Modeling the Formation of Advanced Argillic Lithocaps: Volcanic Vapor Condensation Above Porphyry Intrusions

Hypogene advanced argillic alteration, typically quartz-alunite with halos of kaolinite ± dickite and roots of pyrophyllite ± diaspore, forms in the epithermal environment from condensates of magmatic vapor that contain SO2 and HCl, which exsolved from an underlying intrusive source. The most aggressive, nearly isochemical leaching of the host rock by the most acidic condensate, commonly pH ~1, leaves residual silica that recrystallizes to quartz within the flow channel at a high condensate/rock ratio, forming the core of alteration. The alteration characteristically flares upward along feeder structures, and if a permeable lithologic unit is intersected, the alteration zones mushroom to form a subhorizontal blanket due to lateral flow. Where subsequently mineralized, the residual quartz, commonly with a vuggy texture that reflects the texture of the original lithology, has higher Au (and Cu) grades than the quartz-alunite halo. Tonnage in these high sulfidation systems may develop within the subhorizontal lithocap, although the highest grades are typically confined to the structurally controlled feeders. We modeled a typical volcanic vapor condensate, starting with the measured composition of ≤877°C fumaroles from Satsuma Iwojima rhyolite dome, Japan, as it cooled and reacted with a host rhyolite; the predicted hydrothermal mineralogy reproduces the alteration pattern observed in lithocaps that host high sulfidation deposits. The modeling confirms that aluminum-rich minerals (pyrophyllite, diaspore, locally andalusite) are stable at higher temperature at depth, whereas at lower temperature and shallower depth, Na and K alunite become stable. At the lowest temperature ( 10:1), and where SO2 >>H2S in the original volcanic vapor; this SO2-dominant composition is typical of andesitic to rhyolitic volcanoes. The reason for this mineral transition, and the upward flare (widening) of the alteration zone along structures, is related to the dissociation and increased reactivity of H2SO4 and HCl as the temperature decreases. Below ~200°C, only quartz, pyrite, native S, and anhydrite are stable, hence the formation of the dominant quartz from the silica residue. A further check on our modeling is the observation that the calculated composition of the condensate after reaction with fresh Satsuma Iwojima rhyolite and alteration minerals, and cooling to 100°C, is similar (within a factor of two) to that of acidic springs, with pH ~1, that discharge around Satsuma Iwojima and other active volcanoes. The most extensive lithocap alteration, residual quartz and/or quartz-alunite, is commonly offset from the surface projection of the causative intrusion. This observation can be explained by a combination of two factors—hydrology and temperature. Due to hydraulic gradients at shallow depths in a volcanic edifice, the acidic condensate tends to flow along permeable lithologic units away from the locus of the high-temperature vapor plume, which rises directly over the intrusion. Where lateral flow occurs, the most intense leaching and widespread advanced argillic alteration, which develops largely at temperatures of <200° to 250°C based on our modeling, will form away from the near-surface projection of the intrusion, which is the area of highest surface temperature due to ascent of the vapor plume. This potential for lithocap alteration to form on the shoulder of the underlying causative intrusion must be assessed when exploring for high sulfidation ore; the highest ore grades are commonly related to structures that are proximal to the parent intrusion, but they are unlikely to lie directly over the intrusion. In addition, identifying the most likely location of the source intrusion will help the explorer to efficiently assess any potential for deeper porphyry-style mineralization.

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