Abstract

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 δ18O = 17.1‰, δ34S = 18.8‰), one alunite sample from Romana (δ18O = 12.0‰, δD = –3‰, δ34S = 16.7‰), and quartz from both localities (δ18O = 15.9–22.0‰) 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 (δ18O = 16.6–21.4‰, δD = –43 to –53‰) formed from steam-heated meteoric water having δD = – 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.

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