Argillic alteration zones associated with porphyry and epithermal deposits may contain clays that are antibacterial against antibiotic-resistant human pathogens, thus providing new economic potential for these deposits. This study examined the antibacterial activity of hydrothermal alteration zones in andesite porphyry and volcaniclastic rocks from a sulfur-and pyrite-rich clay deposit in the Oregon Cascades. The deposit is located along a fault zone west of Crater Lake, in Eocene-age volcanic rocks.

Mineralogical relationships were used to deduce emplacement temperatures of the antibacterial clays, and oxygen isotopes of secondary quartz were used to deduce fluid sources. High-sulfidation alteration from magmatic fluids (200°–300°C) produced highly illitic Black clay with elemental sulfur and up to 19 wt % pyrite. Low-sulfidation, mixed magmatic-meteoric fluids (150°–200°C) produced pervasive Blue clay containing reduced Fe-bearing rectorite and minor pyrite (4–5 wt %). White clay containing smectite and kaolinite formed along faults from cooler (≤100°C) meteoric water. Surficial Red clay contains illite-smectite and goethite from the oxidation of pyrite.

Antibacterial activity is greatest in the pyrite-bearing clays but the nonsulfide-bearing White clays also inhibit bacterial growth. Red oxidized clays are not antibacterial. When antibacterial clays are rehydrated in deionized water (100 mg/ml), the pH (<4.5) drives mineral dissolution and metal hydrolysis to produce Fe2+, Al3+, and hydroxyl radicals that kill bacteria. The pH and Eh of the hydrated clay is important for stabilizing these aqueous reactants, therefore we find no antibacterial effect where carbonates or goethite are stable. Accordingly, the antibacterial effect is diminished with burial depth (meters) where the Eh is too low to drive pyrite oxidation.

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