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

doi:10.1016/S0375-6742(00)00153-9

Journal of Geochemical Exploration

Volume 71, Issue 2, November 2000, Pages 209-224

https://doi.org/10.1016/S0375-6742(00)00153-9 Get rights and content

Abstract

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Penedono), together with a deposit linked to sub-vertical structures in silicified metasediments at Três-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 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 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.

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 $350×110×100 m^{3}$ corresponding to some 9.45 Mt of rock removed. The other, Covas, has dimensions of $480×60×80 m^{3}$ 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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The Limarinho gold deposit (northern Portugal), located in the Variscan Iberian Massif, is a system of gold-bearing quartz veins that crosscut a peraluminous synkinematic (syn-D3: 315–310 Ma) Variscan granite. The mineralized structures are subvertical and trend N30–40E parallel to the Penacova-Régua-Verín fault. The quartz veins record two main stages of mineral deposition (I and II), indicating successive periods of vein re-opening, and are crosscut by Stage-III irregular veinlets. Stage-I is characterized by milky quartz (mQ) and arsenopyrite (Apy-1), and produces a greisen-like alteration. Stage-II comprises clear quartz (cQ), K-feldspar, chlorite, rutile and metallic minerals, mainly arsenopyrite (Apy-2 ± Apy-3) and pyrite, with galena, chalcopyrite and sphalerite in lesser amounts. The Stage-III is characterized by chlorite and pyrite. The hydrothermal fluids in stages I and II are low salinity (< 2.4 wt.% NaCleq.) aqueous-carbonic, increasing the CH₄ + N₂ amount toward the Stage-II that indicates mixing with metamorphic fluids. Temperatures of 460 ± 40 °C and pressures from 230 to 350 MPa are inferred for the As–Fe mineralization of Stage-I, the highest P values indicate lithostatic conditions at a depth of around 13 km. The inferred P–T conditions for the Stage-II range from 340 to 425 °C and from 60 to 225 MPa. A decompression of more than 250 MPa through stages I to II, without a substantial cooling of around 150 °C, is observed in accordance with similar Variscan belt gold deposits of Western Europe. Estimated temperatures for Stage-III are from 300 °C to lower than 200 °C, the pressures being around 60 MPa. Late Stage-II and Stage-III are related to aqueous fluids with variable salinity as a result of a mixing between two end-members, a hotter and more saline fluid (6–7 wt.% NaCleq.) and a cooler and dilute one (< 2 wt.% NaCleq.) probably of meteoric origin. Stable isotopic study results also indicate fluid mixing processes. Although the calculated δ¹⁸O values for aqueous carbonic fluids (10.5 to 7.6‰) do not permit to discriminate between magmatic and metamorphic, the δ¹⁸O and δD values obtained for chlorite (4.0 to 5.0‰ and − 24.2 to − 23.1‰, respectively) are more indicative of a meteoric signature. The δ³⁴S values of the fluid in equilibrium with arsenopyrite range from − 10.0 to − 7.8‰. However, the values obtained for pyrite (around − 2.6‰) can be only explained by the incorporation of an isotopically heavier sulfur from a different source, probably sedimentary. A detailed study of the ore has shown two different gold parageneses. The first (Bi–Te–(± S)) comprises native-Au, or maldonite, along with native-Bi, hedleyite and joséite-B. The second ((± Cu)Ag–Pb–Bi–S ± Te) comprises electrum, (± Cu)Ag–Pb–Bi sulfosalts such as lillianite–gustavite s.s., heyrovskyite, vikingite, berryite and matildite, Pb–Bi sulfotellurides and hessite. Lamellar exsolution textures observed in the lillianite–gustavite s.s. indicate initial temperatures above 400 °C. The paragenesis becomes Ag-enricher as temperature goes down. Matildite {111} exsolutions indicate a low limit of mineralization slightly above 200 °C. We propose different mechanisms of gold precipitation: 1) a gold scavenged by Bi-melts in the first case; 2) remobilization of this early gold together with new gold deposition due to sulfur extraction and/or fluid mixing from the hydrothermal fluid during the stage of prevailing sulfide precipitation. We conclude that the existence of at least two gold-bearing hydrothermal events may have enhanced the gold grade in this type of gold deposits.
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The Mokrsko-West deposit is unique among European Variscan gold deposits from the points of view of both the structure (an approx. 200 m thick complex of sheeted, several mm-thick, densely spaced quartz veins) and the economic viability (gold reserves of about 100 t). The deposit is hosted mainly by tonalite of the calc-alkaline Sázava tonalite suite (ca. 354 Ma) of the Central Bohemian Plutonic Complex. Mineralization is characterized by quartz-dominated gangue, no visible hydrothermal alteration, low sulfide content, high fineness native gold accompanied by maldonite, aurostibite, native bismuth and numerous Bi–Te–(S) phases. Five mineralogical stages are described in great detail. Arsenopyrite and chlorite thermometers, mineral phase stabilities and published isotope and fluid inclusion data are used to reconstruct the temperature and compositional evolution of the system. The role of liquid bismuth in the sequestration of gold is also discussed.
The deposit shares the features of both orogenic gold (ORG) and intrusion-related gold (IRG) deposits. The IRG model is advocated by close spatial association between the ore zone and the tonalite host-intrusion, by the absence/scarcity of hydrothermal alteration, by the Au–Bi–Te–As elemental association and by marked thermal gradients from the early to late mineralization stages. The ORG model is advocated by an approx. 15–10 Ma gap between the intrusion of the tonalite-host and the ore formation, by isotope and geochemical evidence for a key role of metamorphic fluids in the mobilization and transfer of many elements/species (inclusive S and Au). The apparently ambiguous classification of the deposit can most probably be attributed to deposit formation at a depth of ≥ 9 km and to setting of the deposit at/inside a large-scale plutonic complex with multiple and prolonged tectonic and intrusive activity.

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

Abstract

Introduction

Section snippets

Regional geology

Methodology

Bulk rock geochemistry

P–T–X evolution and metallogenic implications

Conclusions

Acknowledgements

C. R. Acad. Sci. Paris, Sciences de la terre et des planètes

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Lithos

Tectonophysics

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

The application of a computerised and optimised clathrate stability model to fluid inclusion studies

Bol. Soc. Esp. Miner.

Auriferous mineralisation in some shear zones: a three stage model of metallogenesis

Miner. Deposita

Les minéralisations aurifères de la France dans les cadre géologique autour de 300 Ma

Chron. Rech. Miner.

Gold ores related to shear zones. West Santa Comba-Fervenza area (Galicia, NW Spain): a mineralogical study

Miner. Deposita

Hercynian gold bearing quartz veins from western Europe: the “shear zone model” revisited

Notas sobre uma mina de ouro da serra da Louzã

Bol. Soc. Geol. Portugal, VI

Relações entre os jazigos hipogénicos portugueses de ouro e tungsténio

Bol. Soc. Geol. Portugal

Le district a antimoin-or Durico Beirão (Portugal): evolution paragenetique et geochimique; implications metallogeniques

Miner. Deposita

Simulation des équilibres chimiques dans le système C–O–H. Conséquenses méthodologiques pour les inclusions fluides

Bull. Miner.

Advances in the C–O–H–N–S fluid geochemistry based on micro-Raman spectroscopic analysis of fluid inclusions

Eur. J. Miner.