Paper presented at the Terry Leach Symposium, Sydney, 17 October 2008
Australian Institute of Geoscientists Bulletin 48, pages 25-43.
Influence of magmatic arc geothermal systems on porphyry-epithermal Au-Cu-
Ag exploration models
Greg Corbett
PO Box 282, Willoughby, 2068, Australia
greg@corbettgeology.com
Work that Terry Leach began in the 1970s on the Philippine magmatic arc geothermal systems provides valuable insights into the complex overprinting patterns of alteration and mineralisation recognised in porphyry Cu-Au systems, and the progression to intrusion-related high and low sulphidation epithermal Au-Ag mineralisation. Terry’s use of magmatic arc geothermal analogies is most pertinent as much of the existing geological literature on epithermal Au-Ag has been derived from studies of rift environments which are analogous to only a small portion of epithermal low
sulphidation epithermal Au deposits. Many significant porphyry Cu-Au deposits occur as spine-like intrusion apophyses capping deeper magmatic sources for polyphasal emplacement of intrusions carrying metals and volatiles to higher crustal levels. Porphyry deposits can display distinct
stages of development of hydrothermal alteration, veins and mineralisation. Initial intrusion emplacement and hornfels formation is followed by prograde alteration as: potassic (Kfeldspar, biotite, magnetite), grading outwards to inner propylitic (actinolite, epidote), and outer propylitic (chlorite, zeolites). Early barren A veins (ptygmatic high temperature quartz) form while the intrusion is cooling, and are overprinted by mineralised M veins (quartz, magnetite with pyrite-bornite) which are associated with potassic alteration.Later B veins (open comb quartz with later pyrite- chalcopyrite) display stronger associations with retrograde phyllic (silica, sericite, pyrite) alteration. Sheeted quartz veins develop in dilatant structural sites and favour wall rock porphyry formation, while laminated veins occur in settings of reactivated dilational structures, and stockwork quartz veins may result from polyphasal porphyry emplacement with potential to host elevated Cu-Au grades. Advanced argillic grading to marginal argillic alteration, are interpreted to form by several processes and display varying relationships to porphyry and epithermal Cu- Au-Ag mineralisation, and also contribute towards the development of lithocaps, interpreted by some workers as vectors towards buried porphyry mineralisation. Early ascending vapour-dominated magmatic fluids react with wall rocks to form barren advanced argillic alteration. This differs from alteration associated with high sulphidation epithermal Au deposits, which may be capped by steam heated alteration zones. Collapsing cool low pH waters react with wall rocks to higher temperature and more acidic collapsing waters result in the development of retrograde phyllic alteration, which may also contain advanced argillic portions, and grade to marginal argillic alteration.
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