An intrusion-related gold deposit (IRGD) in the NW of Spain, the Linares deposit: Igneous rocks, veins and related alterations, ore features and fluids involved

Journal of Geochemical Exploration 124 (2013) 101–126

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An intrusion-related gold deposit (IRGD) in the NW of Spain, the Linares deposit: Igneous rocks, veins and related alterations, ore features and fluids involved A. Cepedal a,⁎, M. Fuertes-Fuente a, A. Martín-Izard a, J. García-Nieto b, M.C. Boiron c a b c

Department of Geology, Oviedo University, C/Arias de Velasco s/n, E-33005 Oviedo, Spain Río Narcea Nickel S.A.U. Travesía de San Antonio, 2, E-33120 Pravia, Asturias, Spain UMR 7566 and CREGU, BP 23, 54501 Vadoeuvre-les-Nancy Cedex, France

a r t i c l e

i n f o

Article history: Received 13 December 2011 Accepted 11 August 2012 Available online 21 August 2012 Keywords: Intrusion-related gold Igneous geochemistry Bi melts Fault-valve phenomena Fluid inclusions NW Iberian Massif

a b s t r a c t The Linares deposit is an intrusion-hosted gold deposit located in the NW of the Iberian Variscan Belt. The igneous rocks hosting the ore are the Linares adamellite and the Arganzúa leucogranite. They are high-K, calc-alkaline and slightly peraluminous post-collisional igneous rocks, a mixture of I- and S-types, the Arganzúa leucogranite representing a late-stage fractionated phase. The intrusions have a reduced to intermediate oxidation state which spans the boundary between the ilmenite and magnetite series. The gold mineralization occurs disseminated or along a network of microfractures and veins of variable size, commonly in sheeted vein array. The alteration observed includes both potassic, with K-feldspar and secondary biotite developed mainly in the Arganzúa leucogranite, and sericitic and propylitic alterations. A sericite–chlorite–carbonate alteration, related to hairline fractures, is broadly distributed but rarely pervasive. Wolfram-bearing mineralization is followed by sulphide precipitation, mainly pyrrhotite, chalcopyrite, pyrite, arsenopyrite, molybdenite and galena, with minor löllingite and sphalerite. Gold is commonly associated with Bi–Te-bearing minerals with Bi/Te(S+ Se) ≥ 1, consistent with a pyrrhotite-buffered environment. This is reflected in the strong correlation observed between Au and Bi. The fluids involved in the sulphide precipitation are aqueous-carbonic (CO2 and ±CH4), with low salinity (0.5– 6.3 wt.% NaCl eq.) and variable amounts of other volatiles (N2 and H2S). The fluid inclusion study registered an adiabatic drop from a lithostatic pressure of 1.8–2.6 kbar to a hydrostatic pressure of 0.3–0.9 kbar, at temperatures of 300–400 °C. The fault-valve phenomena could explain the pressure drop producing a volatile release from an early CO2-rich fluid. A later trapped fluid with a wide-ranging CO2/CH4 ratio suggests an isothermal mixture with a CH4(±N2)-rich fluid external to the granitoid (metamorphic). The effervescence and the fluid-mixing could be the possible mechanisms of ore precipitation in this system. The Linares deposit presents a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. The existence of intrusion-related gold systems would have potentially important implications for exploration in the NW of the Iberian Variscides. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Linares deposit is located in the northwest of the Iberian Variscan Massif (Fig. 1), which is the largest outcrop of pre-Permian rocks within the Iberian Peninsula and constitutes the westernmost exposure of the European Variscides. The northwest of the Iberian Massif has been divided into four zones (Farias et al., 1987; Julivert et al., 1972; Pérez-Estaún et al., 1990). The Central-Iberian Zone (CIZ), the allochthonous Galicia–Tras os Montes Zone (GTMZ) and the West Asturian–Leonese Zone (WALZ) represent the hinterland areas. The Cantabrian Zone (CZ), complicated by its peculiar arcuated shape (Ibero-Armorican or Asturian Arc), represents the foreland thrust belt (Fig. 1).

⁎ Corresponding author. Fax: +34 985103103. E-mail address: [email protected] (A. Cepedal). 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2012.08.010

The northwestern part of the Iberian Variscan Massif is a gold district, known since Roman times, where diverse types of hydrothermal gold deposits have been identified (Fig. 1), including different types of skarn (e.g. Carlés, Arcos et al., 1995; Martín-Izard et al., 2000b; El Valle-Boinás, Cepedal, 2001; Cepedal et al., 2000, 2003b, 2006; Ortosa, Fuertes-Fuente et al., 2000), Carlin-like deposits (e.g. Salamón, Crespo et al., 2000; El Valle, Cepedal et al., 2008), intrusion-hosted (e.g. Salave, Gumiel et al., 2008; Martin-Izard and Rodríguez-Terente, 2009) or orogenic gold (Corcoesto, Boiron et al., 1996, 2003; Boixet et al., 2007). Most of these gold deposits are genetically and/or spatially associated with post-orogenic igneous rocks in addition to the NW, NE and E–W fracture systems that affect the Iberian Massif. These fractures both controlled the emplacement of intrusives and also provided conduits for subsequent low temperature gold mineralizing events. Spiering et al. (2000) defined several gold belts delimited by the northeast trending fracture system. More recently, Martínez-Abad et al. (2011) defined the Villalba gold district, which comprises a Au-bearing skarn and

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Fig. 1. A) The Iberian Variscan Massif (García-Sansegundo et al., 2011). CZ: Cantabrian Zone, WALZ: West Asturian Leonese Zone, CIZ: Central Iberian Zone, OMZ: Ossa Morena Zone, and SPZ: South Portuguese Zone. (B) Regional geology of the NW Iberian Peninsula showing the gold belts defined by Spiering et al. (2000) and the situation of the Linares and other gold deposits: (1) El Valle-Boinás, (2) Carlés, (3) Ortosa, (4) Salave, (5) Villalba gold district (Martínez-Abad et al., 2011), and (6) Corcoesto. (C) Geological map of the Linares deposit. Data provided by Río Narcea Gold Mines S.A.

polymetallic mineralizations, and proposed an intrusion-related gold system model (Lang et al., 2000). Most of these deposits were mined areas during Roman times. Since then, mining has been very intermittent and mainly restricted to the first half of the 20th century. Gold exploration in the area was

revived in the mid-1970s, culminating in the discovery of the El Valle-Boinás deposits that, along with the Carlés deposit, was an important gold-producer in Europe until the end of 2006, and is currently working again. Nowadays mining exploration continues in some deposits such as Corcoesto and Salave. The Linares deposit is

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located in the Navelgas gold belt (Fig. 1) which was the most extensively mined during Roman times. Roman workings occur in the NE-trending fracture system that defines this gold belt (18 km wide and 70 km long). Nevertheless, no old mining works, not even Roman ones, appear in the Linares area, and this deposit was discovered during an exploration project of Río Narcea Gold Mines Company (RNGM) in 1996. Nowadays, the belt is being explored for Cu and Au. The Linares deposit consists of an intrusion-hosted gold mineralization that shares many similarities with the intrusion-related gold deposits (IRGDs) (Hart, 2007; Lang and Baker, 2001; Lang et al., 1997, 2000; Mustard, 2001; Thompson and Newberry, 2000; Thompson et al., 1999). A preliminary report of the geological and mineralogical characteristics of the Linares gold mineralization was given by Martín-Izard et al. (2001). In the present paper we include a detailed description of the ore mineralization with special emphasis placed on ore petrography and the associated types of veins and hydrothermal alterations. We report the geochemical characterization of the igneous rocks that host the deposit, in order to provide more data to corroborate the relationship between IRGDs and specific types of igneous rock. Finally we show the fluid inclusion and stable isotope studies carried out on samples from the Linares deposit that allowed us to characterise the hydrothermal fluids involved in the gold mineralization and also to make a contribution to the controversy of fluid reservoirs involved in this type of gold deposit. The Iberian Variscan Massif has recently been compared with the North American Cordillera (Johnston and Gutiérrez-Alonso, 2010). According to these authors, the two orogens share numerous traits such as their geometry, architecture and magmatism. In both, mixed I- and S-type post-collisional magmatism affects the hinterland, and, unusually, compared with other orogenic belts, the foreland regions. In addition, the post-collisional magmatism, which spanned 35 m.y. in the case of the North American Cordillera, and 40 m.y. in the Iberian Massif, is younger toward the foreland regions (Johnston and Gutiérrez-Alonso, 2010). The North American Cordillera comprises some of the most important intrusion-related gold deposits in the world (e.g. Fort Knox, Bakke, 1995; Dublin Gulch, Maloof et al., 2001), being the place where this classification was developed. This is an important fact to be borne in mind for future exploration targets in the NW of Spain. 2. Geological setting The Linares deposit is located in the Narcea Antiform (Julivert, 1971), one of the most extensive exposure of Precambrian rocks in the Iberian Variscan Massif (Fig. 1). The Narcea Antiform is a complex structure that forms the limit between the Cantabrian Zone (CZ), the foreland, and the West Asturian–Leonese Zone (WALZ), the latter representing the transition to the more internal zones of the Variscan orogen of northwestern Iberia. The western limit of the Narcea Antiform is the Allende-Thrust (Fig. 1), a Variscan structure that can be followed for tens of km and has tens of km of displacement (Alonso et al., 1990; Gutiérrez-Alonso, 1992). However, the true boundary between the CZ and WALZ is constituted by the La Espina Thrust (Gutiérrez-Alonso, 1992, 1996), which divided the Narcea Antiform into two sectors due to differences in the rock deformation record in each sector. The CZ constitutes a typical thrust belt with an unmetamorphosed sedimentary pre-Carboniferous Paleozoic sequence composed of up to 7000 m of mostly stable marine platform sediments, thinning to the east and covered by a Carboniferous syn-orogenic sequence of variable thickness (Marcos and Pulgar, 1982). The WALZ is composed of a thick lower pre-orogenic sequence covering the whole of the Cambrian and Ordovician systems, a large part of the Silurian and, locally the Lower Devonian. In both, the CZ and the WALZ, the Paleozoic sequence rests unconformably on upper Proterozoic terrigenous sediments, the Narcea Slates, which outcrop in the Narcea Antiform and are composed mainly of slates and greywackes with turbiditic facies with minor vulcanoclastic

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intercalations, more abundant towards the west. Structurally, the WALZ was affected by three deformation phases (Marcos, 1973; Martínez Catalán et al., 1990; Pérez-Estaún, 1978). The first (D1) gave rise to large recumbent folds and a generalised slaty cleavage or schistosity. The second (D2) was responsible for the appearance of thrust-type structures and associated shear zones (Aller and Bastida, 1993; Bastida et al., 1986), with minor folds that generated a new schistosity or a crenulation cleavage. The third (D3) produces large open folds, approximately homoaxial with the first ones, together with minor folds and the local development of a crenulation cleavage. This zone is affected by conspicuous internal deformation and regional metamorphism which increase progressively towards the west from greenschists to amphibole facies (Marcos, 1973; Pérez-Estaún et al., 1990). This metamorphism is essentially syn-kinematic, although certain thermal metamorphic events may be found linked to the intrusion of granitoids (Bastida et al., 1986; Capdevila, 1969; Suárez et al., 1990). Variscan granitoids are especially abundant in the western part of the WALZ, and, on the basis of their structural relationships, can be divided into two main associations: syn-tectonic granitoids, which intruded syn-kinematically with the D2 phase, and post-tectonic granitoids (Capdevila, 1969; Corretgé et al., 1990). The main petrological features of the granitoids allow these associations to be further subdivided into tonalite–granodiorite–monzogranite intrusions and leucogranite intrusions. Both petrological groups are present in the syn- and post‐tectonic associations, although, in the first case, leucogranites are the volumetrically dominant granitoids, whereas in the second they are the tonalite– granodiorite–monzogranite intrusions. In addition, there are some ultramafic and mafic to intermediate plutonic rocks, mainly post-tectonic and relatively more abundant in the CZ than in the WALZ, that have been interpreted as mantle melts evolved through crystal fractionation and varying degrees of crustal contamination (Suárez et al., 1990). Since the igneous rocks related to the Linares gold deposit are post-orogenic granitoids, we place special emphasis on the post-tectonic association. The dominant petrological group, the tonalite–granodiorite–monzogranite intrusions, comprises biotite–amphibole tonalites, biotite ± amphibole granodiorites and biotite ± moscovite monzogranites. These rocks commonly display features characteristic of I-type high-K calc-alkaline granitoids (Corretgé and Suárez, 1990; Corretgé et al., 1990; Martín-Izard et al., 2000a), and have been interpreted as having derived from the melting of the lower crust with varying degrees of involvement of mantle-derived melts (Gutiérrez-Alonso et al., 2011; Suárez et al., 1992). The leucogranite intrusions are two-mica (Ms > Bt) or muscovite leucomonzogranites, alkali feldspar granites and aplites. These leucogranites are strongly peraluminous with A/CNK ratios ranging between 1.1 and 1.3, and have been interpreted as having derived from the melting of mid-crustal metasedimentary protoliths (Gutiérrez-Alonso et al., 2011, and references therein). The published U–Pb dating of post-orogenic intrusions from the CZ and WALZ (Fernández-Suárez et al., 2000; Gutiérrez-Alonso et al., 2011, and references therein) indicate an age range for this magmatism from 300 to 285 m.y. 3. Sampling and analytical methods Samples were taken from several drill cores from the ore-bearing granitoids of this deposit. The location of these is shown in Fig. 1. Sample suites were chosen for mineralogical and petrographic studies and for geochemical analyses. These samples were studied by transmitted- and/or reflected-light microscopy, SEM-EDS and EPM (Cameca SX100) at Oviedo University (Spain). In all electron microprobe analyses, the standard deviation of results is less than 0.1%. Major and minor elements were determined at 20 kV accelerating potential, 20 nA beam current and an acquisition time of between 10 and 20 s for X-ray Peak and background. In the whole rock samples, major and minor elements were analysed by inductively coupled

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plasma emission spectrometry (ICP-ES) and inductively coupled plasma-mass spectrometry (ICP-MS). All the rock analyses were performed in the ACME Analytical Laboratories Ltd. (Canada). Microthermometric studies were performed on double-polished, 100–300 mm thick plates using a Linkan THMS600 heating-freezing stage at the Ore-deposit Laboratory of the Geology Department (Oviedo University, Spain). The stage was calibrated with melting point standards at Ta >25 °C and synthetic fluid inclusions at Ta b 0 °C. Measurements of phase changes at or below 31 °C are accurate to within ±0.2 °C and high temperature measurements to within ±2.0 °C. The volumetric fraction of the aqueous phase (flw) has been visually estimated by reference to the standard charts of Roedder (1984) and Shepherd et al. (1985). Molar fractions of CO2, CH4, H2S and N2 were determined in individual fluid inclusions by micro-Raman analysis performed with a Labram Raman spectrometer at CREGU (Nancy, France). Bulk composition and molar volume were computed from P–V–T–X properties of individual fluid inclusions in the C–O–H–S system (Bakker, 1997; Dubessy, 1984; Dubessy et al., 1989; Thiery et al., 1994). The analyses for the stable isotope geochemistry were performed at the Stable Isotopes Laboratory of Salamanca University. 4. The Linares deposit 4.1. Local geology The Linares deposit is characterised by the presence of several stocks of granitoids which host the gold mineralization. These granitoids intruded Precambrian and Palaeozoic rocks that are partially covered by unconformable Tertiary sediments (Fig. 1). The Precambrian rocks are represented by the Narcea Slates, a succession of slates and sandstones with intercalations of volcanic porphyroids (Marcos et al., 1980). The Palaeozoic rocks lie in angular uncorformity over the Narcea Slates and consist of a Low Cambrian pre-orogenic sequence, represented by the Cándana Formation (Lotze, 1957) and the Vegadeo Limestones (Zamarreño and Perejón, 1976), and an unconformable Stephanian syn-orogenic sequence. The Cándana Fm. consists of interbedded sandstones, quartzites and slates with some continuous dolomitic interlayer levels at the bottom, up to 30 m in thickness. The upper levels constitute a gradual transition to the Vegadeo Limestones, which in the area, consists of thin bands of dolostones. Both the Precambrian and Cambrian rocks trend N–S, with subvertical dip and frequent folds. A relevant Variscan N–S trending, the La Espina Thrust, affects the area (Fig. 1). This thrust, which as previously mentioned constitutes the true limit between the CZ and the WALZ, has very intense ductile deformation associated with it and a complex history of reactivations (Gutiérrez-Alonso, 1992, 1996), the later stages of thrusting also affecting the Stephanian materials (Marcos et al., 1980). The unconformable tertiary sediments partially cover the area and no evidence exists that an alpine reactivation of the thrust affected them. The Precambrian and Cambrian rocks suffered a weak metamorphism during the Variscan orogeny (Marcos et al., 1980). Furthermore, contact metamorphism took place around the intrusions that developed nodulous slates, with cordierite and minor andalusite (Suárez et al., 1990) and banded and massive hornfels. Barren pyroxene skarn occurs locally in calcareous rocks. 4.2. The igneous rocks The igneous rocks form four small stocks of biotite granitoids in the studied area. The biggest, which we will refer to as the Linares granitoid, outcrops to the south of Arganzúa village in an area of around 1000 by 700 m, partially covered by Tertiary sediments. This stock together with another two smaller stocks, located in the south and in the northwest (Fig. 1), is porphyritic in character due to the presence of K-feldspar megacrystals. The other granitoid, which we will refer to as the Arganzúa granitoid, outcrops as a small stock (200 by 300 m)

close to the biggest one (Fig. 1). This stock lacks K-feldspar megacrystals and has an equigranular character in hand sample. A suite of intermediate to felsic dikes, up to 2 m in thickness, crosscut the granitoids and the surrounding rocks, trending N–S to NE–SW and E–W. The porphyritic Linares granitoid in hand sample is dark-grey in colour with abundant biotite and K-feldspar megacrystals up to 10 cm in size. In thin sections, K-feldspar megacrystals have a poikilitic texture with inclusions of biotite, quartz and plagioclase. Other smaller phenocrysts (up to 0.5 cm) are rounded or irregular quartz, zoned plagioclase (An30–45) and perthitic K-feldspar. The ferromagnesians, which mainly consist of biotite and, to a lesser extent, hornblende, are abundant but smaller than the others. The accessory minerals are apatite and zircon. The age obtained for this Linares granitoid was 290±6 Ma, by the K/Ar method in biotite (Martín-Izard et al., 2001), and 297.3±1.8 Ma from U–Pb in zircon (Gutiérrez-Alonso et al., 2011). This granitoid is crosscut by several subvolcanic porphyritic dikes (Fig. 2A) that are dark in colour in hand sample, with variable amounts of phenocrysts. In thin sections, these rocks have porphyritic to seriated texture with phenocrysts of plagioclase, globular quartz, and biotite, and scarce K-feldspar. The matrix, constituted by the same minerals, is fine or very fine-grained. Accessories are zircon, apatite, and opaque minerals, mainly magnetite and rutile, and very scarce chalcopyrite and pyrite. The equigranular Arganzúa granitoid is, in hand sample, lighter in colour than the porphyritic one, due to the lower amount of biotite and lack of K-feldspar megacrystals. In thin sections, it consists of quartz, zoned plagioclase (An5–20), perthitic K-feldspar and biotite. The accessory minerals are apatite, zircon and garnet (Alm62Sps30Grs6Prp2). One characteristic feature of the equigranular granitoid is the presence of water-clear albite rims between adjacent K-feldspar grains (Fig. 2B and C). Similarly, plagioclase grains adjacent to K-feldspar grains also show albite rims (Fig. 2B). These could have been formed by metasomatic albitization or exsolution. The K-feldspar grains often show a decrease in the number of perthite lamellae near the albite rims, which fits the exsolution model: albite has migrated to the grain boundaries to form the intergranular albite. The exsolution is catalysed by postmagmatic fluids, and strong recrystallization has led to coarsening of the exsolved albite grains of the swapped albite rims. These exsolution–recrystallization-ordering processes probably took place at temperatures around or below 500 °C (see Brown and Parsons, 1989, 1994). Other sub-solidus deformation textures observed are flame-perthites in K-feldspar (Fig. 2), suggesting low-medium grade conditions (400–500 °C) of deformation (Pryer and Robin, 1995). The age obtained for this Arganzúa granitoid by K/Ar dating of biotite was 276 ± 6 Ma (Martín-Izard et al., 2001). Along the drill core the granite sometimes gradually becomes aplitic in character (Fig. 2D). Moreover, the stock is cut by several types of quartz veins that will be described later. Some irregular biotite-rich pegmatitic cavities of small size (up to 1 cm) have been observed associated with quartz veins. These zones typically show an irregular rim constituted by K-feldspar, while the inner part is formed by an intergrowth of biotite, K-feldspar, plagioclase and beryl (Fig. 2E and F). The biotite, which is dark brown or green in colour and sometimes acicular in shape, is altered to muscovite and chlorite. A secondary beryl phosphate (may be moraesite) has been observed associated with this alteration. 4.2.1. Igneous-rock geochemistry Samples of the main stocks (Arganzúa and Linares), the subvolcanic dikes and the aplites were selected from drill-cores and the analysed rocks do not show any signs of hydrothermal alteration, with the exception of the LIN4-254 sample, which presents a slightly feldspathic alteration. The geochemical results are presented in Table 1. The rocks from the equigranular Arganzúa granitoid are high silica (SiO2 > 73 wt.%), low CaO (b0.95 wt.%), low P2O5 (=0.03 wt.%) and low TiO2 (≤0.05 wt.%) (Table 1). In contrast, the rocks from the porphyritic Linares granitoid have lower silica contents (SiO2 b 68.8 wt.%)

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A

105

B Kfs

Kfs Pl

intergranular albite

Kfs

1.6 mm

C

D

0.4 mm

E

F Qz Brl Bt Pl

0.5 mm

0.5 mm

Kfs

Fig. 2. A) Igneous contact between the porphyritic Linares granitoid and an andesite dike. Several samples from the Arganzúa granitoid: B) Albite rims between turbid plagioclase (Pl) and K-feldspar (Kfs) or between adjacent perthitic K-feldspar grains (intergranular albite); C) Flame-shaped albite lamellae in perthitic K-feldspar; D) Aplite dike crosscut by different generations of veins; and E) Appearance of a pegmatoid segregation; F) BSE image of the same area. Biotite (Bt) and beryl (Brl).

and higher CaO (>2.8 wt.%), P2O5 (≥0.16 wt.%) and TiO2 (>0.4 wt.%) contents (Table 1). Moreover, the MgO and FeOt contents are very different between the Arganzúa and Linares granitoids. The MgO + FeOt value ranges from 1 to 1.5 in the former and from 3.2 to 5.2 in the latter, the average MgO/FeOt ratio being 0.05 and 0.32, respectively. The subvolcanic dike shows the lowest silica content and the highest MgO+ FeOt value, according to the abundance of biotite. This rock has been classified as dacite (TAS-diagram of Le Maitre et al., 1989). The granitoids have been classified on a Q–P diagram (Fig. 3A, Debon and Le Fort, 1983). The Linares granitoid plots in the adamellite field, while the Arganzúa one plots in the granite field. The LIN4-254 sample plots separately due to its feldspathic alteration, as mentioned above. All the igneous rocks from the Linares deposits plot in the fields of high-K and calc-alkaline series (Fig. 3A and B). They are slightly peraluminous (A/CNK= 1–1.1, Fig. 3C) and plot in the I + S granitoid field according to Chappell and White (1992). In the diagrams of trace elements versus SiO2 (not shown), these rocks taken collectively show

a negative correlation between Ba, Sr and Zr with SiO2, but Rb is positively correlated with SiO2 in the same way that K2O is with SiO2 (Fig. 3B). These trace elements show the same correlations with the Eu/Eu* ratios (Fig. 4) suggesting that their concentrations are mainly controlled by fractionation of plagioclase and K-feldspar. In the triangular Rb–Ba–Sr diagram from El Bouseily and El Sokkary (1975), the Linares rocks plot in the anomalous granites while the Arganzúa rocks plot in the highly differentiated granite field, reflecting their more evolved nature (Fig. 3D). Chondrite-normalised REE patterns of the Linares rocks are distinct from those of the Arganzúa rocks (Fig. 5A). The Linares samples are enriched in LREE with high (La/Yb)N ratios from 12.5 to 15.9 (Table 1) and small Eu anomalies (Eu/Eu*= 0.72–0.76, Table 1). The dacite has a similar pattern, slightly depleted in HREE. The Arganzúa rocks have flat “bird-wing shape” REE patterns, which are characterised by low (La/Yb)N ratios (from 2.8 to 3.2, Table 1), small negative Nd anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.12–0.08). The

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Table 1 Geochemical analyses of the igneous rocks from the Linares deposit. Sample

wt.% SiO2 TiO2 Al2O3 Fe2O3 FeO MgO MnO CaO Na2O K2O P2O5 H2O/LOI Total Mg + FeOt Mg/FeOt ACNK ΔOxa ppm Rb Ba Sr Cs Be Nb Ta Zr Th Hf U Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (La/Yb)N Eu/Eu* Cr Co Ni Sc V Cu Pb Zn Bi Sn W Mo As Au (ppb) Sb a

Linares granitoid LIN1-17

LIN1-187

LIN1-217

LIN2-95

LIN2-137

67.68 0.49 15.43 0.00 3.91 1.20 0.06 3.06 3.22 4.03 0.18 0.88 99.76 5.11 0.31 1.01 −0.87

68.77 0.47 15.52 1.58 2.03 1.08 0.06 2.88 3.29 4.00 0.17 0.59 100.43 4.69 0.30 1.04 0.29

68.69 0.46 15.40 1.15 2.36 1.09 0.06 2.82 3.19 3.99 0.19 0.77 100.16 4.60 0.31 1.05 0.09

68.26 0.472 15.87 0.38 3.1 1.09 0.062 3.04 3.39 4.02 0.17 0.69 100.53 4.57 0.31 1.03 −0.51

67.98 0.49 15.33 0.95 2.62 1.26 0.06 2.91 3.18 3.58 0.16 0.90 99.40 4.83 0.35 1.06 −0.04

173 683 254 5 4 9.4 1.1 197 12.7 4.8 4.38 21 32.7 70.7 7.87 27 4.98 1.12 4.1 0.62 3.42 0.72 2.15 0.3 1.77 0.261 12.48 0.76 115 6

210 685 253 8.1 3 10.7 1.4 180 15.1 5.1 5.39 23 39.3 84.1 9.33 31.2 5.86 1.31 4.87 0.72 4.18 0.89 2.58 0.362 2.13 0.315 12.47 0.75 114 7 17 8 31 10 49 66 0.32 3 7.1 4 5 14 0.4

200 638 245 5 3 9.2 1.1 191 12.5 4.8 4.69 21 34.7 72 7.87 27.1 5.24 1.11 3.98 0.62 3.6 0.72 2.05 0.309 1.84 0.257 12.74 0.74 124 6

201 761 270 7.5 3 10 1.1 189 15.3 5 4.8 21 42 89.4 10 33.9 5.82 1.27 4.8 0.66 3.61 0.75 2.17 0.306 1.78 0.245 15.94 0.73 99 7

215 581 248 7.5 4 10.9 1.4 187 18 5.8 6.43 23 43.4 95.1 10.5 34.6 6.5 1.4 5.49 0.76 4.48 0.97 2.75 0.379 2.29 0.337 12.81 0.72 123 7

9 38 24 42 0.12 2 63 2 10 0.6

8 31 37 33 44 1 3 18.9 2 12 8 0.3

8 34

8 37

43 67 1.35 3 5.8 2

17 33 0.58 3 7.1 2 12 11 0.3

5 0.3

Dacite

Arganzúa granitoid

LIN3-81

LIN4-172

LIN4-188

LIN4-218

LIN4-254

65.32 0.71 15.78 4.27 3.21 1.82 0.06 3.04 3.12 3.95 0.2 1.5 99.77 9.30 0.24 1.06 0.63

76.80 0.03 13.19 0.00 1.39 0.08 0.01 0.95 3.22 4.74 0.03 0.63 100.33 1.47 0.06 1.09 −2.85

76.44 0.05 13.25 0.00 1.2 0.11 0.02 0.90 3.29 4.92 0.03 0.45 100.41 1.31 0.09 1.07 −2.90

77.33 0.04 12.88 0.00 0.94 0.10 0.01 0.75 2.93 5.30 0.03 0.43 100.49 1.04 0.11 1.08 −2.87

73.98 0.03 14.48 0.00 0.93 0.08 0.01 0.79 3.18 6.92 0.03 0.32 100.47 1.01 0.09 1.02 −2.94

78.09

205.3 743 249.4 14.9 2 11.1 0.8 200.7 20 6 4.7 15.9 40.9 89.8 9.64 37 6.22 1.16 4.25 0.61 3.1 0.56 1.59 0.22 1.49 0.21 18.55 0.69

286 77 46 2.7 7 15.7 4.8 63 22.3 3.4 19.6 30 12.7 30.7 3.55 13.3 4.79 0.19 4.82 0.98 6.1 1.16 3.06 0.454 2.79 0.345 3.08 0.12 203 3

288 93 30 3.7 5 12.8 2.8 65 22.8 3 18.7 53 16.2 38.5 4.89 18.4 6.46 0.197 7.7 1.51 8.69 1.7 4.63 0.647 3.36 0.411 3.26 0.09 131 2 17 5

290 56 26 2.9 5 12.8 3.1 68 19.8 2.9 14.7 46 13.7 32.2 3.87 14.9 5.66 0.168 6.1 1.23 7.56 1.48 3.86 0.552 3.13 0.377 2.96 0.09 89 3

410 58 30 4.4 9 12.7 4.3 64 26.4 3.9 21.6 56 18.6 44.8 5.59 21 7.37 0.215 9.11 1.76 10.5 2.08 5.88 0.827 4.45 0.551 2.82 0.08 192 2

275.2 68 31.3 3.3 9 13.4 6.3 35.5 9.6 3.2 17.5 50 8.6 22.1 2.93 12.3 5.8 0.05 6.83 1.49 7.99 1.36 3.6 0.56 3.69 0.48 1.57 0.02

25.1 8 10 60 4.6 1.7 37 9 364.3 1.4 125.3 6.7

5 144 48 34 11.5 56.7 19 14 173 8.5

74 53 7.61 2 9.9 9 25 128 0.3

Aplite

5

5

129 55

36 59

8.69 27.6 11 258 0.2

7.16 330 5 211 0.3

LIN4-165

12.33 0.25 0.07 0.02 0.49 3.52 4.59 0.01 0.6 99.91 0.34 0.06 1.06 0.86

30.5 0.3 3 7 11 8.4 2 2.6 2 634.9 5.5 11.9 39.1 1.1

ΔOx refers to relative oxidation state calculated using the equation ΔOx = log(Fe2O3/FeO) + 3 + 0.03 ∗ FeOtot (Blevin, 2004).

Arganzúa rocks are slightly enriched in HREE with respect to the Linares rocks, which can be explained by the presence of garnet as an accessory mineral in the former. The aplite shows a more pronounced pattern, more depleted in LREE and with a higher Eu anomaly. The depletion in LREE could be explained by crystallisation of phases such as monazite, allanite or zircon.

On granitoid tectonomagmatic discrimination diagrams (Fig. 5B), both Linares and Arganzúa granitoids are in the post-collisional granite field proposed by Pearce (1996). If the Pearce et al. (1984) discrimination diagram is considered, the Arganzúa granitoid plots in the volcanic-arc field, whereas the Arganzúa rocks mainly plot at the limit between the syn-collision granite and within-plate granite fields.

A. Cepedal et al. / Journal of Geochemical Exploration 124 (2013) 101–126

10Dacite

A K2O (wt.%)

150

adam

LK

Linares granitoid Arganzúa granitoid

B

8Aplite

200 granite

ellite

grano diorite

Q = Si/3 - (K + Na+ 2Ca/3)

250

CA

100

6 Shoshonite series

4

ries aline se Calc-alk High K es ri se line Calc-alka

2

50

Tholeite series

-400

-300

-200

-100

0

100

0 50

60

70

80

SiO2 (wt.%)

P = K - (Na+ Ca)

Rb

3

C

2.6

es

Peraluminous

gra

Metaluminous

D nit

2.2

enc

ia t

ed

1.8

fer

1.4 ly

dif

A/KN

107

Hi gh

1

0.2 0.5

I-Type

I-S

Gr ani tes An o Gr ma ani lo tes us

Peralkaline

0.6

S-Type

1

1.5

2

A/CKN

Ba

Granodiorite Quartz-diorite

Diorite

Sr

Fig. 3. Analyses of the igneous rocks present in the Linares deposit plotted on: A) Q–P diagram. Calk-alkaline (CALK) trends are shown. B) K2O vs. SiO2 binary diagram. C) Shand index diagram. D) Rb–Ba–Sr ternary diagram. Panel a is after Debon and Le Fort (1983). Fields in panel b are from Rickwood (1989). Fields of I and S-type granites in panel c are from Chappell and White (1992).

4.3. Hydrothermal alterations in igneous rocks The Au(±Cu) mineralization occurs disseminated or along a network of microfractures and veins of variable size (from less than 1 mm up to 3 cm), commonly in sheeted vein array. The main vein trending observed in the outcrops is N40–55°E. Another vein system observed is N70–55°W, which is sometimes, but not always, cut by the NE system. Both types of vein carry Au and they are probably contemporary. The alteration observed includes microclinization, sericitic and propylitic alterations Martín-Izard et al. (2001). Minor albitization is locally present, in the envelopes of the veins, although some of the observed textures (intergranular albite, Fig. 2B and C) indicate an exsolution process at higher temperatures as mentioned earlier. 4.3.1. The potassic alteration (microclinization) This alteration is the most important and broadly developed in the Arganzúa granite. It is related to quartz and K-feldspar veins (Fig. 6A), frequently in centimetre scale. Where this alteration is pervasive, the rock becomes reddish to pinkish in hand sample. In thin sections, the rocks mainly consist of microcline masses and interstitial anhedral quartz (Fig. 6B). The hydrothermal K-feldspar is distinguished from the igneous perthitic feldspars because of its porous character. Locally shreddy biotite is formed (Fig. 6A). Where this alteration is less intense the plagioclase crystals are partially replaced by the K-feldspar along grain boundaries and fractures. The relicts of plagioclase not microclinized are frequently affected by subsequent sericitic alteration (Fig. 6C). 4.3.2. The sericitic alteration This alteration has been observed in both granitoids along with the potassic alteration in relation with the same vein: the microclinized envelope of the vein is overprinted and grades outward to the sericitic alteration. This alteration somewhat affects the hydrothermal K-feldspar but, in those areas where it is pervasive, the hydrothermal K-feldspar is completely sericitized. The igneous feldspars are altered to sericite–

moscovite and biotite is replaced by muscovite, rutile and REEminerals (xenotime and Ce- and Nd-monacite). Radiating aggregates of muscovite along with quartz, rutile and carbonate occur filling cavities (Fig. 6B). The abundance of rutile, mainly in the porphyritic Linares granitoid, is noticeable. Rutile forms acicular or prismatic crystals, frequently with overgrowth rims, as will be commented on later. In the Arganzúa granitoid scheelite is normally associated with this alteration (Fig. 6D). The associated sulphides are pyrite, chalcopyrite, molybdenite and arsenopyrite, the latter occurring mainly inside the quartz veins connected with this alteration. 4.3.3. The propylitic alteration This is observed in the more distant zones of the alteration halo around the veins in both granitoids, being more frequent in the Linares stock. This alteration is characterised by the replacement of biotite by chlorite together with rutile and/or titanite, and calcite (Fig. 6E). 4.3.4. The sericite–chlorite–carbonate alteration This alteration is the most broadly distributed both in Arganzúa and in Linares granitoids, although only on few occasions do the igneous rocks appear pervasively altered. It is associated with discontinuous brittle hairline fractures (Fig. 6F and G) that crosscut the igneous rocks and infill cavities. The microfractures commonly cut primary igneous K-feldspar, plagioclase and quartz grains, but also follow planes of weakness such as boundaries between mineral grains and exsolution textures such as perthites or intragranular albite. The plagioclase is replaced mainly at the Ca-richer core of the crystals by a mixture of fine grained chlorite, carbonate and sericite and, often, sulphides. The K-feldspar is weakly altered to sericite, and biotite is replaced by muscovite, rutile and sulphides. Muscovite occurs as fan-shape crystals filling interstices, with scheelite and chlorite. The sulphides associated with this alteration are mainly pyrite, pyrrhotite, chalcopyrite and Bi–S–Te minerals described later and electrum, among others.

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A. Cepedal et al. / Journal of Geochemical Exploration 124 (2013) 101–126

Ba (ppm)

1000

100

10

1 0.01

0.1

1

10

and amphibole hornfels. Potassic alteration of the host rocks is associated with veins of K-feldspar, quartz and, often, biotite that forms nest-like aggregates and is partially replaced by muscovite (Fig. 6H). Tiny crystals of albite and beryl form a narrow rim around these nests. This mineral association is similar to that described for the irregular pegmatitic cavities mentioned before. Scheelite and rutile are also frequent, occurring as coarser and subhedral crystals (Fig. 6H). In addition, amphibolitization is associated with veins of quartz and amphibole, which varies from early ferrohornblende to actinolite. The sulphides present in the hydrothermal alteration of the host-rocks are mainly pyrrhotite, chalcopyrite and pyrite.

Eu/Eu* 4.5. Geochemistry of the mineral-bearing alterations

Sr (ppm)

1000

100

10

1 0.01

0.1

1

10

1

10

Eu/Eu*

Rb (ppm)

1000

100

10

1 0.01

0.1

Eu/Eu* Fig. 4. Eu/Eu* vs. Ba, Rb and Sr plots for the igneous rocks of the Linares deposit (same symbols as Fig. 3).

4.4. Hydrothermal alterations in the host-rocks Host rocks adjacent to the igneous rocks are crosscut by aplitic dikes and quartz veins, locally in high density. These produced silicification and metasomatic alteration, the latter giving rise to biotite

A

4.5.1. Biotite Electron microprobe analyses were carried out on the different types of biotite observed (Table 2): magmatic biotite from the Linares and Arganzúa granitoids, the biotite associated with the potassic alteration in the Arganzúa granitoid and biotite from the altered host rocks that includes the biotite hornfels and the hydrothermal veins described above. Biotite from the igneous rocks is classified as Fe-biotite (Tischendorf et al., 1997), although biotite from Linares has higher (XMg) values and plots close to the Mg-biotite field, whereas the Arganzúa biotite plots close to the end member of siderophyllite. Other differences are the lower amounts of Ti (b2.8 wt.% TiO2), higher amounts of Al (16.5 to 18.3 wt.% Al2O3), and higher F contents (0.26 to 0.84 wt.% F) of the Arganzúa biotite than the Linares biotite (3.6 to 4.5 wt.% TiO2; 13.3 to 14.0 wt.%. Al2O3; 0.14 to 0.3 wt.% F). Moreover, the biotite from potassic alteration of Arganzúa granitoid shows the highest Al contents, with values from 19.0 to 20.5 wt.% Al2O3, and the lowest Ti contents (less than 0.5 wt.% TiO2). The Fe+2–Fe+3–Mg contents in the magmatic biotite from Linares plot above the nickel–nickel oxide (NNO) buffer (Fig. 7A), which defines the boundary between the magnetite and ilmenite series granitoids. On the other hand, the biotite compositions from Arganzúa plot at lower oxygen fugacities, around the quartz–fayalite–magnetite (QFM) buffer (Fig. 7A). However, the compositions of potassic alteration-related biotite in Arganzúa granitoid trend from QFM towards NNO buffers (Fig. 7A), suggesting a relative fO2 increase during the transition from magmatic to hydrothermal conditions. Biotite from the altered host rocks has the highest mole fraction of Mg (XMg) and is classified as Mg-biotite (Table 2). The halogens also vary with the mole fraction of Mg: the Cl content correlates negatively with XMg values, whereas the F content shows a positive correlation. The biotite from amphibolitized host-rocks has the highest XMg values and also the highest F contents (up to 1.4 wt.%), whereas biotite from the potassic alteration of igneous rock shows the highest Cl contents (up to 0.6 wt.%). The positive correlation between high XMg and F

B

Fig. 5. A) Chondrite-normalised REE patterns of the igneous rocks from the Linares deposit. Linares granitoid, pattern area. Arganzúa granitoid, grey area. Square, dacite. Circle, aplite. B) Tectonomagmatic discrimination diagrams of the igneous rocks from the Linares deposit. The Post-COLG field is from Pearce (1996). Normalising values in panel a are from Taylor and McLennan (1985). Panel b is after Pearce et al. (1984).

A. Cepedal et al. / Journal of Geochemical Exploration 124 (2013) 101–126

A

B

109

Ms Kfs Cal

Kfs

Kfs

Bt

Rt+Ms

0.4 mm

D

C

Pl

Sch

Kfs Ms

Pl

Pl

Qtz

0.2 mm

0.4 mm

F

E

Qz

Ttn

Pl

Pl Ttn 0.4 mm

0.5 mm

G

H

Rt Bt Ms Kfs Ab+Brl 0.2 mm

0.4 mm

Fig. 6. A) Core sample from the Arganzúa granitoid with potassic alteration (microclinization and locally shreddy biotite, see arrows) developed along quartz veins that are nearly orthogonal to the core. B) The hydrothermal microcline (Kfs) shows a poikilitic texture. Radiating aggregates of muscovite (Ms) and rutile (Rt) are interstitial to the K-feldspar, along with calcite (Cal). C) Potassic alteration of the Linares granitoid. The plagioclase (Pl) is partially replaced by K-feldspar (Kfs). The plagioclase relicts are sericitized. D) A plagioclase crystal from the Arganzúa granitoid afected by a microvein filled by muscovite (Ms) and scheelite (Sch). A radiating aggregate of muscovite fills a cavity. E) Biotite partially replaced by chlorite and titanite (Ttn) in the propylitic alteration. F) Brittle hairline fractures (arrows) filled by sulphides croscutting the Arganzúa granitoid. The plagioclase crystals (Pl) affected by them appear altered, mostly in the cores. G) Detail of a plagioclase crystal replaced by a mixture of fine grained chlorite, carbonate, sericite and metallic minerals. H) A K-feldspar and quartz vein, with nest of biotite, crosscutting the biotite hornfels. Muscovite partially replaces biotite and a narrow rim of albite (Ab) and beryl (Brl) surrounds this replacement.

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A. Cepedal et al. / Journal of Geochemical Exploration 124 (2013) 101–126

Table 2 Selected electron microprobe analyses of the different types of biotite observed at the Linares deposit. Formulae were calculated on the basis of 22 cation charges by using Mica+ programme (Yavuz, 2003). 1–2, magmatic biotite from Linares granitoid; 3–4, magmatic biotite from Arganzúa granitoid; 5–6, biotite from the potassic alteration of the igneous rocks. Biotite from host-rocks: 7, biotitic hornfels, 8–9, related to K-feldspar veins, 10–11, associated with amphibole veins. The fugacity ratios logf(HF)/f(HCl) were calculated using the equations of Munoz (1992). Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Li2O F Cl O = F,Cl Total Si Al(IV) Al(VI) Ti Fe Mn Mg Lia Ca Na K F Cl OH XMg logXF/XCl logf(HF)/f(HCl) a

1

2

3

4

5

6

7

8

9

10

11

35.365 3.916 14.041 24.552 0.512 8.352 0.037 0.116 9.545 0.598 0.202 0.216 0.134 97.318 2.76 1.24 0.05 0.23 1.60 0.03 0.97 0.19 0.00 0.02 0.95 0.05 0.03 1.92 0.38 0.24 −2.12

35.025 3.848 13.864 24.147 0.399 8.110 0.020 0.098 8.974 0.500 0.136 0.219 0.107 95.233 2.77 1.23 0.07 0.23 1.60 0.03 0.96 0.16 0.00 0.02 0.91 0.03 0.03 1.94 0.37 0.06 −2.29

33.678 1.716 17.473 29.831 0.421 3.087 0.014 0.132 8.787 0.114 0.656 0.127 0.305 95.731 2.71 1.29 0.37 0.10 2.01 0.03 0.37 0.04 0.00 0.02 0.90 0.17 0.02 1.82 0.16 0.98 −0.89

34.916 2.093 17.116 27.729 0.425 4.385 0.006 0.055 9.100 0.469 0.503 0.260 0.270 96.787 2.76 1.24 0.35 0.12 1.83 0.03 0.52 0.15 0.00 0.01 0.92 0.13 0.04 1.84 0.22 0.56 −1.44

32.262 0.105 19.850 28.614 0.423 3.347 0.043 0.065 8.846 0.000 0.226 0.412 0.188 94.005 2.63 1.37 0.54 0.01 1.95 0.03 0.41 0.00 0.00 0.01 0.92 0.06 0.06 1.89 0.17 0.01 −1.90

32.590 0.081 20.153 27.845 0.389 3.176 0.011 0.096 8.931 0.000 0.302 0.336 0.203 93.707 2.65 1.35 0.59 0.01 1.90 0.03 0.39 0.00 0.00 0.02 0.93 0.08 0.05 1.88 0.17 0.23 −1.67

35.176 2.901 17.599 19.600 0.195 9.436 0.020 0.115 9.429 0.544 0.338 0.184 0.184 95.353 2.71 1.29 0.31 0.17 1.26 0.01 1.08 0.17 0.00 0.02 0.93 0.08 0.02 1.89 0.46 0.54 −1.94

36.860 2.910 16.961 14.394 0.301 11.851 0.000 0.160 9.637 1.027 0.509 0.213 0.262 94.561 2.81 1.19 0.33 0.17 0.92 0.02 1.35 0.31 0.00 0.02 0.94 0.12 0.03 1.85 0.60 0.65 −2.04

36.196 2.914 17.658 15.814 0.177 11.608 0.000 0.148 9.720 0.836 0.566 0.053 0.250 95.440 2.75 1.26 0.32 0.17 1.00 0.01 1.31 0.25 0.00 0.02 0.94 0.14 0.01 1.86 0.57 1.30 −1.36

37.942 1.507 12.856 18.081 0.241 13.929 0.020 0.190 9.184 1.337 1.311 0.127 0.581 96.144 2.92 1.08 0.08 0.09 1.16 0.02 1.60 0.41 0.00 0.03 0.90 0.32 0.02 1.66 0.58 1.29 −1.52

38.174 1.450 12.854 17.464 0.235 13.960 0.009 0.180 8.947 1.404 0.871 0.112 0.392 95.268 2.94 1.06 0.11 0.08 1.13 0.02 1.60 0.43 0.00 0.03 0.88 0.21 0.02 1.77 0.59 1.16 −1.65

Calculated after Tindle and Webb (1990).

(wt.%) values is known as “Fe–F avoidance”. Munoz (1984) pointed out that the Cl (wt.%) and F (wt.%) contents of biotite must not be used alone to deduce the relative halogen fugacities in the associated fluids. In the diagram of Fig. 7B, the XMg values versus log(XF/XCl) are plotted, where XF and XCl are, respectively, the mole fraction of F and Cl in the hydroxyl site. The halogen fugacity ratios (fHF/fHCl) for a fluid in equilibrium with biotite, calculated at 400 °C (Munoz, 1992), are also included. In the figure, data of magmatic biotite from Linares and Arganzúa granitoids plot separately. Biotite from Linares granitoid has high XMg values, and the

fluid associated with this biotite shows low log(fHF/fHCl) ratios (−1.8 to −2.4) (Fig. 7B). Biotite from Arganzúa granitoid commonly has lower XMg values than the Linares one, but it appears to have equilibrated with a fluid with relatively higher log(fHF/fHCl) ratios (−0.8 to −1.9). This indicates an increment of the fHF/fHCl ratio with magmatic fractionation. In addition, the significant range of log(XF/XCl) from igneous biotite to potassic alteration-related biotite could represent a crystallisation sequence in the Arganzúa granitoid, suggesting that the fHF/fHCl ratio of the fluids decreases progressively from the late magmatic–hydrothermal

B

A

40

Fig. 7. A) Composition of magmatic biotite, from Linares and Arganzúa granitoids, and the potassic alteration-related biotite in terms of Fe +3–Fe+2–Mg (Wones and Eugster, 1965). FeO and Fe2O3 after Dymek (1983); B) XMg vs. log(XF/XCl) of the different types of biotite from Linares deposit. XMg values are determined from cation fractions and are defined as Mg/(Fe + Mg). The XF and XCl are the mole fractions of F and Cl in the hydroxyl site. Contours are the logarithm of the fluorine–chlorine fugacity ratios fHF/fHCl for a fluid in equilibrium with biotite (Munoz, 1992), calculated at 400 °C.

A. Cepedal et al. / Journal of Geochemical Exploration 124 (2013) 101–126

transition stage to the hydrothermal s.s. stage. This fact could directly affect the solubility of metals such as Mo, Sn, W, Be, Ta, Nb, among others, in the ore-forming fluids. 4.5.2. Muscovite The mineral formulae calculation was done on the basis of 22 oxygens and H2O was calculated after Tindle and Webb (1990). The analyses carried out on the muscovite indicate a great variation of the celadonite proportion (from 1 to 24%) and a correlation between this and the type of muscovite analysed. The muscovite with a chemical composition near igneous muscovite is that related to sericitic alteration of igneous rocks, previously microclinitized or not, and sometimes associated with molybdenite. The highest celadonite proportions (more than 10%) were measured in muscovite associated with the sericite– chlorite–carbonate alteration, both in the fan-shape crystals filling interstices and the muscovite present in the chloritized biotite. 4.5.3. Rutile The electron microprobe analyses on rutile crystals show the presence of substitution of Ti by several elements, the most important being Nb (up to 19 wt.% Nb2O5), W (up to 6 wt.% WO3), Fe (more than 5 wt.% FeO), and Ta (more than 3 wt.% Ta2O5). Other notable impurities are Cr and Sn (up to 2.5 wt.% Cr2O3, and up to 1 wt.% SnO2). Vanadium was only measured in some samples with amounts of less than 0.5 wt.% V2O3. The analyses plotted in a diagram (Sn,Ti, W)–(Nb,Ta)–(Fe,Mn) (not shown) indicated that Nb, Ta and Fe +2 enter the rutile lattice in accordance with the columbite-type substitution ((Fe,Mn)(Nb,Ta)2Ti−3). Nevertheless, the chemical composition of rutile from the altered Linares and Arganzúa granitoids are different. In the latter case, rutile shows the highest values of Nb and Ta, and lowest amount of W (around 2 wt.% WO3), which could be explained by the presence of wolframite and scheelite associated

A

111

with the Arganzúa granitoid. The back-scattered images carried out on rutile crystals show compositional zoning (Fig. 8) that develops complex patterns or, in some cases, is oscillatory. Some rutile crystals show a core of impurity-free rutile (Fig. 8), indicating that the hydrothermal rutile grew over previous crystals. 4.5.4. Titanite Titanite is present in samples of the propylitic alteration affecting the Linares granitoid, along with chlorite, muscovite, rutile and calcite. It occurs in two different forms: 1) as medium-size allotriomorphic crystals filling cavities between quartz and altered feldspar; 2) as fine-size elongated crystals arranged parallel to the cleavage of the chloritized biotite (Fig. 6E). According to the electron microprobe analyses, the main difference between them is the Al-content: in the former it varies between 1.4 and 3 wt.% Al2O3, while in the latter case we can talk of high-Al titanite with values higher than 10 wt.% of Al2O3. In both cases the Fe content is low. Other impurities are Nb, with values from below detection limits (b.d.l.) up to 0.3 wt.% Nb2O5, and Sn, with values from b.d.l. up to 3 wt.% SnO2. This variation in Sn content produces an optical zoning observed in some crystals. Fluorine was also detected (up to 2 wt.% of F) and there is a strong correlation between F and Al contents. The presence of titanite and rutile + calcite suggests near-equilibrium conditions according to the reaction: Titanite þ CO2 ¼ calcite þ rutile þ quartz CaTiSiO5 þ CO2 ¼ CaCO3 þ TiO2 þ SiO2 : Moreover, the Al content of titanite depends on the activity of F relative to CO2 because of equilibrium involving rutile and carbonate (Carswell et al., 1996; Gibert et al., 1990).

B Kfs

bdl WO 3 0.2 wt % Nb 2O5

Ms Rt

0.9 wt % WO3 0.6 wt % Nb 2O5

Chl

5 wt % WO3 0.6 wt % Nb 2O5

Cal

C bdl WO 3 5.8 wt % Nb2O5 0.4 wt % Ta2O5

3.2 wt % WO3 1.2 wt % Nb 2O5

1.7 wt % WO3 9.0 wt % Nb2O5 2.5 wt % Ta2O5

0.9 wt % WO3 19.2 wt % Nb2O5 3.6 wt % Ta2O5

Fig. 8. A) Detail of Fig. 7B photomicrograph: calcite (Cal), rutile (Rt), muscovite (Ms), chlorite (Chl), and K-feldspar (Kfs). B) SEM-image of the rutile grain showing complex zoning. The brighter areas are rich in W and also in other elements such as Nb and Fe, although the behaviour of these latter elements is more erratic. C) SEM-image of two rutile grains from the K-feldspar, quartz and biotite veins in the hornfels, showing oscillatory zoning. Data are from electron microprobe analyses; bdl: below detection limit.

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4.6. Vein description Several aplitic dikes and different types of veins have been observed crosscutting the Arganzúa granitoid along the drill cores. Aplitic dikes seem to be older than the veins, since they are frequently crosscut by different types of vein that have altered and mineralized them (Fig. 2D). The veins have been separated into four types (I to IV) according to characteristics such as geometry and internal features, orientation, type of hydrothermal alteration developed on the selvages and crosscutting relationships. However, conflicting crosscutting relationships are relatively common among veins of Types I to III: one type of vein cutting across another type at one location and the opposite happening at another location indicates that they are broadly contemporaneous. Sulphide and oxide minerals commonly account for less than 5% of the vein. 4.6.1. Type I veins: quartz–K-feldspar veins These veins consist of white or grey quartz, with subordinate K-feldspar (Fig. 6A), and are several millimetres to a few centimetres

thick. The vein margins are sometimes diffuse and there is always an envelope of microclinization with variable thickness that grades outward to sericitic and propylitic alterations. This vein type has been observed affecting both the Linares and Arganzúa granitoids. 4.6.2. Type II veins: fault-fill quartz veins This type includes quartz veins of variable thickness, occurring as isolated veins or as a group with a main vein and parallel or subparallel subordinate veinlets. Under optical microscope, quartz grains exhibit undulose extinction as well as recovery and brecciation textures of diverse intensities. Weakly deformed veins show coarse quartz grains with undulose extinction, and only minor development of subgrains. The subordinate veinlets are often ribbon-shape micromylonite veins infilled by minute fragments of igneous rock enclosing larger ones sometimes altered to sericite–moscovite (Fig. 9C). The above described features suggest that they are related to brittle–ductile shear-zones. The vein selvages are weakly altered and secondary biotite is formed at the savages of the veins and also filling fissures and interstices between

A

B

D Wf

1 cm

Sch

Type II veins

Type III veins

Type IV veins 0.4 mm

C

0.4 mm

E

0.4 mm

Fig. 9. A) A drill core sample from Arganzúa granitoid (LIN4-213.3) crosscut by different vein types B) Scheme of the sample that illustrates the disposition and the vein types observed (more detail in the text). C) Photomicrograph of Type-II veins showing their inner textures: the main vein (top left) consists of polycrystalline quartz with irregular grain boundaries; the subordinate vein (centre) consists of fine-grained recrystallized K-feldspar; the thinnest vein (bottom right) crosscutting twinned K-feldspar crystals and displacing the Carlsbad twin (arrow). CPL. D) Photomicrograph of a Type-III vein. The wolframite (Wf) crystals are attached to the vein margins and, in the inner part, parallel to inclusion trails. A scheelite (Sch) grain also occurs. PPL. E) Same photomicrograph as c with PPL, showing a Type-IV vein infilled by sulphides crosscutting the Type-II veins and altering the Ca-rich core of a plagioclase crystal (arrow).

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grains of other minerals. This biotite is fine-grained, often green, and occurs locally as crystal aggregates with radial patterns. A special signature of these veins is the presence within them of the irregular biotite-rich pegmatitic cavities described in previous sections (Fig. 2E and F). Type II veins have only been observed in the drill-core that crosscuts the inner part of the Arganzúa granitoid. We consider that they could be related to Type I veins (quartz–K-feldspar veins) since both types, I and II, produce potassic alteration. We propose that their different characteristics are due to the fact that Type II veins occur in more internal parts of the Arganzúa stock than Type I, which develop in the marginal parts of this igneous body. 4.6.3. Type III veins: wolframite-bearing quartz veins These veins are conjugated with the previously mentioned veins and are frequently crosscut by them, although, as mentioned earlier, the opposite also occurs (Fig. 9A and B). They consist of quartz with subordinate proportions of wall-rock slivers which are commonly partially replaced by hydrothermal minerals (e.g. muscovite, chlorite) (Fig. 9D). These wall-rock slivers sometimes occur parallel to the vein margins (inclusion bands) or at higher angles to the margins (inclusion trails), suggesting an oblique direction of extension. Quartz consists of polycrystalline inequigranular quartz with irregular, frequently lobate, grain boundaries. The wall-rock selvages are weakly altered, feldspar being altered to sericite and muscovite and biotite to chlorite. These quartz veins are characterised by the presence of wolframite. This mineral occurs as prismatic crystals isolated or in aggregates, attached to the veins margins or inner parts parallel to the inclusion bands or the inclusion trails (Fig. 9D). Wolframite is partially replaced by scheelite and sulphides, mainly pyrite and chalcopyrite. Scheelite also forms an outer rim of fine-grained crystals over the wolframite crystals, or coarser and isolated grains in the same veins. 4.6.4. Type IV veins: brittle hairline fractures These are associated with the Type II veins, crosscutting them or located adjacent to them, and trend discontinuously parallel to the Type III veins (Fig. 9). These veins are typically less than 1 mm in thickness and are infilled by sulphides showing black colour in hand sample. Microscopically the veins show more continuity across the igneous rock, commonly cutting primary igneous feldspar, plagioclase and quartz grains, but also following planes of weakness (Fig. 6F): the boundary between mineral grains, cleavage planes of biotite or feldspars, exsolution textures such as perthites or intragranular albite. Initially, the rock alteration associated with these veins is weak, mainly affecting the Ca-rich core of plagioclase minerals, as previously mentioned (Fig. 6F and G; Fig. 9E), whereas K-feldspar and biotite are mostly unaltered. In this case, the sulphides infilling veins are mainly pyrrhotite, pyrite and chalcopyrite. A higher degree of alteration is related to quartz and carbonate infilling the microveins that develop the sericite–chlorite–carbonate alteration in the host rocks. Mineralization of molybdenite, Bi–Te–S minerals and gold is associated with this stage. In the Linares granitoid the proportion of aplite dikes is lower than in Arganzúa and mainly affect the hornfels. This granitoid is also crosscut by several quartz veins, isolated or in sheeted vein array, with a proportion of metallic minerals of less than 5%, similar to the Arganzúa one. However, some differences between granitoids were observed. In Linares, neither wolframite nor scheelite have been found, which is in accordance with the less differentiated character of this granitoid. The metallic minerals that infill the veins are mainly pyrite, chalcopyrite, pyrrhotite and arsenopyrite. Moreover, quartz veins show less deformation than the Arganzúa ones: Type II veins (fault-fill quartz veins) do not occur, suggesting a more fragile deformation in the Linares granitoid. In addition, the Type IV veins (brittle hairline fractures) neither show the regular trend observed in those of the Arganzúa granitoid, nor their continuity.

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4.7. Ore-mineral paragenesis 4.7.1. W-bearing stage Wolframite was only observed in the Type III veins crosscutting the Arganzúa granite. It occurs as prismatic crystals, isolated or in aggregates, which show growth zoning visible under optical microscopy through the presence of inclusions of gangue minerals parallel to the faces of growth. Wolframite is partially replaced by scheelite and sulphides, mainly pyrite and chalcopyrite, along the growth bands or fractures (Fig. 10A). Scheelite also forms an outer rim of fine-grained crystals (Fig. 10A), or coarser and isolated grains in the same veins (Fig. 9D). The electron microprobe analyses indicated that the wolframite has a dominant ferberite component (Table 3) and a small variation in composition within grains or between different grains (from 87.6 to 84.2 mol% ferberite, Table 3). The analyses show important amounts of Nb (up to 2.3 wt.% Nb2O3), which is negatively correlated with W. In fact, BSE images of wolframite crystals shown oscillatory compositional zoning produced by the variation in Nb and W contents (Fig. 10B). Niobium enters wolframite in solid solution and the correlation observed between (W+6 + Fe+2calc + Mn+2) and (Nb +5 + Ta+5 + Fe+3calc) indicates a couple substitution (Neiva, 2008). Scheelite is more widely distributed than wolframite. As above mentioned, this mineral has been observed along with wolframite in the same veins (Type-III). However, scheelite is very frequently associated with the K-feldspar and sericitic alterations of the igneous rocks, or in the biotite and amphibole hornfels. Scheelite postdates wolframite, according to the overgrowth and replacement textures observed (Fig. 10A). The EMPA shows variable amounts of Fe (up to 6.6 wt.% FeOt), and Mn (up to 1.3 wt.% MnO), the highest values being from scheelite replacing wolframite. Niobium (up to 0.6 wt.% Nb2O3) and Ta (up to 0.08 wt.% Ta2O5) are other impurities. 4.7.2. Sulphide-bearing stage Sulphide minerals are commonly chalcopyrite, pyrrhotite, pyrite, arsenopyrite, bismuthinite and galena, while molybdenite, sphalerite and stannite are less abundant. Other minerals are löllingite, gold and (±Sb)–Bi–Te–S minerals. At least two stages of sulphide precipitation were recognised. The first stage is represented by pyrrhotite, löllingite, arsenopyrite and chalcopyrite. The presence of small inclusions of pyrrhotite and wolframite in some scheelite grains points to the early character of pyrrhotite, which also occurs as blebs along healed fractures in quartz grains. Pyrrhotite often occurs with chalcopyrite and, to a lesser extent, molybdenite. In some samples, pyrrhotite is replaced by pyrite and marcasite, especially in those samples where late oxidation took place, and bird-eyes textures are frequent. Chalcopyrite is the most abundant sulphide and coexists both with pyrrhotite and pyrite within quartz veins or disseminated in the igneous rocks. In the latter case, the sulphides infill Type IV veins or intragranular cavities, in addition to being associated with the alteration of the plagioclase cores or biotite crystals (together with muscovite, chlorite and rutile). In those samples where the sericitic alteration is pervasive, the proportions of sulphides are higher. Early chalcopyrite sometimes contains exsolved sphalerite “stars” that suggest formation temperatures greater than approximately 400 °C (Hutchinson and Scott, 1981). Molybdenite, which is locally abundant, occurs as single flakes within the Type I and II veins with or disseminated in their selvages, sometimes intergrown with muscovite, indicating an intimate temporal relationship. Large molybdenite crystals are curved and overgrown by chalcopyrite (Fig. 10C). Arsenopyrite and löllingite, much less frequent than pyrrhotite and chalcopyrite, are confined to the quartz veins related to the potassic and sericitic alteration in the Linares and Arganzúa granitoids. Early arsenopyrite (Apy I) occurs as single large fractured grains up to several millimetres in size and major as massive aggregates. On a few occasions the crystals revealed an outer rim under optical microscopy. In some

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A

B

C

D

E

F

G

H

Fig. 10. A) Reflected light photomicrograph. Wolframite (Wf) shows growth zonation and is partially replaced by scheelite (Sch), which also forms an outer rim of fine-grained crystals. Sulphides postdate wolframite and locally penetrate along fractures in wolframite crystals or even replaced them. Py: pyrite and Ccp: chalcopyrite. B) BSE-image of wolframite showing oscillatory zonation. There is a very small variation in the amount of iron (Table 3). The zonation is produced by the variation in Nb contents. The darker zones are Nb-rich, while the lighter zones are W-rich and have lower values of Nb, even below the detection limit. Numbers indicate microprobe analyses shown in Table 3. C) Molybdenite (Mol) crystals overgrown by chalcopyrite (Ccp), showing some sphalerite (Sp) exsolutions. Native-Bi (Bi) fills interstices between the molybdenite cleavage planes. D) A brittle hairline fracture (type IV vein) fills with calcite (Cal) and Au–Bi–S–Te minerals (electrum, native-Bi, joséite-B), cutting a molybdenite (Mol)-bearing quartz (Qtz) and K-feldspar (Kfs) vein. E) Pyrrhotite (Po) overgrown and corroded by Sb-bearing native-Bi (Bi) associated with selenian galena. F) An aggregate of bismuth minerals [native-Bi (Bi), bismuthitite (Bis) and joséite-A (JoA)] and a small grain of electrum (El) attached to some molybdenite (Mol) grains. G) Bismuthinite (Bis), with a very porous surface, intergrown with native-Bi (Bi) and associated with a composite grain of hedleyite (Hed), joseíte-B (JoB) and native Bi (Bi). H) Native-Bi (Bi) and native gold (Au) associated with two crystals of löllingite (Lo).

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115

Table 3 Electron microprobe data of wolframite from Linares deposit. Fe+3 and Fe+2 calculated on the basis of a charge balance of 2 cations. Numbers from 1 to 6 are the analyses of Fig. 11B. –: below detection limit. Other analysed elements were Sn and Cr, but always below detection limits.

WO3 CaO TiO2 FeO MnO Nb2O3 Ta2O5 MgO Total W Ti Fe+3 Fe+2 Mn Mg Nb FeWO4 (mol%)

Max

Min

Average

1

2

3

4

5

6

77.656 0.131 0.504 20.386 3.587 2.286 0.440 0.426

73.540 – – 19.039 2.711 – – 0.310

75.781 0.043 0.107 19.699 3.060 1.002 0.027 0.382

77.447 – 0.117 19.303 3.098 0.176 – 0.417 100.558 1.013 0.004

75.766 – 0.051 19.929 2.899 0.855 – 0.392 99.891 0.988 0.002 0.046 0.792 0.123 0.029 0.019 87.2

76.915 – 0.060 19.832 3.147 0.252 – 0.412 100.617 0.998 0.002 0.049 0.781 0.133 0.031 0.006 86.2

73.947 0.072 0.143 20.383 2.940 1.479 – 0.405 99.367 0.956 0.005 0.123 0.727 0.124 0.030 0.033 87.3

74.714 – 0.100 19.750 3.250 2.088 – 0.354 100.255 0.964 0.004 0.049 0.774 0.137 0.026 0.047 85.7

77.425 0.062 – 19.214 3.321 – – 0.359 100.381 1.016

87.6

84.3

0.815 0.132 0.031 0.004 86.0

86.4

samples, small remains of löllingite or inclusions of pyrrhotite appear inside arsenopyrite. Moreover, this arsenopyrite also contains inserts of younger Au–Bi–Te bearing minerals that often outline the löllingite remains. Löllingite also occurs isolated within the quartz veins, as small and idiomorphic crystals (Fig. 10H). A second generation of arsenopyrite (Apy II) forms single grains of rhombohedric shapes up to 1 mm in size or crystalline aggregates intergrown with gangue minerals (quartz). According to the EMPA data, Apy I often have Co in substitution of Fe (up to 5 wt.%), and less frequent Ni (up to 1.2 wt.%). The As content varies from 36 to 39.8 at.% As, the highest values occurring when Ni and Co are present. Löllingite also shows significant Co contents. Arsenopyrite from the rim and Apy II shows a composition varying from 34 to 36.2 at.% As (average 35 at.% As), the Ni and Co contents being negligible. In order to use the arsenopyrite geothermometry (Kretschmar and Scott, 1976; Sharp et al., 1985), only the Apy I with Co below 1 wt.% data were considered, resulting in an interval between 36.0 and 37.5 at.% As (average 36.5 at.% As). To establish a temperature range for these arsenopyrites, it should be assumed that the beginning of this crystallisation took place at the arsenopyrite–pyrrhotite–löllingite buffer fS2 due to pyrrhotite and löllingite occurrence. The derived temperatures range between 550 and 640 °C. The temperatures obtained for Apy II range between 460 and 560 °C, although in this case the fS2

0.814 0.142 0.027 85.1

conditions were higher than the arsenopyrite–pyrrhotite–löllingite buffer. The second stage of sulphide precipitation is associated with the sericite–chlorite–carbonate alteration and is represented by gold, (±Sb)Bi–S–Te minerals and galena. All these minerals associated with calcite (Fig. 10D), muscovite and chlorite infill Type IV veins that crosscut earlier veins or the igneous rock. They also occur in the alteration envelopes around the veins filling intergranular spaces. The more altered the rock is, the higher is the sulphide occurrence. These second-stage sulphides frequently appear at the grain border of earlier sulphides, such as chalcopyrite and pyrrhotite, sometimes replacing them (Fig. 10E), or are attached to molybdenite grains, filling the exfoliation planes of the mineral (Fig. 10F). The most frequent Bi-bearing minerals that have been observed are native-Bi and bismuthinite (Bi2S3), followed by tellurides and sulphotellurides such as hedleyite (Bi7Te3) and joséite-B (Bi4Te2S) Martín-Izard et al. (2001). Other Bi-bearing minerals present in lower proportion are unnamed Bi2Te, joséite-A (Bi4TeS2) and tsumoite (BiTe). Galena (PbS), which is Se-rich (up to 3.4 wt.% Se), is relatively abundant, whereas hessite (Ag2Te) is scarce. Both minerals have been observed in symplectite-like intergrowth, and have a later character in the mineral paragenesis. Representative microprobe data for these minerals are shown in Table 4. The chemical composition of native-Bi shows variable

Table 4 Selected electron microprobe analyses (number of analyses in brackets) of minerals in the assemblages of the Linares deposit. –: below detection limits.

Te Ag S Fe Cu As Se Sb Bi Total

Bi Sb Pb Te Se S

Hedleyite

Unnamed Bi2Te

Joséite-B

Joséite-A

Tsumoite

Native-Bi

Bismuthinite

Selenian galena

Zavaritskite

Bismite

(n = 8)

(n = 2)

(n = 16)

(n = 5)

(n = 3)

(n = 22)

(n = 23)

(n = 8)

(n = 5)

(n = 4)

20.807 – – – – – 0.228 0.219 78.559 99.813

23.045 – – – – – 0.295 0.216 76.492 100.048

21.776 – 2.816 – – – 0.347 0.194 75.290 100.423

11.979 – 6.151 – – – 0.199 0.176 81.064 99.569

40.146 0.142 – – 0.154 – 0.323 0.282 58.777 99.824

– – – 0.238 – – 0.104 6.483 93.446 100.271

– – 85.059 – 0.677 – 9.084 –

0.612 1.217 82.560 1.158 0.388 0.788 – 0.473

94.82

87.196

To 10 atoms 6.91 0.03

To 3 atoms 1.99 0.01

To 7 atoms 4.04 4.01 0.02 0.01

To 2 atoms 0.93 0.01

To 1 atom 0.89 0.99 0.11 0.01

3.00 0.05

0.98 0.02

1.91 0.05 0.98

1.04 0.01

– – – – 0.200 – – 0.679 99.427 100.31

– – 18.485 – – – 0.084 1.380 79.756 99.705

0.178 – 18.541 – 0.102 – 0.075 0.309 80.512 99.717

To 5 atoms 1.96 1.99 0.06 0.01

– 0.285 11.516 – 0.139 – 3.213 – 84.760 99.913 To 2 atoms

1.01 0.97 0.03 1.98

0.01 0.01 2.97

2.99

0.10 0.89

S As Bi Fe Sb Si F Cl

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amounts of Sb, from below the detection limit to 7 wt.% Sb (Table 4). The highest impurities of Sb in bismuth were measured in samples from the Linares granitoid, where some grains show an Sb-enrichment from the inner part to the border (Fig. 10E), suggesting chemical changes during native-Bi precipitation or a reaction between previous grains and Sb-rich fluids. Bismuthinite, which may also contain some antimony (up to 1.4 wt.% Sb), is frequently observed showing a very porous surface (Fig. 10G) with minute grains of native-Bi disseminated within. We propose two possible explanations for this texture: i) a replacement of native-Bi by sulphur-rich fluids, and ii) a myrmekite-like intergrowth formed by decomposition of a previous Bi-rich phase stable at higher temperature. Gold occurs as native-Au or electrum, isolated or, more often, in aggregates associated with Bi–S–Te minerals. This gold-bearing association often forms small (from 3 to 30 μm) and irregular inclusions within arsenopyrite crystals with relicts of löllingite or attached isolated löllingite crystals (Fig. 10H). In far smaller proportions, isolated gold grains have been observed as inclusions in chalcopyrite. Moreover, gold has been observed associated mainly with native-Bi, and, to a far smaller degree, with joséite-B and hedleyite, forming trails of droplets along healed fractures in quartz (Fig. 11). This gold–bismuth association and the observed textures (e.g. trails of droplets) point to a process of gold scavenging from the hydrothermal fluid by a bismuth melt (Douglas et al., 2000; Tooth et al., 2008, 2011), which will be discussed later on. 4.7.3. Gold geochemistry The gold fineness varies from 451 to 1000, the more frequent values being grouped in two different ranges, from 650 to 750 and from 950 to 1000. As a general rule, gold present as isolated grains or in aggregates with non-sulphide minerals [native-Bi, löllingite, hedleyite (Figs. 10H and 12)] has higher fineness that those grains associated with sulphur-bearing minerals. This bimodal gold composition suggests different stages and/or processes of gold precipitation. Moreover, large grains of gold show zones enriched in Ag, next to the contact with sulphide minerals (e.g. arsenopyrite) or in relation to microfractures. In the former case, a redistribution of the elements by diffusion, during and after the crystallisation, could produce a Ag-enriched zone towards the contact between gold and sulphide grains. In the second case it could be due to the interaction between gold and Ag-bearing solution. Apart from electrum, the other silver bearing mineral is hessite, which is associated with galena at the end of sulphide precipitation. Elemental correlation coefficients were calculated from whole-rock geochemistry of the 280 core samples from Linares and Arganzúa stocks (Table 5). The results show that, in the mineralized samples from Arganzúa, gold only correlates with bismuth (R= 0.685) and, in those from Linares, the gold–bismuth correlation is also noticeable and

A

higher. Apart from gold–bismuth, other notable elemental correlations in Arganzúa are antimony with silver and with lead (R= 0.579 and R = 0.776, respectively), and tellurium with bismuth (R= 0.74), although Te-contents are under the detection limit (b 2 ppm) in many of the samples. 5. Fluid characterization: fluid inclusions and oxygen isotopes A fluid inclusion study was done in order to reconstruct the P–T–x evolution of the ore-related fluids. Two hundred and forty fluid inclusions were studied in samples of the hydrothermal quartz that accompanies the sulphides from Type I veins crosscutting the Linares granitoid and from Type I and II veins crosscutting the Arganzúa granitoid. The sampled veins contain the more abundant sulphides (chalcopyrite, arsenopyrite, pyrrhotite and molybdenite) and show alteration halos of K-feldspar overprinted by sericitic alteration. Moreover, these veins or their envelopes are crosscut by the hairline-bearing sulphide veinlets (Type IV veins) that are related to the later chlorite–carbonate–sericitic alteration and to the second stage of sulphide precipitation represented by gold and (±Sb)Bi–S–Te minerals, as described previously. In some of the sampled veins, gold together with (±Sb)Bi–S–Te bearing-minerals occurs as trails of blebs cross cutting quartz crystals (Fig. 11). The fluid inclusion study of the wolframite-bearing Type III veins could not be done due to the lack of suitable fluid inclusions. Taking into account the microscopic observations such as number of phases present at room temperature, vapour/liquid ratios, primary or secondary origin, microthermometric measurements, and Raman analyses of the volatile phase, the fluid inclusions were grouped into five types: Types I, II and III consist of aqueous-carbonic fluid inclusions, whereas Types IV and V are non-aqueous, volatile-rich and aqueous fluid inclusions, respectively. The microthermometric results are summarised in Table 6 and Raman analyses of the volatile phase in Table 7. These types are described below. 5.1. Type I: aqueous-carbonic (CO2) inclusions These have only been found, albeit abundantly, in the quartz veins that crosscut the Linares granitoid. Frequently, they form three-dimensional arrays of abundant fluid inclusions delimiting the euhedral core of quartz crystals and surrounded by a fluid inclusion-poor growth band (Fig. 12A). In other cases, the inclusions appear in small clusters and, more scarcely, isolated. On the basis of the criteria of Roedder (1984), these inclusions are interpreted as being primary. The morphology of the inclusions is mainly in the form of negative crystal together with other polyhedral morphologies such as prismatic and, more rarely, irregular. The sizes vary between 10 and 60 μm.

B

C

Fig. 11. A) Transmitted light photomicrograph of a quartz grain with μm-scale trails of blebs of opaque minerals. B) Reflected light photomicrograph of a detail of the previous photo. Bismuthinite (Bis). C) BSE-image of droplets consisting of native-Bi (Bi) and native gold (Au), with 100 wt.% Au.

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117

A

Vein Margin

B

Type I

Type IIb

Type V

Ccp

50µm

50µm

C

D Type V

Type II

Type III

50µm

50µm

E

F Type IV

Type III

50µm

50µm

Fig. 12. Transmitted light photomicrograph of the fluid inclusion types defined: A) Quartz crystal showing a Type I fluid inclusion-rich core surrounded by a fluid inclusion-poor growth band. B) Type IIb fluid inclusions in trails within a single quartz grain. C) Type II fluid inclusions containing solids: an opaque mineral with triangular section (probably chalcopyrite) and another with laminar habit (probably a mica). D) Intragranular trails intersecting each other of both opaque blebs and Type II fluid inclusions. These trails are crosscut by later Type V fluid inclusion planes. E) Isolated Type III fluid inclusion inside a quartz crystal that exhibits an opaque bleb. F) Curved intragranular healed microfissure close to the border of a quartz grain with Type IV fluid inclusions. Trails of opaque blebs occur in the same quartz crystal.

These inclusions show two or three phases at room temperature (aqueous liquid + carbonic liquid + carbonic vapour) with the volumetric fraction of the aqueous phase (flw) between 0.3 and 0.8, the most frequent interval being from 0.5 to 0.6. The melting temperature of CO2 (TmCO2) ranges from −56.6 to −57.1 °C, with a mode of −56.6 °C. The CO2 homogenization occurs to the vapour state in a temperature range between 25.5 and 28.5 °C (mode around 26.5 °C), and a few of them have a Th CO2 of around 12.5 °C. The melting temperature of clathrate (Tm Cl) is observed between 8 and 9.8 °C. The abovementioned microthermometric data lead us to conclude that the volatile phase is composed of CO2. Where it has been possible to observe the first melting of ice, the temperatures obtained are around the eutectic temperature of the H2O–NaCl system (−20.8 °C; Potter and Brown, 1977). The melting temperature of ice (Tmice) ranges from −4.5 to −2.3 °C. The total homogenization temperatures

(Table 6) range from 200 to 390 °C, the most frequent interval lying between 320 and 360 °C to the liquid state with two modes, around 320 °C and around 355 °C. A lower number of inclusions (20%) homogenise to the vapour state between 310 and 390 °C, with a mode around 390 °C. 5.2. Type II: aqueous-carbonic (CO2 and other volatiles) inclusions These have only been found in the quartz veins that crosscut the Arganzúa granitoid. They are widespread and outline sets of bands that terminate abruptly a few millimetres from the crystal boundary. These bands could represent growth bands of the quartz crystals. Also, they occur as clusters of very abundant fluid inclusions, in many cases in the central part of the quartz grains. Moreover, they appear in trails that occur within single grains and do not cross-cut

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that could be a mica (muscovite) (Fig. 12C), another has a prismatic habit and another is acicular. Raman analysis and SEM-EDS in opened FIs were made in order to identify these solids, but we were unable to obtain good results. Only from SEM-EDS, was K-feldspar successfully identified in some of these inclusions. However, all the analyses performed in the opened FIs detected the presence of Fe and Ca, and nearly always minor quantities of S and Cu, the frequent detection of copper could support the hypothesis that the opaque mineral is chalcopyrite. We did not observe any sign of dissolution of these solids during the heating experiments of the microthermometric analysis and, with the exception of the possible chalcopyrite, they are anomalously large compared to their host inclusion, thus we interpreted them as having been mechanically trapped at the moment of formation of these inclusions. These inclusions show two or three phases at room temperature (aqueous liquid + carbonic liquid+ carbonic vapour) with a wide range of volumetric fraction of the aqueous phase (flw) between 0.05 and 0.9, but the most frequent interval lies between 0.5 and 0.7. The TmCO2 ranges from −61.8 to −57 °C, with a mode around −57.9 °C. The ThCO2 occurs to the vapour state in two ranges of temperatures, in subtype IIa between 17.5 and 24.2 °C (mode around 22 °C) and in subtype IIb between 12.5 and 13.7 °C with a mode around 13 °C. The TmCl is from 9.5 to 13.1 °C, with a mode around 10.2 °C. The first melting of ice was approximately the eutectic temperature of the H2O–NaCl system. The Tmice ranges from −6.8 to −2.6 °C (mode around −3 °C). The total homogenization temperature ranges from 280 to 420 °C, being between 280 and 345 °C to the liquid state with the most frequent interval between 300 and 340 °C (mode around 305 °C). Furthermore, less than 10% of these inclusions homogenise to the vapour state between 310 and 420 °C. The Raman analyses (Table 7) show that CO2 is the main component of the volatile phase, CH4 and N2 are always present and H2S is sometimes present in lower proportions, though not always. However, there are different ranges of concentration between subtypes IIa and IIb (Table 7, Fig. 13A). In subtype IIa, CO2 concentration is higher than 88 mol%, CH4 content ranges from 5.8 to 9 mol%, N2 content is between 1.4 to 2.7 mol% and H2S is nearly always detected, ranging from 0.1 to 0.4 mol%. In subtype IIb, CO2 concentration ranges from 71.7 up to 88.7 mol%, CH4 content is between 8.5 and 26.6 mol%, N2 content ranges from 1.5 to 3 mol% and H2S, when it is present, reaches up to 0.6 mol%. Considering that the fluid inclusion assemblages (FIAs) that

Table 5 Elemental correlation coefficients (R) of selected elements for around 280 rock samples from Linares and Arganzúa granitoids. Linares granitoid

Au Ag As Bi Cu Mo Pb Sb

Au

Ag

As

Bi

Cu

Mo

Pb

Sb

1 0.490 0.358 0.878 0.621 0.427 0.463 0.651

1 0.105 0.768 0.586 0.638 0.696 0.667

1 0.258 0.310 0.017 0.016 0.131

1 0.635 0.668 0.683 0.771

1 0.304 0.328 0.521

1 0.532 0.569

1 0.667

1

Au

Ag

As

Bi

Cu

Mo

Pb

Sb

1 −0.076 −0.091 0.685 0.147 0.217 0.014 −0.023

1 −0.019 −0.085 −0.036 0.063 0.053 0.579

1 −0.079 0.067 −0.090 −0.066 0.192

1 0.125 0.258 0.003 −0.011

1 −0.010 −0.034 0.276

1 0.088 0.144

1 0.776

1

Arganzúa granitoid

Au Ag As Bi Cu Mo Pb Sb

grain boundaries (Fig. 12B). They may represent healed intragranular microcracks. On the basis of the criteria of Roedder (1984), these inclusions are interpreted as being primary and pseudosecondary (e.g. intragranular trails) and, frequently, both occurring in a single crystal. Due to differences, mainly related to the volatile composition of these fluid inclusions, we have preferred to divide this type into two different subtypes denoted IIa and IIb whose differences will be presented further on. The inclusion morphology is mainly as negative crystal, and, more scarcely, elongated and irregular. The size varies over a wide range from less than 5 to more than 30 μm. A noticeable characteristic of these inclusions is that they contain up to four different solids, the most abundant of which are: an opaque mineral that sometimes occurs in triangular sections, indicating that it could be chalcopyrite (Fig. 12C), and another three colourless birefringence minerals with different habits. Of these three, one has a laminar habit, sometimes with a hexagonal contour

Table 6 Summary of microthermometric data for the fluid inclusion types found in mineralized quartz veins from the Linares deposit. Fluid inclusion type and main components

Vein type and occurrence

Sulphide stage

Origin

Flw (%)

Tm CO2

Th CO2

Tm Cl

Tm ice

Th

Type I H2O–CO2–NaCl

Type I LG

1st stage (Ccp, Mol, bPo, Apy)

P

Type II H2O–CO2– (CH4–N2–H2S)–NaCl Type III H2O–CO2–CH4– (N2–H2S)–NaCl and H2O–CH4–CO2– (N2–H2S)–NaCl Type IV N2–CH4

Type I–II AG

1st Stage (Ccp, Mol, bPo, Apy)

P, PS

Related toa Type IV LG

2nd Stage (Au–Bi–S–Te-bearing minerals)

P, PS, S

[0.3, 0.8] 0.6 57 [0.05, 0.9] 0.5 92 [0.5, 0.9] 0.7 48

[−57.1, −56.6] −56.6 40 [−61.8, −57] −57.9 58 [−63, −57.2] −61.3 25

[12.5, 28.5] V 12.5 26.5 22 [12.5, 24.2] V 13 22 24 [6, 19] L,V – 4

[8, 10] 9.3 55 [9.5, 13.1] 10.2 74 [10.4, 15] 12.5 34

[−4.5, −2.3] −3 32 [−6.8, −2.6] −3 36 [−4.8, −1.8] −3 35

[200, 390] L, V 320 (L) 355 (L) 390 (V) 57 [280, 420] L, V 305 (L) 350 (V) 44 [168, 387] L 325 36

Related toa Type IV LG Type I–II LG–AG

2nd Stage (Au–Bi–S–Te-bearing minerals) Not relateda

PS, S

0 0 10 [0.7, 0.9] 0.9 35

– – – – – –

– – – – – –

– – – – – –

– – – [−3, −2] −2 13

[−122.4, −116] V −116 10 [185, 257] L 195 22

Type V H2O–NaCl

S

LG: Linares granitoid; AG Arganzúa granitoid. Ccp: chalcopyrite; Mol: molybdenite; Po: pyrrhotite; Apy: arsenopyrite. Flw: volume fraction of the aqueous phase; P: primary; PS: pseudosecondary; S: secondary; TmCO2: melting temperature of CO2; ThCO2: homogenization temperature of CO2, vapour state (V) and liquid state (L); Tm Cl: melting temperature of clathrate; Tm ice: melting temperature of ice; Th: total homogenization temperatures; L: liquid state; and V: vapour state. All temperatures in °C. Range (first line), mode (second line) and number of measurements (third line, italics) are given for each fluid inclusion type. a See text for further information.

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Table 7 Microthermometric and Raman data, and interpreted bulk composition of the selected fluid inclusions from each type defined in Linares deposit. Microthermometry

Type I

Type II: Subtype IIa

Subtype IIb

Type III

Type IV

Raman data*

TmCO2

ThCO2

TmCl

Tmice

Th

CO2

−56.6 −56.6 −56.6 −56.6 −56.6 −56.6 −56.6 −56.6 −58.3 −57.9 −57.8 −57.6 −58.3 −57.6 −57.6 −57.4 −59.0 −58.0 −58.1 −57.7 −59.0 −59.3 −58.2 −57.8 −58.3 −58.5 −63.0 −58.5 −58.0 −58.3 −57.5 Nv Nv Nv

12.5 V 27 V 27.7 V 26.7 V 25.5 V 26 V 28.5 V 28.5 V 21.5 V 22.1 V 22.5 V 24.2 V 17.5 V 21.5 V 21 V 22 V 13 V 13.7 V 13 V 13 V 13 V 12.5 V 13 V 13 V 13 V 13 V Nv Nv Nv Nv Nv Nv Nv Nv

9.7 9.5 9.1 8.3 9.1 8.8 8.0 8.5 10.4 10.6 10.7 10.3 10.5 10.4 9.8 9.8 12.6 11.4 10.7 10.3 10.8 10.8 10.3 10.8 10.7 10.5 12.5 14.0 14.3 14.3 13.0 10.5 10.4 10.4

−3.0 −3.0 −3.0 −3.0 −3.0 −3.7 −3.0 −3.0 −3.0 −3.0 −3.0 −3.0 −3.2 −3.0 −3.0 −3.0 −4.4 −3.0 −3.0 −3.3 −4.8 −4.8 −3.0 −2.6 −2.0 −3.0 −3.0 −4.0 −3.7 −3.7 −4.0 −3.0 −3.0 −3.0

320 V 320 L 310 L 345 L 348 L 385 V 310 V 390 V 305 L 342 L 314 L 290a 395a 316 L 315 L 335 L 305 L 305 L 329 L 315 L 305 L 315 L 345 L 342 L 332 L 300 L 255 L 360a 300 L 326 L 340a 355 L 250a 304 L −117.6 V −117.6 V −122 V −116 V

100 100 100 100 100 100 100 100 88.7 90.0 91.2 90.7 88.9 89.9 90.9 91.9 71.7 81.8 84.8 88.4 72.5 83.5 88.1 88.7 85.3 87.8 5.3 40.7 46.3 49.5 61.7 69.3 77.1 87.3 nd nd nd nd

CH4

9.0 7.7 6.2 5.8 8.3 8.5 7.6 6.4 26.6 16.0 13.0 9.5 25.1 15.0 10.1 8.5 11.8 9.9 85.7 56.7 51.1 48.0 35.5 21.3 16.1 10.3 35.8 31.6 31.4 32.6

Bulk composition N2

2.1 2.3 2.6 3.0 2.2 1.6 1.5 1.7 1.7 1.8 1.8 1.9 2.0 1.4 1.6 2.7 2.7 2.1 9.0 1.7 2.2 1.7 1.9 9.4 6.8 2.4 64.2 68.4 68.6 67.4

H2S

0.2 nd nd 0.5 0.6 nd nd nd nd 0.4 0.4 0.3 0.4 0.2 0.2 0.1 0.2 0.1 nd 0.9 0.4 0.8 0.9 nd nd nd nd nd nd nd

H2O

CO2

89.6 90.6 87.6 84.2 88.4 85.0 78.5 83.3 87.1 85.6 80.8 75.4 66.7 85.1 73.6 73.9 90.8 91.4 92.7 91.7 92.5 92.4 92.3 92.2 92.4 92.2 94.2 91.8 91.8 91.8 89.0 94.0 95.5 94.1

8.6 7.4 10.0 12.6 9.2 12.3 18.3 13.7 9.0 11.2 16.0 21.0 28.0 11.2 21.7 22.0 6.1 6.1 4.8 6.7 4.4 4.8 6.5 6.6 6.5 5.0 0.4 3.5 4.0 4.1 7.1 4.0 3.0 4.5

CH4

0.8 0.8 1.0 1.3 2.5 0.9 1.7 1.4 1.5 0.7 0.4 0.5 0.7 0.4 0.5 0.4 0.6 0.3 3.1 3.0 2.6 2.5 3.0 0.6 0.1 0.2 35.8 31.6 31.4 32.6

N2

0.2 0.2 0.4 0.6 0.6 0.2 0.3 0.4 0.1 0.1 b0.1 0.1 b0.1 b0.1 b0.1 0.1 0.1 b0.1 0.3 0.1 0.1 0.1 0.1 0.2 b0.1 b0.1 64.2 68.4 68.6 67.4

NaCl 1.8 2.0 2.4 3.2 2.4 2.7 3.2 3.0 2.9 2.2 1.8 1.7 2.2 2.6 2.7 2.3 1.5 1.7 2.1 1.0 2.4 2.3 0.7 0.7 0.4 2.5 2.0 1.6 1.5 1.5 0.8 1.2 1.4 1.2

Compositions are given in mol%; TmCO2: melting temperature of CO2; ThCO2: homogenization temperature of CO2; Tm Cl: melting temperature of clathrate; Tm ice: melting temperature of ice; Th: total homogenization temperatures; L: liquid state; and V: vapour state. All temperatures in °C. H2S is not included in bulk composition because its concentration is always below 0.1 mol%; Nv: not visible; and nd: not detected. * Type I was not analysed by Raman (see text for more details). a Decrepitation temperature.

are undoubtedly primary in origin correspond to subtype IIa, and FIAs that may be pseudosecondary in origin always belong to subtype IIb, it is likely that those of subtype IIb were trapped later than, but quite close in time to subtype IIa. Thus, we can summarise that FIAs of pseudosecondary character (subtype IIb) have higher CH4 and lower CO2 contents in their volatile phase (Fig. 13A). 5.3. Type III: aqueous-carbonic (≪ CO2 and other volatile) inclusions These were found in quartz grains of veins that crosscut the Linares granitoid with a notable abundance in Au–Bi–Te blebs. These opaque blebs form planar arrays crosscutting quartz grains (Fig. 11) and are accompanied by these fluid inclusions occurring in the same way. Thus, both opaque blebs and fluid inclusions occur frequently, as curved intragranular trails with random orientations that intersect each other (Fig. 12D) and as transgranular trails. Sometimes, these fluid inclusions appear isolated or in small clusters inside a quartz crystal as do opaque blebs (Fig. 12E). On the basis of Roedder's (1984) criteria, these inclusions are interpreted as pseudosecondary and secondary. Nevertheless, the textural relationships described above support that the FIs and the Au–Bi–Te blebs are cogenetic. The morphology of the inclusions is mainly as negative crystal, and, more scarcely, prismatic and irregular. The size varies between 10 and 40 μm. Occasionally they have a tiny opaque mineral that

does not melt during heating experiments, hence it may be trapped. These inclusions show two phases at room temperature and, more rarely, three phases with a range of volumetric fraction of the aqueous phase (flw) between 0.5 and 0.9, with a mode of 0.7. Despite the fact that the melting of CO2 is easily measured in several of these FIs (TmCO2 ranges from −63 to −57.2 °C, with a mode around −61.3 °C), the ThCO2 is rarely observed, having only been recorded in four cases (10.7 and 18.2 °C to the liquid state; 6 and 19 °C to the vapour state). Nevertheless, in many of these FIs the presence of a volatile component is only detected from clathrate formation. The TmCl is from 10.4 to 15 °C, with a mode around 12.5 °C. The first melting of ice is around the eutectic temperature of the H2O–NaCl system. The Tmice ranges from − 4.8 to − 1.8 °C (mode around − 3 °C). These fluid inclusions show a broad range of total homogenization temperatures of between 168 and 387 °C to the liquid state, the most frequent interval lying between 300 and 340 °C, with a mode around 325 °C (Table 6). The Raman analyses show that CH4 and CO2 are the main components of the volatile phase, N2 and H2S are present in lower proportions (Table 7). The CO2/CH4 ratio is very variable from 0.06 to 8, the most frequent value being around 1 (Fig. 13A). Thus, CO2 concentration ranges from 5 up to 87 mol%, CH4 content is between 10 and 85 mol% and N2 content ranges from 1.6 to 9.5 mol%. The H2S, when detected, ranges from 0.4 to 0.9 mol%, these FIs reaching the highest H2S values measured (Fig. 13A).

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A

B

Fig. 13. Compositional ternary plots of the different fluid inclusion types measured in the Linares deposit: A) volatile-rich phase B) bulk fluid inclusion.

5.4. Type IV: volatile-rich non-aqueous fluid inclusions These are scarce and have only found in the quartz veins with extremely abundant Au–Bi–Te blebs crosscutting the Linares granitoid. According to terminology of Simmons and Richter (1976) and Kranz (1983), these FIs occur in curved intragranular healed microfissures sometimes together with opaque blebs (Fig. 12F) and often as intergranular inclusions decorating grain boundaries. Van den Kerkhof and Hein (2001) point out that the latter occurrence may be related to two processes: (1) healing of extensional grain boundary cracks and (2) fluid collection during grain boundary migration, this occurrence being particularly common in rocks which underwent intensive dynamic re-crystallisation. We consider that they could be pseudosecondary and/ or secondary in origin according to Roedder's criteria. Occasionally, they appear in intragranular healed microfissures in the same quartz crystal in which Type III are present but healing different microcracks. The chronological relationship between both types is uncertain because of the lack of crosscutting criteria. They are irregular in morphology with a variable size from 5 up to 30 μm. At room temperature they are monophasic and, during freezing, a meniscus can be observed below − 130 °C. The only phase change that could be measured was the total homogenization of these inclusions to the vapour state between − 116 and − 122.4 °C. The Raman analyses (Table 7) show that N2 is the main component of the volatile phase and CH4 is always present in lower proportions. The N2 concentration ranges from 64 to 68 mol%, and the CH4 content is between 31 and 35 mol% (Fig. 13A). 5.5. Type V: aqueous inclusions

measurements and the Raman analyses of the volatile phase using a clathrate stability model (Bakker, 1998; Bakker et al., 1996; Dubessy et al., 1992) and the Q2 and ICE computer programmes of the CLATHRATES package (Bakker, 1997). Bulk compositions of the fluids have been plotted in a H2O–CO2– 10(CH4 + N2) ternary diagram (Fig. 13B) and are shown in Table 7. Early aqueous-carbonic fluids (i.e. Type I, subtype IIa) are characterised by H2O content in the range of 66 to 90 mol%, a CO2 content of between 8 and 28 mol%, low contents of CH4 and N2 from undetected levels up to 3 mol% (Table 7), and with low salinity between 1.7 and 3.2 mol% (3.5–6 wt.% NaCl, Fig. 14). The earlier fluid circulating in veins of the Arganzúa granitoid (i.e. subtype IIa) is somewhat different from that flowing through the Linares granitoid (i.e. Type I), the former being slightly richer in volatiles, mainly because of the presence of measurable CH4 and N2 (Fig. 13B). Fluid trapped by subtype IIb fluid inclusions has higher water content (91–92.7 mol%), lower volatile concentration (4.8–6.5 mol% CO2, 0.4–1.6 mol% CH4 + N2) and salinity (0.4–2.5 mol%). As we mentioned previously, the petrography of Type II fluid inclusions indicates that subtype IIb fluid was trapped somewhat later than subtype IIa fluid, though close in time. Thus, we propose that these subtypes record the evolution (from subtype IIa to subtype IIb) of a single fluid which progressively lost volatiles with more or less constant salinity (Fig. 14). Type III fluid inclusions trapped the aqueous-carbonic fluid clearly involved in the deposition of Au–Bi–Te blebs. This is the aqueous-carbonic fluid that reaches the highest water content (95 mol%), hence the lowest volatile content (less than 4 mol%), the salinity also being low (0.8 and 2 mol%) (Fig. 14). It is noticeable that the main volatiles (i.e. CO2 and CH4) have very variable concentrations, from 0.4 mol% CO2 and 3 mol%

These occur in transgranular healed microfissures of quartz grains (Fig. 12D) from veinlets crosscutting both granitoids. We therefore consider them as secondary. They are irregular in morphology, varying in size from 10 to 20 μm. They have two phases at room temperature and the volumetric fraction of the aqueous phase is between 0.7 and 0.9. The first melting of ice is around the eutectic temperature of the H2O–NaCl system. The final melting of ice ranges between −3 and −2 °C, giving an estimated salinity of between 3.4 and 4.9 wt.% NaCl according to Bodnar and Vityk (1994). Total homogenization temperatures vary between 185 and 257 °C with a mode around 195 °C, to the liquid state. 5.6. Global fluid composition Bulk composition and density of a selection of representative fluid inclusions of each type were calculated from microthermometric

Fig. 14. Salinity vs. homogenization temperature of the fluid inclusions measured in the Linares deposits.

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CH4 to 4.5 mol% CO2 and 0.2 mol% CH4, yet this fluid attains the highest methane contents of the aqueous-carbonic fluids found in this study. In addition, a circulation of a non‐aqueous fluid made up of nitrogen and, to a far lesser degree, methane (68 mol% N2 and 32 mol% CH4) is trapped by Type IV fluid inclusions that were also found in spatial association with the opaque blebs (Fig. 13D). From fluid inclusion petrography, either the link of this N2–CH4 fluid with the deposition Au–Bi–Te blebs or the chronological position with respect to Type III fluid is uncertain, as was mentioned above. Finally, Type V fluid inclusions record the circulation of a low salinity aqueous fluid (3.5–5 wt.% NaCl) that may not be associated with any type of mineralization in accordance with the fluid inclusion petrography. 5.7. P–T reconstruction A reconstruction of the pressure–temperature regime recording the ore-bearing veins has been carried out based on fluid inclusion isochores for each fluid circulation event in the Arganzúa and Linares granitoids (Fig. 15A and B). Isochores were calculated with the ISOC computer programme of the FLUIDS package (Bakker, 2003; Bakker and Brown, 2003) using the state equation of Bowers and Helgelson (1983) and Bakker (1999) for the volatile-bearing fluid inclusions (Types I, II, III, IV) and the Zhang and Frantz's (1987) EOSs for the aqueous fluid inclusions (Type V). Quartz veins crosscutting the Arganzúa granitoid show the circulation of a fluid represented by Type II fluid inclusions. The earlier stage (Stage 1) is represented by dense aqueous-carbonic subtype IIa fluid inclusions (density varies between 0.77 and 0.89 g/cm 3) with a minimal trapping temperature range from 300 to 340 °C, which corresponds to minimal trapping pressures ranging from 1.4 to 2.1 kbar (Fig. 15A). This fluid evolved releasing volatiles and decreasing in density (0.5 to 0.7 g/cm3), as evidenced by subtype IIb fluid inclusions. Hence, minimum trapping conditions give a similar temperature range (300– 340 °C), however, correspond to lower pressures of between 0.1 and 0.5 kbar. This would characterise a Stage 2 (Fig. 15A). As a constraint upon the real trapping temperature, the arsenopyrite geothermometre gives a range of temperature from 460 °C up to 640 °C. Thus, from the type II isochores the pressure corresponding to such a high temperature has an unrealistic value (>4 kbar), unsupported by the regional geology. The fluid inclusions that trapped the fluid related to the arsenopyrite precipitation may not have been found. Hence, we decided to use chlorite as a geothermometre (Cathelineau, 1988; Kranidiotis and MacLean,

A

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1987). The selected chlorites were those formed in the alteration haloes of sulphide-bearing veins filled by the quartz crystals where Type IIa and Type IIb fluid inclusions were studied. A quite similar temperature interval of between 355 and 370 °C was obtained from Kranidiotis and MacLean (1987) and Cathelineau (1988) chlorite geothermometres. Considering this interval and Type II isochores, two ranges of corresponding pressures are obtained, 1.8–2.7 kbar from subtype IIa and 0.3–0.9 kbar from subtype IIb. Thus, a noteworthy decompression of around 1700 bar at possibly quite constant temperature seems to have happened during the formation of the sulphide-bearing quartz veins that crosscut the Arganzúa granitoid. A pressure interval of 1.8– 2.6 kbar probably represents a lithostatic pressure, and assuming an average density for the rock column of 2.6 g/cm3, such pressure may indicate a relatively deep structural level of between 7 and 10 km. However, the pressure range of 0.3–0.9 kbar almost certainly corresponds to hydrostatic pressure. Pressure lower than 500 bar is ordinarily assumed to be hydrostatic. In fact, if it were lithostatic it would imply an unrealistic change in structural level considering the close temporal relationship between subtype IIa and IIb fluid inclusions. As an approximation, a water column with a density of 1 g/cm3 at the maximum structural level obtained (10 km) gives a hydrostatic pressure of 980 bar, which is roughly in agreement with the higher pressure obtained from subtype IIb fluid (875 bar). Thus, the striking decompression of around 1.7 kbar from Stage 1 to Stage 2 could be explained by successive pressure drops within these veins from lithostatic to hydrostatic conditions during a broader decompression process, producing connectivity in the vein system. Quartz veins crosscutting the Linares granitoid record the circulation of a fluid represented by Type I fluid inclusions. This is an aqueouscarbonic fluid with density varying between 0.5 and 0.7 g/cm3 under minimum trapping temperatures of between 320 and 355 °C and an interval of minimum trapping pressures between 0.24 and 0.8 kbar (Fig. 15B). The chlorite geothermometry (Cathelineau, 1988; Kranidiotis and MacLean, 1987) was also used with the chlorite from the alteration selvages of the Type I quartz veins that crosscut the Linares stock, where the Type I fluid inclusions were studied. The calculated temperature range (330–350 °C) is quite similar to that of the homogenization temperature in Type I fluid inclusions, thus the minimum P–T conditions mentioned before are probably close to the real trapping conditions. Later on, a higher density (0.7 and 0.9 g/cm3) aqueous-carbonic fluid (Type III fluid inclusions) cogenetic with trails of Au–Bi–Te blebs flows through these veins under minimum trapping conditions of 320±20 °C and a corresponding range of pressure between 0.5 and 1.1 kbar (Fig. 15B). The

B

Fig. 15. P–T reconstruction diagram with the representative isochores of the different fluid inclusion types identified in the Linares deposit. The grey areas are the minimum P–T conditions (Ph–Th) for the different stages of trapping. Pattern areas indicate the temperature ranges obtained from chlorite geothermometry (more details in text): A) data from veins of Arganzúa stock; B) data from veins of Linares stock.

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fluid inclusions found in veins of the Linares granitoid do not show any evidence of the high pressure Stage 1 and subsequent decompression established in Arganzúa quartz-veins. In fact, the earlier fluid circulation in Linares was probably under a hydrostatic pressure similar to that of Stage 2 in Arganzúa. A fluid cogenetic with the Au–Bi–Te blebs subsequently circulated under reasonably similar minimum P–T conditions to that of Stage 2, since the slightly higher pressure (Δ300 bar) could be explained by self-sealing processes of the veins that lead to a local increase in fluid pressure. In addition, a non-aqueous fluid (Type IV fluid inclusions) consisting of a mixture of N2 and CH4 of very low density (around 0.05 g/cm3) flowed through veins of the Linares granitoid. The isochores of this fluid have gentle slopes giving pressures not higher than 250 bar. As we commented before, this Type IV fluid evidently circulated later than Type I fluid, though the chronological relationship between Type IV and Type III fluid is doubtful. According to the low pressure obtained, the Type IV fluid probably circulated later but we cannot state whether it was related with the latest stage of Au–Bi–Te bleb deposition. Finally, veins in both granitoids record a later stage fluid represented by Type V fluid inclusion. This is a low salinity aqueous fluid that may not be associated with any type of mineralization, with Th around 195 °C and corresponding Ph around 10 bar (Fig. 15). 5.8. Oxygen isotope compositions Quartz samples from the veins crosscutting both, the Linares and Arganzúa granitoids, were taken to estimate the oxygen isotopic composition of the hydrothermal fluids. The measured δ 18O(SMOW) values in quartz are similar in all the sampled veins and there is not distinction between the veins hosted by the Linares granitoid and those hosted by the Arganzúa granitoid. In fact, the range of the Linares-hosted veins is from + 11.6 to + 12.0‰ and that of the Arganzúa-hosted veins is between + 11.3 and + 11.7‰. For assessing the δ 18O of the hydrothermal fluid, we used a temperature range of between 300 and 450 °C in accordance with the fluid inclusion and chlorite geothermometric data, the estimated values varying from + 6.8 to + 7.9‰ and from + 5.6 to + 6.7‰, respectively (Matsuhisa et al., 1979). Such values fall in the range of typical magmatic fluids, however metamorphic fluids cannot discard. 6. Summary and conclusions The Linares deposit consists of an intrusion-hosted gold mineralization located in the Narcea Antiform, one of the more extensive exposures of Precambrian rocks in the Variscan Iberian Massif. This antiform constitutes the limit between the foreland areas, represented by the Cantabrian Zone, and the West Asturian–Leonese Zone. The latter represents the transition to the hinterland areas of the orogen. The boundary between both zones is formed by the La Espina Thrust, a regional thrust with very intense associated ductile deformation and a complex history of reactivations. The area comprises a magmatic complex with several stocks, the biggest ones being the Linares and Arganzúa stocks, and numerous dacite and aplite dikes that crosscut igneous and host rocks. The Linares stock is a biotite-rich, hornblende-bearing porphyritic adamellite, with megacrystals of K-feldspar. The Arganzúa stock is a granite with a lower proportion of biotite and garnet as accessories. The geochemical data revealed a suite of high-K and calc-alkaline igneous rocks, slightly peraluminous (A/CKN = 1.01–1.09), the Arganzúa granite being the most evolved member of this magmatic suite. This rock is depleted in LREE with respect to the Linares one and shows high negative Eu anomalies. Moreover, the negative correlation between Ba, Sr and Zr with SiO2 and the Eu/Eu* ratios indicates differentiation mainly by mineral fractionation. According to the Chappell and White (1992) diagram, all the igneous rocks from the Linares deposit can be considered as intermediate between I-

and S-type granites (Fig. 3C). The relative oxidation state of both granitoids is different. According to the ΔOx index (Table 1), the Linares granitoid has a character between moderately oxidised and moderately reduced (Blevin, 2004). This is also in accordance with the composition of magmatic biotite (Fig. 7A). The values of the ΔOx index obtained for the Arganzúa igneous rocks (Table 1) are not representative of their actual oxidation state due to their low FeOtot content (Blevin, 2004). However, the composition from magmatic biotite indicates a relatively more reduced character for this granitoid. In the Rb vs. Y + Nb tectonomagmatic discrimination diagram of Pearce (1996) all rocks plot in the post-collisional granitoid field. The field relationship between the igneous rocks and the upper Palaeozoic host-rocks (i.e. those stocks crosscutting the Stephanian syn-orogenic conglomerates) and the lack of important deformation textures support their post-collisional character. The geochronological data for the Linares granitoid (297.3± 1.8 Ma from U–Pb in zircon, GutiérrezAlonso et al., 2011, and 290 ± 6 Ma, from K/Ar in biotite, Martín-Izard et al., 2001) indicates that the intrusion of this body took place in early Permian. There are no field relationships between Linares and Arganzúa granitoids that show the relationship between them. The only age data that exists for the Arganzúa granitoid is 276± 6 Ma, from K/Ar in biotite (Martín-Izard et al., 2001). This data, although younger than the timing interval given for the post-orogenic magmatism in this area (300 to 285 m.y., Gutiérrez-Alonso et al., 2011), suggests that the Arganzúa granitoid intruded after the Linares one. It would be necessary to obtain a U–Pb age for the Arganzúa granitoid, too, in order to constrain the interval of time between the two intrusions. Apart from that, we consider that the Arganzúa leucogranite cannot be included among the muscovite–leucogranite granitoids of the post-tectonic granitoid association mentioned in the geological setting section. The rocks from the Arganzúa leucogranite are not muscovite-bearing rocks and are slightly peraluminous (A/CNK = 1– 1.1), whereas the muscovite-bearing leucogranites are strongly peraluminous (A/CNK = 1.1–1.3). As indicated above, we consider both stocks to be the result of the evolution of a similar magma, the Arganzúa leucogranite being a more evolved phase. The gold mineralization is present in both granitoids, in relation to quartz veins and hydrothermal alteration of the igneous rocks. Five different types of quartz veins have been defined. They commonly occur in sheeted vein arrays, generally in millimetre scale, or thicker isolated veins (up to 3 cm). Two conjugated systems are also frequent. The outcrop vein-trending measurements showed a main N40–55°E system and a subordinate N70–55°W system that is cut by the NE system, although the opposite also occurs, so they are contemporary. This is coincident with the main regional strain in late Carboniferous times. In any case, interconnected, multidirectional quartz vein stockworks, which are typical of porphyry deposits, were observed. The sulphide content within the veins is typically low (b 5 vol.%). The hydrothermal alteration is generally restricted to the envelope of quartz veins and is rarely pervasive. Nevertheless, the more altered the rock is, the higher is the sulphide occurrence and, in addition, the gold grade. The hydrothermal alteration types observed and the related metallic associations are basically the same in both granitoids, although there are some differences between the two in terms of types of veins, hydrothermal alterations and mineral associations. In the Arganzúa granitoid two sets of conjugated veins constituted by Type III veins, with textures suggesting oblique extension, and Type II veins, with variable grade of internal deformation, have been observed (Fig. 9). The internal features and disposition of the veins indicate that they formed in the transition between ductile–fragile conditions. In the Linares granitoid, Type II veins have not been observed, which could indicate more fragile conditions during the hydrothermal stage. Moreover, the Arganzúa granitoid presents abundant aplitic dikes that evolve laterally to quartz veins, and also pegmatoid segregations,

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suggesting transition from late-stage magmatic to hydrothermal processes. A weak and early albitization is only observed in the Arganzúa granitoid and consists in the development of intergranular albite and albite rims around plagioclase crystals. However, the potassic alteration was more intense, above all in the Arganzúa granitoid, although in the Linares granitoid it is also present. This alteration mainly consists in microclinization, although shreddy biotite associated with quartz and K-feldspar veins was locally formed in the Arganzúa granitoid. The accompanying ore minerals are pyrrhotite, chalcopyrite, arsenopyrite, molybdenite and ±scheelite. Subsequent sericitic alteration took place in relation to the same veins, overprinting the potassic alteration and grade outwards to the vein, or in relation to later quartz veins. Muscovite, sericite, carbonate, ±scheelite and rutile, with minor REE-minerals, replace biotite and plagioclase crystals or fill interstices. K-feldspar is also affected when the alteration is pervasive. The related sulphides are pyrite and chalcopyrite, while arsenopyrite is mainly in the quartz veins. A sericite–chlorite–carbonate alteration is broadly distributed although only on few occasions do the igneous rocks appear pervasively altered. It is associated with Type IV veins (hairline fractures), which are typically filled by sulphides and crosscut previous veins running through the igneous rock. In the Arganzúa granitoid, these Type IV veins often occur associated with the conjugated Type II and III veins: they cut, in a regular pattern, the Type II veins and trend subparallel to the oblique-extension direction of the Type III veins (Fig. 9). This mode of occurrence suggests a continuity in the fracturation process and the hydrothermal alteration. Tungsten-bearing minerals, wolframite, with noticeable amounts of Ta and Nb, and scheelite, only occur in the Arganzúa granitoid. We consider that this could be due to the evolved character of this granite (Fig. 3D). Moreover, biotite geochemistry in this rock suggests moderately high fluorine fugacity in the last-stage magmatic fluid, thus this halogen could have had a role in the transport of the above mentioned incompatible elements. In the Linares granitoid, tungsten is present only as a trace element in minerals such as rutile (Fig. 8B1). Wolframite occurs in early quartz veins that produced a very weak alteration of the host rocks. This mineral is replaced by scheelite and sulphides. The precipitation of scheelite after wolframite implied an increase in the Ca/Fe ratio and an increase in the pH value of the ore-forming fluid, as a consequence of the plagioclase alteration during the subsequent potassic and sericitic alteration that liberates Ca and some of the Na, and the hydrolysis of feldspar. The sulphide precipitation begins with pyrrhotite and löllingite, which appear as inclusions within arsenopyrite crystals and are partially replaced, followed by chalcopyrite, arsenopyrite, pyrite and molybdenite. Pyrrhotite is replaced by pyrite and marcasite. Nevertheless this mineral continued forming, along with chalcopyrite instead of pyrite, where the fO2 conditions were lower. There is no evidence of gold precipitation during this first sulphide stage. Gold occurs as inclusions within arsenopyrite and, to a far lesser degree, in chalcopyrite, although no correlation exists between Au and As and only in Linares granitoid a poor correlation does exist between Au and Cu. However, gold is commonly associated with Bi–Te-bearing minerals, which is reflected in the strong correlation between Au and Bi observed in the Linares and Arganzúa granitoids. In terms of oxygen fugacity, the speciation of Bi-tellurides that accompanied gold with Bi/Te(S+ Se)≥ 1 is consistent with a pyrrhotite-buffered environment (Cook and Ciobanu, 2004). The fluids involved in the sulphide precipitation are aqueous-carbonic (CO2 and ±CH4), with low salinity (0.5–6.3 wt.% NaCl eq.) and variable amounts of other volatiles (N2 and H2S). Different types of fluid inclusion have been defined, mainly on the basis of the proportion and composition of the volatile phase. The combination of the fluid inclusion study and chlorite geothermometry in Arganzúa samples indicates initial temperature conditions of between 300 and 400 °C and pressures of between 1.8 and 2.6 kbar for an aqueous-carbonic fluid with a relatively high proportion of volatiles (20–30 mol%) that mainly consists of carbon dioxide and lesser amounts of methane and nitrogen. This range of pressure

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would represent lithosthatic conditions corresponding to a relatively deep structural level of between 7 and 10 km. The samples registered an adiabatic drop of pressure over a range of 0.3–0.9 kbar, normally considered hydrostatic pressures. The fluid evolved, probably through loss of volatiles (mainly CO2) to a more water-rich (>90 mol% H2O) and slightly saline fluid (Fig. 14). According to Fournier (1999), pressure can increase until rapid fluid-release events force brittle fracturing, sometimes producing pressure changes from lithostatic to hydrostatic gradients. Such pressure changes are analogous to those described in the fault-valve model defined by Sibson and Poulson (1988) during the formation of extensional veins. These authors point out that these systems record cyclic fluctuations in fluid pressure from supralithostatic to hydrostatic because the opening of the veins is attributable to the prefailure stage of supralithostatic fluid pressure and deposition is attributable to the immediate postfailure fluid discharge, therefore the self-sealing of the veins leads to reaccumulation of fluid pressure and a repetition of the cycle. Mutual crosscutting relationships between vein sets, as also observed in the Linares deposit, are a natural consequence of the cyclicity of the process. If it took place in the studied veins, such depressurization could have forced the earlier H2O–CO2 fluid (Type IIa) to intersect its solvus and to release a CO2-rich phase. This would be a low density vapour that could physically separate and escape from the immediate environment. A possible consequence of the latter process, aggravated by the tendency of the water-rich fluid to wet growing phases, is that CO2-rich, vapour-like fluid may be under-represented or even absent from the resulting fluid inclusion assemblage (Wilkinson, 2001). This may be a possible explanation for the lack of fluid inclusions representative of the CO2-rich phase in the studied quartz crystals, and hence we can only speak about a Type II fluid evolution characterised by the loss of volatiles. Consequently, the fault-valve phenomena and therefore effervescence could be a possible mechanism of ore precipitation. The fluid inclusions studied in the veins that crosscut the Linares stock only record the conditions of the stage II defined in the Arganzúa granitoid, e.g., minimum trapping conditions of 320 to 355 °C of temperature and 0.3 to 0.8 kbar of pressure. The high-pressure stage mentioned above is not registered. Moreover, there are some differences in the fluid composition. In the Linares stock, the early trapped fluid only contains CO2 in the carbonic phase (Fig. 13A). The lack of methane in this fluid could be a consequence of the more oxidised character of the Linares granitoid than the Arganzúa one. However, the trapped fluid in the Type III inclusions, which is cogenetic with the precipitation of the Au–Bi–Te assemblages, includes CH4, N2 and H2S in the vapour phase. The presence of fluid inclusions with a CH4-rich volatile phase suggests more reduced conditions, which are in accordance with the speciation of Bi-tellurides that accompanied gold, as mentioned above. In addition, as was mentioned previously, the striking difference of the aqueous-carbonic Type III fluid is a wide-ranging CO2/CH4 ratio (Fig. 13A). This may be as a result of isothermal mixing of the fluid of stage II with a hot low-salinity methane-bearing aqueous fluid, the source of which could be external to the granitoid. In this case, the methane source would be the surrounding carbonaceous matter-bearing host metasediments. Release of nitrogen during destruction of organic matter and/or by breakdown of NH4-bearing biotite during prograde or retrograde processes in metamorphic rocks may explain the presence of N2-gas in fluid inclusions (Bebout et al., 1999). The non-aqueous fluid trapped in Type IV, with a N2/CH4 ratio of 2:1, supports this idea. These fluid inclusions, which were also found in spatial association with the Au–Bi–Te blebs (Fig. 12D), often occur as intergranular inclusions decorating grain boundaries. Non-aqueous N2–CH4 fluid inclusions occurring in the same way had already been observed in other gold-bearing deposits from the Río Narcea Gold Belt: the El Valle-Boinás Cu–Au skarn and the Ortosa Au skarn (Cepedal, 2001; Cepedal et al., 2003a). Bearing in mind above mentioned points, the isothermal fluid-mixing would have been a possible mechanism for Au–Bi–Te bleb deposition in veins

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crosscutting the Linares stock. This would explain the dilution of the earlier fluid (i.e. less volatile content and salinity) and the higher CH4-content of its volatile phase. Despite Type III fluid inclusions not having been found in Arganzúa quartz veins during this research, we do not discard the possibility that fluid-mixing processes would have also affected the Arganzúa stock. If this is the case, the fault-valve phenomena and subsequent fluid-mixing would have been a possible mechanism of ore precipitation in this system. A conspicuous feature of the ore-related fluids found in this deposit is their low salinity which, together with the reduced conditions established, imply that the gold ligands may have been bisulphide complex (Seward, 1991), besides the low quantities of H2S detected in the volatile phase by RAMAN. A decrease in the sulphur activity in solution may act as mechanism for gold deposition. Moreover, a liquid bismuth collector model (Douglas et al., 2000; Tooth et al., 2008, 2011) can explain the strong association observed between Au and Bi in the Linares deposit, as was previously mentioned. According to this model, given low sulphur fugacity and temperatures exceeding 271 °C, bismuth may precipitate from hydrothermal fluids as a liquid. The strong partition of gold (up to 20 wt.% Au) into liquid bismuth from the coexisting hydrothermal fluid (Douglas et al., 2000) can produce the deposition of gold, even when that fluid is significantly gold undersaturated (Tooth et al., 2008). We consider that the gold mineralization is temporally related to the youngest Arganzúa leucogranite, whereas the oldest Linares stock would have a passive behaviour, hosting the mineralization. The distribution of the hydrothermal alteration observed in the igneous rocks, with the potassic alteration mainly developed in the Arganzúa stock and the sericitic and propylitic alterations more frequent in the Linares one in addition to the composition, evolution and trapping conditions of fluids involved in this system identify the Arganzúa stock as the causative intrusion of the gold mineralization. The Linares deposit shares many similarities in terms of alteration and mineralization assemblages, geochemistry and fluid inclusions with intrusion-related gold deposits (IRGDs) (Hart, 2007; Lang and Baker, 2001; Lang et al., 1997, 2000; Mustard, 2001; Thompson and Newberry, 2000; Thompson et al., 1999): – Igneous rock characteristics. The composition of the igneous rocks that host the gold mineralization is typical of those associated with IRGDs. These are highly differentiated (adamellite to granite), subalcalic, calc-alkaline and slightly peraluminous intrusions. Although IRGDs are mostly related to metaluminous intrusions, peraluminous intrusions are also usually present (e.g. Yukon, Gordey and Anderson, 1993; Coulson et al., 2001). The intrusions have a reduced to intermediate oxidation state which spans the boundary between the ilmenite and magnetite series. Moreover, this magmatism displays mixed characteristics between I- and S-types, generated in a post-collisional tectonic setting by the melting of the lower crust with varying degrees of involvement of mantle derived melts. Importance of late-stage fractionated phases, and the transition from magmatic to hydrothermal processes, with abundance of aplitic dikes that prograde into quartz veins. – Vein features. Sheeted veins arrays, sometimes conjugated veins, but at all events interconnected, multidirectional stockworks characteristic of porphyry deposits were observed. – Fluids. Aqueous-carbonic fluids are typical in IRGSs. They are CO2rich fluids. The abundance of CH4 and other volatiles such as N2 and H2S are more common in IRGD deposits formed at higher pressures (from 5 to 10 km) as occurs in the Linares deposit. The temperatures and pressures obtained for the gold mineralization in the Linares deposit are consistent with those established for IRGDs, where the precipitation of gold and related metals span broad ranges of b200 to >600 °C and b 0.5 to >3 kbar. In the IRGD model a predominantly magmatic source for CO2-rich

fluids is favoured and the potential contribution to ore formation by metamorphic fluids remains to be checked. The δ18Ofluid values calculated from the quartz veins suggest that the fluid implicated could be magmatic in origin. Nevertheless, these δ18O data do not allow us to distinguish between magmatic and metamorphic fluids. Thus, we cannot discard the role of the metamorphic fluids in the gold mineralization in the Linares deposit. The presence of a volatile-rich fluid with high CH4 and N2 contents in this deposit suggests the influx of an external fluid produced during the metamorphism and replacement of the host rocks. – The mineral paragenesis and metal geochemistry. Early precipitation of W-bearing minerals (wolframite and scheelite) in K-feldspar– quartz veins, followed by a later episode of Au–Bi–Te precipitation associated with sericite, chlorite–carbonate alteration that infills fractures and replaces the earlier veins. The low sulphide content (b5 vol.%), with a reduced ore mineral assemblage comprised of arsenopyrite, pyrrhotite, pyrite and chalcopyrite, and bismuth minerals with Bi/Te(S+Se)≥1, consistent with strongly to moderately reduced conditions. Intrusion-related gold deposits (IRGD) have achieved recognition as a different style of gold mineralization with significant gold endowment (i.e. multi-million oz deposits). Hence, the analogies of the Linares deposit with IRGDs encourage exploration in similar settings in the Navelgas gold belt and other gold belts of the NW of the Iberian Peninsula. Acknowledgements The authors thank Río Narcea Gold Mines for data provided and financial support. This work has also been financed by the Science and Innovation Ministry of Spain (project CGL-2011-23219). References Aller, J., Bastida, F., 1993. Anatomy of Mondonedo Nappe basal shear zone (NW Spain). Journal of Structural Geology 15, 1405–1419. Alonso, J.L., Aller, J., Bastida, F., Marcos, A., Marquínez, J., Pérez-Estaún, A., Pulgar, J.A., 1990. Mapa geológico de España, 1:200000, Hoja n.° 2 (Avilés), Instituto Tecnológico Geominero de España, Madrid. Arcos, D., Soler, A., Delgado, J., 1995. Gold–copper deposit related with the Carlés granodiorite (NW Spain). In: Pasava, J., Kribek, B., Zak, K. (Eds.), Mineral Deposits: From Their Origin to Environmental Impacts. Balkema, Rotterdam, pp. 411–414. Bakke, A.A., 1995. The Fort Knox “porphyry” gold deposit: Structurally controlled stockwork and shear quartz vein, sulphide-poor mineralization hosted by Late Cretaceous pluton, east-central Alaska. In: Schroeter, T.G. (Ed.), Porphyry Deposits of the Northwestern Cordillera of North America: Canadian Institute of Mining and Metallurgy, vol. 46, pp. 795–802. Bakker, R.J., 1997. Clathrates: computer programs to calculate fluid inclusion V–X properties using clathrate melting temperatures. Computers & Geosciences 23, 1–18. Bakker, R.J., 1998. Improvements in clathrate modelling II: The H2O–CO2–CH4–N2–C2H6 fluid system. In: Henriet, J.P., Mienert, J. (Eds.), Gas Hydrates: Relevance to World Margin Stability and Climate Change: Geological Society, London, Special Publications, 137, pp. 75–105. Bakker, R.J., 1999. Adaption of Bowers & Helgeson (1983) equation of state to isochore and fugacity coefficient calculation in the H2O–CO2–CH4–N2–NaCl fluid system. Chemical Geology 154, 225–236. Bakker, R.J., 2003. Package FLUIDS 1. New computer programs for the analysis of fluid inclusion data and for modelling bulk fluid properties. Chemical Geology 194, 3–23. Bakker, R.J., Brown, P., 2003. Computer modelling in fluid inclusion research. In: Samson, I., Anderson, A., Marshall, D. (Eds.), Fluid Inclusions: Analysis and Interpretation: Mineralogical Association of Canada, Short Course, 32, pp. 175–212. Bakker, R.J., Dubessy, J., Cathelineau, M., 1996. Improvements in clathrate modelling I: the H2O–CO2 system with various salts. Geochimica et Cosmochimica Acta 60, 1657–1681. Bastida, F., Martínez Catalán, J.R., Pulgar, J.A., 1986. Structural, metamorphic and magmatic history of the Mondonedo nappe (Hercynian belt, NW Spain). Journal of Structural Geology 8, 415–430. Bebout, G.E., Cooper, D.C., Bradley, A.D., Sadofsky, S.J., 1999. Nitrogen-isotope record of fluid–rock interactions in the Skiddaw aureole and granite, English Lake District. American Mineralogist 84, 1495–1505. Blevin, P.L., 2004. Redox and compositional parameters for interpreting the granitoid metallogeny of eastern Australia: implications for gold-rich ore systems. Resource Geology 54 (3), 241–252. Bodnar, R.J., Vityk, M.O., 1994. Interpretation of microthermometric data for H2O–NaCl fluid inclusions. In: De Vivo, B., Frezzotti, M.L. (Eds.), Fluid Inclusions in Minerals. :

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