 1995). The orogen has a fan-like configuration with megafolds and overthrusts facing toward the external Devonian–Carboniferous basins. In the Hesperic Massif, the inner zones of the orogen are represented by the
Methodology
To determine the metal concentration in different lithologies the rock samples were examined petrographically and those with evidence of hydrothermal alteration were excluded. Samples were crushed (2–3 kg), split, and then pulverised to 300 mesh; 30 gram aliquots were analysed by Instrumental Neutron Activation Analysis (INAA) for Au and other elements.
The characteristics of the quartz veins was determined by optical microscopy of thick and thin sections, and in greater detail by
Bulk rock geochemistry
The granites from the studied area have gold contents of <6 ppb irrespective of their petrography. However, in the metasediments of TMSD and CSD surrounding the TM and VPA gold deposits, the lithogeochemical study carried out by Ribeiro, 1998, Ribeiro et al., 1999, reveals that the average metal content of these metasediments is much higher than the average crustal values (Table 1). For example the average Au concentration in the TMSD is 9.8 ppb and in CSD is 11.2 ppb, while the crustal average is
P–T–X evolution and metallogenic implications
The P–T properties of representative C–H–O inclusions from selected quartz were modelled for the system H2O–CO2–CH4–NaCl using the data obtained from microthermometric studies and from Raman analyses. Sets of isochores give an estimate of the minimum pressure of trapping at a given temperature, and by considering other temperature and/or pressure indicators (stability of biotite, chlorite geothermometry) it was possible to place some constraints on the P–T conditions of fluid trapping. A P–T
Conclusions
The integrated interdisciplinary study of fluid circulation in Três-Minas, Vila Pouca de Aguiar, Penedono and Grovelas deposits, including pre-ore stages, leads to the conclusion that these deposits were formed in response to fluid percolation at the crustal scale during the late Carboniferous uplift of the Variscan belt. Several conclusions are proposed:
- 1.
The anomalous gold content of the Paleozoic metasediments was remobilised during late retrograde metamorphism and deformation, especially
Acknowledgements
This study has been supported by the “Rede de Formação-Investigação França-Portugal (ICCTI/“Ministère Français de La Recherche”) RFR 37/96 and “Acordo de Cooperação Luso-Britânico CRUP/British Council B-16/97”. The authors thank T. Shepperd, V. Bouchot and an anonymous reviewer for their pertinent comments.
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