FORMATION OF ADVANCED ARGILLIC LITHOCAPS OVER PORPHYRY SYSTEMS, AND IMPLICATIONS FOR EXPLORATION

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, all of which exsolved from an underlying intrusive source. The most aggressive, nearly isochemical leaching of host rock by acidic condensate, commonly pH~1, leaves residual silica that recrystallizes to quartz close to the flow channel, forming the core of alteration. The alteration flares upward along feeder structures, and if a lithologic unit is intersected and condensate flows laterally, a subhorizontal blanket of alteration is formed, termed a lithocap; the residual quartz can host subsequently formed Au-Cu ore with high-sulfidation state sulfides.

Modelling of the condensation of typical volcanic vapor and its cooling and reaction with rhyolite reproduces the alteration patterns observed. Pyrophyllite and diaspore are stable at higher temperatures at depth; at lower temperatures and more shallow conditions, Na- and K-alunite become stable, eventually followed by residual quartz at higher condensate:rock ratios (>10:1). The reason for this transition, and the upward flare of the alteration zone along structures, is caused by the increased dissociation of HCl and H₂SO₄ as the temperature decreases; only quartz, pyrite, anhydrite and native S are stable below ~200^o C.

The most extensive lithocap alteration, residual quartz and/or quartz-alunite, is commonly observed to be offset from the surface projection of the causative intrusion. This is due to a combination of two factors. Lateral flow of acidic condensate away from the high-temperature vapor plume that forms directly over the intrusion is caused by hydraulic gradients at shallow depths around a volcanic edifice. Thus, the most intense and abundant advanced argillic alteration, which forms at lower temperatures, tends to occur away from the near-surface projection of the intrusion. The potential for lithocap alteration to be offset from the intrusion must be determined when assessing the likely location of the causative intrusion, which may potentially be related to porphyry-style mineralization.