Of particular use, however, are gold geochemistry and magnetometry.
Appreciable depletion of gold has not been recorded from leached cappings
above gold-rich porphyry deposits subjected to temperate, arid or tropical
climatic conditions; indeed enrichment took place at Bingham (Boutwell,
1905) and Ok Tedi (Danti et al, 1988). Consequently, gold values in bedrock,
talus fines, and soils closely reflect hypo-gene gold contents of underlying
porphyry deposits (e.g., Learned and Boisson, 1973; Lowder and Dow, 1978;
Vila et a/., 1991). The -80 mesh (<177 urn) fraction of stream-sediment
samples is also an effective medium for detection of gold-rich porphyry
deposits, given the fine-grained nature of the native metal. Panned-
concentrate (heavy-mineral) geochemistry is also often applicable, especially
where placer gold is associated with gold-rich porphyry copper deposits
(e.g., Bingham, Panguna, Ok Tedi).
The high magnetite contents of many gold-rich porphyry deposits enable
detection by ground and in some cases, as at Island Copper (Cargill et al.,
1976), even airborne magnetic surveys (Fig. 6). Nevertheless, magnetic
susceptibilities can be reduced dramatically by both hypogene (intermediate
argillic) and supergene martitization of magnetite.
CONCLUDING REMARKS
Gold-rich porphyry copper deposits are not a discrete ore deposit type;
instead they represent the gold-rich end of the porphyry copper spectrum
(Fig. 5) (e.g., Cox and Singer, 1988). However, besides the obvious
enrichment in gold and general (but far from universal) impoverishment in
molybdenum (Fig. 5), gold-rich porphyry copper deposits do not possess any
unique features that distinguish them from porphyry copper-molybdenum
deposits. Nevertheless, more than 80% (Table I) of gold-rich porphyry
copper deposits are appreciably richer in hydrothermal magnetite (±
hematite) than other porphyry copper deposits. As well, they tend to display
calc-silicate in addition to K-silicate alteration of centrally located ore zones,
as exemplified by the presence of amphibole, pyroxene and (or) garnet.
Furthermore, sericitic alteration overprinting or alongside copper-gold zones
appears to be less widespread than in gold-poor deposits