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

Chemical and short‐wave infrared characteristics of white mica associated with the Gajah Tidur porphyry copper system at the deep Grasberg Cu—Au—(Mo) deposit, Indonesia

“…As mentioned earlier, the MQP assemblage was formed at lower temperature than the MAC assemblage. Hedenquist and Arribas (2022) indicate that cooling of a muscovite‐stable fluid alone may result in pyrophyllite growth. On the other hand, pyrophyllite may also be the result of increasingly acid alteration such as present at Butte (Reed and Palandri, 2006; Reed et al ., 2013).…”

“…Sericitic alteration commonly overprints earlier biotite‐K‐feldspar (potassic) alteration in a porphyry system (Sillitoe, 2010), and mainly comprises white mica that is deposited by cooling hydrothermal fluids (Hemley and Jones, 1964; Harris and Golding, 2002; Reed et al ., 2013) resulting in a hydrolytic reaction at temperatures of 300–500°C (Seedorff et al ., 2005; Halley et al ., 2015). As “sericite” is a simplified name applied in a field context to fine‐grained‐light‐colored sheet silicates (Alva‐Jimenez et al ., 2020; Hedenquist and Arribas, 2022), we refer to all high temperature (≥300°C) hydrothermal light‐colored sheet silicates in this study as white micas.…”

Linking top and subsoil types, alteration and degassing processes at Rotokawa geothermal field, New Zealand

“…Many of the depressions in the western part of the DOC thermal area (e.g., at site 9) and southwest of domain 1 (e.g., sites 3 and 4) contain abundant mud pots that are dominated by fine-grained sediments suspended in the water columns accumulating on their floors. These mud-rich waters are the products of rock dissolution by hot, acidic fluids (Delmelle and Bernard, 1994;Hedenquist and Taran, 2013;Hedenquist et al, 2017;Hedenquist et al, 2021), and their finely laminated sediments are known to contain amorphous silica along with sulphates such as gypsum, barite, anatase, native sulphur, and pyrite (Rodgers et al, 2002;Rodgers et al, 2004).…”

The Hydrothermal Alteration of the Cordón de Inacaliri Volcanic Complex in the Framework of the Hidden Geothermal Systems within the Pabelloncito Graben (Northern Chile)

“…This alteration is defined by the assemblage (iv) kaolinite + hematite + halloysite + smectite + I/S (~85%Sm) + illite [4,[16][17][18] and is documented in the CIWS along the ENE transversal lineament and as traces in the alunite + gypsum domain (domain 2), while developing in the active zones of the CIES on steaming grounds associated with quartz deposits (domain 5) or kaolinite + alunite + opal-CT + anatase (domain 1).…”

“…A steam-heated blanket is formed from reactive H 2 S-bearing fluids in the vadose zone above the groundwater table, and it can occur in the framework of high-(HS), intermediate-(IS) or low-sulfidation (LS) epithermal deposits [15][16][17] and/or geothermal fields [18,19]. This blanket is typically friable and consists primarily of fine-grained alunite, kaolinite and cristobalite, with different mineral associations mostly controlled by temperature (60-150 • C), pH and redox conditions [3][4][5], while the groundwater table, limiting the subhorizontal bases of the steam-heated zones, typically produces fine-grained porous to massive silicic rocks.…”