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Mineralogical and Stable Isotope Studies of Kaolin Deposits: Shallow Epithermal Systems of Western Sardinia, Italy

R. Simeone

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

View larger version (9K): [in a new window]   FIG. 1. Location of the main kaolinite-rich epithermal systems of Sardinia hosted in Oligocene-Miocene calc-alkaline volcanic rocks.

View larger version (74K): [in a new window]   FIG. 2. Geologic and alteration sketch map of the Tresnuraghes epithermal system (this study).

View larger version (16K): [in a new window]   FIG. 3. Schematic section through the Tresnuraghes epithermal district, showing alteration zonation at the studied sites. For legend see Figure 2.

View larger version (139K): [in a new window]   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.

View larger version (89K): [in a new window]   FIG. 5. Geologic and alteration sketch map of the Romana epithermal system (this study).

View larger version (31K): [in a new window]   FIG. 6. Schematic section through the Romana epithermal district, showing alteration zonation at the studied sites. For legend see Figure 5.

View larger version (38K): [in a new window]   FIG. 7. Plot of 34S vs. 18O values for sulfate minerals and 34S values for pyrite from Tresnuraghes and Romana (single 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 34S values reflect condensation of magmatic gas into magmatic fluids and low 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 34S of pyrite differs significantly from 34S of alunite, which argues against a supergene origin for alunite by weathering of pyrite.

View larger version (25K): [in a new window]   FIG. 8. A. Plot of 18O vs. 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 (D ~–20 ± 10) are from Giggenbach (1992) and Taylor (1986). Mediterranean magmatic waters (D = 10 ± 10) 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 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).

View larger version (57K): [in a new window]   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.