Advanced Argillic Alteration
Advanced argillic alteration is believed to be widespread in the upper,
commonly volcanic-hosted parts of gold-rich porphyry systems. This
alteration is partly preserved in eight of the deposits considered (Table I).
Chalcedonic quartz, alunite-natroalunite, pyrophyllite, diaspore, and kaolinite
± dickite are abundant component minerals. Barite and native sulphur appear
also as late-stage, open-space fillings. Pyrite-rich, high-sulphidation sulphide
assemblages containing abundant enargite and luzonite + covellite may be
present, especially in structurally controlled replacement bodies of
chalcedonic quartz ± alunite (e.g., Guinaoang: Sillitoe and Angeles, 1985).
Pyrophyllite-clay zones are normally metal-poor and typified by only a few
percent disseminated pyrite (e.g., Island Copper: Perello et al, 1989).
Advanced argillic zones are invariably the last to be active during the
hydrothermal decline of porphyry systems, although the timing of their
initiation remains obscure (Fig. 3). Advanced argillic assemblages clearly
overprint intermediate argillic and K-silicate alteration, and at Marte (Vila et
al., 1991) and Guinaoang (Sillitoe and Angeles, 1985), structurally
localized, advanced argillic alteration extends for tens to hundreds of metres
beneath the subhorizontal, roughly planar bases of advanced argillic zones
(Fig. 2). At Guinaoang, hypogene chalco-cite and covellite, rather than
copper sulphosaits, are present in these advanced argillic roots (Sillitoe and
Angeles, 1985). The interface between advanced argillic alteration and
underlying alteration types may be designated as the transition between
epithermal and subvolcanic porphyry environments.
HYDROTHERMAL BRECCIAS
Hydrothermal breccias are commonly associated with gold-rich porphyry
deposits. Where generated early and of magmatic-hydrothermal
(orthomagmatic) origin, breccias show K-silicate alteration and may carry
higher copper and gold contents than surrounding stockwork and
disseminated zones. Examples are Panguna (Clark, 1990) and Endeavour 27
at Goonumbla (Jones, 1985). Inter-mineral to late-mineral breccias (Fig. 2)
may also be well mineralized — as at Lepanto Far Southeast (Garcia, 1991)
and Galore Creek (writer's unpublished report, 1991), but they are
commonly of sub-ore grade or barren. In contrast, end-stage diatreme
breccias, associated plug domes (Fig. 2), and volumetrically minor pebble
dykes are generally barren (Sillitoe, 1985); however, at Lepanto, there is a
major enargite-gold orebody immediately beneath a flared diatreme contact
(Sillitoe and Gappe, 1984; Concepcion and Cinco, 1989; Garcia, 1991).