DIVERSITY OF INTENSE ACID (ADVANCED ARGILLIC) ALTERATION

Advanced argillic alteration (AAA; leaching of bases by acidic fluids, local Al mobility) occurs in many hydrothermal systems and exhibits considerable variation in mineralogy, geometry, and associated elements. Most study has emphasized AAA in high-sulfidation epithermal/porphyry systems where S-rich magmatic gases play a key role, and in near-surface settings related to oxidation of H₂S in gases, or pre-existing sulfides to form acid groundwaters. Although these types are most common, our work on Fe-oxide(-Cu-Au) [=IOCG] systems, comparative study of western US deposits, and a literature review show that AAA exhibits widely varying metal ratios, sulfur contents, and styles – features that reflect contrasting origins and have exploration and environmental significance.

AAA is typically silica-saturated and ranges from low-T (25-250ºC) kaolinite-stable to high-T (400-600ºC) andalusite-stable assemblages; these features and accessory phases vary with setting. AAA varies markedly in oxidation state, sulfidation state and sulfide contents ranging from (1) high-sulfidation, S-rich (5-20% pyrite) in porphyry and epithermal systems, to (2) lower sulfidation S-poor (0-2% pyrite, common hypogene Fe oxides) in several settings including IOCG systems, to (3) to sulfate-bearing (typ. alunite) sulfide-poor, oxidized assemblages in near surface systems. Bulk metal contents and ratios vary widely (e.g., Ag:Au in high sulfidation AAA varies from <1 to >100; high to absent As, Hg); in some cases metals are leached or absent (e.g., IOCGs, some pyrophyllite deposits).These characteristics require different fluids and processes – including but not limited to the familiar SO₂-driven (porphyry, epithermal), steam-heated (geothermal), and weathering-related mechanisms.

As is well established, SO₂-rich hydrothermal fluids of magmatic derivation are most common; yet even they exhibit large differences in element enrichments requiring that other factors be important including differences in magmatic compositions and vapor vs. brine transport. Low sulfur systems require other acid sources, in IOCG and perhaps other settings this is likely HCl and metal chlorides, factors that in turn will govern distinctive element enrichments. Supported by NSF EAR08-38157 and USGS MRERP 08HQGR0060.