Gold-bearing hedenbergite skarns from the SW contact of the Andorra granite (Central Pyrenees, Spain)

Mineral. Deposita 25 [Suppl] S 59-$68 (1990) MINERALIUM DEPOSITA

�9 Springer-Verlag 1990

Gold-bearing hedenbergite skarns from the SW contact of the Andorra granite (Central Pyrenees, Spain) A. Soler 1, C. Ayora 3, E. Cardellach 2, and J. Delgado 1

1 Departament de Cristallografia, Mineralogia i Dip6sits Minerals, Universitat de Barcelona 2 Departament de Geologia, Universitat Aut6noma de Barcelona 3 Institut da Ciencies de la Terra (J. Almera), CSIC, Barcelona, Spain

Received: December 1989/Accepted: August 1, 1990

Abstract. Several varieties of skarn outcrop have been found to develop along the contact between the Andorra granite and the Devonian limestones. The skarns contain variable amounts of gold ranging up to 5 g/t, always associated with sulphides. The sulphides in the skarn include arsenopyrite and pyrrhotite with lesser amounts of chalcopyrite, galena, sphalerite and Bi-minerals. Geothermometric and geobarometric data indicate the skarns formed at about 2 kbar and temperatures ranging from 500 ~ to 350 ~ from CO2-free polysaline brines at a slightly acidic pH and oxygen fugacity which decreases with time from the pyrite-pyrrhotite-magnetite towards the QFM buffer. Available data on gold solubility suggest that sufficient quantities of gold to form an ore deposit could have been transported as AuCI~- at the high temperatures and salinities under which the skarns formed. Both gold deposition and sulphide precipitation could have occurred due to a decrease in temperature and/or oxygen fugacity.

Although skarn lithologies are not an important source of gold worldwide, gold can be obtained as a byproduct of other metals, especially copper (Shimazaki 1980). In fact, a great number of gold-bearing skarns are reported in the literature, generally associated with intrusions of acidic to intermediate composition (Boyle 1979; Meiner 1989). In Japan, gold is preferentially concentrated in the epidote-rich oxidized skarns, at the contact of magnetite- bearing granitic intrusions (Sato 1980). On the other hand, reduced skarns with hedenbergite and sulphides as the dominant minerals have also been reported to contain significant gold grades (Boyle 1979; Meinert 1989; Ewers and Sun 1989). The skarns described by Brown and Nes- bitt (1987) in the Tombstone Mountains, Yukon, and those mined at the Nickel Plate Mine, British Columbia (Lamb et al. 1957) with an average grade of 13.7 g/t, show a mineral association similar to the skarns described in Central Pyrenees. Although skarns, many of which host important tungsten deposits such as Salau

Costabona (Soler 1977; Guy 1979; Guy et al. 1988; Fonteilles et al. 1989), are a common feature associated with the Hercynian intrusions of the Pyrenees, significant gold contents have not been reported until now in these lithologies. The Hercynian gold occurrences which have been described in the Pyrenees consist of disseminations in pre-Caradocian metapelites or of remobilizations in fractures and thrusts (Ayora and Casas 1986). To our knowledge, the mineralizations described here are, to- gether with massive arsenopyrite bodies (Soler and Ayora 1989), the first gold-bearing skarns described in the western Mediterranean Hercynian orogen. They represent a new type of hydrothermal remobilization of gold, in a region where a considerable number of gold occurrences are contained in the series.

Geological setting

The hedenbergite and pyrrhotite skarns are situated along the SW contact of the Andorra granite. They occur whithin a fragment of the Hercynian orogen which outcrops in the axial zone of Pyrenees. The region is made up primarily of Paleozoic metasediments which are intruded Hercynian granites (Fig. 1). The lower Paleozoic series, ranging in age from pre-Caradocian to Silurian, are composed mainly of siliciclastic sediments (clay, sandstones, black shales), whereas the Devonian consists primarily of limestones and marls (Hartevelt 1970).

The first Hercynian tectonic event to be recognized is an episode of E-W trending folds, with associated axial plane foliation and low-grade metamorphism characterized by the extensive development of chlorite and muscovite. A number of Hercynian thrusts have developed within the Silurian black shales and the Devonian marls, resulting in a thick pile of repeated Devonian limestones and marls, interlayered with thin wedges of black shales units. Graphite has not been found in the carbonates although it is present within the balck shale units. The limestones are the host rocks for the contact metamorphic calcsilicates and skarns which developed later. The

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STUDIED AREA

northern edge of this sequence of rocks is marked by an Hercynian out-of-sequence thrust, that rises over the pre- Caradocian sediments (Casas et al. 1989). The Andorra granite intrudes the structures described above. It is a homogeneous stock made up primarily of biotite-bearing granodiorite and hornblende-biotite-bearing granodiorite, accompanied locally by more highly differentiated granitic facies (Soler and Enrique 1989). The age of the intrusion has been estimated by Vitrac-Michard and AI- 16gre (1975) to be 275 Ma based on Rb/Sr isotopic data (r = 1.39 x 10 11 year- 1). A set of basic dykes (lampro- phyres of Zwart (1965)) crosscut the granites.

Like the rest of the Hercynian granites of the region (Autran 1980), all of the described igneous intrusives belong to the calcalkaline suite. In particular, the Andorra- Mont Lluis granite is interpreted by Vitrac-Michard and Allegre (1975) to be the result of mixing between mantle- derived and crustally derived material. The intrusion of the granite produced a 300 m wide contact aureole of thermal metamorphism (Fig. 1). Andalusite and/or sillimanite are the most characteristic silicates in the pelitic lithologies close to the contact, whereas biotite and cordierite developed further away. In the impure limestone beds, contact metamorphism results in the growth of fine-grained silicates (garnet, idocrase, diopside, epidote, biotite and K feldspar).

The contact between the granite and the Devonian carbonates is sharp, mostly cutting across bedding planes, thus suggesting that the intrusion took place under conditions of brittle deformation. The intrusion was emplaced close to 600 ~ and 2 kbar as indicated by the coexistence of andalusite, sillimanite, K-feldspar, muscovite and quartz, assuming pure water to be the fluid phase (Kerrick 1972). The assumption of a slightly less water-rich fluid phase results in a slight drop in the temperature to 550 ~ and an increase in pressure to 2.3 kbar.

Pyrrhotite skarn

Hedenbergite skarn

Granodiorite

Contact Metamorphism

Limestones

Sfliciclastics Fig. l. Geological map of the SW contact of the Andorra granite showing the skarn outcroppings

A thin band of massive fine-grained calcsilicates is found to be restricted to the contact between limestones and granite. Only where centimentre to metre wide granitic dykes intrude the carbonates along bedding plane joints, metre to tens-of metres thick skarns are developed. The granitic dykes are typically transformed en- tirely into endoskarns, while the carbonate rocks are con- verted into exoskarns. In our opinion, the presence of this particular structural setting is a favourable exploration guide for skarns in the region.

Accordingly to their mineralogy, the skarns of the SW contact of the Andorra granite are crudely zoned from north to south: (1) idocrase with wollastonite skarns; (2) hedenbergite skarns; (3) pyrrhotite skarns; (4) arsenopyrite skarns; and (5) magnetite skarns. Gold has been detected in arsenopyrite, pyrrhotite and hedenbergite skarns, whereas magnetite and idocrase skarns are barren, although in one locality the later contain a tungsten mineralization. The gold-bearing skarns which have been recognized are tens of metres in size and contain grades which are irregularly distributed and considerably lower in the pyrrhotite skarns. Nevertheless, the great number of recognized sites, despite the lack of mining works and the limited outcropping conditions, make the area potentially interesting as an exploration target. The aim of this paper is to describe the most important geological and geochemical features of the hedenbergite and pyrrhotite skarns, as well as discussing criteria to distin- guish potentially higher-grade lithologies based on their ore-forming conditions.

Hedenbergite skarns

Six outcrops of hedenbergite skarns ranging from hec- tometres to metres in width have been recognized. They

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Fig. 2. Geological cross-section of a hedenbergite skarn, as well as a detail of the most interesting mineral stage (labelled as) showing the skarn crosscutting the carbonate bedding. l - limestone; 2 - granodiorite; 3 - fine-grained calcsilicate; 4 - hedenbergite, garnet and quartz + calcite + sulphide; 5 - epidote; 6 - early quartz + calcite; 7 - hedenbergite; 8 - garnet; 9 - quartz + calcite + chlorite + sulphides

Contact Metasomatism metamorphism Exoskarn

Calcite Diopside Idiocrase Biotite Quartz Hedenbergite Garnet m m

AUanite Epidote Actinolite Chlorite Phyrrhotite L611ingite Arsenopyrite Chalcopyrite Sphalerite Cosalite STeBi 2 phase Galena Native bismutl" Native gold Pyrite Endoskarn

Albite 1 K.felspar Muscovite m Calcite m m m

Chlorite m m m m

fitanite m

Fig. 3. Mineral paragenesis of hedenbergite skarns

immmm

m m

m

m

mmm

m m m m

are located at the contact between the limestones and the granodiorite and are labelled as Sk- l l , Sk-124, Sk-413, Sk-475, Sk-478 and Sk-480 in Fig. 1. Dykes ofgranodior- ite also intrude through the limestone along bedding planes and generate small skarn bodies (Fig. 2). The skarns are made up dominantly of calcsilicates with dis- seminated sulphides. The mineral paragenesis is shown in Fig. 3.

Calcsilicate stage

All the outcrops show a more or less complete zonation. The various silicates correspond to different metasomatic

fronts in the same sense as those described by Guy (1979) in the Costabona skarn.

1. An early zone of quartz and calcite is found at the contact with the limestone. Wollastonite has never been observed. 2. The hedenbergite zone is the most abundant, making up as much as 90% of the volume of the skarn. It is composed of radiating aggregates of decimeter-size crystals of hedenbergite, which range in composition from Wo45 Di2o Hd35 to Wo43 Di 2 Hd55. 3. Garnet with a core composition Ad22 Gr75 AI2 and a rim composition of Adloo Gro Alo is found replacing hedenbergite, both as cross-cutting veinlets or as poikilo- blasts surrounding hedenbergite. The andraditic garnets are Sn-bearing. Epidote is occasionally found as a later mineral.

Sulphide stage

l. All the skarns are crosscut by veinlets of a later paragenesis of quartz, calcite and chlorite. Gold and the rest of sulphides are found associated with this paragenesis. 2. The sulphides are irregularly distributed in the calcsilicates, never forming the massive bodies characteristic of the arsenopyrite and pyrrhotite skarns in the area. In the hedenbergite skarns, the arsenopyrite occurs as isolated crystals in an aggregate of pyrrhotite. A core of corroded 1611ingite can be found in some skarns, indicating that the reaction 1611ingite plus pyrrhotite to form arsenopyrite took place. The minerals of the As-S-Fe system are fol- lowed by a polymetallic association which includes galena, sphalerite, chatcopyrite, molybdenite and locally cosalite and an unnamed phase of composition STeBi 2 . Graphite accompanies this later association of sulphides. Magnetite is also present in one skarn. Silver-bearing gold grains can be found accompanying all the sulphides of this stage. Gold grades from random sampling in several outcrops are variable, never reaching 6 g/t.

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Fluid inclusions

The silicate minerals which occur in the skarns described above are not transparent enough to enable a systematic fluid inclusion study. Nevertheless, 30 inclusions ranging in size from 5 to 30 jam have been observed in quartz of the latest quartz, calcite, chlorite and sulphide stage. They are aqueous two-phase inclusions (degree of fill- ings0.8) with irregular shapes which form secondary arrays crosscutting the quartz-grain boundaries.

After freezing the inclusions, the first melting temperature of a solid (eutectic temperature) has been observed to range between 64 ~ and 54~ indicating the existence of a polysaline system made up of primarily CaC12, NaC1 and HzO (Crawford 1981). The small size of the inclusions precludes the hydrohalite melting from being observed and thus it is not possible to estimate the pro- portions of the different salts in the inclusions. The last crystal of ice melts (Tml) at-19.5 +_2.5 ~ in most of the inclusions, indicating salinities as high as 22.3_+2 w% eq. NaCI (5M NaCI). It was not possible to detect either two phases of CO 2 or clathrate, CO2 4.75 H 2 0 , during the microthermometrical measurements. Crushing-stage tests in glycerine also gave no evidence of an inmiscible CO2-rich phase. This suggests that CO2 fugacities were very low in the fluids which filled the inclusions. There- fore because of the low CO2 contents and because of the lack of experimental data on salt systems other than NaC1-H20, the calculations which follow treat the NaC1- H 2 0 system.

Homogenization temperatures (Tn) scatter over a range of a hundred degrees, but the lack of correlation between Tmi and T, suggests that the changes in density are due to post-trapping recrystallization, rather than to the existence of different fluids. The majority of measurements can be grouped at 190_20~ and this temperature has been considered as representative in the fluid density calculation (1.05 g/cm 3 ).

The corresponding isochore (Vm=20.08 cm3/mol, P(bar) = 10.60 T(K)-4910) has been calculated according to the data of Potter and Brown (1977), computerized in the HALWAT program by Nicholls and Crawford (1985). Accounting for the standard deviation of the T, values, and for a confining pressure of about 2 kbar, the trapping temperature of the fluids can be estimated to be close to 380_+ 30 ~

From textural criteria, the fluid inclusions examined here appear to be secondary with regard to quartz crys- tallization. We still considered them, however, to represent fluids related to the formation of the quartz-chlorite- sulphide stage, for the following reasons: (1) the temperature and chemistry of the fluids are very similar to those reported in skarn processes (Einaudi et al. 1981; Kwak 1986; Guy 1989; Ewers and Sun 1989); (2) the quartz- chlorite-sulphide stage is the last recognized stage of mineralization and no evidence of later hydrothermal processes has been observed in this particular outcrop; (3) the temperature deduced from microthermometrical data is consistent with the temperature of formation of the quartz-chlorite-sulphide stage deduced from other inde-

pendent thermometric criteria (i.e. mineral equilibria, isotope sulphide pairs, see below).

Pyrrhotite skarns

The four recognized outcrops of this type of skarn are labelled as Sk-337, Sk-356, Sk-360, Sk-392 and Sk-869 in Fig. 1. The outcrops consist primarily of exoskarns situated at the contact between the granite and the limestone, particularly where this contact is perpendicular to the carbonate bedding planes. Widths of the outcrops are a metre to a hectometere in size, although limits of the outcrops cannot be clearly observed in most cases. The skarns consist of a core of massive pyrrhotite enveloped by fine-grained silicates.

Calcsilicate stage

1. Idocrase forms aggregates of idiomorphic crystals dis- playing a well-developed optical zonation corresponding to A1-Ti substitution. Wollastonite relicts have been observed within the idocrase in some skarns. 2. Garnet of composition Ad49 Gr49 A12 replaces idocrase and locally diopside of composition Wo43 Di43 Hd 6.

Sulphide stage

1. As in the above described skarns, a later paragenesis of quartz, calcite, chlorite, titanite and sulphides replaces the calsilicate along veinlets. 2. The sulphide stage is characterized by massive aggregates of pyrrhotite accompanied by minor amounts of other sulphides. Arsenopyrite crystals occur rarely and, as in their occurrence in the hedenbergite skarns, they contain a corroded core of 1611ingite (Fig. 4). Minor amounts of pyrite, chalcopyrite, galena, sphalerite, native bismuth and the STeBi 2 phase can also be found dissem- inated in pyrrhotite. No gold has been microscopically observed in this stage, and gold grades of samples from several outcrops are always below 0.8 g/t. Although a

Fig. 4. Photomicrograph of an arsenopyrite crystal in pyrrhotite, including a corroded 1611ingitc core

more systematic sampling should be carried out, the pre- liminary results suggest that this type of skarn tends to be much lower grade than the arsenopyrite and hedenbergite skarns.

Ore-forming conditions

As is evident from the mineral described above, the skarns were formed under a wide range of conditions. The skarns evolved from calcsilicates to a final association of quartz, calcite, sulphides and hydrated silicates (chlorite), classically known as hydrothermal or retro- grade skarn facies (Einaudi et al. 1981; Kwak 1986).

An upper limit for the temperature of the early silicates in the hedenbergite skarn can be established by the absence ofwollastonite forming from quartz and calcite. No data on the Xco 2 of the coexisting fluid has been obtained, but assuming an upper limit of 0.1, common in many skarns (Einaudi et al. 1981) and more specifically in the skarns from the Pyrenees (Guy 1989), a maximum temperature of 550 ~ can be estimated (Fig. 5). On the contrary, the presence of wollastonite within the idocrase of some pyrrhotite skarn indicates that the fluids which formed these skarns were either characterized by higher temperatures or a lower Xco 2.

In addition to the fluid inclusion microthermometry, data on arsenopyrite and chlorite thermochemistry, have been obtained. The As/S ratio of the arsenopyrite coexisting with other phases in the system As-S-Fe has been experimentally callibrated as a geothermometer (Kretschmar and Scott 1976) and successfully applied to several types of ore deposits (Berglund and Ekstrom 1978; Gole 1980; Lowell and Gasparrini 1982). A confining pressure of 2 kbar raises the temperature at which mineral equilibrate by less than 10 ~ (Sharp et al. 1985), whithin analytical error. Therefore, the pressure effect has not been considered. Arsenopyrite from several localities of both types of skarns has been analyzed by means of EDS using the standards ASP-57, ASP-200 (Kretschmar and Scott 1976) and RB-ORL (MoElo et al. 1985). Co, Ni, Cu and Sb have not been detected. The isopleths of atomic As are projected onto a temperature versus sulphur fugacity diagram (Fig. 6).

Conditions close to the pyrrhotite + 1611ingite + arsenopyrite equilibria have been assumed for the arsenopyrite which contains relicts of 1611ingite. As 1611ingite was considered as pure As2Fe in the experimental calibration, it is not possible to confirm, from the S content of this mineral, that this assemblage was actually in equilibrium. In the skarns where 1611ingite is absent, the stability limit of this mineral and the presence of bismuth and the absence of bismuthinite, constrains the log as2-T field en- closed in the arsenopyrite isopleths (Fig. 6).

As indicated in Fig. 6, the arsenopyrite from the pyrrhotite skarns shows temperatures of 440_40~ whereas the arsenopyrite from the hedenbergite skarns displays distinctly lower temperatures, of the order of 370+_50~ This temperature is consistent with that which is deduced from galena-sphalerite isotopic geothermometry (Soler 1990). The presence ofcosalite, a mineral which is unstable above 425 ~ (Craig and Bar-

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400

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Fig. 5. Diagram of the mineral equilibria constraining the conditions of formation of the calcsilicate stage in the skarns. Data for the mineral equilibria are from Powell and Holland (1988) and Holland and Powell (1990)

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Fig. 6. Isopleths of atomic As in arsenopyrite in a sulphur fugacity versus temperature diagram. Data for the mineral equilibria are from Barton and Skinner (1979). Kretschmar and Scott (1976) and Craig and Barton (1973). L o - 1611ingite; p o - pyrrhotite; a s p y -

arsenopyrite; p y - pyrite; l - liquid As; c o - cosalite; g a - galena; b n - bornite; c y p - chalcopyrite

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Table 1. Microprobe analyses of chlorite from hedenbergite (Hd) and pyrrhotite (Po) skarns

Sample skarn 187-1 187-2 187-3 187-4 187-5 187-6 187-7 187-8 187-9 187-10 Hd Hd Hd Hd Hd Hd Hd Hd Hd Hd

% Weight FeO 33.54 33.43 34.25 34.75 38.92 39.31 39.26 40.17 38.95 39.59 NaO 0.00 0.03 0.00 0.01 0.05 0.02 0.04 0,00 0.01 0.01 K20 0.02 0.03 0.00 0.03 0.00 0.00 0.04 0,00 0.06 0.05 MnO 0.87 0.77 0.82 0.79 1.73 1.48 1.52 1.33 1.67 1.44 MgO 9.42 9.89 9.11 8.23 4.01 3.90 3.21 3.18 2.90 2.65 CaO 0.13 0.10 0.03 0.04 0.00 0.00 0.03 0.01 0.21 0.05 SiO 2 24.97 25.39 24.51 23.91 23.00 23.15 22.89 23.33 22.87 22.58 TiO 2 0.07 0.06 0.05 0.09 0.01 0.06 0.00 0.07 0.09 0.06 AI20 2 17.80 17.71 18.15 18.47 20.61 20.90 20.63 21.28 21.13 21.24

Structural formula Si(T) 2.81 2.83 2.77 2.73 2.63 2.63 2.65 2.64 2.64 2.61 AI(T) 1.19 1.17 1.23 1.27 1.37 1.37 1.36 1.36 1.36 1.39 Total(T) 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00

AI(O) 1.18 1.16 1.18 1.22 1.41 1.43 1.46 1.48 1.51 1.51 Mg(O) 1.67 1.72 1.61 1.48 0.85 0.80 0.70 0.66 0.66 0.60 Fe+2(O) 3.16 3.12 3.23 3.32 3.72 3.74 3.79 3.80 3.76 3.83 Total(O) 6.01 6.00 6.02 6.02 5.98 5.97 5.95 5.94 5.93 5.94

Sample skarn 126-1 126-2 126-3 392-1 392-2 392-3 392-4 392-5 369-1 369-2 369-3 Hd Hd Hd Po Po Po Po Po Po Po Po

% Weight FeO 28.90 29.62 29.01 40.07 40.74 28.62 37.58 36.72 41.83 42.73 39.72 NaO 0.00 0.02 0.02 0.00 0.04 0.04 0.01 0.06 0.00 0.00 0.05 K20 0.00 0.00 0.01 0.09 0.07 0.05 0.16 0.03 0.00 0.07 0.00 MnO 2.55 2.26 2.50 0,21 0.41 1.84 1.38 0.16 0.00 0.00 0.00 MgO 11.19 11.42 10.93 1.17 0.87 12.07 5.23 4.43 3.51 2.64 3.53 CaO 0.11 0.22 0.16 0.03 0.07 0.04 0.07 0.05 0.11 0.09 0.00 SiO 2 24.88 25.42 24.85 21.74 21.29 25.61 23.69 22.41 25.81 30.02 24.55 TiO 2 0.11 0.03 0.03 0.03 0.06 0.00 0.03 0.00 0.00 0.10 0.06 Al203 19,08 18.67 18.10 24.13 24.01 19.65 20.27 24.14 17.79 11.48 20.41

Structural formula Si(T) 2,75 2.79 2.80 2.60 2.74 2.79 2.78 2.91 2.93 3.50 2.78 AI(T) 1,25 1.21 1.20 1.40 1.26 1.21 1.22 1.09 1.07 0.50 1.22 Total(T) 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00

AI(O) 1.24 1.21 1.20 1.53 1.34 1.33 1.30 1.14 1.31 1.08 1.50 Mg(O) 2.09 2.08 2.07 0.64 0.92 0.84 0.89 1.12 0.59 0.46 0.60 Fe+2(O) 2.68 2.72 2.73 3.76 3.69 3.77 3.77 3.72 3.97 4.17 3.76 Total(O) 6.01 6.01 6.00 5.93 5.95 5.94 5.96 5.98 5.87 5.71 5.86

"lhble 2. Parametres of M6ssbauer spectra and resulting Fe 3 +/(Fe 3 + +Fe 2+)

Sample skarn Fe 3 f Fe 2 § Fe 3 + / Fe(t)

I.S. Q.S. I.S. Q.S.

368 Pyrrhotite 0.41(1) 0.84(2) 1.11(1) 2.63(1) 0.197(5) 187d Hedenbergite 0.35(I) 0.86(2) 1.10(2) 2.60(3) 0.239(8)

ton 1973), in the hedenbergite skarns also supports lower temperatures for this type of skarn. Lower temperatures for the silicate stage in this type of skarn are also indicated by the early presence of quar tz and calcite instead of wollastonite.

Wi th in the same range of su lphur fugacity, the temperatures of the massive arsenopyri te skarns of the area plot close to 450 _+ 50 ~ (Soler and Ayora 1989), between

the temperatures calculated for the hedenbergi te and pyrrhot i te skarns.

Chlorites from the sulphide stage in the hedenbergi te and pyrrhot i te skarns have been analysed by convent ional electron microprobe with a wave dispersive system. Based on the results presented in Table 1, the chlorites can be considered to belong to the chamosi te- ta lcbruci te series (Stoessell 1984). M6ssbauer spectra for a selected chlorite from each type of skarn have been obta ined at room temperature , with conven t iona l equ ipmen t and ad- j u s tmen t condi t ions described in Soler and Ayora (1989). In agreement with l i terature references (Coey et al. 1974; Blaauw et al. 1980) and with the chlorite spectra from the arsenopyri te skarns (Soler and Ayora 1989), no Fe 3+ have been detected in tetrahedral sites. A mean Fe 3+/ (Fe 2 + + Fe 3 + ) value of 0.2 (Table 2), coinciding with that for the arsenopyri te skarn, has been assumed in the calculation.

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T: 400"C

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Po

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Aspy*As Lo

' = [ ] I .

11 HEDENBERGITE SKARP

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X i X r

Mg Hm

1 -30 -25 -20 IogoO 2

Fig. 7, Chlorite compositions and observed sulphide associations on a sulphur versus oxygen fugacity diagram at 400 ~ Data for mineral equilibria from Barton and Skinner (1979), Kretschmar and Scott (1976) and Ohmoto and Kerrich (1977). Fugacity coefficient for CO z from SUPCRT data base. Q F M - quartz + fayalite + magnetite; p y - pyrite; a s p y - arsenopyrite; p o - pyrrhotite; 1o - 1611in- gite; r n g - magnetite; h m - hematite

Thermochemical calculations for chlorite are based on a six-component solid-solution model (Walshe 1986). Modifications of the molar fraction and activity of some components have been introduced based on the absence o f F e 3 + in the tetrahedral sites, and in agreement with the results for natural assemblages (Soler and Ayora 1989). At a particular temperature, the oxygen fugacity of the fluid has been calculated assuming equilibria between Fe z +- and Fe 3 +-bearing components using the chemical analyses and the Fe 3 +/(Fe 2 + + Fe 3 + ) ratio as input data. The sulphur fugacity of the fluid has been derived assuming equilibria between the Fe 2 +-bearing component and the iron sulphide. Further discussion, as well as some details of the calculations, may be found in Soler and Ayora (1989).

As shown in Fig. 7, the chlorite analyses indicate that oxygen fugacities fall between the pyrite-pyrrhotite-magnetite (PPM) and the quarzt-fayalite-magnetite (QFM) buffers. In the hedenbergite skarns, and especially in the arsenopyrite skarns (Soler and Ayora 1989), where textural evidence is available, the chlorite plots indicate a

- 2 6

- 2 8

- 3 O

_0" - 3 2

- -34

- 3 6

10.0 1.0 / / 03

i / i / / AuCI~ /

/ / / PPM

/ ~ / / ~ ~ / 0.01

/ ~ i / ~ . "~ / ~.1.0 " 4 j<. . oF. / --~ / , / " ~ , Alpy~ Lt~+Po

/ ~ ~ \ 0.1 /

/ ~ / ~u (Hr

/ ~ ~'~ "~" ~O,01 / Jr /

E ~- 0001

4 5 6 7 pH

Fig. 8. Solubility contours of gold (in ppb) as Au(HS) 2 (data from Schenberger and Barnes 1989) and as AuCI s (data from Nikolaeva et al. 1972; referred to in the op. cit.) in a oxygen fugacity versus pH diagram constructed at 350~ [C1]=5 M; aHzS buffered by the pyrrhotite+magnetite equilibrium. The a r r o w describes the varia- tion of ore-forming conditions with time. Data from Montoya and Hemley (1975) and Barton and Skinner (1979). P P M - pyrite+ pyrrhotite + magnetite; Q F M - quartz + fayalite + magnetite; m s -

muscovite; k f - K-feldspar; q - quartz; a s p y - arsenopyrite; l o -

1611ingite; p o - pyrrhotite

decrease in the oxygen fugacity of the system from PPM to QFM buffer with time. The association of the latter chlorites with the sulphide stage (between the Bi-Bi2S 3 and the arsenopyrite-1611ingite-pyrrhotite equilibria), as well as locally with graphite, confirms their lower oxygen fugacity. On the other hand, chlorites from the pyrrhotite skarns, which plot close to the Q F M buffer, do not dis- play an evolution with time.

Transport and deposition of gold

The effect of the chemical composition of the fluid phase on gold solubility will be discussed first at a constant temperature of 350 ~ (Fig. 8), because it is the highest temperature at which experimental data is available (Shenberger and Barnes 1989). A pH near 5 has been assumed based on the coexistence of quartz, muscovite and K-feldspar as common minerals in the metasomatic silicate association (the equilibrium constant has been taken from the experimental data of Montoya and Hem- ley [1975]). The solubility data of calcite (Bowers et al. 1984), a mineral also found in the ore association, also indicates a similar range of pH. The activity of the electrolytes in solution is assumed to be 5 M NaC1 equiva- lent, based on measurements from one hedenbergite skarn sample (see fluid inclusions). This represents an intermediate value which falls in the large gap of salinities reported in skarns (Einaudi etal . 1981; Kwak 1986). Nevertheless, the influence of total salinity on gold solubility is discussed below. The proport ion of the different electrolytes are taken from the analyses of leachates from the Costabona skarn (Guy 1989). However, variations in the proportion of the cations have a very small effect on

S 66

the gold solubility. The activity coefficients of the species in solution have been obtained from the extended Debye- Huckel equation proposed by Helgeson and Kirkham (1974).

According to experimental and literature data referred to in Shenberger and Barnes (1989), at the conditions deduced for the chlorite-quartz-ore stage, gold can be transported as both As(HS)2 and AuC12 complexes (Fig. 8). By comparing the grades required to form a deposit (Barnes 1979) and the base metal contents in solution, it appears that the amount of gold present in solution may be sufficient provided that an efficient and long-lived precipitation process occurs.

The decrease in the oxygen fugacity established above from the PPM to the QFM buffer causes a ten-fold drop in the gold solubility in both the sulphur and the chloride gold complexes and may provide an effective precipitation mechanism. On the contrary, small changes in pH do not greatly affect gold solubility. This model of decrease of oxygen fugacity as a cause of gold precipitation is also supported by the association of the gold mineralization with sulphides and locally with graphite.

On the other hand, most of the skarn formation described above, including the stage with which gold is associated, took place at temperatures above 350~ If the gold solubility data is extrapolated above 350 ~ (Fig. 9), it appears that AuC12 is by far the most dominant complex accounting for gold in solution, particularly in highly concentrated brines, such as those commonly found in skarns. Because of the temperature-dependence of the AuC12 complex, cooling of the solution may be the most efficient mechanism of gold deposition.

Changes in the chloride content also affect gold solubility (6[AU]ppm:2k[Cl]mo16[C1]mol, being k=2.1 • 10 -3 at the conditions established above). Thus, if we suppose a solution with 7M or with 3M of total salinity (both reported commonly in skarns) gold solubility would in-

0

- 1

~0 v - 2

< - 3

_ / - 4

-5 i l , 150 200 250 300 350 400

tempera ture (~ Fig. 9. Temperature dependence of gold solubility as Au(HS)2 and AuC] 2 complexes (data from Schemberger and Barnes 1989; and from Nikolaeva et al. 1972, referred to in the op. cit.). [C1]=5 M; oxygen and sulfur fugacity buffered by the pyri te+pyrrhoti te+ magnetite equilibria; pH buffered by the muscovite + K-feldspar + quartz equilibria

crease to twice or decrease to 1/3 respectively, but the above discussion on the gold precipitation mechanism is equally valid. It also becomes apparent from the solubility data that a dilution of the total CI in the solution could cause precipitation of gold, although, in this case, this is a less efficient mechanism than a temperature and oxygen fugacity drop.

As many metals can be also transported as chloride complexes, (Helgeson 1969; Crerar and Barnes 1976; Se- ward 1976, 1984; Barnes 1979) and show a similar de- creasing solubility with temperature pattern, gold will also be precipitated with all the metal sulphides in this case. Although further experimental work on gold solubility at high temperatures is required, chloride complexes appear to be responsible for gold transport at high temperatures and salinities. Deposition is probably due to a drop in temperature and/or a decrease in the oxygen fugacity, with both mechanisms leading to the close association of gold and metal sulphides.

Conclusions

Several gold-bearing skarns have been discovered along the SW contact of the Andorra granite. They are the first of this type of occurrence cited in the Hercynian outcrops of the Pyrenees. Despite their relatively small size, the large number of outcrops within a small area suggest that they deserve further investigation. It appears that perpendicular contacts between the limestone bedding and the intrusive contacts can be used as a prospecting criterium. The many granite dykes which crosscut the limestone bedding in these particular areas are also favorable targets.

The gold-bearing skarns are characterized by two stages: an early silicate stage and a later one which is rich in sulphides. The silicates found, such as hedenbergite, are typical of calcic-reduced skarns. Gold is found associated with sulphides, arsenides, native Bi and tellurides. The association of Au with Fe-rich hedenbergite and with bismuth minerals and tellurides, appears to be a common feature of Au skarns (Meinert 1989).

As established by mineral equilibria thermochemistry and fluid inclusion microthermometry, the sulphide association formed at about 2 kbar and temperatures of 500 ~ to 400 ~ The ore-forming fluid was a polysaline brine, with a pH close to the quartz+muscovite+K feldspar equilibrium and a sulphur fugacity ranging between the Bi-Bi2S3 and the arsenopyrite-1611ingite-pyrrhotite uni- variant curves. The oxygen fugacity of the fluid evolved from the pyrite-pyrrhotite-magnetite buffer in the early stages of skarn formation to the quartz-fayalite-magnetite buffer during the sulphide-forming stage, as indicated by the chlorite compositions and the sulphide and graphite stabilities which are determined.

According to the available experimental data on gold solubility, it appears that enough gold may be present in solution for a deposit to form at temperatures close to 350 ~ At temperatures above 350 ~ and the high salinities estimated for these skarns, the AuCI 2 complex is most likely to be the dominant gold-bearing complex.

S 67

Ei ther a decrease in t e m p e r a t u r e a n d / o r oxygen fugac i ty o f the f luid m a y have caused gold depos i t ion . These re- suits are no t in ag reemen t wi th the cr i ter ia o f M e ine r t (1989), who suggests for this type o f ska rn a m e c h a n i s m o f go ld p rec ip i t a t i on based on the o x i d a t i o n o f the Au(H2S ) - complexes , changes in p H and T being less i m p o r t a n t . In the case o f the skarns f rom the Cen t ra l Pyrenees , the s imi lar i ty in the b e h a v i o u r o f so lubi l i ty and tha t o f the base meta l s p resen t as ch lor ide complexes wou ld accoun t for the close a s soc ia t ion o f gold and sulphides . This cr i ter ia is in ag reemen t with the observa- t ional fea tures and should be cons ide red in the p rospec t - ing o f this type o f deposi t .

Acknowledgements. This research has been financed by the CICYT Project PB86-0572. Arsenopyrite standards have been kindly provided by S.D. Scott (University of Toronto) and Y. Moelo (CNRS Orleans). We also would like to thank C. Steefel (Yale University) and one of the anonymous reviewers whose suggestions helped to improve the manuscript.

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Documents

Gold-bearing hedenbergite skarns from the SW contact of the Andorra granite (Central Pyrenees, Spain)