Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals

Abstract Advanced argillic minerals, as defined, include alunite and anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, topaz), diaspore. One or more of these minerals form in five distinctly different geologic environments hydrolytic alteration, with pH 4–5 to <1, most at depths <500 m. (1) Where an intrusion-related hydrothermal system, typical that associated porphyry Cu ± Au deposits, evolves white-mica stability, continued ascent cooling the white-mica–stable liquid results pyrophyllite (± diaspore) becoming stable near base lithocap. (2) A well-understood hypogene environment formation is vapor condensation volcanic vents, where magmatic SO2 HCl condense into local groundwater produce H2SO4 HCl-rich solutions a 1–1.5. Close isochemical dissolution host rock occurs because high solubility Al Fe hydroxides <2, except for SiO2 component, which remains siliceous residue relatively low SiO2. This residual quartz, commonly vuggy texture, largely barren metals metal content high-temperature but low-pressure vapor. Rock causes acidic solution increase, such kaolinite (or dickite higher temperatures) become stable, forming halo quartz. initially forms lithocap horizon permeable lithologic units are intersected by feeder structure, may mineralized if subsequent ascends this level precipitates copper gold. (3) Boiling generates CO2 H2S. condenses above water table, atmospheric O2 vadose (unsaturated) zone oxidation H2S sulfuric acid, steam-heated acid-sulfate 2–3. In environment, horizons table <100°C. Silica derived within will precipitate amorphous silica condensate follows hydraulic gradient, causing opal replacement aquifer. (4) By contrast, below carbonic acid (H2CO3), leading 4–5. marginal carapace condensate, temperatures up 150°–170°C, acts diluent ascending parental NaCl liquid. intermediate alteration clays, kaolinite, Fe-Mn carbonates; can be present several hundreds meters, potentially mistaken having been caused supergene overprint. (5) The final setting supergene, posthydrothermal weathering mainly pyrite, locally creating <1 concentrations alunite, oxyhydroxides. genetic framework advanced (and intermediate) provides basis interpret mineralogy, combination textures morphology plus zonation, including overprint one style on another. used help focus exploration assessment ore epithermal, porphyry, volcanic-hosted massive sulfide.