3. The stocks are foci of more extensive hydrothermal systems, within which
gold deposits of several different kinds — high and low-sulphidation
epithermal veins, skarns and replacements in carbonate and non-carbonate
rocks may also be present
4. Gold-rich porphyry copper deposits are emplaced in volcano-plutonic arcs,
which are generated during and, in small part, immediately following active
subduction of oceanic lithosphere.
5. Generation of gold-rich porphyry copper deposits is believed to be
independent of the composition and thickness of underlying crust
6. Gold-rich porphyry copper deposits, like gold-poor porphyry systems, are
predominantly of Tertiary age
7. Stocks in gold-rich porphyry deposits belong exclusively to I-type and
magnetite-series suites
8. However, if gold is incorporated principally in magmatic sulphides rather
than in magnetite, highly oxidized magmas should favor generation of gold-
rich deposits
9. Five principal alteration types are developed in and around gold-rich
porphyry copper deposits
10. Potassium-silicate (K-silicate) alteration is typically characterized by the
presence of replacement and vein-filling biotite
11. Calcic systems also commonly contain epidote, as an integral component
of K-silicate as well as propylitic assemblages.
12. Hydrothermal magnetite, attaining 5-10 vol.% in some deposits, is
typically associated with synchronous or later copper-iron sulphides; it
appears as disseminated grains, patches and veinlets with and without quartz
13. In bornite-rich zones, bornite and gold are characteristically intergrown
14. Gold enrichment is abnormal in leached cappings over gold-rich
porphyry deposits
15. Epithermal gold deposits, especially the high-sulphidation, acid-sulphate
type, are commonplace in areas with an appropriate erosion level for
exposure and preservation of gold-rich porphyry deposits