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Economic Geology; January 2005; v. 100; no. 1; p. 115-130; DOI: 10.2113/100.1.0115
© 2005 Society of Economic Geologists
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Mineralogical and Stable Isotope Studies of Kaolin Deposits: Shallow Epithermal Systems of Western Sardinia, Italy

R. Simeone{dagger}

Via Corvetto 33, 09014 Carloforte, Italy

J. H. Dilles

Department of Geosciences, Oregon State University, Corvallis, Oregon 97331

G. Padalino

Dipartimento di Geoingegneria e Tecnologie Ambientali, Università di Cagliari, Italy

M. Palomba

Istituto di Geologia Ambientale e Geoingegneria del C.N.R., Sez. di Cagliari, Università di Cagliari, Italy

{dagger} Corresponding author: e-mail, rossana.simeone{at}tiscali.it


   Abstract
Top
 Abstract
Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 
Large kaolin deposits hosted by Miocene silicic pyroclastic rocks in northwestern Sardinia represent hydrothermal alteration formed within 200 m of the Miocene paleosurface. Boiling hydrothermal fluids ascended steeply dipping faults that are enveloped by altered rock. The broadly stratiform kaolin deposits constitute advanced argillic alteration that was produced in a steam-heated zone near the paleosurface overlying the deeper hydrothermal systems. The deeper zones represent two distinct types of epithermal systems: weakly acidic (inferred low-sulfidation) systems at Tresnuraghes and acidic (high-sulfidation) systems at Romana.

Tresnuraghes is characterized at depth by chalcedony ± quartz ± barite veins within a 50-m-wide zone of K-feldspar-quartz-illite alteration and overlying local occurrences of chalcedony sinter, which define the paleosurface. Kaolin deposits near the paleosurface are characterized by zonation outward and downward from an inner shallow zone of kaolinite 1T-opal ± dickite ± alunite (<20-µm-diam grains) to an outer deeper kaolinite 1M-montmorillonite-cristobalite. This zonation indicates formation by descending acidic fluids. The system evolved from ascending weakly acidic or neutral fluids that boiled to produce H2S-rich vapor, which condensed and oxidized within the near-surface vadose zone to form steam-heated acid-sulfate waters and kaolin alteration.

At Romana, veins at depth contain chalcedony or quartz and minor pyrite and are enclosed in up to 20-m-wide zones of kaolinite 1T-quartz alteration. Near hydrothermal vents along the paleosurface, chalcedonic silica is enclosed within a zone of kaolinite 1T-alunite (<50-µm-diam grains)-quartz-opal ± dickite ± cristobalite. Kaolin quarries near the paleosurface display outward and downward zoning to kaolinite 1T-opal ± cristobalite and then to montmorillonite-kaolinite 1T ± opal, consistent with formation by descending low pH fluid. The siliceous and advanced argillic alteration along steep conduits formed from acidic ascending magmatic-hydrothermal fluids, whereas the near-surface kaolin formed from steam-heated meteoric waters.

Alteration mineral assemblages and stable isotope data provide evidence of the temperature and source of hydrothermal fluids. Barite from Tresnuraghes (average {delta}18O = 17.1{per thousand}, {delta}34S = 18.8{per thousand}), one alunite sample from Romana ({delta}18O = 12.0{per thousand}, {delta}D = –3{per thousand}, {delta}34S = 16.7{per thousand}), and quartz from both localities ({delta}18O = 15.9–22.0{per thousand}) formed in hydrothermal feeders. Source fluids were likely mixtures of meteoric water and minor magmatic fluid, similar to other epithermal systems. Kaolinite-dickite minerals from the kaolin deposits ({delta}18O = 16.6–21.4{per thousand}, {delta}D = –43 to –53{per thousand}) formed from steam-heated meteoric water having {delta}D = – 20 per mil, consistent with the presence of anomalous Hg and fine-grained Na- and Fe-poor alunite. The laterally extensive kaolin deposits in Sardinia, and possibly similar deposits elsewhere in the world, appear to represent the uppermost parts of large hydrothermal systems that may be prospects for gold at depth.


   Introduction
Top
 Abstract
 Introduction
Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 
IN SARDINIA, epithermal systems are hosted in and formed contemporaneously with Oligocene-Miocene calc-alkaline volcanic rocks that were erupted in a convergent margin setting. The volcanic products crop out along a 200-km-long, north-south–trending rift structure in western Sardinia. Two low-sulfidation epithermal precious metal prospects have been identified to date (Osilo and Montiferro) and one high-sulfidation deposit (Furtei; Fig. 1; Garbarino et al., 1991; Dessì et al., 1996; Ruggieri et al., 1997, Simeone and Simmons, 1999). Tertiary volcanic rocks are also host to kaolin deposits of hydrothermal origin (Murray, 1988; Garbarino et al., 1994) that formed synchronously with volcanism. The major kaolin deposits occur at Romana, Tresnuraghes, and Furtei (Fig. 1). The only operating kaolin mine is at the Locchera quarry in the Romana area. Kaolinite together with silica minerals and alunite in these deposits comprise the typical minerals of advanced argillic alteration. Similar alteration systems elsewhere in the world form in three different environments: in the halo of high-sulfidation gold (Cu-As-Au) deposits formed by hypogene magmatic-hydrothermal activity (Hayba et al., 1985; Stoffregen, 1987; Arribas et al., 1995), in the vadose zone in the steam-heated environments of high- and low-sulfidation systems (Schoen et al., 1974), and in the supergene environment above the water table (Sillitoe, 1993). Distinguishing between these environments is important because of their different genetic and spatial relationships to gold mineralization. Whereas Cu-Au mineralization is associated with the hypogene magmatic hydrothermal environment, the steam-heated and supergene environments are less commonly associated with the deposition of precious metals (Sillitoe, 1993). At Furtei, the kaolin deposits are genetically and spatially associated with a high-sulfidation gold deposit, and the geochemical and mineralogical evidence indicates a hypogene origin for the kaolinite (Marini et al., 1992; Ruggieri et al., 1997).


Figure 011601
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  		FIG. 1. Location of the main kaolinite-rich epithermal systems of Sardinia hosted in Oligocene-Miocene calc-alkaline volcanic rocks.

 
Previous studies of the Sardinian kaolinite deposits have examined geological, mineralogical, and geochemical features (Carboni et al., 1984; Ligas et al., 1996) with an emphasis on the properties of the clay minerals, their possible industrial use and economic potential. These areas have only recently been recognized as potential gold prospects (Dessì et al., 1996), and they are currently being investigated by the Sardinia Gold Mining Company (Gold Mines of Sardinia, 1999). This paper reports the results of mineralogical and stable isotope research undertaken to define the environment in the formation of the Romana and Tresnuraghes kaolin deposits.


   Regional Geology
Top
 Abstract
 Introduction
 Regional Geology
Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 
The Romana and Tresnuraghes areas are located in northwest Sardinia. Host rocks of the kaolin deposits consist of a sequence of Miocene volcanic rocks (Savelli et al., 1979; Beccaluva et al., 1989). These rocks were emplaced during the Oligocene-Miocene when the Sardinia-Corsica microplate separated from the paleo-European margin and rotated counterclockwise. Arc-related magmatism with calc-alkaline affinity evolved in response to a northwest-dipping subduction zone located east of Sardinia (Beccaluva et al., 1994).

Pre-Tertiary rocks are not exposed in these areas but are likely Hercynian-age granitoids, which are unconformably overlain by Carboniferous-Permian volcano-sedimentary rocks and Mesozoic carbonate rocks elsewhere in the region (Carmignani et al., 1989, 1992).

Younger rocks in the region are composed of late Miocene sedimentary sequences made up of volcaniclastic and marine deposits (Cherchi and Montadert, 1982), and these are unconformably overlain by Pliocene-Quaternary basalts.


   Tresnuraghes Area
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 
Local geology
In the Tresnuraghes area several kaolin deposits hosted by rhyolitic ignimbrites have been exploited by recent mining (Fig. 2). The ignimbrites consist of two main facies, with the older unit affected by hydrothermal alteration. The lithologies consist principally of pumice and ash and are mostly unwelded, with phenocrysts of plagioclase (andesine), minor quartz, and K-feldspar, and rare biotite in a cryptocrystalline matrix. A younger pyroclastic flow is strongly welded and rarely altered (Garbarino and Palomba, 1990). Miocene marine sedimentary rocks (Langhian-Serravalian) overlie the volcanic rocks and consist of conglomerate, sandstone, marl, and limestone (Cherchi, 1974). Quaternary basalt flows overlie the ignimbrites and Miocene sediments. Emplacement of both the Tertiary and Quaternary volcanic rocks was controlled by northeast-southwest– and north-northwest–south-southeast–trending rift-related faults. Late east-west–trending faults commonly displace the altered units.


Figure 011702
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  		FIG. 2. Geologic and alteration sketch map of the Tresnuraghes epithermal system (this study).

 
Hydrothermal alteration
The three larger kaolin deposits in the Tresnuraghes area (Salamura, Punta Allò, and Patalza) are located within an area of hydrothermal alteration that can be mapped over approximately 8 km2. The quality of the kaolin in these deposits is low and precludes commercial extraction. The primary texture of the pyroclastic rocks exposed in each quarry is commonly completely obscured but may be preserved in areas of less intense alteration. The intensity of hydrothermal alteration is variable and confined to less than 200 m below the present surface (Fig. 3). The common hydrothermal minerals, as determined by X-ray diffraction studies (App.), include kaolinite, alunite, silica minerals (opal CT, cristobalite, chalcedony and quartz), K-feldspar, montmorillonite, and minor illite, and barite. Kaolinite occurs in two types: poorly ordered (1Md, disordered, pseudomonoclinic) and well-ordered (1T, ordered triclinic; Deer et al., 1962). A total of six alteration types (Table 1) can be differentiated based on the spatial zoning and mineral association.


Figure 011703
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  		FIG. 3. Schematic section through the Tresnuraghes epithermal district, showing alteration zonation at the studied sites. For legend see Figure 2.

 

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  		TABLE 1 TABLE 1. Hydrothermal Alteration Features at Tresnuraghes

 
The hydrothermal alteration is zoned laterally from kaolinite-alunite-opal, close to vertical fractures that occur in the central part of the quarries, outward through to kaolinite-montmorillonite-cristobalite, and finally to fresh rock. The vertical zonation in the quarries is characterized by opal at the top, kaolinite-alunite-opal in the central part, and montmorillonite in the lower part accompanied by a decrease in the intensity of alteration downward. This zonation suggests a major structural control of the fluid flow and minor lithological control.

Silicified rock, siliceous veins, and sinter: Silica minerals at Tresnuraghes occur in veins and sinter and as pervasive silicification of the pyroclastic rocks. These include opal (as opal CT), chalcedony, and microcrystalline quartz. Opal and some chalcedony also closely associated with kaolinite. Opaline layers a few centimeters thick occur at the top of the alteration zones. At Porto Alabe, a chalcedony sinter, more than 10 m thick, is exposed in a gravel pit. It is characterized by well-developed horizontal laminations with columnar structures perpendicular to and between laminations (possibly evidence of bacterial activity during silica deposition). Plant stems 1 cm in diameter and up to 5 cm in length were also recognized. These stems are unequivocal evidence of a sinter origin (White et al., 1989). At Nuraghe Tepporo, a silicified breccia crops out discontinuously along an east-west–trending fracture. It is characterized by angular centimeter-sized clasts of white opal in a red opaline matrix.

Quartz and chalcedony chiefly occur in veins that appear to underlie opal-, alunite-, and kaolinite-bearing zones and are here referred to as siliceous feeders. At Ischia Ruggia, veins and silicified zones of wall rock up to 3 m thick occur in north-south–trending structures displaced by late east-west faults. The veins are composed of chalcedony and quartz. At San Marco, stockwork veinlets of chalcedony and barite are hosted by a silicified pyroclastic flow. Barite commonly fills vugs in the chalcedony veinlets.

Rock samples from the different silicified bodies contain high Hg (up to 800 ppb) and As (up to 200 ppm). Enrichments in Hg and As are associated with anomalous Au in some rock samples at Porto Alabe (up to 0.5 ppm Au) and at San Marco (up to 0.1 ppm Au). Silicified rock at San Marco is also characterized by high Ba, ranging from 100 to 1,000 ppm (Gold Mines of Sardinia, 1999).

Kaolinite 1T-opal ± quartz ± dickite ± alunite: This alteration is pervasive in the Salamura, Punta Allò, and Nuraghe Tepporo areas and is typical of the kaolin quarries. It extends vertically up to 20 m and laterally for less than 30 m at Nuraghe Tepporo and up to 30 m vertically and up to 100 m laterally at Salamura and Punta Allò quarries (Fig. 3). Differential thermal analyses (Garbarino et al., 1991a) indicate a bulk composition of up to 50 wt percent silica minerals and up to 50 wt percent of kaolinite group minerals + alunite. Alunite and dickite increase in abundance where the alteration is more intense and destroys the texture of the primary ignimbrite. Alunite and kaolinite with lesser opal CT and quartz form very fine grained powdery aggregates and pervasively replace feldspar phenocrysts and matrix. These mixtures are white to tan in hand sample, but in thin section, most alunite-quartz and alunite-kaolinite mixtures are light to medium reddish-brown and consist of grains that are <20 µm in diameter. Individual crystals commonly cannot be discerned (Fig. 4B, sample A-6). Opal occurs as aggregates within alunite and kaolinite and also in veinlets that crosscut the kaolin deposits. Fine-grained (<20-µm) opal-alunite mixtures are also found at Punta Allò. Alunite is K rich and Fe and P poor. Reconstructed alunite compositions from <20-µm-diam grains mixed with quartz, opal, and kaolinite from Tresnuraghes are shown in Table 2.


Figure 011904
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  		FIG. 4. Photomicrographs of alunite from Romana and Trenuraghes. A. Alunite lining 1-mm-diam vug (main photo, crossed polars, sample Romana 7). Grains of alunite are 10 to 50 µ m in diam and project into cavity (inset, plane-polarized light). Dark background is medium reddish brown kaolinite mineral with minor quartz and alunite. B. Tresnuraghes sample A-6 (plane-polarized light), showing light to medium reddish brown mixture of alunite (<20-µ m-diam kaolinite group minerals (kaol) intergrown with and filling spaces between euhedral quartz grains 50 to 200 µ m in diam. Most of sample A-6 is composed mainly of kaolinite, quartz, and alunite.

 

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  		TABLE 2 TABLE 2. Alunite Chemical Analyses (wt %)

 
Kaolinite 1Md-kaolinite 1T-opal: At the Patalza quarry, where this alteration is observed, a primary ignimbrite texture is preserved, suggesting an association with lower intensity of alteration. The intensity of the alteration decreases from the top to the bottom of the quarry (depth 20 m). Poorly ordered kaolinite (1Md) is dominant compared to the well-ordered kaolinite (1T), and alunite is absent. Supergene Fe oxides and hydroxides, commonly associated with opal and montmorillonite, fill fractures in the shallower part of the quarry (10-m depth). Fe oxides and hydroxides also extend at surface horizontally away (up to 50 m) from the central part of the quarry.

Kaolinite 1Md-montmorillonite-cristobalite: This alteration assemblage forms a wide halo extending up to 500 m from the quarries.

K-feldspar-quartz-illite: This alteration occurs in the ignimbrites as a selvage to the siliceous (quartz-chalcedony) veins that crop out at Ischia Ruggia and extends for several meters (up to 100 m) away from the veins.

Montmorillonite-quartz-illite: This alteration occurs as a pervasive outer alteration halo peripheral to the siliceous veins and inner K-feldspar-quartz-illite alteration halo at Ischia Ruggia (from 50–500 m). Rock texture is preserved, and primary feldspar occurs as relics. Similar alteration occurs as an envelope to the Porto Alabe sinter.


   Romana Area
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 
Local geology
In the Romana district, the Miocene calc-alkaline volcanic sequence comprises basal units of andesitic to dacitic domes and lava flows that crop out east of the study area (Fig. 5). This sequence is overlain by numerous rhyolitic to rhyodacitic pyroclastic flows (Ligas et al., 1997). These pyroclastic rocks are characterized by a vitroclastic texture with lithic and pumice clasts and plagioclase, biotite, and quartz phenocrysts in a cryptocrystalline matrix. The Romana kaolin deposits are hosted by these rocks. Late flow domes, ranging from andesite to rhyodacite, complete the volcanic sequence. Miocene sedimentary rocks, which overlie the volcanic rocks, are composed of sandy marls and compact fossil-bearing limestone beds.


Figure 012005
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  		FIG. 5. Geologic and alteration sketch map of the Romana epithermal system (this study).

 
Tertiary tectonic activity produced major structures trending east-northeast and west-northwest and minor faults trending north-northeast and north-northwest. These steeply inclined faults were the principal conduits for hydrothermal fluid flow, whereas lateral flow was controlled by lithological permeability.

Hydrothermal alteration
Hydrothermally altered rocks in the Romana area cover 4 km2. There are four major kaolin quarries: Locchera, Scanu 1, Scanu 2, and Donigazza (Fig. 5), and Locchera is the largest kaolin deposit in Italy with measured reserves of approximately 100,000 t and production of 20,000 t/y.

Common minerals are kaolinite 1T, dickite, and silica minerals (from opal CT to chalcedony to quartz). Alunite occurs in the Locchera and Scanu quarries and as a minor component at D’Algata. Montmorillonite occurs in the less altered areas and as the capping alteration at the Scanu quarry. The only sulfide present is pyrite. The altered rock extends over a vertical interval of at least 300 m (Fig. 6).


Figure 012106
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  		FIG. 6. Schematic section through the Romana epithermal district, showing alteration zonation at the studied sites. For legend see Figure 5.

 
Five principal alteration types were distinguished on the basis of mineral association (Table 3). The spatial distribution of these alteration types is shown in map view in Figure 5 and in cross section in Figure 6.


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  		TABLE 3 TABLE 3. Hydrothermal Alteration Features at Romana

 
Silicified rock and siliceous veins: Silicified bodies crop out in the Rattari, Locchera, and D’Algata areas. These include massive and layered near-surface bodies as well as veins of quartz and chalcedony that are inferred to be deeper siliceous feeders. At Locchera, massive silicified bodies occur along the intersection of north-south– and east-west–trending faults, with chalcedony destroying the primary rock texture. At Rattari, a relict texture of the host rock is preserved in the silicified bodies, suggesting that this formed by silica replacement processes. Chalcedony veins occur in north-south– and east-west–striking structures along the north side of Rattari Hill. At D’Algata a sequence of terraces formed by centimeter-scale siliceous layers occurs near the surface. Laminations in the siliceous deposits are accentuated by alternating black and yellow pyrite-rich layers. Quartz veins striking north-south occur beneath the terraces, but the paragenetic relationship is not clear. Rock samples of silicified bodies at Romana have high Hg and As contents (up to 200 ppb and up to 100 ppm, respectively). Rattari and Locchera silicified bodies also have anomalous concentrations of Cu (up to 150 ppm) and Au (up to 750 ppb; Gold Mines of Sardinia, 1999).

Kaolinite 1T-quartz: This is a pervasive alteration that occurs as an envelope of up to 20 m in width around the silicified bodies. Kaolinite is present throughout but decreases in abundance away from the kaolinite-quartz zone and grades outward into a kaolinite-montmorillonite ± opal zone.

Kaolinite 1T-alunite-quartz-opal ± dickite ± cristobalite: This mineral association is found at the Locchera quarry. Alteration is pervasive and intense and appears to be controlled by subvertical faults. At present the quarry face is 50 m high. Within the exposures no vertical zonation of the minerals has been observed except for a few silicified layers (1 m thick) that occur at different levels in the quarry. However, alunite decreases in abundance laterally away from the controlling fluid conduits. Alunite is intergrown with kaolinite and quartz and produces a white to yellow aspect to the rock. The alunite-kaolinite mixtures fill and surround euhedral prismatic quartz grains up to 0.3 mm long. Locally, vuggy cavities where feldspar has been removed by hydrothermal alteration are lined with a <0.3-mm-thick zone of euhedral rhombic alunite crystals (<10–50 µm in diam) projecting into vugs (Fig. 4A). Electron microprobe analyses of the alunite indicate that it is K rich and contain 4 to 8 mole percent end-member natroalunite, about 1 mole percent phosphorus in the aluminum site, and about 1 mole percent jarosite component (e.g., Fe/Fe + Al; Table 2). Centimeter-wide veins of dickite crosscut this alteration from the center to the margin of the quarries. Pervasive supergene minerals consisting of Fe and Mn oxides occur in the shallower part of the deposit (up to 4–5 m below the surface) and fill fractures.

At the Scanu quarry a similar association occurs. The alteration consists of quartz-opal-kaolinite ± cristobalite and rare alunite. A 3-m-thick montmorillonite-rich bed occurs as a horizontal layer at the surface and is interpreted to be late and unrelated to the main hydrothermal alteration event.

Kaolinite 1T-opal-montmorillonite ± cristobalite: At the Donigazza quarry vertical zonation of hydrothermal minerals is observed, within the 20-m-high quarry face. The intensity of alteration decreases with depth. The upper part (<10 m in thickness) of the alteration zone is characterized by pervasive kaolinite-opal-cristobalite, but the primary rock texture is preserved. At the base of the quarry face the alteration grades into opal-kaolinite-montmorillonite with relatively high silica content. Primary quartz and plagioclases are preserved.

Kaolinite 1T-montmorillonite-opal: This alteration occurs as a pervasive zone in the pyroclastic units of Romana. The alteration is more intense close to the silicified bodies and the kaolin deposits. Montmorillonite-opal occurs in veinlets filling fractures that crosscut the kaolinite 1T-montmorillonite-opal alteration and is commonly associated with Fe oxides and Fe hydroxides.


Figure 012207
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  		FIG. 7. Plot of {delta}34S vs. {delta}18O values for sulfate minerals and {delta}34S values for pyrite from Tresnuraghes and Romana (single {delta}18O of Romana alunite represents bulk mineral). Positions of magmatic gas, theoretical steam-heated alunite, and theoretical magmatic-hydrothermal alunite at indicated temperatures are from Rye et al. (1992). The position of the magmatic-hydrothermal alunite field has been recalculated to reflect local magmatic and meteoric waters (see text and Fig. 8), where high {delta}34S values reflect condensation of magmatic gas into magmatic fluids and low {delta}34S reflects condensation into mixed magmatic-meteoric fluids (cf. Rye et al., 1992). The outlined areas show fields of natural alunites from steam-heated environments (Cactus, Tolfa, and Marysvale) and magmatic-hydrothermal environments (Red Mount, Summitville, Julcani, and Rodalquilar) from data in Rye et al. (1992) and Rye (1993) and references therein. Note that the {delta}34S of pyrite differs significantly from {delta}34S of alunite, which argues against a supergene origin for alunite by weathering of pyrite.

 

Figure 012408
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  		FIG. 8. A. Plot of {delta}18O vs. {delta}D values of kaolinite minerals and alunite. Positions of local present-day and inferred Miocene meteoric water, local Miocene magmatic water, and 18O-enriched Miocene meteoric water are discussed in text. Supergene alunite OH (SAOH) field is from Rye et al. (1992). Primary magmatic water fields for arc-type magmatic water ({delta}D ~–20 ± 10{per thousand}) are from Giggenbach (1992) and Taylor (1986). Mediterranean magmatic waters ({delta}D = 10 ± 10{per thousand}) are modeled after D-enriched magmatic water from Sardinia (cf. Giggenbach, 1997). Isotopic compositions of kaolinite and dickite (symbols on right) are in equilibrium with local Miocene meteoric water indicated by shaded field on left, calculated at 25° to 50°C except for one Romana dickite that is in equilibrium at 120°C. Shaded fields on right show the calculated compositions of kaolinite-dickite at 25° to 50°C in equilibrium with local Miocene meteoric water (linked with tie-line), and the compositions of kaolinite-dickite in equlibrium with 18O-enriched meteoric water (w/r = 1) at 50° to 90°C (see Table 4 and text). Note that Romana alunite could be in equilibrium with either magmatic water at ~220°C or with meteoric waters at 90° to 160°C. Water/rock exchange paths in increments of 10, 1, 0.1, and 0.05 mass ratios are indicated for reaction of Miocene meteoric water with volcanic rocks and carbonate rocks at 250°C (see Appen.). B. The {delta}18O compositions of quartz and barite, together with calculated equilibrium water compositions at 100° and 160°C, characteristic of hydrothermal feeders (see text and Table 4).

 

   Stable Isotopes
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
Environment of Formation
 Conclusions
 APPENDIX
 References
 
Stable isotope studies of kaolinite, dickite, silica minerals, alunite, barite, and pyrite were performed on samples from the Romana and Tresnuraghes areas using methods described in the Appendix. The isotopic data are listed in Table 4 and plotted in Figures 7 and 8. The estimated isotopic composition of the coexisting water is presented in a later section, but it should be noted here that the hydrothermal minerals may have formed at temperatures up to 220°C, based on comparisons with formation temperatures for similar mineral assemblages in active geothermal systems (e.g., Reyes, 1990; see below). These comparisons, together with isotopic data, are the basis for a genetic model of the Romana and Tresnuraghes hydrothermal systems.


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  		TABLE 4 TABLE 4. Isotopic Compositions of Minerals and Calculated Compositions of Coexisting Aqueous Species

 
Sulfur-bearing minerals
Two barite samples and one alunite sample from Tresnuraghes, and one pyrite sample and two alunite samples from Romana, were analyzed (Table 4). The {delta}34S value of the Romana pyrite is –1.1 per mil, similar to magmatic sulfur (–3 to +3{per thousand}; Rye et al., 1992). Alunite is finely intergrown with kaolinite and quartz, so sulfur isotope determinations are from mixtures. The {delta}34S values of the two alunite samples from Romana are 5.0 and 12.0 per mil; alunite from Tresnuraghes has a {delta}34S value of 12.0 per mil. The {delta}34S values of the two barite samples from Tresnuraghes are 17.6 and 19.0 per mil. The {delta}34S values of barite and alunite differ significantly from that of pyrite, so alunite cannot be formed solely from the weathering of pyrite (cf. Field and Lombardi, 1972; Field and Gustafson, 1976; Rye et al., 1992). The difference between the {delta}34S values of pyrite and alunite are less than would be expected for low-temperature (<200°C) isotopic equilibrium (Ohmoto and Lasaga, 1982), so these minerals are not in isotopic equilibrium.

The combined oxygen and sulfur isotope compositions of alunite have been used by Rye et al. (1992) to distinguish between alunite (and barite) of steam-heated and magmatic-hydrothermal origins. The {delta}18O values of two Tresnuraghes barite samples are 15.4 and 18.9 per mil. The {delta}34S and {delta}18O values of the barite are similar to those of alunite in well-documented epithermal systems formed at 200°C or less and having H2S/SO2–4 ratios in the hydrothermal fluid of about unity (Fig. 7; Rye et al., 1992).

The {delta}18O value of a mixture of alunite > kaolinite > quartz in volume proportions of ca. 60/30/10 from sample Rom-7 is 16.7 per mil, from which we can estimate a range of {delta}18O for the combined oxygen in the sulfate and hydroxyl sites of alunite of about 16 to 17.5 per mil. This estimate is based on the proportions of the three minerals, the constant alunite-kaolinite fractionation factor of 3.3 per mil in the range 25° to 100°C, a quartz-alunite fractionation of 10 per mil at 25° to 100°C, and alunite composition in which oxygen is 4/7 in the sulfate site and 3/7 in the hydroxyl site (Table 4, Fig. 4A). We assume that the {delta}18O composition of the water in equilibrium with the assemblage can be modeled using the alunite-water fractionation factor of Stoffregen et al. (1994). They report that the exchange rates for oxygen in the hydroxyl site and the sulfate site are the same within error, which permits us to estimate the total alunite-water fractionation factors as the sum of the fractionation factors for the hydroxyl and sulphate sites multiplied by the respective mole fractions of oxygen in these sites. When this estimated {delta}18O value and the corresponding {delta}34S value of 12.0 per mil are plotted in Figure 7, the composition lies close to that of low-temperature alunite from steam-heated environments, such as Tolfa, Italy (Lombardi and Sheppard, 1977).

Oxygen and hydrogen isotopes of kaolinite minerals
The {delta}18O values of kaolinite (n = 3) and dickite (n = 3) have a range of 16.6 to 21.4 per mil, whereas the {delta}D values fall in a narrow range of –43 to–53 per mil. The compositions lie on a {delta}18O versus {delta}D plot between the "kaolinite line" of Savin and Lee (1988) and the field of Tolfa kaolinites (Fig. 8). Three kaolinite values from Romana lie very close to the kaolinite line, consistent with the formation of kaolinite during weathering (e.g., soil formation) or supergene processes in equilibrium with meteoric waters at ambient temperatures of ca. 25°C. Dickite, has slightly lower values of {delta}18O of 16.6 to 19.0 per mil and higher values of {delta}D of –43 to –47 per mil, compared to the Romana kaolinites (Table 4, Fig. 7). The single alunite sample Rom7 is a mixture of alunite-kaolinite-quartz from Romana and has a {delta}18O value of 16.7 per mil and a {delta}D value of –3 per mil. Therefore, the alunite and dickite are not in isotopic equilibrium, which suggests that they formed at different times or one of the minerals later exchanged with a water of different isotopic composition or temperature.

Oxygen in silica minerals and barite
Hydrothermal quartz from Romana and Tresnuraghes has {delta}18O values ranging from 15.9 to 22.0 per mil (Table 4, Fig. 8). The {delta}18O value of unaltered, optically clear sanidine from the rhyolitic host rock is 6.7 per mil, which should be similar to the original bulk magmatic {delta}18O value of rhyolite and quartz in rhyolite. This value of 6.7 per mil is lower than the {delta}18O values of 8.5 to 13 per mil reported for 18O-enriched Quaternary silicic volcanic rocks in the Latium area of Italy (Turi and Taylor, 1976; Ferrara et al., 1985). Thus, the hydrothermal quartz is strongly enriched in 18O relative to unaltered host rock. A slightly lower {delta}18O value of 13.0 per mil was measured in opal from Tresnuraghes.


   Environment of Formation
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
Conclusions
 APPENDIX
 References
 
The geology, hydrothermal mineral associations, and stable isotope data all indicate a shallow and low-temperature depositional environment for both Tresnuraghes and Romana (Browne, 1978, 1991; Sillitoe, 1993). Maximum vertical relief of rock exposures in both areas is ca. 250 m, and the deepest samples come from about 250 m below the kaolinite zones formed near or at the ancient paleosurface. In this environment hydrostatic conditions prevailed and therefore the pressure ranged from 1 bar at surface to 23 bars at 250-m depth. The maximum temperature of fluids under boiling conditions would have been 100° and 220°C, respectively.

The surface exposures at Tresnuraghes and Romana generally lack sulfides, and there is no drill information, so the sulfidation state of the hydrothermal fluids cannot be directly determined. However, we can estimate other characteristics of the hydrothermal systems based on the silicate and sulfate mineralogy.

Tresnuraghes
At Tresnuraghes the occurrence of kaolinite associated with alunite indicates an acid environment with a pH between 2 and 4 (Hemley et al., 1969; Stoffregen, 1987). By comparison with active geothermal systems, the kaolinite-opal association suggests temperatures up to 120°C (Reyes, 1990). At low pH, hydrogen ions inhibit the polymerization of dissolved silica (Fournier, 1985). Therefore, neutralization of the fluids was required to cause silica precipitation in the form of opal (Sillitoe, 1993).

The mineral assemblage grades horizontally from proximal kaolinite 1Md and kaolinite 1T-opal into distal kaolinite 1 Md-montmorillonite-cristobalite. Although there are no geologic data to indicate the significance of the crystal structure of kaolinite (cf. Bailey, 1993), the disordered monoclinic form (1Md) most likely occurs at lower temperature than the ordered triclinic form (1T), by analogy with the muscovite structure. Montmorillonite forms at temperatures up to 140°C and at a pH of ca. 5 to 6 in modern geothermal systems (Reyes, 1990; Simmons and Browne, 1998), and therefore may reflect alteration by fluids that had previously been partly neutralized by reaction with the host rock. Because montmorillonite-bearing assemblages occur at depth in the quarries, we infer that fluid flow, cooling, and neutralization was lateral and downward (cf. Schoen et al., 1974). This may explain the shallow depth extent of these deposits.

Based on the S-O-H isotopes and the alteration zoning at Tresnuraghes the kaolinite-opal ± alunite deposits likely formed in a steam-heated environment where hot spring-derived, H2S-rich steam condensed in the near-surface vadose zone, mixed with local ground water, cooled, oxidized, acidified, and descended (cf. Schoen et al., 1974).

The fine-grained nature of alunite (<20 µm) and its intimate intergrowth with light brown kaolinite and opal are also consistent with a steam-heated environment. Thompson et al. (1999) noted that low-temperature alunite of both steam-heated and supergene origin is typically <50 µm in diameter, whereas coarser alunite (50–100 µm) is characteristic of high-sulfidation quartz-alunite assemblages formed at >100°C (Hedenquist et al., 2000). The low Na, P, and Fe contents and high K contents of the Tresnuraghes alunite contrast with mixed Na-K-alunite with substantial P in quartz-alunite alteration associated with porphyry copper deposits (Watanabe and Hedenquist, 2001; Lipske, 2002) where temperatures exceed 100°C.

The fluids that formed the central and deeper siliceous feeders were hotter and geochemically different from the fluids that formed the overlying and distal kaolin deposits. The occurrence of K-feldspar-quartz-illite alteration proximal to the vein systems with quartz-chalcedony and local barite suggests circulation of weakly acidic to near-neutral pH fluid and temperature higher than 220°C (cf. Reyes, 1990). Sinter, which occurs at Tresnuraghes, commonly forms from boiled, alkali-chloride waters where the paleowater table intersects the paleosurface (Hayba et al., 1985; Browne, 1991). Thus, the fluid-dominated hydrothermal system of Tresnuraghes most likely represents an upflow zone of neutral alkali-chloride fluid that is overlain by a steam-heated, acid-sulfate environment containing kaolinite deposits. The Tresnuraghes system can be classified as adularia-sericite type and, due to the lack of sulfides, is inferred to have formed under low-sulfidation conditions based on common sulfide-silicate associations (Hayba et al., 1985).

Rock samples from the kaolin quarries show an enrichment of As and Hg in the siliceous feeders with generally low amounts of chloride-complexed metals such as Pb, Sb, and Zn values (Gold Mines of Sardinia, 1999). Because Hg is readily transported in the vapor phase and As is not (Barnes and Seward, 1997), the Hg-As association suggests fluctuation from aqueous transport of As in the hydrothermal fluid to vapor transport of Hg in the steam-heated vadose environment. Possibly, this is the result of a late steam-heated overprint upon an earlier and underlying As-rich rock produced by condensed fluids.


Figure 012609
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  		FIG. 9. Models for the epithermal systems related to northern Sardinian kaolinite deposits (see text). A. Tresnuraghes deposits are related to ascending weakly acidic to near-neutral pH (low-sulfidation) fluids that produced local sinter on the paleosurface and extensive steam-heated zones near the paleosurface at <120°C. B. Romana deposits are related to ascending low pH, high-sulfidation magmatic-hydrothermal fluids that produced kaolinite at <120°C by mixing with surface waters and in steam-heated zones. In both cases, magmatic-hydrothermal fluid components are required, and quartz and barite in the central vein feeder zones formed at elevated temperatures of ~100° to ~220°C.

 
Stable isotope data also support the interpretation that there were two subenvironments at Tresnuraghes. The two dickite samples from the blanketlike kaolinite-opal-alunite zone have isotopic compositions in equilibrium with meteoric water having a {delta}D value of –20 per mil at temperatures of 40° to 50°C (Table 4, Fig. 8). These data suggest dickite formation from local meteoric water at about 25° to 50°C, but dickite typically forms at a temperature of >120°C in geothermal systems (Reyes, 1990). At 120°C, Tresnuraghes dickite would be in equilibrium with fluid having a {delta}18O value of 8 per mil and a {delta}D value of –25 per mil, similar to arc-type magmatic water. A magmatic water origin is considered unlikely because of the low-temperature and near-surface environment. Exchange between meteoric water and Paleozoic carbonate rocks at 250°C could also have caused 18O enrichment of water to {delta}18O values of 8 per mil but is also considered unlikely because it requires a water/rock mass ratio of about 0.3 (Fig. 8), which is lower than the water/rock ratio required for strong hydrothermal alteration (cf. Taylor, 1979). Our preferred model is that the dickite likely originally formed at >120°C from meteoric water with a small component of magmatic water, but reequilibrated isotopically at 25° to 50°C in the steam-heated environment. The isotopic data do not rule out a supergene origin for the dickite and associated kaolinite by acid produced by weathering of pyrite at 25°C. However, we know of no geologic examples of dickite that has formed via supergene processes. Furthermore, the {delta}34S value of 12.0 per mil for a single alunite sample from the kaolinite-alunite-opal assemblage is unlikely to have been produced only by weathering of pyrite with a {delta}34S value of ~O per mil.

Barite from central siliceous feeders a Tresnuraghes plots on the {delta}34S versus {delta}18O diagram in the field of magmatic-hydrothermal alunite and sulfate produced by ~200°C fluids with H2S/SO4 ratios of about unity (Fig. 7; Rye et al., 1992). Equilibrium between barite and quartz and the local meteoric water would imply that these two minerals formed at temperatures >75° and >100°C, respectively (Fig. 8B). At 100°C the quartz from the sinter sample would be in equilibrium with meteoric water with a {delta}18O value of –3.7 per mil, similar to the dickite minerals (Table 4). We assume a higher temperature for the deeper quartz veinlets and calculate equilibrium {delta}18O values for water of –0.3 to + 2.8 per mil at 160°C (e.g., >57-m depth at hydrostatic pressure) and 3.8 to 6.9 per mil at 220°C (>250-m depth). These 18O-enriched compositions would indicate a component of magmatic water but also could be interpreted as meteoric water enriched by exchange with Paleozoic carbonate or Cenozoic volcanic rocks at water/rock ratios of ~5 to 0.1 (Fig. 8A).

Romana
Blanketlike kaolin deposits at Romana are characterized by hydrothermal alteration proximal to hydrothermal veins and feeders with the association kaolinite 1T-alunite-quartz-opal ± dickite ± cristobalite. The mineralogy suggests a formation temperature of <120°C for kaolinite-rich (1T) assemblages and 120° to 200°C for kaolinite- and dickite-bearing assemblages based on comparisons with active geothermal systems (Reyes, 1990). However, similar to Tresnuraghes, the S-O-H data indicate temperatures of <120°C. The coexistence of cristobalite, opal, and quartz in these deposits could be explained by rapid change of fluid-flow rate, or temperature along the flowpath or with time (Dove and Rimstidt, 1994), or partial transformation of amorphous silica to quartz and chalcedony (Fournier, 1985). The latter requires that proximal fluids had a temperature below 120°C. More distal kaolinite 1T-rich alteration at Romana has a steam-heated origin at <50°C, similar to Tresnuraghes, and is consistent with the fine-grain size (<50 µm) and K-rich composition of coexisting alunite. At the Scanu and Donigazza quarries, the vertical zonation of the alteration minerals is similar to that observed in the Tresnuraghes quarries and supports the origin of the kaolin deposits by descending fluids.

Silicified feeders, including silicified rocks and the quartz-, chalcedony-, and pyrite-bearing veins, at Romana are enveloped in rock that has been altered to kaolinite 1T, alunite, quartz, and local dickite and therefore formed from fluids with lower pH than the fluids that produced the quartz-chalcedony veins enveloped by K-feldspar-illite at Tresnuraghes. Hypogene assemblages with kaolinite, alunite, and pyrite require both low pH (<2) and a high-sulfidation state (cf. fig. 5 of Heald et al., 1987). The siliceous layers in the D’Algata body are similar to the shallow parts of high-sulfidation epithermal systems where geothermal water (Browne, 1991) discharged on the surface after extensive lateral flow and dilution (Sillitoe, 1993). Deeper parts of hydrothermal systems at Romana are locally exposed in the 250 m of relief (Fig. 5). High Hg and As values of silicified bodies are consistent with fluctuating vapor- and liquid-dominated conditions, as at Tresnuraghes. The Rattari and Locchera silicified bodies contain weakly anomalous Cu and Au, similar to the common Cu-As-Au metal association in the high-sulfidation environment (Hayba et al., 1985). Based on the mineralogical and geochemical features and the S-O-H data, Romana is a shallow epithermal system in which ascending acidic fluids produced selvages of kaolinite-rich alteration along hydrothermal feeders that were overlain by near-surface blanketlike kaolin deposits. The deep silicified feeders and hydrothermally altered rocks therefore are interpreted to have formed in an acid-sulfate (Heald et al., 1987) or high-sulfidation epithermal environment (Stoffregen, 1987).

The hydrogen and oxygen isotope values of three Romana kaolinites and one dickite from the blanketlike near-surface kaolin deposits plot along the kaolinite line and are consistent with either a steam-heated or supergene origin because they are in isotopic equilibirium at 25° to 50°C with meteoric water having a {delta}D value of –20 per mil. The sulfur isotope values of alunite ({delta}34S = 5 and 12{per thousand}) are consistent with partial oxidation of H2S at <100°C, similar to that observed in other steam-heated localities such as Tolfa, Italy. The sulfur isotope values of alunite also differ from Romana pyrite ({delta}34S = –1.1{per thousand}), and therefore all sulfate cannot be derived from supergene oxidation of pyrite. The alunite with the {delta}34S of 12 per mil can be interpreted to be of magmatic-hydrothermal origin because it is in isotopic equilibrium at 100° and 220°C with water having {delta}18O values of 0.3 and 8.8 per mil, respectively, and a {delta}D value of –1 per mil (Table 4 shows the calculation for 100°C). The {delta}18O of this fluid is consistent with a mixture of magmatic and meteoric water, and the {delta}D is similar to local magmatic waters sampled from volcanoes in Italy and the Mediterranean basin (Fig. 8A; {delta}D ~ 0 ± 10{per thousand}; Panichi and Noto, 1992; D’Amore and Bolognesi, 1994; Giggenbach, 1997). Quartz from the central zone of silicified rock at D’Algata and kaolinite-alunite-opal ± dickite alteration at Locchera has {delta}18O values with isotopic equilibrium temperatures higher than 120°C. At 160°C this quartz would have been in equilibrium with water having {delta}18O values of –1.9 and +4.2 per mil, respectively, which is consistent with the central silicified feeders having formed from a mixture of meteoric and minor magmatic waters (Fig. 8B, Table 4).

Isotopic composition of Miocene meteoric water
The Sardinian kaolinites and dickites are calculated to have formed in isotopic equilibrium with meteoric water having a {delta}D value of –20 per mil, which is higher than present-day Sardinian meteoric water ({delta}D = –38 to –54{per thousand}; Caboi et al., 1994). The Sardinian kaolin minerals most likely formed from Miocene meteoric water, which is significantly heavier than present waters, and their formation is possibly also attributable to the wetter or cooler climate of the western Mediterranean in the Miocene than the climate of today. This situation is similar to the one proposed by Arribas et al. (1995) for the Miocene Rodalquilar epithermal Au district of southeastern Spain.


   Conclusions
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
APPENDIX
 References
 
The Tresnuraghes and Romana hydrothermal systems are hosted in silicic pyroclastic rocks associated with local silicic domes of Miocene age. The fluid flow was structurally controlled by upflow along steeply dipping faults and lateral flow away from vents immediately below the water table (Figs. 3, 6). The hydrothermal activity in these areas shows mineralogical and geochemical features (Au, Hg, As) typical of a shallow epithermal environment close to the paleosurface. Alteration conditions of <220°C at <23 bars are implied by the vertical distribution of alteration zones in rock exposures with up to 250 m of relief. In the Tresnuraghes area silicified bodies include fault-filling quartz-chalcedony-barite veins within volcanic rock altered to K-feldspar-quartz-illite and shallow chalcedonic sinter, which formed at estimated temperatures of ~220° and 100°C, respectively. The overlying blanketlike kaolin deposits formed by descent of low-temperature acidic water from the surface, as suggested by the mineral zonation from inner kaolinite 1T ± dickite to kaolinite 1Md-montmorillonite in the quarries associated with alunite and opal, and anomalous Hg and As. The geology and isotopic data indicate kaolin formation in a steam-heated environment from meteoric water at temperatures of <120°C and mainly at 25° to 50°C. The siliceous feeders, representing the deeper part of the Tresnuraghes hydrothermal system, resemble the upper parts of some near-neutral pH, low-sulfidation epithermal systems in which silica-barite veins and adularia-sericite alteration occur along feeder faults (cf. Hayba et al., 1985). Boiling near the water table and in hot springs produced steam containing H2S, which ascended, condensed in the vadose zone, mixed with meteoric water, cooled, oxidized, and moved laterally and downward to produce extensive kaolinite-opal ± alunite deposits (Fig. 9).

In the Romana area silicified bodies (chalcedony or quartz ± pyrite) with locally anomalous As, Hg, Cu, and Au formed along faults at 100°C and possibly as high as 220°C, from ascending acidic water. These bodies are flanked and overlain by rock alteration zoned from inner kaolinite 1T-alunite-quartz ± dickite at Locchera quarry to outer kaolinite 1T-quartz. The S-O-H isotope composition of alunite and quartz from the central silicified zone suggests contributions of magmatic-hydrothermal sulfur and water, whereas the data for near-surface kaolinite, dickite, and alunite suggest equilibration at 25° to 50°C with meteoric water. The mineral zonation in the kaolin quarries from inner kaolinite 1T + opal to montmorillonite suggests that fluids descended from the surface and were gradually neutralized by host rocks. Mineralogical and isotopic features of Romana suggest the silicified feeders represent the shallow part of a boiling acidic and high-sulfidation hydrothermal system, similar to the quartz-alunite veins in the Virginia City area, Nevada (Vikre, 1998), Nansatsu district, Japan (Hedenquist et al., 1994), and San Juan Mountains, Colorado (Larson and Taylor, 1987). The uppermost part of the system formed kaolin and dickite at temperatures up to ~120°C in an environment that likely included both steam-heated conditions and local mixing and cooling of meteoric and magmatic fluids.

The Sardinian kaolin deposits are remarkable for the large amount of clay and alunite produced from feldspar and volcanic glass by hydrolysis, which requires large inputs of acid and sulfur from magma-derived fluids. The {delta}18O of the kaolinite minerals in the blanketlike kaolin deposits is higher than other steam-heated alunite-bearing occurrences such as Marysvale, Utah (Cunningham et al., 1984). The 18O-enriched kaolin minerals in the Sardinian deposits suggest that the kaolin alteration occurred at very low temperatures of ~25° to 50°C from fluids that originated in the steam-heated environment but were subsequently cooled (cf. Rye et al., 1992). The stable isotope data argue against a supergene (weathering) origin for the kaolin and alunite and suggest that the low-temperature steam-heated waters were modified Miocene meteoric waters. These waters had a {delta}D of –20 per mil, which is distinguishable from modern meteoric water with a {delta}D of –38 to –54 per mil (Caboi et al., 1994). The isotopic data suggest a contribution of magmatic sulfur, oxygen, and hydrogen in both the Tresnuraghes and the Romana systems along steeply dipping faults flanked by steam-heated environments.

Future studies of these deposits are needed to understand in more detail the paragenesis of different silica minerals and to correlate trace metal enrichments with sulfide and ore mineral occurrences in the hydrothermal environment. At present, the lack of information on sulfide minerals precludes rigorous definition of the sulfidation state. However it is evident that the current exposures of kaolin deposits represent the ancient Miocene paleosurface and steam-heated environment, with only limited exposure of the underlying fluid-dominated epithermal environment. Hence, the deeper parts of these systems may be a prospect for precious metals.


   APPENDIX
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
References
 
Analytical Methods
   Sampling and analytic procedures
Mapping of features of altered rocks was done concurrently with sampling for isotopic analysis. Alteration associations were delineated by field mapping, X-ray diffraction analyses, and petrographic studies.

One hundred and twenty samples were collected for X-ray diffraction analyses. The mineralogical composition of the specimens was determined using a Rigaku geigerflex diffractometer at 30 kV, 30 mA, Cu tube, and Ni filter. The structural order of the kaolinite phases was determined on the basis of peak morphology, taking into account both intensity and neatness of the basal reflection or the presence of near-continuous diffraction bands. All the peaks obtained were compared with the JCPDS (Joint Committee on Powder Diffraction) datafile (1985).

Twenty samples of mineral separates of quartz, kaolinite, barite, alunite, and pyrite were prepared for {delta}18O, {delta}D and/or {delta}34S analyses by standard techniques in the laboratories of Oregon State University. Oxygen gas was extracted from silicate via laser fluorination with ClF3 following the method of Sharp (1990). Hydrogen gas was liberated via heating to 1,400°C in vacuum and reduction of water via uranium at 650°C (Bigeleisen et al., 1952). Sulfur was extracted from sulfide via a cupric oxide combustion method and from sulfate via conversion to Ag2S, using the method of Thode et al. (1961), followed by cupric oxide combustion.

Oxygen and hydrogen isotopes were analyzed in the laboratories of Washington State University. Oxygen isotope analyses of sulfate were done in the U.S. Geological Survey laboratories at Denver, using graphite-furnace reduction and continuous flow mass spectrometry. Replicate analyses of NBS and laboratory standards yielded reproducibilites of ±0.2, ±4, and ±0.1 per mil for {delta}18O, {delta}D, and {delta}34S, respectively. The {delta}18O and {delta}D analyses are reported per mil relative to V-SMOW. Analyses of NBS-30 biotite standard gave a value of {delta}D = –65 per mil and {delta}18O = +5.1 per mil. The {delta}34S values are reported relative to Canyon Diablo Troilite.

   Calculation of water-rock exchange
Calculations of equilibrium isotopic compositions of water and water/rock ratios were made following the standard methodology and equations of Taylor (1979, 1997) and Field and Fifarek (1985) at 250°C (lower temperatures would yield final water compositions little-shifted from starting composition). Isotopic exchange reactions for hydrogen are governed by Formula

where {delta}DH2Of is the final water composition, {delta}D H2Oi is the initial water composition, {delta}Drocki is the initial rock compostion, {Delta}r-w is the rock-water isotopic fractionation, and w/r is the atomic water/rock ratio for hydrogen [= water/rock mass ratio x (wt % H in water)/(wt % H in rock)]. Equation (1) allows calculation of final isotopic composition of waters reacted with rock at various w/r ratios. Analogous equations are used for oxygen.

Initial parent Miocene meteoric water with {delta}18O of –3.75 per mil and {delta}D of –20 per mil is reacted with carbonate (50 wt % O and 0.1 wt % H having the isotopic composition {delta}18O of 23{per thousand} and {delta}D of –60{per thousand}), and all exchangeable oxygen and hydrogen, respectively, are assumed to be in calcite ({Delta}r-w ~1,000 ln {alpha} calcite-water = 2.78 x 106/(T°K)2 – 2.89{per thousand}; Friedman and O’Neil, 1977) and illite/muscovite/smectite (1,000 ln {alpha} mica-water = –25{per thousand}; Sheppard and Gilg, 1996). Similarly, meteoric water is reacted with volcanic rock (50 wt % O and 0.1 wt % H having the isotopic composition {delta}18O of 10{per thousand} and {delta}D of –10{per thousand}), where we assume that oxygen contained in volcanic glass, its devitrification products, alkali feldspar, and Na-rich plagioclase can by modeled by K-feldspar (1,000 ln {alpha} K-feldspar-water = 2.39 x 106/(T°K)2 – 2.51; Matsuhisa et al., 1979). Hydrogen fractionation in volcanic rocks is also assumed to be governed by illite/muscovite/smectite mineral fractionation.


   Acknowledgments
 
This research was supported by C.N.R., Istituto di Geologia Ambientale e Geoingegneria, Sez. di Cagliari, Italy, and M.I.U.R. (60% grants to G.P.). We thank Sardinia Gold Mining Company for providing results from their research. We thank Lihua Zhang, who assisted with oxygen and hydrogen analyses, and Cyrus Field and Robert Rye, who provided sulfur and oxygen isotope data on barite, respectively. We have greatly benefited from discussions with Field, Zhang, and Rye, and constructive reviews by Antonio Arribas, Jr., Jeffrey Hedenquist, and Mark Hannington.

May 10, 2002; November 4, 2004

May 10, 2002; November 4, 2004


   References
Top
 Abstract
 Introduction
 Regional Geology
 Tresnuraghes Area
 Romana Area
 Stable Isotopes
 Environment of Formation
 Conclusions
 APPENDIX
 References
 

Arribas, A. Jr., Cunningham, C.G., Rytuba, J.J., Rye, R.O., Kelly, W.C., Podwysocki, M.H., McKee, E.H., and Tosdal, R.M., 1995, Geology, geochronology, fluid inclusions, and isotope geochemistry of the Rodalquilar gold-alunite deposit, Spain: Economic Geology, v. 90, p. 795–822.[Abstract][ISI][GeoRef]

Bailey, S.W., 1993, Review of the structural relationships of the kaolin minerals, in Murrary, H.H., Bunday, W.M., and Harvey, C.C., eds., Kaolin genesis and utilization: Boulder, CO, Clay Minerals Society, Special Publication 1, p. 25–42.

Barnes, H.L., and Seward, T.M., 1997, Geothermal systems and mercury deposits, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits, 3rd ed.: New York, John Wiley, p. 699–736.

Beccaluva, L., Brotzu, P., Mac